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John Kienholz and Ike Edeogu
DisclaimerThis publication contains a discussion of issues related to development of pre-cooling and handling of freshproduce. The purpose of the publication is to provide information that will help guide the development of pre-cooling and handling systems for small market gardeners. It is not intended for any other purpose. While everyeffort has been made to ensure that the information in this publication is correct at the time of printing,neither the Crown, its agents or employees, nor the authors of this publication assume any responsibility orliability whatsoever for any financial loss or damage suffered as a result of the publication or use of thesematerials or as a result of any changes in the law after publication of these materials.
It is recommended that the professional services of an engineer be sought in designing facilities for post-harvest cooling, storage and handling of fresh produce.
Published by:
Alberta Agriculture, Food and Rural DevelopmentInformation Packaging Centre7000 - 113 StreetEdmonton, AlbertaCanada T6H 5T6
Editor: Chris KaulbarsGraphic Designer: John GillmoreElectronic Publishing Operator: Carolyn Boechler
Copyright ©2002 Her Majesty the Queen in Right of Alberta.All rights reserved.
No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or byany means, electronic, mechanical photocopying, recording or otherwise without written permission from theInformation Packaging Centre, Alberta Agriculture, Food and Rural Development.
Copies of this publication may be obtained from:
Publications OfficeAlberta, Agriculture, Food and Rural Development7000 - 113 StreetEdmonton, AlbertaCanada T6H 5T6Phone: 1-800-292-5697 (toll free in Canada)
(780) 427-0391
1
Introduction
Pre-cooling is the key component in thepreservation of quality for perishable freshproduce in post-harvest systems. Pre-cooling isalso very closely linked to the other operationssuch as handling and storage.
The practice of pre-cooling fruits and vegetablesafter harvest has existed for many years. In thattime, several methods and techniques of pre-cooling were developed, primarily to meetrequirements of the large producers and marketsin the United States and Europe.
Pre-coolingInterest in pre-cooling fresh fruits andvegetables is increasing in Alberta. Theseproducts need an improved shelf life and mustbe of high quality to enter and compete inretail markets. Many Alberta marketgardeners, however, do not pre-cool theirproduce. Often, they do not pre-cool becauseof the initial costs involved, the problem indeciding which pre-cooling method to use andnot knowing how to effectively set up a pre-cooling process.
This publication discusses the various coolingtechnologies and the basis for selecting andsetting up systems that will meet the needs of awide range of market gardeners.
What is Pre-cooling?Pre-cooling is the rapid removal of heat fromfreshly harvested produce. This process istypically done before the produce is shipped tomarket or put into cold storage.
Although produce may be pre-cooled in a coldstorage facility, pre-cooling differs from cold
storage. In cold storage, the temperature issimply maintained at a predetermined lowtemperature. If the cold storage facility is todouble as a pre-cooling facility, higherrefrigeration capacity is required as well asappropriate provisions for pre-cooling andhandling of the produce.
Why Pre-cool Fresh Produce?Fresh produce starts to deteriorate immediatelyfollowing harvest. Respiration due to enzymaticoxidation in the growing produce continues afterharvest. This process results in the consumptionof sugars, starches and moisture withoutreplenishment by the plant.
Carbon dioxide and other gases along with heatare generated in the process. If the heat is notremoved, the process is accelerated. Growth ofmolds and the loss of moisture from theproduce are also accelerated by heat.
Bruising of the produce further accelerates theseprocesses, resulting in the loss of texture,firmness, colour, flavour and appearance. Inaddition, some nutritional value may also belost. When these losses occur, the produce isgenerally considered to have lost its freshnessand quality.
Rapid lowering of produce temperature and thenmaintaining it at a constant low temperatureminimizes the enzymatic and other processesthat cause these losses. Pre-cooling as quicklyas practical is therefore a very importantrequirement for maintaining optimum producequality, especially for those types with naturallyhigh respiration rates. Other benefits resultingfrom pre-cooling fruits and vegetables includethe following:
2
• minimized production lossesWeather variability during the harvest season cancause produce to mature earlier or later thanplanned. If the produce is not harvested at theoptimum time, losses will occur. Late harvestingmay also occur if market opportunities are notavailable at the desired time. Pre-cooling andcold storage are valuable tools that better allowproduce to be harvested on time and sold whenmarkets become available.
• improved economics of harvestoperations
The daily harvest may be increased with theassurance that produce quality will be preserved.This confidence permits harvesting to be doneover a longer period, thus spreading outworkloads. Daily harvest hours could also beextended because the effect of ambienttemperatures on the produce would belessened. This flexibility results in better use ofequipment and personnel.
• minimized losses during marketingSome types of produce spoil very quickly if notpre-cooled immediately after harvest. Thesetypes have to be sold within a day or two ofharvesting to be of acceptable quality.Strawberries are a good example. Otherexamples are leafy vegetables that requiretrimming to maintain a fresh appearance. Pre-cooling extends the shelf life and thus theopportunity for sale before the produce is nolonger marketable. See Table 1.
• improved utilization by consumerConsumers can be supplied with top qualityproduce with a longer shelf life through pre-cooling. This extended shelf life lessens theurgency to consume or process produce quicklyafter it is purchased. Consumers are more likelyto make larger purchases, enjoy lower handlingcosts and have more timely product utilization.
• expanded market opportunitiesRetail marketers require produce of the highestquality that has longest practical shelf life. Ahighly perishable product that is not pre-cooled,kept cold and handled with care will lose qualityand have difficulty competing with a qualityimported product. It is also difficult for the lowerquality product to meet grading standards. Pre-cooling is, therefore, the key for perishableproduce to enter and compete in retail markets.
Factors Contributing to SpoilageDifferent types of fruits and vegetables spoilnaturally at different rates. Table I shows thelength of time fresh produce (not rapidly pre-cooled) will last in a refrigerator at 4 degreescentigrade.
Rapid pre-cooling and storage at a stabletemperature will extend this shelf life for mostproduce subject to rapid spoilage. For higherstorage temperatures, it is a generally acceptedrule of thumb that “deterioration of freshproduce doubles for every 10 degreescentigrade above the optimum storagetemperature.” Types of produce with highspoilage rates are often of higher value thanthose with low spoilage rates. This aspect makescontrol of spoilage an especially important factorfor them.
Produce such as onions, potatoes and wintersquash spoil rapidly if not cured as soon aspossible after harvest. Curing at an elevatedtemperature heals cuts and bruises and forms atight outer skin that resists further deterioration.
Atmospheres in storages also affect the spoilagerate. Excess carbon dioxide and insufficientoxygen result in rapid deterioration and offflavours. Ethylene produced by some fruitsaccelerates the ripening of many fruits. It istherefore important to consider the gases thatwill be associated with the cold storage.
3
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51 skeeL -atadon- 33 nroCteewS yad1nahtssel
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Significant moisture loss affects theappearance and firmness of produce. Table 2shows the maximum amount of moisture theproduce may lose before losing its appearanceand firmness. Room cooling, forced air coolingand vacuum cooling all remove some moisturefrom produce during pre-cooling. The amountof moisture lost will depend on design andoperation of the system. More moisture willbe lost if the produce is held for extendedperiods before cooling.
Water used in direct contact with the producecan affect produce quality as well. Rapiddeterioration and off flavours can be triggered bywater containing iron, high levels of minerals,bacteria and other organic material.
Contaminated water used on fresh produce canalso mean that people who consume it becomeill. The cleanliness, quality and sanitation ofwater used in produce facilities is thereforevery important.
4
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31 ibarlhoK -atadon- 13 )retniw(hsauqS -atadon-
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51 skeeL 0.7 33 nroCteewS 0.7
61 ecutteL 7.3 43 drahCssiwS -atadon-
71 snoinO 0.01 53 seotamoT 0.7
81 spinsraP 0.7 63 inihccuZ -atadon-
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In preparing fresh produce for long termstorage, remove free surface moisture on theproduct. This moisture can support the growthof spoilage bacteria. This situation isparticularly true in bruised and cut areas ofproduce such as root crops. If mechanicaldamage is minimized and the tops areremoved with a clean cut, the removal of freemoisture is easier.
The spoilage rate of produce and itscontributing factors are the most importantconsiderations when selecting the type of pre-cooling and handling system to be used.
5
Methods for Pre-CoolingProduce
Considerable loss in quality and shelf life canoccur as a result of holding harvested producein the field before pre-cooling. All methodsrequire sufficient refrigeration capacity toreduce the temperature of the produce withinthe required time plus the ability to removethe normal heat gain in the facility. Table 3outlines the recommended methods for pre-cooling fruits and vegetables and is followedby a brief description of each of the pre-cooling methods.
Pre-cooling rapidly lowers the temperature offreshly harvested produce and is doneimmediately following harvest to minimizespoilage. There are five principal methods ofpre-cooling fresh produce:
• room cooling• forced-air cooling• hydro-cooling• ice cooling• vacuum cooling
6
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Produce is loaded into a refrigerated room ortransportation truck. Cold air is circulated withinthe room and around the produce by therefrigeration fan/s. This is a slow method of pre-cooling because the cold air does not circulatereadily through the produce (Figure A). Thissituation is particulary true for packaged
• room coolingRoom cooling is one method used for producesensitive to free moisture or surface moisture.Because this type of cooling is slow, roomcooling is only appropriate for very smallamounts of produce or produce that does notdeteriorate rapidly.
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produce. Room cooling systems are also usedfor the curing of produce such as winter squash.
• forced-air coolingForced air cooling is used mainly for bulkproduce and palletized produce. It is the mostversatile and widely used of all cooling methods.In forced air cooling, chilled air is forced to flowaround each piece of produce. This closecontact of chilled air with the produce results inrapid, even cooling throughout the mass ofproduce.
For piled bulk produce, such as carrots andpotatoes, air ducts are used to distribute thechilled air through the produce. For palletizedproduce, pallets loaded with bulk orcontainerized produce are aligned with air ducts(plenums) that direct chilled air through them.
The air can be channeled to flow eitherhorizontally or vertically. In a horizontal flowsystem, the air is forced to flow horizontally fromone side of the pallet load to the other
through holes in the sides of the pallet bin orcontainers (see Figure B). Only two sides thatare opposite can be open in the pallet bin orcontainers. In stacking containers, the sideholes must line up for the air to pass fromone side of the stack to the other. In thissystem, the top and bottom of the pallet orcontainers must be sealed to prevent air frombypassing the produce.
In a vertical flow system, the air is forced to flowvertically from the bottom to the top of the palletthrough holes in the bottom of the pallet, andcontainers if used, then out the top(see Figure C). In this system, the sides must besealed to prevent the air from bypassing theproduce. Also, if containers are used, the holesin the tops and bottoms of the containers mustline up, so the air can travel vertically from onecontainer to the next. This method is faster thanroom cooling because a flow of chilled air is indirect contact with the produce.
Figure A. Room cooling is a slow method.
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In these systems, condensation on theproduce can be minimized by a simple coverplaced on top of the stack of containers,which prevents the entry of ambient air duringhandling.
To minimize moisture loss in room and forcedair cooling, the evaporator of the refrigerationsystem should have as large a surface area aspractical. Also, the evaporator should not be runat a temperature much below freezing as thispractice causes undue drying of the air.Excessive air flow should also be avoided tominimize moisture loss.
A bed of chunk or cube ice can be used toprovide cooling in place of a refrigerationsystem. The temperature of the air comingthrough a bed of melting ice will be in the rangeof 1 to 4 degrees centigrade and near 100 percent humidity. See the section on “Ice, A ColdSource for Pre-cooling.”
Room and forced air cooling systems are alsoused for the higher temperature curing ofproducts such as onions, potatoes and squash,before cooling for extended storage.
Some market gardeners own a cold storagefacility. Typically, these cold rooms are designedto remove heat from produce and othersources in 24 hours or maybe even longer.With increased refrigeration capacity and asystem to force air through the produce, thesefacilities can also be used to rapidly pre-coolproduce.
Mobile versions of forced air systems can beused in the field as an integral part of theharvest operations. Because produce can beloaded into the cooler as soon as it is harvested,cooling can be done within minutes of harvest.By minimizing the holding time before cooling,these systems can be very effective andcompetitive with other cooling systems that arecurrently considered as being fast. Mobilesystems can also serve as refrigerated shippingsystems for local needs.
• hydro-coolingIn hydrocooling, cold water is used to pre-coolproduce. Produce may be immersed in a tank ofcirculating cold water or as in other techniques,cold water is sprinkled or sprayed over theproduce. The equipment used for hydrocooling isoften equipped with a water chiller.
Figure B. Forced horizontal air flow. Figure C. Forced vertical air flow.
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This method of pre-cooling is faster thanforced air cooling because water in directcontact with the produce has a higher heatremoval capacity than air. If ice is available, itcan be used to chill the water. See the sectionon “Ice, A Cold Source for Pre-cooling.”
However, not all types of produce can behydrocooled. Strawberries and cauliflower, forexample, should not be cooled by this methodbecause it results in rapid spoilage of theproduce. Hydro-cooling requires particularattention to water quality and sanitation.Chemicals may be used to kill molds andbacteria in the water and on the produce.
• ice coolingIn ice cooling, crushed or fine granular ice isused to cool the produce. The ice is eitherpacked around produce in cartons or sacks, or itis made into a slurry with water and injected intowaxed cartons packed with produce. The icethen fills the voids around the produce.
Ice cooling is faster than hydrocooling becausecontact with the produce is good, and ice has ahigher heat removal capacity than water. It takes144 BTUs of energy to melt 1 pound of ice. Thisamount will theoretically lower the temperatureof 4 pounds of produce by at least 36 degreesFahrenheit.
Ice can also provide cooling for extendedperiods. It is often a marketing requirement thatthere be ice remaining in the sacks or cartons ofproduce at the time it is received by the retailer.As in hydro-cooling, ice cooling requiresparticular attention to water quality andsanitation. See the section on “Ice, A ColdSource for Pre-cooling.”
• vacuum coolingIn vacuum cooling, containers of produce areput in a vacuum chamber. Air is drawn out of thechamber creating a high vacuum. Under the highvacuum, the boiling temperature of water issubstantially lowered.
For example, water at the freezing point boilswhen the pressure is reduced to 0.08859 psiabsolute. Normal atmospheric pressure is about14 psi absolute. As a result, a small amount ofwater in the warm produce evaporates causing acorresponding lowering of the producetemperature. In some systems, the produce ismisted with water to minimize weight loss.
Vacuum cooling is the fastest and mostuniform method of pre-cooling because of thehigh amount of heat required for evaporatingthe water (about 1000 BTUs per pound ofwater evaporated). It is, however, one of themost expensive systems to set up.
Because vacuum cooling is a batch process,sufficient produce must be harvested to fill thevacuum cooler before cooling can begin. For thesystem to be effective, it is imperative that theproduce not be held any longer than absolutelynecessary after harvest and before cooling. As aresult, harvesting and vacuum cooling systemsmust be very carefully matched.
Other sources of information:North Carolina State University atwww.bae.ncsu.edu/programs/extension/publicat/postharv/index.html
10
Ice, A Cold Source forPre-cooling
basis for low cost pre-cooling of a wide rangeof fresh produce grown by market gardeners.
If cube or chunk ice is to be used for directicing of produce, it should be crushed first.Crushed ice can also be used in a slurry withwater for injection into cases of produce.
If ice is to provide refrigeration capacity for aforced air system, the air must be able to flowuniformly and easily around each piece of ice ina bin for the air to be chilled efficiently. Cube orchunk ice is best for this purpose. Flake andcrushed ice tend to nest in a way that restrictsthe flow of air.
Pressure drop through an ice bin can be reducedby increasing the area of the bed. The reducedpressure requires less fan energy and, thus, lessheat is generated by the fan.
Data and Rules of Thumbfor Developing anIce-based System• Air required for pre-cooling is about 2 cubic
feet per minute per pound of produce. Allowfor at least 2 inches water pressure to movethe air through the system.
• Air velocity through the floor should be keptbelow 200 feet per minute to avoid unduepressure drop.
• Specific heat of air is 0.23 BTUs per poundper degree Fahrenheit.
• 1 pound of air is about 14 cubic feet.• The bulk density of cube ice is about 32.8
pounds per cubic foot.
Ice has characteristics that make it veryeffective in pre-cooling fruits and vegetables,especially for small-scale market gardeners.These characteristics include its versatility as acold source for several pre-cooling methods,its thermal storage capacity and its portability.
Ice may be considered stored refrigeration. Asmall ice-making machine can produce icecontinuously for use at a high rate during a shortperiod. The capital cost of the refrigerationsystem can therefore be a fraction of the cost ofa conventional refrigeration system of the samecapacity.
Cooling provided by melting ice is naturallyregulated at just above freezing with thehumidity of air around the ice near 100 percent. The ice can be used several ways: directlyon the produce, or for chilling the water forhydro-cooling, or in an ice bed to chill the air forforced air cooling.
Ice is also very adaptable to pre-cooling in thefield as an integral part of the harvest operation.Residual ice in a mobile ice based cooler canalso help maintain the temperature of produceduring transport. These characteristics make iceideal for use in cooling most fresh produce.
Many market gardeners do not pre-cool theirproduce because of the economics involved. Byusing ice as a cold source, the fixed cost ofrefrigeration can be substantially reduced, andthe benefits of quality produce realized. Theadaptability of ice to provide refrigeration forboth wet and dry types of pre-cooling can be the
11
• Latent heat of ice is 144 BTUs per pound.• Latent heat of vaporization of water is about
1000 BTUs per pound.• Specific heat of water is 1 BTU per pound
per degree Fahrenheit.• Maximum depth of ice should not exceed
about 18 inches to avoid undue pressuredrop.
• Minimum depth of ice at the start of acooling cycle should not be less than 8inches to ensure adequate cooling during thetime of high temperature difference acrossthe bed.
• Ice bin floors should be 60 per cent open tocontain the ice while minimizing pressureloss.
• The shape and size of ice bin floor openingsshould be such as to contain the shape andsize of ice used.
• Cube or chunk ice should be about 1 inchacross to minimize pressure loss and providesufficient surface area for good cooling.
• It takes about 1 pound of ice to cool 2 to3 pounds of produce. The actual amountdepends mainly on temperature of theproduce, how well the cooler is insulatedand sealed and the ambient temperature.
• Specific heat of high moisture produce isabout 0.85 BTUs per pound per degreeFahrenheit
Example CalculationsHere are sample calculations for an ice-basedcooling system. The system will be designed for700 pounds of produce. The produce will becooled from 70 degrees F to 34 degrees, adecrease in temperature of 36 degrees F.
– 700 pounds of produce requires 700/3 =233 pounds of ice.
– 233 pounds of ice occupies 233/32.8 =7.1 cubic feet.
– 700 pounds of produce requires 700 X 2= 1400 cubic feet per minute of air.
– the minimum area of ice bed is1400/200 = 7 square feet.
– if the initial depth of the ice is to be 10inches (10/12 of a foot), the bin floorarea will be 7.1/(10/12) = 8.52 squarefeet.
Other Interesting Calculations andInformationThe area of an ice bin floor is determined bythe air velocity through it and the amount of icerequired. In sizing an ice bed, determine thetotal amount of ice required for one load ofproduce. If 280 pounds of ice are required, thisamount will fill a volume of 280/32.8 = 8.53cubic feet. If the depth of ice is initially 1 foot,this amount will require an area of 8.53/1 =8.53 square feet. This area could be a bin, forexample, that is 2.5' X 3.41' = 8.53 squarefeet.
Tests done by Ike Edeogu in 1998/99 showedthat air can be cooled from 70 degrees F to 34degrees F, a total drop of 36 degrees, with an8 inch deep bed of cube or chunk ice using anair flow of about 200 cubic feet per minute persquare foot of ice bed area, with a pressuredrop of 1.41 inches of water.
To calculate the cooling capacity of 1 squarefoot of ice bin loaded with cube ice to a depthof 8 inches, make the following calculations. Ifthe maximum 200 cubic feet per minute persquare foot of air is used, the air will weigh200/14 = 14.28 pounds. The amount of heatrequired to heat 14.28 pounds of air by 36degrees would be 14.28 X 36 X 0.23 =118.24 BTUs per minute or about 118.24 X60 = 7094 BTUs per hour per square foot.
To lower the temperature of 1 pound ofproduce from 70 degrees F to 34 degrees F, orby a total of 36 degrees, would theoreticallyrequire 1 X 36 X 0.85 = 30.6 BTUs. This total
12
would be the equivalent of 30.6/144 =0.2125 pounds of ice. The ratio of produce toice would therefore be 1:0.215. Changing thisfigure to the ratio of ice to produce wouldtheoretically be 1:4.706. A portion of ice,however, is used to offset heat gains from
infiltration, fans and lights and through thewalls and doors of the facility. The actual ratioof ice to produce used is therefore closer to1:3. Also, an allowance needs to be made forsome ice to cool the unit down.
Index of ManufacturersHoshizaki America, Inc.818 Highway 74 SouthPeachtree, Georgia 30269
IMI Cornelius, Inc.One Cornelius PlaceAnoka, MN 55303-1592
Manitowoc Ice Inc.Sub. of Manitowoc Food Service Group Inc.P.O. Box 1720, Manitowoc, WI 54221
Mile High Equipment Company11100 East 45 AvenueDenver, Colorado 80239
Scotsman Ice Systems775 Corporate Woods ParkwayVernon Hills, Illinois 60061
Kold-Draft1525 East Lake RoadErie, PA 16511
13
Storage Systems
required temperature drop. The specific heatof fresh produce is about 0.85 BTUs perpound per degree Fahrenheit. For example, ifthe harvest rate is 1000 pounds per hour, andthe cooling is to be done in 0.5 hour and thetemperature drop is 40 degrees Fahrenheit,then the system will require an extra 1000 X40 X 0.85 / 0.5 = 68000 BTUs per hour.Consider ice to be a source of refrigeration. Itcan dramatically reduce the size of therefrigeration system required by spreading theextra refrigeration load (required for makingice) out over a long period.
Storage systems provide a stable, controlledenvironment that minimizes deterioration ofthe fresh produce. These systems can also beadapted to curing and pre-cooling. See Series6000 plans from Canada Plan Service on theInternet at http://www.cps.gov.on.ca/ forstorage systems.
If the storage facilities are to be used for rapidpre-cooling, the capacity of the refrigerationsystem must be increased. The amount ofincrease will be determined by the rate ofharvest, the desired cooling time and the
14
Handling Systems
minimum. If produce can be packed intomarket-ready containers in the field andimmediately pre-cooled, the handling costscan be minimized. In addition, deteriorationresulting from holding non-pre-cooledproduce, as well as contamination andbruising, can all be kept to a minimum.Market gardeners may find that the additionalcosts of harvest operations, to accommodatepre-cooling and to reduce contamination andbruising, can be balanced out against theresulting lower handling and producedeterioration costs.
Handling of produce can be done in bulk, inpallet bins and pallets of boxes, trays, baskets,trays of baskets, porous sacks and pails.
• bulk systemsBulk systems can be the cause of considerabledamage to the produce if the systems are notcarefully designed and operated. Loading andunloading equipment should lower produce asgently as possible to avoid bruising and shouldavoid exposing the produce to sharp edges.Also, be aware of pressure damage due toproduce piled to excessive depths.
Bulk systems are very cost effective if damage tothe produce can be minimized. Bulk handlingsystems can also readily accommodateexpansion, again, if damage to the produce isminimized.
• pallet-based systemsPallet-based systems are very versatile andexpandable. They can be used in handling,curing, pre-cooling, storage and shippingsystems from field to market. Pallet bins canbe stacked high without causing pressuredamage to the produce. Different lots of
Handling systems must be appropriate, notonly for the produce, but also for the pre-cooling, storage, shipping and marketingsystems. Handling systems do not add value tothe product, but they do have a major effecton the maintenance of produce quality andthe cost of post-harvest operations.
The potential for system scale up should also beconsidered so that the process can be done withminimal changes to the basic system.Equipment and containers used for handlingshould be easily cleaned and sanitized.
Containers used in handling systems mustaccommodate the type of cooling to be used aswell as the harvesting, handling and shippingoperations. In some cases, the containers areused at the retail level as well. Containersrequire at least 15 per cent open area for the airto enter and 15 per cent open area for theoutlet located opposite the inlet. If thecontainers are to be stacked, the outlets mustline up with the inlets to permit air to flowthrough the stack. Other openings in thecontainers can result in air bypassing theproduce if the flow is not restricted.
Machine-harvested root crops are generallyhandled in bulk from the field. After washing andgrading, these crops may be handled with palletsystems. Corn and some squash may also behandled in bulk from the field.
Green produce and fruit are generally handledwith pallet systems from field to market. Somevery small operations may use individualcontainers.
As with all handling systems, the actualhandling of produce should be kept to a
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produce can also be kept separate foreffective inventory control. Because of thesebenefits, pallets are the preferred way ofhandling the wide range of market gardenproduce.
Pallet bins can be used as an integral part ofharvest operations for transport, curing, pre-cooling and storage. In some instances, palletbins of produce can be shipped directly tomarket.
Pallet bins used for produce curing and pre-cooling must have perforations. It is best to haveslotted holes in only the bottoms of the pallets.This configuration will accommodate verticalforced air, hydro-cooling and ice based pre-cooling systems. Also, natural convection isbetter accommodated with slot-type perforationsin the pallet bottom.
Palletized containers of produce can be handledand managed in a way similar to pallet bins.Totes, trays or cases of produce can be stackedon the pallets in the field. If the produce is to becooled with forced air, the containers and palletsmust have openings that line up when thecontainers are stacked on the pallets. Thisalignment is to permit the air to be forcedthrough each of the containers and thus aroundall the produce.
For vertical flow systems, both the bottom andtop of the containers should have at least 15per cent of the area open. It is much better tohave 30 per cent openings if the strength ofthe containers is not seriously compromised.For horizontal flow systems, opposite sides ofthe containers should each have 15 per centopenings. Forced air pre-cooling can be donein the field to minimize the time from harvestto cooling, thus optimizing the effects of pre-cooling.
• individual containersIndividual container handling systems aregenerally only appropriate for very smalloperations. Cases, totes, sacks, pails andbaskets can be used in harvest, handling,shipping and marketing operations. They can behandled individually or with the aid of a handcart.
If these containers are to be used in coolingsystems, they must have at least 15 per cent ofthe bottoms open to allow for air flow and/ordrainage. As with palletized systems used withforced air cooling, individual containers mustpermit the flow of air from one container to thenext if forced air cooling is to be effective.Containers used for cooling berries should haveslotted holes instead of round ones to minimizeair blockage by the berries.
• open-top totes and containersIf open-top totes or containers are to be used ina forced-air cooling system, the top of one toteor container needs to seal against the bottom ofthe one above it.
If this configuration is not feasible, a “mask”(see Figure 9) can be used. A mask is a sheet ofmaterial with openings that sits on top of a rowof totes or containers. The openings are locatedover the totes or containers in such a way sothat air travels around the produce to chill it. Thenext row of totes or containers is then stacked,so they are centred over these openings in themask. This structure forces the chilled air topass through the containers and not aroundthem.
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Developing an IntegratedSystem
Determining the best method for pre-coolingfresh produce is essential for an effective,economic operation. Each type of fruit andvegetable has a preferred method for pre-cooling.
The preferred method for a particular type ofproduce, however, may not be the mosteconomic for a particular gardener. Most marketgardens produce more than one type ofproduce. It is therefore important, in manycases, to choose systems that are sufficientlyversatile for a wide range of produce.
Developing an integrated post-harvest systemrequires a detailed knowledge of each type ofproduce, especially the factors associated withproduce spoilage and deterioration. Developingan integrated system also means having athorough knowledge of harvest systems,handling methods, pre-cooling, curing, washing,grading, storage and marketing. With thisinformation, the various types of produce can begrouped together into sub-systems and thenindividual systems.
The following possible process steps can beused as a guide in deciding on the bestconfiguration for a system. Look at each stepand the possible methods for each function foreach type of produce. Then, list the methods inorder of preference for each type of produce.Include potential new types of produce andupgraded processes.
1. Harvesting2. Methods for handling at various process
stages from field to market3. Washing4. Curing5. Pre-cooling6. Storage7. Washing8. Grading9. Packaging10. Handling method11. Shipping12. Marketing
Use the Figure D flow chart “general flow optionsfor post-harvest operations” as an aid inidentifying the necessary steps for each type ofproduce. Note: each step has several methodsas possible options.
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Figure D. General flow options for post-harvest operations.
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Arrange the types of produce in order ofpriority for each of the steps. This approachwill help define the types of sub-systems touse.
In most cases, a pallet-based handling systemwill be used. Small operations can start withcontainers suitable for stacking on pallets, with alater upgrade to the pallet system. Largeoperations that handle produce in bulk from thefield often use pallet systems after washing orgrading or for shipping of packaged produce.
Handling systems must have at least the samecapacity as the harvest systems. The samehandling systems used for harvest are oftenused for curing, pre-cooling and storage.
Washing systems are generally specific to certaingroups of produce. Grading is generally anintegral part of the final wash systems, andpackaging is generally an integral part of thegrading systems.
The pre-cooling systems used, as discussedearlier, depend on the type of product andhandling system used. The associated curingand storage requirements are similar to thosefor pre-cooling. Pre-cooling systems for highlyperishable produce require refrigerationcapacity that is sufficient to cool the produceat the rate it is harvested. Less perishableproduce can be pre-cooled over a longerperiod, but the pre-cooling should becompleted at least within a day of harvest tosecure optimum quality.
Consider the use of ice as a source ofrefrigeration, especially for highly diverse typesof operations. Ice can be used in forced aircooling, hydro cooling, top icing, slurry icing andchilling of wash water. It can also providerefrigeration for the short term storage andtransport of highly perishable produce.
Curing systems are fairly specific to producetypes, and heat is often required. In somecases, curing may even be done in the field.
Storage systems need to take into account thecompatibility of produce. This situation includestemperature, humidity, air flow, sensitivity tostorage gasses as well as production of odoursdetrimental to other produce. In most cases,storages used by market gardens need to bedivided into rooms with separate controls fortemperature and humidity. However, the samehandling and shipping systems are often usedfor produce from separate storages.
Two main objectives in setting up the system:
1. Minimize deterioration of product2. Minimize handling
Other objectives:
• Have designated and centralized handlingareas for moving produce between thevarious operations.
• Minimize the storage of equipment andmaterials in the produce storage andprocessing areas.
• Lay out systems and equipment so thatmaintenance and cleanup are easily carriedout.
• Provide ample drainage inside and outsidethe facility.
• Provide for expansion of storages andupgrading of processing areas.
• Floors and walls must be easy to clean andsanitize.
• Optimize use of refrigeration capacity.
These pointers should aid in the developmentof effective, efficient post-harvest systems formarket gardeners in Alberta.
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Build Your Own Ice-basedCooling System
more easily in a central facility. Ice left in thewall can more likely be utilized with greatereffectiveness in a central facility as well.
System SpecificationsThe horizontal ice bin system unit is designedto handle a maximum of about 700 pounds ofproduct. This amount of product will requireabout 300 pounds of ice to lower thetemperature by 50 degrees Fahrenheit.
A generous ice bin area of about 12 squarefeet is available in the system described. Theporous floor should be at least 60 per centopen. The openings should be about 3/8"across to minimize ice loss. The materialshould be corrosion resistant or have acorrosion resistant coating.
An air flow of about 1400 cubic feet per minuteat 2 inches water pressure is provided forcooling by a centrifugal fan driven by a gasolineengine. The fan has an inlet on one side only.The engine is rated at about 4 horse power. Thebelt drive should be carefully considered. Thebelt drive could have a ratio, for example, ofabout 2.5 to 1 for the fan to run at 1200 RPMwith the engine running at 3000 RPM. At thisspeed, small light engines run fairly smoothunder load.
All areas are insulated with 2 inch extrudedrigid polystyrene foam. Separate exterior doorsprovide access to the produce area, enginecompartment and ice bin. The tail gate of thetruck is left on to provide a standing area for
System DescriptionThe ice-based cooling system described in thissection is intended for use in the field as anintegral part of harvest operations. It is designedto be mounted on a pick-up truck, but could bemodified to mount on a trailer if required.
The unit consists of an enclosure divided intoproduce, ice bin and engine/fan compartments.A fan forces air down through a bed of ice, alonga plenum on the floor, up through containers ofproduce in the produce compartment and backthrough the fan. Air exits the ice bed at or below35 degrees Fahrenheit and near 100 per centrelative humidity, making it ideal for chillingfragile produce. Once the ice starts to melt at32 degrees Fahrenheit, the air temperatureremains relatively constant, making temperaturecontrols unnecessary.
Figures 1 through 17 on the following pagesprovide detail on each step of the horizontalice bin system construction.
An alternative to the horizontal ice bin systemis a vertical wall ice bin. This system may beappropriate for batch cooling in a centralfacility. Vertical ice walls require less airpressure due to the distance the air travelsthrough the ice, so the temperature isoptimized and consistent.
Because the ice must be loaded in at the topof the wall, an ice machine can be located toload directly into a feed hopper from above, ifthe unit is in a central facility. Also, regularagitation of the ice in the wall can be done
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loading and unloading the produce. Drains forwater from the melting ice are provided in theice bin area, which requires appropriatelylocated drain holes in the truck box.
Containers used in the system mustaccommodate the vertical flow of air throughthem. This approach means that the holes areonly in the bottom of the containers and in thetop when applicable. A mask on the floorplenum directs chilled air up through thebottoms of the containers. A mask is a sheetof material, such as plywood, that has cut-outs that the containers are stacked on todirect the flow of air through only the bottomsof the containers.
A relatively air tight seal is also requiredbetween stacked containers to ensure apositive flow of air through the produce in allthe containers. This flow may be achieved withjust the containers if a relatively tight seal canbe made between the top and bottom of thestacked containers. A second method makesuse of a mask similar to the one on the floor.This mask is then installed between each layerof containers.
FabricationFabrication of the cooler requires good basicshop skills. Plans for the cooler have been keptgeneral because of the wide range of truck boxconfigurations, fans, engines, fabricationmethods and construction materials that couldbe used. Basic concepts of the system shouldbe adhered to so the unit will function efficiently.The unit requires the following:
• A recirculating and positive vertical flow ofcooling air from the fan, through the ice bin,through the produce and back through thefan.
• Fan located so air is supplied to it from theproduce area and not the ice bin area. Heatis added to the air by the fan in the processof compressing and moving the air. It is alsomore difficult to achieve a uniform flow ofair through the produce if the high velocityair from the fan discharge is close to theproduce air inlet.
• Insulate and seal refrigerated compartmentsand fan to minimize heat gain and optimizecooling effect of ice.
• A generous ice floor area to minimize airpressure required.
• Containers that are easy to handle, providefor vertical air flow only and can be easilycleaned and stacked.
Construction of the cooler is described inFigures 1 through 17. Materials open to theice bin and produce compartments should beselected to avoid contamination of theproduce. Pressure-treated wood or exposedfiberglass insulation are to be avoided.Consider cedar for ice bin and produce frames.Clean and coat all metal components tominimize rust.
Modifications may be required to the topportion of the unit if the unit is to be removedand installed often. For example, corner jacksor lifting brackets may be installed.
Operation1. Load empty containers into produce
compartment.2. Determine the amount of ice required and
load into ice bin. Use cube or cylindricalice. Flake ice and fine granular types of icerestrict the flow of air. Use about 1 poundof ice cubes for every 2 to 3 pounds ofproduce – more on hot days less on colddays.
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3. Move cooler to a location in the field thatis central to harvest operations.
4. Unload containers.5. Ensure ice bin and produce doors are
closed; start fan to cool down the unit.6. Harvest produce and fill containers.7. Load containers of produce into cooler as
they are filled, so they can be cooling assoon as possible.
8. Monitor ice bin and adjust ice levels ifrequired. Some areas may melt fasterthan others. Stack the ice deeper in theseareas.
9. Continue loading until unit is full or harvestis done.
10. Continue cooling until the top containersare chilled. At this point, the ice should benearly used up. Air pressure drop throughthe system at the start will be due to thedepth of ice, while near the end ofloading, the pressure drop will be duemainly to the depth of product.
11. Use the cooler unit to transport and storethe cooled produce. If the produce is to beheld in the cooler for a substantial time,more ice will be required, and the fan willneed to be run periodically to maintain thetemperature. Be aware that somecirculation of air will result from convectiveforces.
12. For high harvest rates where a central pre-cooler is available, only a portion of thecooling may be done in the mobile cooler.The mobile cooler can be unloaded intothe central facility, allowing the mobileunit to return to the field to supportharvest operations.
MaintenanceAfter use, clean and dry out the mobile cooler.Store the unit inside if possible, so the doorscan be left open to dry and air out. When theunit is to be stored for long periods, make surethat the gasoline is drained completely from thetank and that the engine has clean oil in it.
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Figure 1. Ice-based cooling system.
Figure 2. Produce/ice bin access view.
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Figure 3. Engine/fan side view.
Figure 4. Truck box insulation and drains.
Notes: Insulate floor with 2" rigid extruded foam. Cut 1" x 1" groove in foam as shown to drain water from melted ice. Locatethe groove about 5" in front of the wheel wells. Also provide drainage from these holes from the truck box. Also insulate wallsand wheel wells of truck box. Protection of the back edge of the floor insulation may be done with a length of 2" x 1½" wood.
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Figure 5. Ice bin frame.
Notes: The ice bin fills the space in front of the wheel wells. It is made of 2 x 4s and angle iron. The angle iron is1½" x 1½" x 1/8". Fabricate as shown to fit in front of the wheel wells with insulation installed in truck box. Allow at least ¾"on the front and sides of the frame for clearance and the ½" thick walls of the ice bin. Allow ½" on the back of the frame forthe ½" thick rear wall of the ice bin. The angle iron floor frame can simply rest on top of the 2 x 4s because the ice bin wallswill keep it in place when they are installed.
Figure 6. Assembled ice bin.
Notes: The grate or mesh to be used for the floor of the ice bin should have openings about 3/8" across to minimize loss of iceand yet let the air pass freely. The mesh should be cut to fit inside the angle iron frame. ½" plywood can be used for the icebin walls. The top of the bin should be made so that it will be level with the top of the truck box when the bin is installed. Therear wall of the ice bin must have a cut out at the bottom that is 3½" high by 4" less than the distance between the insulatedwheel wells. The top of the bin requires reinforcing with a 2 x 4 installed as shown. The bin is now ready for installation in thefront of the truck box.
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Figure 8. Produce frame.
Notes: The produce frame holds the produce off the floor so that the refrigerated air can be forced up through the produce.The frame is made from 2 x 4s with a length of angle iron (1½" x 1½" x 1/8") at the front. Notch the angle iron flush into the2 x 4s. It is suggested that the frame members be held together with 3" deck screws. Do not make the frame too long as itwill prevent closure of the tail gate.
Figure 7. Installation of ice bin.
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Figure 9. Produce mask installation.
Notes: The produce mask forces the cooling air to pass through the bottoms of the produce containers and not around theoutsides of them. The mask can be made from ½" plywood. The shape, size and arrangements of the openings will bedetermined by the containers that will be used and their arrangement.
Figure 10. Installation of produce floor.
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Figure 11. Cooler frame.
Notes: The cooler frame is made of 1½" x 2" and 2 x 4 spruce with a 1½" x 1½" x 1/8" angle iron header above the rear doors.3" deck screws are recommended for fastening the frame together. The frame must be no longer than 8 feet to accommodatethe plywood cladding. The overall height of the frame should be 4 feet to accommodate the 4 foot width of plywood claddingrequired. Take care to ensure the frame seals against the top of the ice bin wall. Also ensure that the back of the frame will belocated so that the doors can be installed to provide a seal at the tail gate. The angle iron header at the upper back of theframe requires recessing so that it is flush with the wood members. Install a foam seal between the frame and truck box.Attach the frame at the truck stake pockets. Note that some truck boxes are tapered from front to back.
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Figure 12. Bulkhead wall.
Notes: The bulkhead wall can be made from 3/8" plywood. It will require notching to fit around the framing members. The holefor the inlet of the fan should be accurately cut so that it provides a seal without restricting air flow. Install a welded meshscreen with ½" openings over the fan inlet hole.
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Figure 13. Engine compartment.
Notes: The engine room cladding must be installed so that fumes and spilled oil and gasoline will not get into the rest of theunit. Cut and install the floor first. Note that the back edge is attached to the underside of the frame member. Cut and positionthe back panel. Accurately locate and cut the hole for the fan. Install the panel and seal all joints.
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Figure 14. Installation of cladding.
Notes: Cladding should be 3/8" or preferably ½" plywood. Install sides first. At the door openings for the engine and ice bin,ensure that the cladding covers only ½" of the width of the door frame member. This step is to provide stops for the doors toseal against. Use deck screws to fasten the cladding. Install the front cladding. Fabricate and install the front and rear dripcaps. Install roof cladding using a flexible caulking to seal the joints. Also complete sealing of the engine compartment.
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Figure 15. Fan and engine installation.
Notes: Prepare the engine compartment by sealing and painting with an oil-based paint. A galvanized metal pan may beinstalled on the floor of the compartment to contain spills of fuel and oil.Fan installation – Insulate the fan housing with self-adhesive foil-back foam insulation. The shaft of the fan will need to bereplaced with a longer one. Make the shaft long enough to replace the existing shaft plus reach to within a half inch of thefront cladding of the engine compartment. Slide a large felt washer and back-up collar on the shaft to provide a seal betweenthe fan housing and shaft. Slide the fan pulley and flange bearing on the shaft. Install the fan assembly, securing it on theframe members provided. Seal around the fan inlet and outlet. Slide the belt on the shaft. Align shaft and install flange bearingon front wall of engine compartment.Engine installation – Fabricate hinged engine mount and drill for engine mount bolts. Install hinged engine mount. Installengine and belt tensioning bolt. Align and secure pulleys and install and tighten belt.
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Figure 16. Door installation.
Notes: Engine room and ice bin doors – fabricate from ½" plywood and install with continuous type hinges. The ice bin doorrequires a seal and at least two latch points to contain the air pressure in the ice bin area. Use a type of latch that will notwork its way open during travel.Rear produce doors – fabricate door frames from 1½" x 2" spruce. Orient the 2" dimension to accommodate the 2" insulation.Install ½" plywood cladding. Note overlap where doors meet in the centre. Install hinges and hang doors. A system will berequired to latch the last door closed to the header. The system will also be required to latch the two doors together at a pointabout one foot from the bottom of the doors. See truck body suppliers and manufacturers for hardware.
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Figure 17. Installation of insulation.
Notes: Use 2" extruded rigid foam and hold it in place with adhesive designed for the purpose. Insulate all wall and door areasexcept the bulkhead in both the ice bin and produce areas. The engine room area of the bulkhead around the fan should alsobe insulated. This insulation may need to be done on the engine room side of the bulkhead.
