Cold Preservation of Meat Products

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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