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Cold preservation of meat products

Meat itself is not a living organism but it is subject to endogenic enzymatic activity, or proteolysis, which causes muscle tissue to mature, become tender and develop a typicaltaste. This process is retarded by cold.

Due to its chemical composition which is rich in proteins, lipids and water, meat is aparticularly favourable substrate for the growth of microorganisms. The lipidic content alsomakes it very sensitive to oxidation.

Healthy animals, hygienically slaughtered after resting and fasting, provide a practicallyaseptic meat. However, following slaughter the evisceration and dressing operationsinevitably produce microbial contamination in depth and especially on the surface, throughcontact with equipment, tools, hands and clothes, despite all precautions.

Again, micro-organism growth is a temperature-dependent process. To avoid it, it isabsolutely essential to reduce the temperature of the meat, especially on the surface,immediately after dressing. Cooling must therefore be carried out in the slaughterhouseitself. This operation is known as primary chilling.

Meat loses weight through surface evaporation. This process depends on differences intemperature and relative humidity between the meat and the environment.

Slaughter operations and carcass dressing separate the parts of the animal which havedistinct histological properties and are intended for different uses. The carcass itself incorporates mainly muscles, bones, fat and connective tissue. The offal includes someedible organs, while some glands are used in pharmaceutical preparations. These differentparts must be subjected to varying cooling conditions according to their susceptibility tomicrobial growth, to temperature effects and to the risk of surface dehydration.

CHILLING

To prevent or even to reduce the deterioration process, particularly microorganismdevelopment, chilling has to be carried out quickly after carcass dousing at the end of theslaughter process and the chilled state has to be maintained until the meat is processed for consumption.

Chilling can be defined as the fundamental operation in applying cold to meat to reduce itstemperature quickly. This is done in a cold chamber with intensive air draught or movement.

Rapid cooling of the meat surface not only slows and nearly stops the development of surface micro-organisms but also reduces weight loss and discoloration of the surfaceowing to haemoglobin oxidation. Different systems of primary chilling are in use (includingimmersion in iced water, especially for poultry) but air chilling is the most common.

The cold chambers where chilling takes place must have a low air temperature, a high air speed, a high relative humidity and a high refrigerating capacity.


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Air temperature must be in the region of 0C, with no decrease below -1C, which couldfreeze the meat surface and impair its appearance.

Air speed can range from 0.25 to 3.0 m/s. However, for economical reasons the mostcommon speeds in use are from 0.75 to 1.5 m/s in the empty section of the cold chamber.

Air speed over the carcasses will be much higher because of the reduction in air circulation.Increased air speed reduces the cooling period but it has a limit as there is a thresholdabove which fan-power consumption increases more than the chilling rate, resulting in anincrease in operational costs. Also, the higher the air speed the greater the weight loss.

Relative humidity during the chilling operation should be kept fairly high to preventexcessive weight loss. The recommended rate is between 90 and 95 percent, though this isthe most difficult factor to control.

Primary chilling is completed when the warmest point of the carcass has reached atemperature of about 7C (3C for edible offal). With current technology these temperaturescan be arrived at in 16 24 hours in small carcasses and in less than 48 hours in largecarcasses (centre of the hind leg). Average and surface temperatures are obviously muchlower, reaching 0C on the surface within four hours; this is important to slow microbialproliferation.

Quick chilling has its problems, cold shortening being the most common. Cold shorteningcan often be seen in beef and mutton, when the meat, still in its pre-rigor phase, reachestemperatures of 10C or lower. These conditions cause irreversible contractions of themuscle tissue which toughen the meat even after prolonged ripening.

Quick primary chilling also signifies an increase in investment and higher operational costs.

The chilling period can be reduced by lowering the air temperature (surface freezing risks)or increasing air speed (higher operational costs) or both. Occasionally cold chambers arerefrigerated in advance to reach lower temperatures than those in operation (-5C/-6C for beef; -10C/-12C for pork), taking advantage of thermal inertia to offset the effect of warmmeat loads.

Quick primary chilling can be performed in small chambers or in cooling tunnels. In coldchambers it is carried out in two or three phases. During the first phase the air temperatureis maintained at about 0C, carefully controlling the risk of superficial freezing while air movement is maintained at a high level. For large carcasses, after 10 12 hours the air circulation inside the store is reduced, maintaining temperature and humidity conditions; thissecond phase lasts another six to 10 hours. After this period the meat is transferred to coldstorage chambers where the carcass temperature is stabilized, concluding the third phase.

Small cold chambers used for chilling must be designed so their capacity can be filled in twohours at the slaughterhouse's normal work rate. The number of chambers should besufficient for a peak working day. Particular care should be taken that warm humidcarcasses are placed behind those already chilled or in the process of being chilled so thatthe air, which is still cold, reaches them and there is no risk of superficial condensation.


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Cooling tunnels used for chilling meat are usually of the continuous type. Here again meatis subjected to a two-phase process, with conditions similar to the cold chamber. However the temperature can be as low as -5C for a short time. Beef carcasses can reach anaverage temperature of about 15C in a four-hour period, while pork and mutton reach thesame temperature in two to two and a half hours. Surface temperature decreases to 4 5C. During the second phase, conditions are less exacting, and an average temperature of about 4C is stabilized after 15 16 hours in a secondary refrigerating chamber. This methodis used in high-capacity slaughterhouses particularly for pig carcasses; for beef and muttonslower cooling is recommended because of the dangers of cold shortening.

CHILLED STORAGE

Stored chilled meat is mainly intended to serve as buffer stock between production andshipment and/or consumption. During storage, ageing (ripening) of the meat also occurs,progressively increasing tenderness and developing taste through the proteolytic activity of meat enzymes. Ageing depends on temperature and can be accelerated by increasing it,but for hygienic reasons it is recommended that 4C be used with a relative humidity of 85 95 percent. In these conditions ageing takes place in a few hours for poultry, two to four days for pork, four days for mutton and two weeks for beef. It can thus be considered ascomplementary treatment only for the last two products.

When chilled meat is stored for long periods a lower temperature without the risk of freezingshould be used; normally 0C is a reasonable choice, though (as shown in Table 1)conditions differ according to the type of meat.

A temperature of about 4C is used in butcher shops (for final ageing, due to the difficulty of maintaining lower temperatures as the cold store rooms are small). Relative humidity isbetween 80 and 90 percent, which is a compromise between weight loss and microbial

development; 80 percent is normally used for carcasses and quarters, and 85 90 percentfor small meat cuts.

The preservation of edible offal requires different conditions: -1C rather than 0C and arelative humidity close to saturation to avoid surface blemishes. Organs intended for therapeutic purposes, such as thyroid, pancreas, ovaries, pituitary and so on, must befrozen immediately to preserve their active principles.

Table 1 gives the maximum storage time in which the products can be kept safe and keeptheir commercial quality during the subsequent marketing period, even if this is short and infavourable climatic conditions.

However, there is wastage and some loss of quality and nutritive value when a meatcarcass is stored for the whole period indicated in Table 1. It is therefore recommended thatthe storage time should not exceed by much the ripening period required for the differenttypes of meat.

Air circulation inside industrial chambers should be at a rate of 20 35 times per hour thevolume of the empty cold room. When the chambers are used to store offal it is advisable touse natural air circulation to maintain high humidity levels.


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Carcasses should be hung on rails in such a way that they are aligned in the direction of air circulation, avoiding contact with each other (see Figures 1 and 2).

Whenever a new product at a temperature different from that of the store is placed in storethe product should be distributed around the room rather than concentrated in one place.

INCOMPATIBILITIES

In many countries a deficient cold chain and insufficient cold storage capacity sometimesmake it necessary to store different products in the same room. These products may beincompatible because they require different storage temperatures or they present some riskof tainting, through the transfer of aromas from one to another. The first type of risk is not aproblem with animal products as they keep reasonably well under similar temperatureconditions (obviously reference is made here only to chilled storage). Nevertheless thelowest recommended temperature without risk of superficial freezing should always beused.

Tainting is likely when meats or other animal products are stored with odorous fruits likeoranges or apples, and the risk is more severe when there is mixed storage with potatoes.However, it is unusual to store meats with vegetables, so precautions are mainly neededwhen storing mixed animal products. There is some danger of cross action between beef and bacon; cheese will taint beef, mutton and pork.

Tainting must be guarded against not only during mixed storage but also when using a coldchamber which has previously stored produce with a strong tainting potential. Chambersmust be thoroughly cleaned before any other product is stored.

COMPLEMENTARY TREATMENT

Like the ageing of meat during cold storage, such as when butchers keep their stock at 4Cand 85 95 percent relative humidity, other complementary treatments are also used tolengthen the storage period, maintaining quality and reducing the risk of microbial spoilage.

A modified atmosphere is one of the treatments used nowadays for animal products, thoughnot to such an extent as for fruit and vegetables. This technology employs a gas-composedatmosphere which is different from the normal (i.e. 21 percent O 2, 79 percent N 2 and minor contents of other gases).

A more common complementary treatment for meat storage is the vacuum packaging of

boneless meat cuts. Special extremely airtight (oxygen-tight) synthetic films have beendeveloped which can be heatsealed after removing the air around the packed meat cut,thus keeping it practically out of contact with the surrounding atmosphere. Providedhygienic slaughter and cutting methods are used, the shelf-life of meat packed in this wayand stored under 0/-1C can be remarkably extended (up to eight weeks for beef, four weeks for lamb and two to three weeks for pork), which is important for the export of boneless chilled meat from meatproducing countries. This type of packaging is widely usedfor shipments of dried beef and mutton.


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In special cases radiation is used as a complementary treatment to extend the shelf-life of chilled meat carcasses. However, this treatment is subject to national food legislation and isnot allowed in many countries.

UV light (200 320 nm) is also used to reduce surface microbial contamination on meat andmeat products. As the cuts have irregular shapes it is rather difficult to achieve the sameradiation intensity, so it is normal procedure to irradiate the most contaminated zones.Radiation intensity produced by a 30W UV lamp is enough for every 10 12 m 2 of floor space in a slaughterhouse or cold chamber.


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FIGURE 1 Height of suspension and spacing for half carcasses on hooks


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FIGURE 2 Height of suspension and spacing for quarters

Ionizing radiation is a promising complementary treatment for chilled meat preservation.Low doses of radiation are sufficient to reduce microbial contamination and the bestprospects are for packaged meats which cannot be recontaminated. Ionizing radiation canalso be employed to destroy trichinae ( Trichinella spiralis ) in pork meats.


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FREEZING

Freezing is usually limited to meat to be used as buffer stock, frequently intended for exportor for storage with a view to later processing.

When the preservation period is longer than that acceptable for chilled meat, freezing mustbe used to minimize any physical, biochemical and microbiological changes affecting qualityin storage. During freezing most of the water content of the meat, about 80 percent,solidifies into pure ice crystals, accompanied by a separation of dissolved solids.

A product can be considered frozen when its centre has a temperature of -12C or less. Toreach this temperature the product passes through the temperature range of maximumcrystallization (from -1 to -5C). The speed of freezing is a very important factor as frozenmeat quality depends mainly on the size of the ice crystal formed: the lower the speed of freezing the larger the size of the crystals.

Slow freezing facilitates the separation of solution and the migration of water out of themuscle cells which is subsequently frozen, forming rather large crystals. Quick freezingconversely produces many small ice crystals, mainly formed within the muscle cells, andreduces water migration and separation of solution. It is obvious that the latter technologywill preserve the meat closer to its original quality and, particularly during thawing, moistureloss will generally be lower.

The International Institute of Refrigeration (IIR) expresses the freezing speed as the velocitywith which a temperature front moves through the body of the product (cm/h). Good resultsare attained when the speed is from 2 to 5 cm/h. Slow freezing is considered to be below 1cm/h and quick freezing above 5 cm/h.

Meat can be treated before freezing, generally being refrigerated to a chilled condition.Cutting into quarters is usual, particularly for large animals, and the fat is removed fromsome parts because though it prevents surface desiccation it reduces the heat transfer rate,and is susceptible to damage during frozen storage.

The relationship between thickness and freezing speed favours cutting and deboning beforefreezing, either as lean meat packaged in cardboard boxes or cut into individual portions.This has many advantages:

the mass to be frozen is reduced by 30 percent or more; storage density is increased by 100 percent; handling operations are easier; deboning after thawing, which causes hygienic and exudative problems, is avoided.

Freezing is performed in tunnels or in chambers with intense air circulation called blastchambers. Air temperatures should be in the range of -30 to -35C; sometimes -40C isused. Air is circulated at high speed, from 2 to 4 m/s and up to 6 m/s. An air circulationcoefficient of 150 300 is used inside freezing chambers. Relative humidity is maintained at95 percent or above.


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In these conditions half beef carcasses or quarters are frozen in about 16 20 hours, cut-upmeat in cardboard boxes measuring 543416 cm in about four hours and smallprepackaged cuts in about one hour.

Small boxes and cuts, particularly of offal, are sometimes frozen in surface contact freezers(plate freezers): the product is pressed between two metallic plates cooled by directexpansion refrigerant. For items 3 5 cm thick, freezing time is as low as two to three hours.

After freezing carcasses and quarters must be protected with plastic film, usually under cloth or jute fabric. Meat cuts are covered with plastic film, or vacuum-packed in plasticbags; they are placed inside cardboard boxes and usually frozen in these.

When meat cuts are prepackaged without vacuum, air pockets must be avoided. A 2-cmspace should be left in the upper part of the box to allow for expansion. Superficial fatshould be eliminated before freezing to reduce the development of rancidity during storage.

TABLE 2. Practical storage life of meat and meat products

ProductsPractical storage life in months

-18 C -25 C -30 CBeef carcass 12 18 24Roasts, steaks, packaged 12 18 24Ground meat, packaged, (unsalted) 10 >12 >12Veal carcass 9 12 24Roasts, chops 9 10 12 12Lamb carcass 9 12 24Roasts, chops 10 12 24Pork carcass 6 12 15

Roasts, chops 6 12 15Ground sausage 6 10Bacon (green, unsmoked) 2 4 6 12Lard 9 12 12Poultry, chicken and turkeys, eviscerated,well packaged 12 24 24Fried chicken 6 9 12Offal, edible 4

From: Recommendations for the processing and handling of frozen foods, International Institute of Refrigeration, Paris, 1972.

CONDITIONS OF FROZEN STORAGEMeats properly frozen are transferred from the freezer to storage chambers wheretemperature, relative humidity and air circulation should be adequate and can be tightlycontrolled. In particular fluctuations in temperature must be kept to a very narrow timeinterval.


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As there is a certain degree of quality deterioration, even at very low temperatures, storagelife is limited. The usual temperatures are in the range of -18 to -25C for periods of preservation of one year or more. However, each type of meat requires specific conditions.Table 2 gives some approximate data regarding these. The higher the relative humidity thebetter: a range of 95 98 percent prevents meat dehydration.

For frozen meats and other animal products storage incompatibility is low. The temperaturelevel needed in the chamber is similar for all of them, and tainting is negligible owing to thelow temperature and to the fact that most of the products are in adequately protectivepackages.

The main problem with frozen storage is deterioration in organoleptic quality. There may bechanges in meat texture, fat can become granular and crumble, and there can also be somediscoloration of the meat. Fat modification induced by air oxygen produces rancidity andacidity, and a disagreeable taste. Microbial enzymes also remain active, especially thosethat attack the fat.

As in chilled storage, there are also weight losses through evaporation. This can be seen asfreezer burn, i.e. superficial desiccated areas which can occur even in packaged meatswhen the packaging film is loose and temperature fluctuates inside the chamber. Weightloss, which can be between 1 and 4 percent in unpacked meat, favours organolepticdeterioration. The surface of the meat grows dry and porous, encouraging rancidity andtransfer of aromas. In addition, the area of surface sublimation of frozen meat is very large:12 m 2/t for beef quarters, 11 m 2/t for pork and 20 m 2/t for mutton.

PRACTICAL STORAGE LIFE (PSL)

PSL is the storage period from the time of freezing for as long as the product maintains its

organoleptic and nutritive characteristics and is suitable for human consumption or for further processing.

PSL relies on high quality raw material, good industrial practice, including hygiene, and theuse of a reasonably constant temperature. PSL is therefore clearly dependent on PPPfactors product, processing and packaging.

Processing refers mainly to preliminary treatment and the freezing operation. Slaughtering,dressing of carcasses, preliminary chilling, cutting and deboning, and prepackaging of smallcuts, must be conducted hygienically and by skilled labour.

In addition to personal hygiene, and cleaning and disinfection programmes inslaughterhouses, chilling facilities and cutting rooms, particular care should be taken whencutting and deboning and packaging, keeping contamination of the meat to a minimum.Carcasses should preferably be cut while hanging or on regularly cleaned surfaces, withtools frequently sterilized during operation and the meat stored in clean containers. Thepackaging material should be of good quality and clean.

Packaging is intended to preserve products from microbial contamination, from dehydrationand from environmental factors that affect quality and nutrition. The materials used, besides


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being specifically for food, must be chemically inert and prevent the transfer of foreignodours or flavours. They must be stable at low and high temperatures, elastic, tear-resistant, and proof against water vapour, oxygen and volatile substances. They must offer protection against light, particularly UV light. Moreover they should be adaptable to differentautomatic packaging systems, of an appropriate size and shape for easy storing anddistribution, and ready for opening.

Plastic films and papers and cardboard lined with plastic film are often used. For specialpackaging, different plastic films can be combined to take advantage of the main propertiesof each. Plastics of interest to the meat industry for cold storage are:

polyamide (PA) polyethylene (PE) polyester (polyterephthalic acid ester) (PET/PETP) polyvinylchloride (PVC) polyvinyliden chloride (PVDC).

There is a daily quality loss in frozen meat storage that is cumulative, i.e. the total qualityloss through freezing, storage, transport and distribution can be calculated by adding thelosses at each step of the process. The tolerance of a product to a fixed temperature andtime storage can be determined and expressed in figures. Figures or graphs representingpractical storage life in different conditions can be established through time tolerance andtolerance (TTT) studies. These are run at at least three temperatures, around thosenormally used for storing the product under test (-18, -25 and -30C for instance) andshow the quality relationship to timetemperature conditions.

As temperature fluctuations highly influence the final quality of the frozen product, itsrefrigeration history should be known. This together with the TTT characteristics of theproduct will allow the residual storage time to be calculated.

The rotation of stock throughout the cold chain should be organized according to the first in-first out (FIFO) rule: the first lots to be stored are the first to be unloaded.

THAWING

Thawing is another critical phase in the freezing process as it involves a change from icecrystals to melted water, which is reabsorbed, and microbial reactivation.

If heat is applied to the frozen product its surface becomes warm enough to transfer heat tothe inside and create conditions of temperature and humidity suitable for microbialdevelopment. Low temperature thawing, below 5C, reduces the risk of microbial growthand produces a slow thawing rate which guarantees efficient reabsorption of the meltedwater.

It is recommended that carcasses be thawed at 4 to 6C, in a hanging position and withoutany covering (plastic or jute is removed), inside a cold chamber with a reasonably low levelof air circulation - about 0.2 m/s. Relative humidity must be kept low at the beginning (70percent) to avoid frost forming on the meat surface, with an increase at the end of the


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thawing period up to 90 95 percent. In these conditions thawing of beef carcasses lastsabout four to five days and of smaller carcasses one to three days. It must take place ininstallations specifically designed for this purpose.

Offal is not particularly influenced by the manner of thawing, but it is advisable to follow thesame method.

Thawing is considered finished when the temperature of the meat is about 0 to -1C.

When frozen meats are further processed they may in some cases be used directly in thefrozen state. The consumer can start cooking small prepackaged cuts without prior thawing.

New quick thawing systems that satisfy hygienic requirements are now being used in themeat industry. Rapid thawing tunnels for carcasses, microwave ovens and tunnels andvacuum steam-heated autoclaves are some of the novelties. Thawed meats deterioratequickly and must be kept at about 0C and consumed as soon as possible.

Obviously a badly conducted freezing operation and/or frozen storage period (whichincludes transport and distribution) with irregular storage conditions will produce disorders inmeat which become immediately apparent after thawing. Exudation indicates histologicaldamage by ice crystals; other undesirable changes have already been mentioned.

Planning of cold storage

A cold store essentially consists of a number of refrigerated chambers which are able tochill, freeze and store any perishable product. Certain general substructural conditions mustbe fulfilled to construct a cold store successfully: site selection for easy access by road andtrain; terrain of good mechanical resistance and without problems of surface water; goodsupply of potable and industrial water and electricity; drainage facilities. As well, the localavailability of labour (technical personnel, skilled labour for maintenance and generallabourers) should be investigated.

Although a cold store is essentially an area where products are preserved, plus a site for the machinery room, it is obvious that complementary spaces are necessary: office,laboratory, public services, toilets and cloakroom, spare parts room and workshop, andpackaging material storage.

Some other services may be found annexed to a cold store, such as cutting and deboning,salting, meat products manufacture, packaging, and a sales office.

Before planning a cold store it is important to define operational and technicalspecifications. These are strictly dependent on products, stores, storage conditions,environment, energy and personnel. The publication Guide to a refrigerated store (IIR,1976) includes an exhaustive checklist worthy of use at this planning stage.

GENERAL ARRANGEMENTS


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The general features of a cold store operational programme (products, chilling and chilledstorage, freezing and frozen storage, cutting, deboning and packaging, stocks, dailymovements) include total capacity, number and size of rooms, refrigeration system, storageand handling equipment and access facilities.

The relative positioning of the different parts will condition the refrigeration system chosen.The site of the cold chambers should be decided once the sizes are known, but as ageneral rule they should be in the shade of direct sunlight.

The land area must be large enough for the store, its annexes and areas for traffic, parkingand possible future enlargement. A land area about six to ten times the area of the coveredsurface will suffice.

There is a general trend to construct single-storey cold stores, in spite of the relatively highsurface: volume ratio influencing heat losses. The single storey has many advantages:lighter construction; span and pillar height can be increased; building on lower resistancesoils is possible; internal mechanical transport is easier. Mechanical handling with forklifttrucks allows the building of stores of great height, reducing the costs of construction for agiven total volume.

The greater the height of the chambers the better, limited only by the mechanical means of stacking and by the mechanical resistance either of the packaging material or of theunpackaged merchandise. In fact chilled meat carcasses, which cannot be stacked, limit theheight of the chamber as they are hung from rails (see Figures 1 and 3). Chilled quartersand cuts can be mechanically handled for storage without stacking (see Figures 2 and 4).

The length and width of the chambers are determined by the total amount of merchandiseto be handled, how it is handled (hanging from rails, forklift trucks), the number of chambers

and the dimensions of basic handling elements.FIGURE 3 Storage of carcasses on hooks


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There is no advantage in building many chambers of a small size, particularly for meatproducts. Thermal and hygrometric requirements are not so strict as to justify a lot of rooms:the accuracy of the measuring instruments and the regulation of conditions inside thechamber always produce higher deviations than those of ideal storage conditions for different products. This is particularly true for frozen products.

Chilled meats of different species (beef, pork or mutton) can be stored in the same room asthey do not present temperature or tainting incompatibilities. Frozen meats have even lessproblems. Stores for refrigerated products are usually more divided and often of lower height than those for frozen goods.

A design that opts for fewer, larger chambers represents in the first place an economy inconstruction costs as many divisional walls and doors are eliminated. Refrigeration andcontrol equipment is simplified and reduced, affecting investment and running costs. Largechambers allow easier control of temperature and relative humidity and also better use of storage space. Only in very particular situations should the cold store be designed withmore than five or six cold chambers.

FIGURE 4 Stacking in racks

Store capacity is the total amount of produce to be stored. If the total volume of thechambers is filled, the quantity of produce by unit of volume will express storage density.

Several parameters must be defined within a cold store. The total volume is the spacecomprised within the floor, roof and walls of the building. The gross volume is the totalvolume in which produce can be stored, that is excluding other spaces not for storage. Thenet volume represents the space where produce is stacked, excluding those spacesoccupied by pillars, coolers, ducts, air circulation and traffic passages inside the chambers


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that are included in the gross volume. Storage density referred to as net volume isexpressed in kg/useful m 3, but is most commonly referred to as gross volume.

An index of how reasonably and economically the cold store has been designed is the grossvolume divided by the total volume. It must be in the range of 0.50 to 0.80.

Similarly gross volume is about 50 percent greater than net volume, and gross area (sameconcept as volume) is about 25 percent greater than net area.

The extent of occupation is the ratio between the actual quantity of produce in storage at agiven moment and that which can be stored. Equally the extent of utilization is the averageof the extent of occupation during a given period usually a year, but it can also be per month.

MANAGEMENT

The operation of cold rooms must take into account the storage requirements of theproduce, rules for loading, maintenance and hygiene, and the running and maintenance of the refrigeration equipment.

The loading plan will depend on the type of cold chamber, whether it is for preliminarychilling, for chilled storage or frozen storage.

Preliminary chilling, which is done in the slaughterhouse itself, depends on the slaughteringrate as this determines the amount of meat to be chilled hourly. To avoid keeping the storedoor continuously open, freshly killed warm carcasses are separated into lots andintroduced into the chilling chamber every half hour, for instance.

Carcass movement inside the store should be designed in such a way that warm and wetmeat faces the air from already chilled carcasses.

Overhead rails must be placed so carcasses are oriented in the sense of air circulation, andto prevent them from touching each other.

It is sometimes advisable to divide the total chilling capacity among a few chambers,computed on two hours' slaughtering, when the capacity is high enough (10 40 t/day).

Another option for high slaughtering capacity (over 40 t/day) is the continuous chillingsystem, where carcasses pass through a chilling tunnel transported by a mechanicalconveyor for two to four hours and are then put in a cold chamber to undergo final chilling.

Chilling facilities should be systematically emptied after chilling and before preparing for thefollowing day's production.

Already chilled carcasses are placed in a refrigerated store. The storage rooms should be atleast equal in capacity to the chilling rooms.


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If fresh meats are intended for quick distribution they should not leave the chilling facilitiesuntil the temperature of the warmest point is below 7C, meaning an average temperatureof about 2C.

The storage density of carcasses on hooks is dependent on the spacing of the rails, theheight of suspension and the size of the carcasses, and very little on the unit weight of thecarcasses. Figures 3 and 4 show details of carcass disposition and loading plans. Tables 3and 4 give some data on storage densities.

Meat is frozen in full carcasses, or in halves for small and medium-size animals and inquarters for large animals. The disposition of meat in a freezing chamber or tunnel is similar to that in the chilling operation. However, frozen meat is not stored hanging from rails butloose on the floor, on racked pallets or in boxes when it is cut up and/or deboned.

Figure 5 gives a diagram for loading on pallets for long-term frozen storage and the correctsystem for loading. Table 4 gives some useful data on stacking densities for frozen meat.

TABLE 3. Density of storage of hanging carcasses

BEEF. Weight: 300 to 400 kg

- in half-carcasses hanging from a high-level rail

Height of rail from ground 3.80 to 4.00 m

Point of hook 3.00 to 3.40 m

perpendicular to the track 450 to 600 kg/m

(3 half-carcasses per linear m, separation of tracks) 1.00 m (minimum)

parallel to the track (2 carcasses on the same track) 430 to 500 kg/m

separation of tracks 0.90 m (minimum)- in quarters, height of rail 2.60 to 3.00 m

Point of hook 1.90 m above ground

perpendicular to the track:

- 4 quarters rear 400 kg/m

- 4 quarters front 3.00

separation of tracks 1.00 m

parallel to the track mean of 200 to

250 kg

HORSE

parallel to the track 435 kg/m


VEAL. Weight: 45 to 80 kg

- hung on 4-toothed, with extension, or 3 veal by the tibia

2 to 3 veal per truck 135 to 240 kg/m


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separation of tracks 0.90 to 1.00 m(min)

- hung bar with hooks

1 to each 2 hooks 70 to 120 kg/m

number of hooks 3/m

MUTTON (or lamb). Weight: 15 to 30 kg

- on extension (4 mutton 15 to 20 kg in a circle of 0.70 m) 85 to 115 kg/m

- on hangers (3 to linear m) 45 to 90 kg/m

- in groups of 8 superimposed carcasses 290 to 400 kg/m

separation of rails 0.80 m

- on special aerial chassis with 10 hooks

(10 carcasses/linear m) 150 to 300 kg/m

separation between bars 1.40 to 2.00 m

bars with double hangers 0.50 m

bars in relation to the wall 0.50 m

bars grouped in lots for the despatch hall 0.50 m

PORK. Weight: 80 kg

- on a runner, with extension, with 4 hooks 400 to 600 kg/m

- 4 carcasses in a circle of 1.00 m 300 to 400 kg/m


- separately hung on a runner with gambrel-separator 250 to 400 kg/m

3 to 5 carcasses/m. Separation of tracks 0.90 to 1.00 m

- on hangers, carcass per hook 100 to 150 kg/mseparation 3/m

OFFAL

These are on wall hooks spaced at 125 mm or disposed in tubs of 30litres.

- hung on bars, wall hooks, superimposed, mounted on trucks on theground 250 to 300 kg/m2

From: FAO Agricultural Services Bulletin 19/2, Rome, FAO, 1984 .

TABLE 4. Density of storage of meat products

PRODUCT UNIT PACKAGING PALLETIZATION

STACKING STORAGEDENSITY

Nature Size (cm)

Mass Palletsize

Number

Pallets/stack

Stack

Nettma

Volume/

Nett mass

Effect Gross vol.


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Gross

(kg)

Nett (k g)

(cm) packages/

pallet

height

(m)

ss/stac

k (kg)

stack (m 3 )

ivevolum

e (kg/m

3 )

Accessibility

PALLETIZEDSTORAGE

Per Numb

er layer

of layers

Poor ( 10

rows)

Good (2

rows)

Meatfrozen bone

d

boxwoode

nfibrebo

ard

643619603215

27.5

2523

100120100120

48611

33

5.10

5.70

2400

4554

6.1717.087

390640

230380

125200

carcass

beef hindquarters onpalletwith racks on

corner posts

100120

172 15 23.56

2400 9.487 250 150 80

Meatfresh carcass beef hindquartershanging on point withracks on corner posts

100120270

8 2 5.401

280 7.087 180 110 60

NONPALLETIZED STORAGE

Meatfrozen

boned

boxwoode

nfibrebo

ard

643619603215

2.5 2523570790

460630

370510

carcass BEEF hind or forequarters with 10 ribs

320/350

260/280

210/230

forequarters with 5 ribs 280 220 180

PORK in half-carcasses 350/380280/300

230/250

pork loins 350/420280/340

230/270

MUTTON in whole carcasses 230/240180/190

150/160

POULTRYGAME

freshfroze

n

in baskets 150/200120/160

100/130

in boxes 250/350200/280

160/130

From: FAO Agricultural Services Bulletin 19/2. FAO, Rome, 1984.

Fresh chilled meat carcasses are not wrapped, but small meat cuts are usuallyprepackaged under vacuum which favours stacking as the small packets are stored andhandled inside rectangular boxes or cartons.


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Frozen carcasses and quarters are enclosed in polyethylene film and a cotton stockinette toprotect against soiling and contamination; in this way they can be stored in piles or palletboxes. Frozen small cuts are prepackaged like chilled cuts.

Meats packed in boxes and frozen meat on pallets are usually stacked. Stacking methodsand height depend on several factors: resistance of the package, handling techniques andthermal state.

The resistance of the package dictates the total weight the lowest can support. Carcassesof frozen meat can be stacked in bulk to heights of 8 10 m. When stored on pallets theheight of the pallet load for carcasses can reach 1.9 m (Figure 6). Pallets can be equippedwith corner supports of detachable metallic framework, placed around the load andtransferring the weight from the upper pallet to the lower.

Packaged and frozen meat is usually handled mechanically, combining forklift trucks withpallets. Besides the rapidity of operation (10 20 t/h per truck) and the great height of thestacks, any part of the stack is easily accessible, which reduces the need for lateralgangways.

With mechanical handling pallets are easily moved within the store. A layout plan for palletization should be drawn up to control loading and stocks and each cold chamber should have a control table (see record sheets in the annex) by which any pallet or stockedproduct can be identified.

AIR CIRCULATION AND CHANGES

Conditions for air circulation in completely filled chambers or tunnels have been given inChapter 2 for chilling, freezing and storage. However, a cold room takes some time to fill

(six to seven hours for primary chilling rooms) so for part of this time it is only partly filled;nevertheless air circulation must be correct.

To maintain the circulation of air in a partly filled room the stack alignment must beperpendicular to the direction of air movement and the stacks placed close to the cooler.Fans must be operating when the refrigeration system is running and it is advisable to stopthem only during the defrosting period. Two-speed fans should be used to adjust to air circulation needs in the room. Stacking must follow exactly the layout prescribed, respectingloading limits and allowing space between the stacks and walls, and below the pallets.

FIGURE 5 Store loading plan


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Correct system for alley loading is 1, 2, 3

The palletization layout plan must take account of distances between store elements. Theyare in the range of 5 10 cm between pallets, 15 20 cm along the walls and a stacking limitof 40 60 cm below the ceiling. The gangways for forklift truck circulation depend on the typeof truck, but are in the range of 2.15 to 3.0 m.


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Air circulation inside the store is expressed by the air speed (m/s) through an empty cross-section of the store and also by the chamber coefficient of air circulation, which is thenumber of times the air equivalent to the total internal volume of the empty chamber passesthrough the cooler in one hour. Both are obviously related, but the latter is more commonlyused for chambers than for tunnels as it gives a clearer idea of air movement.

FIGURE 6 Forklift truck moving carcasses

Air change refers to replacing the air inside the chamber with clean external air, thusavoiding an accumulation of undesirable odours or volatile components. Air changes neednot be considered in frozen storage as frozen produce does not give off odours but it mustbe taken into account for chilled products, particularly if they are mixed. It is especiallyimportant for meat products that the external air comes from a clean source, with nonoxious components, and it should be passed through a filter fine enough to retain at leastdust. Air should be changed as seldom as possible as any change disturbs the storageconditions and increases running costs: the external air has to be dried and cooled and thefrost deposit increased on the evaporator. The outside air should be cooled anddehumidified before entering the chamber, or it should at least go immediately over thecooler.

It is possible to calculate the volume of introduced air required to reduce contaminantconcentration to a safe level but in general it should be equal to five times the volume of theroom to reduce contaminants by 99 percent (although it has been claimed that if this volumeis less than eight to ten times the volume of the room the dilution of contaminants may beincomplete).

Air change can be achieved in a small room by opening the door, but this is a cumbersomemethod, alternating with brief periods of running the refrigeration machinery to restore


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storage temperature. For large rooms of over 1 000 m 3 it is necessary to employ fans toinduce change and drive the outside air over the cooler.

It follows that every time the cold store door is opened an air exchange is generated byinside and outside air density differences. Although this exchange is sometimes beneficial,when the door is open too long or the type of product stored does not need air purification itclearly becomes an extra running cost and has to be prevented.

UNLOADING COLD ROOMS

When products leave the cold room there is a risk of condensation of atmospheric humidityon the cold surfaces, making them wet and liable to microbial development. This candamage the packaging materials or spoil their appearance, which can sometimes beirreversible if carton or cardboard packaging is used.

Condensation occurs when the dew point of the ambient air is higher than the surfacetemperature of the product or its packaging. At this point it is necessary to take someprecautions, such as proceeding on a progressive rewarming pattern, or promoting quickevaporation of the moisture formed by introducing slightly warmed dry air, or covering theproduct with awning or, finally, using packaging materials that resist the adverse effect of the moisture.

The graph in Figure 7 shows how to calculate if there is any risk of condensation whenmoving a product from storage to a different ambient temperature.

For efficient management and stock control, any unloading operation must obviously bewritten on the control table for the chamber, including all relevant data (see loading andunloading record sheet in the annex).

HYGIENE AND DISINFECTION

Cold chambers intended for meat chilling and chilled storage must be kept in a strictlyhygienic condition as microbial invasion is a grave risk. The following operations areessential:

immediately eliminate all waste in a cold room; each time a room is emptied, or after rewarming rooms at low temperature, wash

floors and walls with detergent and hot water, rinse them with clean water, and spraywith a solution containing active chlorine (0.3 percent);

clean pallets and storage containers every four months; disinfect chilled storage rooms for 48 hours at least twice a year and frozen productrooms when they are emptied;

before storing animal products in rooms that have contained strongly odorous fruitsand vegetables, deodorize by washing, prolonged ventilation and finally sprayingwith a solution containing ammonium salts.


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Hygiene must be maintained after storage, during transport and distribution. Transport over long distances should be in refrigerated vehicles, which must be cleaned and disinfectedafter every day's duty.

FIGURE 7 Conditions for condensation on the surface of cold produce

From Recommended conditions for cold storage of perishable produce .International Institute of Refrigeration, Paris, 1967.

HANDLING METHODS AND EQUIPMENT

There are two very distinct systems of meat handling. The first is for carcasses or large cutsduring chilling and chilled storage. The meat is hung on overhead rails of appropriateheight, which can be pushed by hand or can be mechanically transported. The latter is more


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common in continuous systems and high-capacity stores. The second method uses forklifttrucks and is employed for frozen meats of different shapes, chilled or frozen packaged cutsin geometrical containers and even chilled carcasses of small animals or quarters of largeanimals. These are hung from specifically designed crates (see Figure 6) which can bestacked in the same way as pallets.

Internal transport must be rationally linked to that used for the reception of goods and for distribution.

When the chilling and chilled storage facilities are annexed to a slaughterhouse the type of rail and the height of chambers and doors are planned accordingly. The design of thechambers must take into account the supporting rails and beams, adapting the coolers for good air circulation.

The distribution of rails inside a chamber must allow for easy control of turnover with as littlehandling as possible. The number of rails and their location are dependent on the room'sstock rotation, but keeping in mind that the length of storage for chilled meat is usuallyrather short.

For pallets and similar stacking elements the layout of the chamber is based on the palletmodule, including the size of the pallet, tolerance of air circulation and ease of manoeuvre.Different lot sizes may require different spacing of gangways.

Pallets, which can be made of different materials, are becoming standardized, the mostusual dimensions being 0.801.001.20 m. The shorter and longer dimensions can beincreased by 5 and 15 cm respectively to set up the recommended pallet module.

Stacking width is influenced by the width of the gangway and the length of the pallets. The

width of the gangway depends on the forklift truck used and the depth of the palletsdepends on stock rotation the slower the rotation the deeper the pallets. Pallet stackingdepth is three to four pallets for a high rotation and seven to eight pallets for a low rotation.

FIGURE 8 Types of pallet used for forklift truck handling and stacking


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Several layers of boxes can be used on a pallet, the number being determined mainly bythe mechanical resistance of the packages and their shape for ease of piling. Five to sixlayers are usual and sometimes seven are possible. The number of pallets in a pile is alsodependent on the mechanical resistance of the packages and on the type and reach of theforklift truck used for stacking. A stacking height of two to four pallets is the most common,but for large stores with a low rotation up to five pallets would be suitable.


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To avoid any problem of box resistance, fixed racks made of suitably resistant material canbe installed in which pallet loads can be placed by the forklift truck (see Figure 9). The onlyinconvenience, besides higher installation costs, is that they occupy a fixed space. When itis necessary to save most of the space occupied by the gangways it can be worth installingmovable racks, though installation is rather expensive.

FIGURE 9 Rotating truck operation

There is a trend toward the total automatization of stacking systems, combined withcomputer-controlled stock direction and checking; obviously the increased investment costsmust be taken into account.

Forklift trucks can be powered electrically, by LPG (liquefied petroleum gas) or petrol. Eachhas advantages and disadvantages. Petrol-operated trucks emit dangerous fumes so theyare not recommended for meat cold stores. LPG-operated trucks produce moderate fumesand are noisy; their only advantage over electrically operated trucks is that they are faster.Electrically operated trucks need their batteries recharged every working period or alternating batteries when operated for long periods.


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Automatic transmission is essential so the trucks can be started and stopped accurately,and also for exact placing. Automatic transmission is incorporated in all modern machines.

Special trucks are used in cold stores where space saving has resulted in movementproblems. Rotating and retractable trucks (Figure 9) are an example, but their cost rangesfrom twice to four times that of a normal truck.

Sometimes conveyors are used for internal transport, as they are less expensive thantrucks. As with overhead rails, they are useful for continuous transport with no crossingtraffic.

DESIGN

The distribution of the cold store and its flow pattern are determined by their relation to therest of the operations and the sequence they have to follow. There are some essentialpoints to be considered.

A cold store basically incorporates a reception room, where fresh meat is received andinspected at a controlled room temperature between 8 and 12C, dispatch and holdingrooms at about 2 4 C, and one or several cold chambers for meat and offal at appropriatestorage temperature. It may incorporate a cutting and deboning room at a temperaturebetween 8 and 12C, a packaging room and a sales room, both with a low temperatureand dew point at about 5 to 7C. For freezing and frozen storage there are freezing tunnelsor rooms at temperatures ranging from -30 to -45C, and frozen storage chambers at atemperature to suit the intended storage period. General requirements are a machine room,offices and cloakroom.

The main objective in designing a cold store is to avoid unused space, so the location of

labour premises, offices, etc., must be considered when filling up gaps.

Cold chambers will face directly either the holding room or the cutting and packaging room.Corridors for traffic should be reduced to a minimum. The use of large anterooms kept at anintermediate temperature between ambient and storage is now obsolete, but these may beof interest in hot and humid climates to avoid condensation on the product.

The location of the machinery room is of paramount importance. It should be as close to thecold rooms as possible and especially to the cooling equipment. This is one of the designdifficulties to be resolved if future extension is planned. The room must be readilyaccessible from outside.

The width of the corridors will depend on the normal traffic. If it is heavy, corridors will bedesigned for two-way truck movement - one-way is possible only when two trucks do notcross during transport operations. Corridor width ranges from 2.00 m for one-way to 3.60 mfor two-way traffic. Width is also affected by the size of the forklift truck, i.e. the load it isable to transport, within the range of 1 to 3 tonnes. The general trend is to build trafficcorridors which are wide enough for two loaded trucks to pass, even when unit loadsoccupy part of them while waiting for handling. In normal operation areas a width of 4 5 mis recommended.


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In one-way corridors any right-angle turn that might prove difficult for truck movementshould have the width increased.

Figure 10 gives some basic measurements that concern the manoeuvrability of the usualtype of forklift truck.

Door dimensions must be in relation to transport, the type and size of forklift truck and thewidth of the traffic corridor, if this is not wide enough for a right-angle turn the door must besufficiently wide to allow an inclined entrance. Door width for mechanical handling rangesbetween 1.80 and 2.10 m. Carcasses laid on pallets need a 2.50 m door and up to 2.80 mwhen the corridor is not wide enough for a 90 degree turn.

FIGURE 10 Manoeuvrability of forklift trucks

Door height will be determined by the height of the load: the pallet unit is usually from 2.20to 2.80 m for normal trucks and up to 3.30 m for high stacking trucks. As height favours theentrance of warm humid air when the door is open, it should be kept to a minimum.

With carcass handling door height is obviously dictated by overhead rails and themechanism that opens and closes the doors.

Door must be thermally isolated to the same extent as the walls. The insulation is placedwithin a rigid frame which can withstand tough handling without distortion. The doors must


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close tightly against the framework, exerting high pressure on a dense elastic rubber-stripfilling with a very regular contact surface; the closing system must be strongenough to pressthe door against the strip and keep it in that position. For cold rooms at temperatures below0C the strip must be electrically heated to avoid ice accumulation.

Although there are different ways of opening doors (rotating on hinges, sliding horizontallyor vertically), the type most used in cold rooms is the one that opens horizontally.

For quick opening and closing (in the range of few seconds) doors should be mechanicallyor pneumatically operated. The opening is activated by a photo-electric cell or morecommonly by a switch pull placed close to the door which can be operated by the driver without moving from the truck. An automatic controller fixes the time necessary to passthrough the door and closes it.

FIGURE 11 Air curtain and flexible plastic doors preventing thermal exchange

Every time a door opens an intense exchange of inside and ambient air takes place. Whenopenings are frequent the heat and humidity loads in the cold room may be high,considerably increasing refrigeration and frost deposit on coolers. To reduce air exchange,


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flexible transparent plastic doors, transparent swinging doors or air curtains are used. Thefirst two types need constant maintenance as they often break when undergoing heavyduty; in addition they come in contact with merchandise and labourers' clothes, which ishygienically dangerous.

Some precaution in design will help to reduce air exchange. The number of doors should bereduced to a minimum and they should be for one-way traffic; they should not be placedopposite each other, and should face away from prevailing winds.

The air curtain system consists of a powerful blower, usually sited above the door, whichinduces a strong horizontal or vertical air draught (Figure 11). This causes dynamicpressure which balances the density effect of the inside cold air and prevents cold air leaving and warm air entering the store. The capacity of the blower depends on the door size. For instance, for a door 2 m high the air flow should be 1 500 m 3/h, and for a 4m door about 3 000 m 3/h. The angle of the air jet depends on the temperature difference betweenthe cold store and the corridor. As a guide the angle will range from five to 15 degrees; thegreater the temperature difference the higher the angle.

The efficiency of the air curtain system is considered to be between 60 and 70 percent.

LOADING DOCKS

Loading docks ease the handling and transfer of pallets to and from the cold stores andtransport vehicles, so most stores are provided with loading/ unloading docks adapted toroad or railway transport. For road transport the problem is to determine the height of thedock to correspond with average vehicle height: for trucks it will be about 1.40 m, but for distribution vans it will be as low as 60 cm. Moreover when the vehicle is loaded or unloaded its height changes, and this is particularly awkward when the forklift truck has to

enter it. Levelling facilities will adjust the dock to any vehicle height; the dock and truckplatform thus corresponding at any time of the loading/ unloading operation (Figure 12).

Docks for railway transport can be built to a standard height.

The length of a loading bank should allow the simultaneous handling of an adequatenumber of vehicles; it will depend on the size of the cold store and its rotation of storedproduce, which also influence the depth of the bank. The minimum recommended depth is 6m, but one of 8 10 m is considered to be more suitable.

Loading docks are usually under cover, sometimes simply an extended canopy open allaround and sometimes enclosed with a surrounding wall and doors. The choice of open or enclosed docks is mainly influenced by climate and the handling system employed.

Enclosed docks are usually cooled and they should be used where temperature andhumidity are high, and when the merchandise is handled excessively with a long exposurein conditions that are very different from those of storage. Any delay in transfer from trucksto cold store in an open dock is obviously more detrimental than in a cooled enclosed dock.


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Cooled loading docks must be insulated and are equipped with a refrigeration system; thefloor should be heated to prevent condensation.

FIGURE 12 Dock levelling system

The height of the canopy is determined by the height of the store doors plus themechanisms above the lintels for door opening and/or air curtain.

Where for economy of handling two pallets are superimposed for transfer to the cold store,this unit load height will decide the free height of the loading dock roof.

Cooled dock doors should be equipped with a perimeter cushion seal to adjust the rear of the truck to the loading door, reducing the cold air leakage. This system is usually providedwith a displacement mechanism which, together with the levelling device, will ease handlingand the maintenance of the loading dock temperature (see Figure 13).

Part of the loading dock is sometimes utilized to house offices, cloakrooms and, particularly,a supervisor's room for the direct control of the handling operations as efficient direction inloading is essential. Battery changing facilities may also be built into the dock at a point

which does not interfere with traffic; adequate ventilation should be provided to prevent gasaccumulation.

When the temperature of a cooled dock is very exacting warming rooms for personnel areincorporated in it.

FIGURE 13 Docking facilities


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Another solution for covering the loading dock is to locate the administrative offices above it.Good insulation of the office floors is necessary when the temperature of the enclosed dockis low.

TRANSPORT

The transport of meat in a chilled or frozen state must be undertaken at a controlledtemperature so as not to exceed the threshold that encourages microbial development or starts thawing or recrystallization in frozen meats.

Transport vehicles are classified in three categories: insulated, refrigerated andmechanically refrigerated.

Insulated vehicles should be used only for short distances and for short periods of distribution when not much door opening is involved.

Both types of refrigerated vehicle are employed for long hauls. Ice, solid carbon dioxide andeutectic mixtures add enormous weight to the load to be transported, reducing the space for merchandise. The same is true of the liquid nitrogen refrigeration system for long journeysas the dead weight of the cylinders containing liquid nitrogen will be too high. Onlymechanically refrigerated vehicles should be considered at present for long-distance

transport.

Insulation should be thick enough to give low values for the overall heat transfer coefficientwithout reducing load space. For warm countries reinforced insulation leading to an overallheat transfer coefficient under 0.4 W/m2C is recommended.

To reduce transport time and cope more easily with vehicle movement to and from coldstores large traffic areas are recommended. The size of the forecourt of the loading docks


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will depend on traffic and the size of the trucks, but it should be at least 35 m wide whenhigh traffic is expected and trucks can line up at right angles to the dock.

Trucks with an overhead rail should be used for carcass transport. Rails should bestandardized to ease transfer from the cold store to the truck, eliminating additionalhandling. Chilled carcasses must not be piled on the floor.

FIGURE 14 Meat processing plant flow sheet, including freezing, cutting, deboning, packagingand storage

Hygiene is vital in meat transport. Vehicles must be thoroughly cleaned and disinfectedimmediately after unloading or at least before loading. Floor, walls, racks and hooks must

be faultlessly clean. Interior finishing should be washable and waterproof.

Offal must be transported in hermetic containers made of material that is easy to clean.Poultry should be packaged individually.

LAYOUT OF SLAUGHTERHOUSE AND CUTTING ROOM


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The slaughterhouse and cutting room, together or autonomously, are closely related to coldstorage chambers so they condition the general layout of the industrial plant. When they areintegrated, the design is more complicated, due to the many possibilities of flow pattern.

FIGURE 15 Plant distribution for a general store

After dressing and dousing, carcasses will generally pass quickly to chilling rooms or tunnels. As they do not enter the chilling room continuously a certain area should beprovided where carcasses can be accumulated; its surface will depend on the killing rateand the chilling programme.

The doors of the chilling rooms will open on to this area. There may be only one chilling

room in small slaughterhouses and from two to four in others; the total capacity must sufficefor peak day slaughtering.

After four hours in the chilling room carcasses will move to the storage rooms. Storagecapacity will be at least equal to that of the chilling room or higher if some buffer stock isexpected. The number of rooms can be reduced depending on total capacity, the meatdistribution programme and whether the cutting room is incorporated in the slaughterhouse.


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Part of the already chilled meat may be utilized for cutting and packaging, or deboning andpackaging, and part for freezing. The distribution programme will indicate the averagequantities for these different treatments and these will determine the capacity and size of the different sections.

Meats processed in the cutting and packaging rooms may be distributed fresh or chilled, or be frozen for frozen storage or transport to other frozen storage centres.

One loading dock enclosed and refrigerated to a temperature low enough to prevent surfacethawing, about -10C, can handle the shipment of all types of cuts and boxed meats. Chilledcarcasses and quarters will be shifted by overhead rails and chilled package cuts andfrozen meats will be handled on forklift trucks.

Figure 14 is a flow sheet explaining the functioning of the meat complex and Figure 15suggests the layout.

Refrigeration equipment

Internal storage conditions in the meat cold store are characterized by three factors:temperature, humidity and air circulation rate. These are governed by functioningrefrigeration equipment and can be modified by changing the operating conditions of thatequipment. Changes in operating parameters can be made to achieve different storageconditions or to counteract naturally occurring variations in external conditions.

The refrigeration plant has to rely on sensing elements to control internal storage conditionsthat reflect any deviation and on command mechanisms that can be activated to offset thealerted change. Sensing elements and command mechanisms are linked either manually,with operators periodically taking readings of the sensing elements and accordinglyactivating the command mechanisms, or automatically, usually when an electric signal fromthe sensing element activates the command mechanism. Between these two extremes of regulation there are some intermediate operating methods.

Within the sensing, command and functioning circuit of a refrigeration plant there are manyelements, mainly electric (relays, disjunctors, contactors), which are not specific torefrigeration equipment but they must be familiar to the maintenance personnel for effectivetroubleshooting.

Automatic refrigeration is somewhat more complicated than manual as it has to incorporatecertain devices for the easier handling of the refrigerating fluid (refrigerant).

Generally speaking the physical conditions (temperature, humidity, and to a certain degreeair speed) established in any determined area of a cold chamber are not strictly constantover a period but oscillate between a superior and an inferior value, these changesrecurring periodically in a somewhat regular pattern. This cycle can be divided into twoperiods: during the first the refrigerant distribution equipment functions, and eventually therefrigerating installation; in the second period this installation is off- cycle. This on -offoperating mode is usual in conventional refrigerating plants.


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Each physical condition to be controlled has its own peculiarities so it would be useful tolook at each one in detail.

TEMPERATURE

Temperature distribution inside a cold chamber is one of the most difficult features tocontrol. As the temperature differs in the various parts of the chamber its distributiondepends on correct design, on the stacking patterns chosen and on air circulation rate.

The space (and product) temperature is directly controlled by locating the thermostat in theinterior space of the cold room (or on the product itself, but this is not the case for meatproducts). However it may be controlled indirectly by clamping the sensor of the thermostatto the evaporator so that the thermostat controls the evaporator temperature. The former method is indicated where close control of the space and product temperatures isnecessary, as in chilled meat storage. The second method is better when it is necessary toensure total defrosting of the evaporator, even if fluctuations in temperature are minor. It isemployed for operations above freezing.

When the temperature is controlled by a thermostat or similar device the sensor elementmust be placed in such a way that it controls a temperature close to the averagetemperature of the chamber. To determine this position ordinary thermometers aredistributed inside the chamber and, after a few readings, the point with the mostrepresentative temperature is easily located. When the temperature in the cold room hasreached an equilibrium with the stored produce (which may take from one day to a week),the temperature of the air is practically equal to that of the produce.

Heat dispersal through the floor and walls influences the temperature distribution inside thestore; an appropriate insulation thickness and correct installation will favour even

distribution. If in spite of this insulation differences in temperature in the store become quiteextreme, faulty stacking should be considered responsible.

The thermometer or the sensor of the thermostat that measures the temperature of themicro-environment of the store should be placed at mid-height on a wall far from doors andopenings, a few centimetres away from and not directly touching the surface and, whenever possible, in the middle of one of the longer sides of the room.

When several coolers are used to refrigerate the chamber the sensor should be placed atan equal distance from them, usually on the opposite wall.

If the temperature to be maintained is close to 0C, with a risk of freezing the storedproduce, the coldest place should be chosen for the sensor, sometimes near the cooler, butin any case close to the floor.

For frozen products the same principles apply, though small temperature variations are nota major problem as some large frozen storage chambers where the refrigeration plant is runcontinuously during the night and stopped during the day allow for a certain oscillation intemperature.


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The position of the sensor and the reading and command instruments has to be such as toallow easy access even when the chamber is completely full of produce.

A direct-reading thermometer (mercury or alcohol, graduated to 1/5C) should be placedbeside the sensor of the thermostat to be read in the morning and evening.

Thermometers used for temperature monitoring and refrigeration plant operation should beplaced where temperature control is considered most necessary. However it is advisable tomonitor temperatures at more than one location, particularly in large chambers. Themeasurement of product temperature is also recommended.

Storage conditions should be verified at least once a day.

The thermostat, which is the device to control room temperature, consists of a sensor (working in the same way as the thermometer) and an emissary (electric contactor) thattransfers the information given by the sensor. It works on an on-off basis; modulated controlis not used in refrigerating plants. The instant that the refrigeration plant must work or stopis determined by the sensor, but it gives an indication only, which is transmitted by theemissary to the electrically acting element, the compressor, for automatic operation.Thermostats control the temperature level of a refrigerated space by starting and stoppingthe compressor driving motor.

Once the maximum temperature level is reached inside the cold chamber the thermostatcloses the electric circuit that starts the driving motor, and when the minimum temperaturelevel is reached it opens the circuit stopping the motor. These cut-out points are fixed sothat the thermostat can maintain temperature conditions inside a refrigerated space. Thespan between the two limit values is called the temperature differential and it representsthe maximum difference that occurs. As long as the temperature of the refrigerated space is

kept within the fluctuation limits, the functioning of the refrigeration machinery does notchange.

For space temperature control, when the sensing element is located on a wall far from thecooler, the differential is ordinarily 2 3.5C, but sometimes for chilled produce smalldifferentials are established (about 0.5C). When the sensor is clamped to the evaporator,controlling indirectly the space and product temperature through the evaporator temperaturecontrol, the differential used must be larger, from 8 to 10C or even more, to avoid short-cycling of the equipment.

When the thermostat controls the space temperature directly its average temperature isapproximately halfway between the cut-in and cut-out temperatures.

Correct adjustment of the temperature differential is essential if the refrigeration plant is tooperate efficiently. When it is too small there will be a tendency to short-cycle, starting andstopping frequently, which affects the working life of the equipment. When the differential istoo large the on and off cycles will be too long, resulting in excessively large fluctuations inspace temperature.


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As well as the equipment differential the effective differential, which indicates the actualfluctuation in storage temperature, must be considered, as it is always larger due to theinertia of the machinery.

Although the range of the system is defined as the difference between the cut -in and cut-out temperatures, it must not be confused with the differential. The values of thetemperature difference in both cases coincide but the range affects the temperature level atwhich the control is operating, while the temperature differential does not affect this level.The range and the differential are adjusted and controlled with only one thermostat placedin the refrigerated space, but the adjustment of one implies the modification of the other. For temperature ranges some tolerance must be allowed, taking into account that the accuracyof automatic equipment is generally in the range of 1 1.5C.

The more sophisticated control systems use electrical sensing elements (electric resistance,thermistors) and electronic controllers, and the output of the controller is fed to a servosystem to operate pneumatic or electric valves.

The temperatures of the cold rooms can be recorded intermittently or continually; thereading may be either directly transmitted to the control room or checked by the supervisor who notes down the reading in each room once or several times daily.

Thermometers should be calibrated annually, but sophisticated measuring systems needmore frequent calibration; thus, automatic devices should be verified at least once a week.

Reading points must be easily accessible and thermometers should be protected againstshock.

In summary, thermostats are used for automatic control of the temperature level in the cold

room, their function being to start and stop the refrigeration plant by controlling the electricmotors driving the compressor and the condenser fan, and also activating the solenoidvalve.

Maintenance will prevent the malfunctioning of a thermostat. With the pressure bellowstype, if the sensing element has lost its charge it has to be recharged whenever possibleotherwise the thermostat must be changed. If electrical contacts are poor owing to a worn or corroded contact point then they should be replaced, though in an emergency they can becleaned to continue operation. Poor electrical connections must be cleaned and tightened.

RELATIVE HUMIDITY

The relative humidity in a cold room is an indication of the equilibrium between the water evaporated from the stored produce and its removal from the air by the evaporator.

Relative humidity influences loss in meat weight during storage. This mass loss can be of considerable importance economically (less weight, spoiled appearance) and nutritionally.

The recommended levels of relative humidity for refrigerated storage provide adequateprotection against micro-organism development, but they generate a certain loss of mass


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due to water evaporation that is accepted as unavoidable. Total losses are at a minimumwithin a relatively narrow range of relative humidity, say 80 90 percent.

Relative humidity inside a cold store is governed by many factors: quantity of product instore, type and method of packaging, stacking patterns, air motion, system running time,type of refrigeration system control, temperature difference, amount of exposed productsurface, heat and water vapour infiltration, outside air conditions, and length of the workingcycle of the refrigeration installation.

Of these, temperature difference (TD) is the most important. It should not be confused withthe evaporator differential; TD is the difference between the temperature of the air enteringthe evaporator and the temperature of saturation of the refrigerant corresponding to thepressure at the evaporator outlet. The TD of an evaporator can be chosen in function of theproduce, the evaporator geometry, the operating time of the refrigeration machinery,evaporator frost deposit and the type of refrigerant feeding into the evaporator. The smaller the difference in temperature between the evaporator surface and the space, the higher therelative humidity in the cold chamber (the contrary also being true). Notwithstanding other

factors the following figures give an approximate guide for TD in evaporator design toachieve the desired relative humidity when the cooler works under forced convectionconditions.

Design TD (C) 4.0 5.5 5.5 6.5 6.5 8.0 8 9 9 10

Relative humidity (%) 95 91 90 86 85 81 80 76 75 70

The IIR publication Packing station for fruits and vegetables (1973) includes a table for theestimation of the relative humidity in a cold store from the air cooler surface temperatureand storage space temperature. For chambers with storage capacities between 500 and 1000 tonnes relative humidity should remain sufficiently high (about 88 percent or more)when TD is about 10C or lower. Where the capacity is below 500 tonnes there is a rapidfall in relative humidity.

Obviously the difference in temperature between the evaporator and the space is directlyrelated to the size of the evaporator compared to the amount of heat that must be removed.To increase the cooling load of a given evaporator surface the TD value must be increased,and, whenever possible, the air speed over the evaporator.

Evaporators with large surface areas are more expensive and occupy more space in thestore, so it is more costly to construct a cold store operating at a high relative humidity. This,no doubt, is why most cold stores present humidity problems.

However, a large evaporator surface is not enough to achieve a high relative humidity. Theoperating efficiency of the evaporator is as important, and is influenced by design, air distribution, refrigerant feeding system, distribution and control; e.g. a constant pressurevalve will avoid too low an evaporating temperature.

Weight loss is not only influenced by the space relative humidity but also by air circulationand length of the operating cycles of the refrigeration machinery, which in turn influence the


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relative humidity. These factors interact and are finally influenced by the design andoperation of the cold store. Here is some practical advice concerning design and operationto reduce weight loss:

store at the lowest temperature compatible with the produce; keep TD low; run the fans of the air cooler under the control of the compressor unit; slightly overinsulate and keep insulation in good condition; do not plan on introducing water by spraying or sprinkling; limit the duration of storage, organizing produce rotation; keep the store 50 100 percent full; avoid fluctuations of temperature in the store; limit the duration and frequency of door opening; service the door gaskets; moisten pallets and water-absorbing packages before they are stored; check the room atmosphere regularly to control irregular dehydration; wrap the produce, using hermetic and close-fitting wrapping for frozen produce.

Different procedures to control and maintain relative humidity in the store are only effectivewithin certain limits. For instance, hermetic wrapping of frozen products may induceprogressive ice growth in the interior of the package, the rate of growth depending on thestorage temperature and on the range and frequency of temperature fluctuation. Iceformation can be reduced by lowering the storage temperature and temperature differentialand by limiting storage temperature variations.

There are ways to raise the relative humidity inside a cold room, but they are onlytemporary: water sprinkling or spraying; ice spraying or dusting; water vapour emission;passing air flow along a packed tower. These must be considered only as emergency or supplementary operations, as most of the incorporated water goes straight to theevaporator, uselessly increasing energy consumption and requiring more frequentdefrosting of the evaporator.

When using water sprinkling or packed towers to humidify the atmosphere, care should betaken that water droplets evaporate before reaching the product stacked in front of thecooler. The humidifying equipment and its control system should be incorporated in theoverall air circulation system.

A system capable of supplying 0.25 g of water per kg of refrigeration capacity (1 g/kcalrefrigeration capacity) should maintain a relative humidity of about 95 percent.

Sometimes in winter or during cold weather too high a humidity can cause problems. As thecooling equipment has to function at much less than its actual capacity working periodsbecome very short and cycling-off periods very long; this allows the humidity level to riseand reach values that can be dangerous for the proper storage of meat. The atmosphere of the chilled storage chamber may have a relative humidity of 92 95 percent or even higher during most of the off-cycle which favours bacterial growth, making the meat sticky andsmelly. Excess humidity can be avoided by: dividing the evaporator into several elements;reducing the superficial temperature of the evaporator; prolonging the working time of the


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refrigeration equipment; or, finally, by direct dehumidification of the atmosphere by passingit through a water absorbent.

The extension of the refrigeration working period implies that some heat has to beintroduced into the cold store to increase the naturally occurring heat load. This can bedone by using electric resistances or dehumidifiers in small mobile units designed for coldchambers with high humidity problems. The evaporator of the equipment condenses theexcess atmospheric humidity and consequently the air passing over the condenser isheated, leading to the on-cycling of the main refrigerating plant. The electric resistances or the dehumidifiers are governed by a hygrostat, though a couple of hours' daily operationshould be enough to keep humidity at the desired level.

The heat furnished by electric resistances represents additional energy consumption, as theheating power should be about 20 percent of refrigeration power. The small dehumidifierson the contrary consume as little as one-fifth of the total heating power, and besidesremoving humidity from the air they reduce the amount of water that condenses (andeventually frosts) on the surface of the cold room evaporator.

The cooling surface of the evaporator can be reduced by dividing and isolating some of theelements. This lengthens the operating time and also reduces refrigerant evaporationtemperature. Both factors favour the reduction of relative humidity.

The humidity level in the cold room can be measured and controlled by using a hygrostat or hygrometer; all instruments should have a fast response. There are different types of hygrometer currently in use.

Hair hygrometers. They are simple and robust but not accurate, and can beregulated between 30 and 90 percent. They are easily soiled and must be

periodically checked. Main sources of failure are friction, ageing and dirt. Wet and dry bulb hygrometers, based on the temperature depression of the wetbulb. They usually consist of two gauge thermometers, calibrated to no more than0.2C. The thermometers can be replaced by thermocouples and thermistors,allowing remote reading.

Hygroscopic plastic membrane hygrometers, based on the changes of tensionproduced in the membrane by relative humidity variations. They are not accurateand have great tension hysteresis.

Surface film hygrometers, which work on the principle that the electrical resistance of a thin film of lithium chloride, for instance, or any other hygroscopic salt, is a functionof the concentration of the solution which is related to space humidity. The solutionhas to be regenerated periodically and is sensitive to ageing and dilution.

Hygrometers are often unstable and can easily become out of order; they are very sensitiveto dust, their accuracy decreasing when dirt accumulates on the wet parts. An intensemaintenance programme must be established and they must be tested periodically, usingthe wet and dry bulb psychrometer, keeping the wick clean and fitting the thermometer bulbtightly; the air velocity over the wick must be at least 30 cm/s. If a recording type hygrometer is used, the recording can be centralized in the control room. Automatic humidity devicesthat indicate and/or record must be regularly calibrated.


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Careful records should be kept to ensure conditions are correct and also to gain experiencein storage operation. Relative humidity at temperatures below 0C necessitates specialtechniques and precautions.

AIR CIRCULATION

Air is the secondary refrigerant that removes heat from the produce and its surroundingsand carries it to the evaporator where it is cooled and discharged again into the refrigeratedspace. Heat, both sensible and latent due to water vapour condensation, is absorbed by theevaporator surface.

Air movement inside the cold chamber serves two main functions: first, the atmospherictemperature and relative humidity are homogenized to keep them reasonably uniform;second, evaporator efficiency is improved, in that the heat transfer coefficient is increasedas air