"i .. Cooling Horticultural Commodities James E Thompson, E Gordon Mitchell, and Robert E Kasmire COOLING 97 Controlling product temperature and reducing the amount of time that product is at less-than-optimal temperatures are the most impor- tant methods of slowing quality loss in perishables (see chapter 4). Postharvest temperature management begins with planning the har- vest and field handling. Some products are so sensitive to temperature abuse that they should not be harvested when temperatures are too warm. For example, table grapes show signs of stem shrivel at about 2% weight loss, and if stem quality is to be maintained at consumer level, the fruit should not be subject to more than about 0.5% weight loss between harvest and the beginning of cooling. Although grapes can be held for more than 8 hours at 20DC (68DF) before cooling, at 30DC (86DF) cooling should begin within 1.5 hours after picking (fig. 11.1). A few growers harvest at night to prevent exposure to excessive heat after harvest. Other methods of protecting product from temperature-caused damage are to . Make frequent trips between the field and the cooler to minimize temporary field storage. . Pack in light-colored containers. . Cover containers with lids if left in direct sun. . Use a shaded area for temporary field storage. Remember, shade cast by a tree moves with the sun during the day. I . For short trips, use covered trucks to transport product to cooler. Long trips need refrigerated trucks. . Begin cooling as soon as possible after product arrivep at the coo]- ing facility. Some commodities can withstand a fairly long time between har- vest and cooling. For example, apples placed in controllFd atmosphere storage often do not reach optimal storage temperature until several days after harvest; exported California oranges may not reach best storage temperature until they have been at sea for several days. Prod- ucts that do not require fast cooling generally have slowlrespiration rates, low moisture loss (transpiration) rates, and are often grown in climates with mild temperatures. The first part of this chapter describes the variety of cooling sys- tems available for horticultural commodities and the iss~es that need to be understood in their use. The second part describesla systematic approach for selecting a cooling system for a particular operation. COOLING METHODS " Initial cooling of horticultural products to near their opt~mal storage "- temperature can be done with several cooling methods, including room cooling, forced-air cooling, hydrocooling, paCkage ~ ing, and vacuum cooling. Mechanical refrigeration in ships or ref igerated marine containers may be used for cooling a few commo ities during transport. A few cooling methods (e.g., room cooling, fo ced-air cool- ing, and hydrocooling) are used with a wide range of co~modities. Some commodities can be cooled by several methods, bu~ most com- modities respond best to one or two cooling methods. Most users are concerned with the time to "complete C t oling," which usually means the time to reach a desired temperat re before
Transcript
"i ..
CoolingHorticulturalCommodities
James E Thompson, E Gordon Mitchell,
and Robert E Kasmire
COOLING 97
Controlling product temperature and reducing the amount of timethat product is at less-than-optimal temperatures are the most impor-tant methods of slowing quality loss in perishables (see chapter 4).Postharvest temperature management begins with planning the har-vest and field handling. Some products are so sensitive to temperatureabuse that they should not be harvested when temperatures are toowarm. For example, table grapes show signs of stem shrivel at about2% weight loss, and if stem quality is to be maintained at consumerlevel, the fruit should not be subject to more than about 0.5% weightloss between harvest and the beginning of cooling. Although grapescan be held for more than 8 hours at 20DC (68DF) before cooling, at30DC (86DF) cooling should begin within 1.5 hours after picking (fig.11.1). A few growers harvest at night to prevent exposure to excessiveheat after harvest.
Other methods of protecting product from temperature-causeddamage are to
. Make frequent trips between the field and the cooler to minimizetemporary field storage.
. Pack in light-colored containers.
. Cover containers with lids if left in direct sun.
. Use a shaded area for temporary field storage. Remember, shadecast by a tree moves with the sun during the day. I
. For short trips, use covered trucks to transport product to cooler.Long trips need refrigerated trucks.
. Begin cooling as soon as possible after product arrivep at the coo]-ing facility.
Some commodities can withstand a fairly long time between har-
vest and cooling. For example, apples placed in controllFd atmospherestorage often do not reach optimal storage temperature until severaldays after harvest; exported California oranges may not reach beststorage temperature until they have been at sea for several days. Prod-
ucts that do not require fast cooling generally have slowlrespirationrates, low moisture loss (transpiration) rates, and are often grown inclimates with mild temperatures.
The first part of this chapter describes the variety of cooling sys-tems available for horticultural commodities and the iss~es that needto be understood in their use. The second part describesla systematicapproach for selecting a cooling system for a particular operation.
COOLING METHODS
" Initial cooling of horticultural products to near their opt~mal storage"- temperature can be done with several cooling methods, including
ing, andvacuum cooling. Mechanical refrigeration in ships or ref igeratedmarine containers may be used for cooling a few commo ities duringtransport. A few cooling methods (e.g., room cooling, fo ced-air cool-ing, and hydrocooling) are used with a wide range of co~modities.Some commodities can be cooled by several methods, bu~ most com-modities respond best to one or two cooling methods.
Most users are concerned with the time to "complete Ct
oling,"which usually means the time to reach a desired temperat re before
98 CHAPTER II
Figure 11.1
Effectof grape temperatureat harveston the time neededfor the berriesto lose0.5% of their harvestedweight.
Tablegrapes12
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60 90 100 of70 80
Air and grape temperature
Figure 11.2
Typicalcooling curvefor perishableproducts.Coolingtimes are typicalfor largefruit, like peaches,exposedto moderateamountsof airflow.
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/ Initial product temperature
20 _/
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5-;~~10~ti'""t:J0ct 5
/Average product temperature¥
Air temperature
4 8 12 16 20
Hours of cooling
transfer to storage or transport. Yet coolingtimes are usually reported as "half-cooling" or"seven-eighths-cooling" times. Half-coolingtime is the time to cool the product halfwayfrom its initial temperature to the tempera-ture of the cooling medium. Seven-eighthscooling time is three times longer than halfcooling. and is the time needed for the prod-uct temperature to drop by seven-eighths ofthe difference between the initial producttemperature and the temperature of the cool-ing medium. Both of these cooling times areconstant values for a given package type in agiven cooling system and are not affected by
varying initial product te eratures or vary-ing cooling medium temp rature.
As horticultural produc s cool, their rateof temperature drop slows as cooling pro-gresses. For example, for eaches with aninitial pulp temperature 0 20°C (68°F), half-cooling them in a forced-a r cooler at DoC(32°F) (i.e., cooling them 0 lOoC [50°F])takes 4 hours. It takes an dditional 4 hoursto cool them to soC (41°F and 4 more hours
to reach seven-eighths coo ing (about 2.5°C[365°F]) (fig. 11.2). Seve -eighths-cooling,or three half-cooling times (in this example,12 hours), is often used as a reference cool-ing time.
Both initial product tern erature andcoolant temperature influe ce cooling timeof a product. The peaches n the examplementioned above might be harvested in themorning, but by late aftem on on a hot Cali-fornia day they could have ulp temperaturesnear 40°C (lO4°F), in whi case, with DoC
(32°F) cooling air, one add tional 4-hourhalf-cooling period (16 ho rs total time)would be required to reach the same 25°C(36.5°F) pulp temperature s fruit harvestedin the morning. If the cooli g air temperaturewas 1.2°C (about 34°F), co ling peacheswith initial temperature of DOCwould alsorequire 4 half-cooling perio s, or 16 hours.
In room and forced-air oolers, the prod-uct closest to the cold air c ols noticeablyfaster than the product fart est from the coldair. Coolers should be man ged so that thewarmest product reaches a ceptably lowtemperatures before the co ling process ishalted.
DC
ROOM COOLING
This widely used cooling m thod involvesplacing field or shipping co tainers of pro-duce into a cold room. Its ost common use
is for products with a relati ely long storagelife that are stored in the sa e room in which
they are cooled. Examples i clude cut flow-ers before packing, potatoes sweet potatoes,citrus fruits, apples, and pe s.
In room cooling, cold air rom the evapo-rator coils sweeps past the p oduce contain-ers and slowly cools the pro uct (fig. 11.3).The main advantage of roo cooling is thatproduce can be cooled and s ored in the sameroom without the need for t ansfer. Its disad-
vantages are that it is too slo for most com-modities; it may initially req ire empty floor
Figure 11.3
Diagrammaticview of air path during room cooling of producein bins.Air circulating through the room passesover surfacesand through forkliftopeningsin returning to the cooling coils.In this systemthe air takesthepath of leastresistancein moving past the product.Cooling from the sur-face to the centerof bins is largelyby conduction.
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area between stacked containers for air chan-
nels to speed cooling and subsequent rehan-dling after cooling is finished; and for someproducts, it can result in excessive water losscompared with faster cooling systems. Roomcooling requires days for packed product toreach desired temperature, but it can be fasterfor unpacked products with good exposure tothe cold air. For example, bunched flowers inbuckets can cool in 15 minutes, but the sameflowers packed in boxes and loaded on a pal-let take days to cool.
For best results, containers should be
stacked so that the moving cold air can con-tact all container surfaces. Total fan airflow
should be at least 0.3 m3/min per ton of prod-uct storage capacity (100 cErn/ton) for ade-quate heat removal. (For several commonlyused airflow systems for room cooling, seefig. 12.3 in chapter 12.) Well-vented contain-ers with vent alignment between containersgreatly speed room cooling by allowing airmovement through containers. After coolingis complete, airflow can be reduced to 20 to40% of that needed for initial cooling.
COOLING 99
Cooling baysFor both cooling and storage, a single largeroom is divided into bays by in taIling parti-tions partway into the room fro each side(fig. 11.4). Air supply channels direct the airinto the back of each bay. Whe a single bayis filled with warm product, su ply ducts areopened to direct a large volume of cold airbehind the product. Air return ccurs downthe center forklift aisle. When ooling iscompleted, the air supply is re ced in thatone bay to create the desired st rage condi-tions. If each cooling bay has a eparate coldair supply, cold product in one ay is notwarmed by warm product in ot er bays.
FORCED-AIR COOLING
Forced-air cooling is adaptable 0 a widerrange of commodities than any other coolingmethod. It is much faster than oom coolingbecause it causes cold air to mo e through,rather than around, containers. This allowscold air to be in direct contact ith warm
product. With proper design, fa t, uniformcooling can be achieved throug stacks ofpallet bins or unitized pallet loads of con-tainers. Water loss varies with t e moisture
loss characteristics of individua productsand can range from virtually no e to 1 to 2%of initial weight. Forced-air is t e mostwidely adaptable and fastest co ling methodfor small-scale operations.
The speed of forced-air coolig is con-trolled by the volume of cold ai passing overthe product. Maximum feasible coolingrequires about 0.001 to 0.002 I 3-sec-I-kg-Jof product (1 to 2 cfm/lb). Rate. greater thanthis only slightly reduce coolin time, but asthe air volume increases, the static pressurerequired greatly increases, raisi g the energyconsumption of the fan. Some products canwithstand slower cooling and u e air vol-umes of 0.00025 to 0.0005m3-s cl-kg-l ofproduct (0.25 to 0.5 cfm/lb). St tic pressureneeded to produce the desired a'rflow is verydependent on container vent de ign and theuse of interior packaging materi Is. A com-plete description of this issue is included inthe UC ANR publication Comm rcial Coolingof Fruits, Vegetables, and Flowers (Thompsonet al. 1998).
Various airflow designs can
1
used,
depending on specific needs. C nvertingexisting cooling facilities to forc d-air cool-ing is often simple and inexpen ive if enough
I/IiII
100 CHAPTER II
Figure 11.4
Topview of a cold room divided into cooling bays.
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Evaporator""'/ Top view
Figure 11.5_--on
Diagrammaticview of a forced-air cooling tunnel. Eitherbins or pal-letized containerscan be placedto form a tunnel from which air isexhausted.Thenegativepressurethen causescold air from the roomtopassthrough ventilation slotsto directlycontact the warm product.
re~rigeration capacity is available. A well-designed forced-air cooler is a separate roomfrJm cold storage rooms.
Tunnel-type forced-air coolingIn tunnel-type forced-air cooling, the most-used forced-air cooling system, a row of pal-letized containers or bins is placed on eitherside of an exhaust fan, leaving an aislebetween the rows. The aisle and the openend are then covered to create an air plenum
tunnel (fig. 11.5). The exhaust fan creates
--------
Figure 11.6--- n- on
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Forced-aircooling tunnel is in operafon, cooling pack-aged produceon unitized pallets.Air-circulatingfan cir-culatesair through fruit and over co ling coils.Canvasplenum cover is designedto fit varyi g cooling loads.
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low air pressure within the tu nel. Cold airfrom the room moves throug the openingsin or between containers tow' rd the low-
pressure zone, sweeping heat away from theproduct. The exhaust fan is u ually a perma-nent unit that also circulates ir over the
refrigeration coils and return it to the coldroom (fig. 11.6). The exhaust fan can also bea portable unit that is placed a direct thewarm exhaust air toward the ir return of
the cold room or refrigeratio evaporators.
Cold wall
This forced-air cooling system uses a perma-nent air plenum equipped wit exhaust fans(fig. 11.7). The air plenum is £len located atone end or side of a cold roo ,with the
exhaust fans designed to mov air over therefrigeration coils. Because op nings are locat-ed along the room side of the lenum, againstwhich stacks or pallet loads a containers canbe placed (fig. 11.8), this met ad is not oftenused for products in bins. Var'ous damperdesigns can be used to ensure that airflow isblocked except when a pallet s in place. Eachpallet starts cooling as soon a it is in place,so there is no need to await d liveries to
complete a tunnel. Shelves m y be built sothat several layers of pallets c n be cooled.Different packages and even p rtial pallets
Figure 11.7
Crosssectionof a cold-wall type forced-air cooler.
Damperisopenedwhen,palletis pushedagainstbumper
~ Evaporator coils
can be accommodated by proper design of the
damper system. This is a benefit in operationsthat handle a range of commodities. Each pal-let must be independently monitored fortemperature and promptly moved from thecooler as soon as it is cold in order to avoid
desiccation from continued rapid airflow overthe product.
Serpentine coolingThe serpentine system is used for forced-aircooling of produce in bins. Bins must havebottom ventilation slots. Ventilation on the
sidewalls of the bins does not aid cooling,
although it probably does not hinder iteither. The system requires modification ofthe cold-wall design to allow the forkliftopenings between bins to be used as air sup-ply and return plenums. The cold air movesvertically through the product in each bin,causing a slight pressure difference betweenplenums (fig. 11.9). Bins may be stacked upto six rows deep against the cold wall, depend-ing on the cooling speed desired and theavailable airflow. The airflow capacity of thesmall forklift opening plenums usually limitsairflow to less than 0.0005 m3osec-lokg-l of
product (0.5 cfmllb), and cooling is usuallyslower than tunnel coolers. To achieve the
desired airflow pattern, openings are placedin the cold wall to match alternate forklift
openings, starting one bin up from the floor.On the room side of the bins, these same
openings are then blocked (fig. 11.10). Airflows into an open slot between bins andpasses up or down through one bin of prod-
COOLING 101
Figure 11.8--- ,- - -- -~
~
---
Cold-wall type forced-air coolerfor use ith stacksofflower containers.Openend-ventsalia air to be pulledthrough the containersfor rapid cooling but allow clos-ing during shipment.
uct to reach the return plenum.
this system
requires no space between rows f bins, andbins can be stacked in even num ers as highas the cold store or forklifts can r ach,
Forced-air evaporative coolinIn forced-air evaporative cooling, the air iscooled with an evaporative coole instead ofwith mechanical refrigeration. If esignedand operated correctly, an evapor tive coolerproduces air a few degrees above he outsidewet bulb temperature, and the co ler air isabove 90% RH. In most areas of alifornia,
product temperatures of 16° to 2 °C (60° to70°F) can be achieved. A typical rced-airevaporative cooler is shown in fig re ll.ll,This cooling method may be ade ate forsome products that are best held t moderatetemperatures such as tomatoes or for prod-ucts that are marketed quickly aft r harvest.In most cases, growers can build t eir ownforced-air evaporative coolers. Th yaremuch more energy-efficient than echanicalrefrigeration, and if properly desi ned theyprovide high-humidity cooling air
Container ventingEffective container venting is esse tial forforced-air cooling to work efficie tly. Coldair must be able to pass through a 1 parts ofa container. For this to happen, c ntainervents must remain unblocked afte stacking.If containers are palletized in-regi ter(aligned with one another), conta ner sideor end vents will suffice, provided they areproperly located in relation to tra s, pads,
...
102 CHAPTER / /
Figure 11.9
Patternof airflow in a serpentineforced-air cooling system.Thissystemisspecificfor cooling fruit in field bins.Byblocking alternate forklift open-ings on cold-wall and room sides,with fans operating,air is forcedtopassvertically through bins to cool fruit.
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and other packaging. If cross-stacking isused, matching side and end vents is essen-tial. For the 400 by 300 mm (or 16 by 12in) container cross-stacked on the 1,200 by1,000 mm (or 48 by 40 in) pallet, verticalvent slots on 100-mm (4-in) centers around
the container perimeter should be consid-ered, because they remain matched whencross-stacked (fig. 11.12).
Too little venting restricts airflow; toomuch venting weakens the container. A rea-sonable compromise appears to be about 5to 6% side or end wall venting. A few largevents are more effective than many smallvents for speeding the cooling rate. Locatingvents midway from top to bottom is ade-quate unless trays or other packing materialsisolate some of the product. Vertical slots atleast 12 mm (1;2in) wide are better than
round vents. Vent design should minimizethe effect of product blocking vents. Anytype of unvented bag, liner, or verticaldivider inside the package may block ventsand reduce airflow. If a solid liner is used,
slow but acceptable cooling times can beobtained if the box is designed so that coldair can flow over and under each box, and
boxes are fairly shallow.Flower boxes often have closable vents.
This allows closing after cooling so that
'T
Figure 11.10
Serpentineforced-air cooler in operati n. Plasticstrapsare placedover everyother forklift op ning from bot-tom to top.Thesecloseoff the openin s in the roomside of the cooler.Air entering the op n channelsthenmust moveup and down through the roduct to returnto the cold wall. Notethat bins canb tightly stackedinrows sinceno centerairflow plenum i needed.
flowers shipped on unrefriger
1
ted transportcan maintain temperatures 10 ger than ifshipped in a box with open v nts.
Cooling in transportMarine containers and break- ulk ships haveenough airflow and refrigerati n capacity toachieve slow forced-air coolin . In-transit
cooling is used for products p oduced inareas without cooling facilitie . But it is usu-ally better to cool the produc before it isloaded in the ship or contain r, since mosttransport modes do not have he extra refrig-eration capacity needed for fa t cooling.
Refrigerated ships and mo t marine con-tainers have a bottom-delive air supply sys-tem. Air travels from the refri eration systemto a floor plenum. It then fIo s upwardthrough the boxes and return to the refrig-eration system in the space a ove the prod-uct. To work well, boxes mus have vent
IpSt-
holes on the top and bottom, and the ventholes must align if boxes are cross-stacked,The floor must be covered to prevent airfrom bypassing the load,
Refrigerated highway trailers with a top-~n
Jrndin
Figure 11.11
Cutawayview of an evaporativeforced-aircooler.Air is cooledby passingthrough the wet pad before it passesthrough packagesand around theproduct.
Porouspad
rov
Water is collectedand recirculated
Figure 11.12
Recommended box vent design to allow good airfow or water flow while
maintaining package strength.
Shoulderventsallow vertical airflow
Panelvents are
away fromcornersandnarrow
Ventsare aligned toallow horizontal air-
flow through crossstackedboxes
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Figure 11.13
Side view of a batch-type hydrocooler for pallet bins,
Water distribution pan'o\
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5-Refrigerationsystem\
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Water pump\\
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',- Water reservoir/
COOLING 103
delivery air supply system do n~t haveenough airflow to allow transpqrt cooling.
HYDROCOOLING
Cold water is an effective met od for quick-ly cooling a wide range of frui and vegeta-bles in containers or in bulk (f gs. 11.13and 11.14). Typical seven-eigh hs coolingtimes are 10 minutes for small diameter
products like cherries and up t 1 hour forlarge products such as melons, Hydrocoolerscan use either an immersion 0 a shower
system to bring products in co tact withthe cold water. Hydrocooling a oids waterloss and may even add water t a slightlywilted commodity, as is often one withleafy green vegetables. Hydroc olers can beportable, extending the coolin season.Containers used in hydrocooli g must bewater-tolerant.
In a typical shower-type hyd cooler, coldwater is pumped to an overhead perforateddistribution pan, The water sho ers over thecommodity, which may be in bi s or boxes, orloose on i conveyer belt. The w ter leavingthe product may be filtered to r move debris,then passed over refrigeration c ils (or ice),where it is recooled. The refrige ation coils arelocated under or beside the con eyer or abovethe shower pan, Some commod'ties, such asleafy vegetables and cherries, ar sensitive towater-beating damage. For prod cts like these,the distribution pan should be 0 more than20 cm (8 in) above the exposed product.These products can also be cool d in animmersi.on hydro cooler.
Efficient cooling depends u on adequatewater flow over the product su face. Forproduct in bins or boxes, wate flows of 13.6to 17.0 l-sec-lom-2 (20 to 25 ga minlft2) ofsurface area are generally used. Bin hydro-coolers are often designed to a commodatetwo-high stacking of bins on t e conveyors.Bulk product in shallow layers n a conveyorbelt requires 4.75 to 6.80 losec- °m-2 (7 to10 gallminlft2) , Water is usuall cooled bymechanical refrigeration, but i e may beused if it can be broken into fi t-sized piecesand added fast enough to prod ce adequatecooling, In some areas, clean ell water iscold enough to do initial cooli g or evencomplete cooling. Hydrocooler should bedrained and cleaned at least da Iy or beequipped with special filters to clean thewater. Low concentrations (10 to 150 ppm)
104 CHAPTER II
Figure 11.14
Conveyor-typebin hydrocoolerin operation. Icewater is pumpedinto thetop pan,where it runsdown through the produCtin the bin. Dwell time inthe cooler is controlled by conveyorspeed.
of active chlorine are usually used to disin-fect the water and minimize the spread ofpostharvest decay of products.
Hydrocooling has some potentiallimita-tions, The product and any packages andpacking materials must be tolerant of wet-ting, and they must also be tolerant of chlo-rine (apricots sometimes show chlorine dam-age) or other chemicals that are used tosanitize the hydro cooling water. Shower-panholes must be cleaned regularly to avoidplugging, which causes uneven water flowover the product. Arriving warm producemay have to remain at ambient temperaturesfor some time when the hydrocooler is oper-ating at peak capacity Cooled product mustbe moved quickly to a cold room or elserapid rewarming occurs. Hydrocooling oper-ations can also require rehandling of the pal-let bins before packing or storage.
Hydrocooling can be energy-efficient pro-vided that the hydrocooler is operated con-tinuously at maximum capacity and is insidea cold room or an insulated enclosure.
Shower-type hydrocoolers (conveyor orbatch units) are the most commonly usedhydrocooling systems, but immersion hydro-coolers are sometimes used. Fresh-cut vegeta-bles are commonly cooled as they are con-veyed in a water flume. In this case, theproduct, normally in bulk, is in direct contactwith the cold water as it moves through along tank of cold water, This method is bestsuited for products that do not float. Because
slow cooling would result if t e product sim-ply moved with the water, im ersion hydro-coolers convey product again t the directionof water flow and often have system for agi-tating the water. Conveyors ust be designedfor positive movement of the roductthrough and out of the water.
PACKAGE-ICING
Some commodities are cooled by fillingpacked containers with crush d or flaked ice.Initially, the direct contact bet een productand ice causes fast cooling. H wever, as theice in contact with the produ t melts, thecooling rate slows considerab y The constantsupply of meltwater keeps a h'gh RH aroundthe product. Liquid ice, a slur of ice andwater, distributes ice through ut the box,achieving better contact with he product(figs. 11.15 and 11.16). Ice ca be producedduring off-peak hours when e ectricity ischeapest and stored for dayti e use.
Package-icing requires exp nsive, water-tolerant packages. The packa es should befairly tight but should have e ough holes todrain meltwater. In small ope ations the iceis hand-raked or shoveled int containers.
Large operations use liquid-ic machines toautomatically ice pallet loads f packed car-tons. The process, which req ires only a fewminutes, is used for cooling s me field-packed vegetables, particularl broccoli. Theiced packages should be plac d into a coldroom after filling to minimize ice melt.
The product must be toler nt of pro-longed exposure to wet cond tions at ODC(32aF). Some low-density pr ducts haveexcess space in which to loa ice within thepackage, and ice not melted uring coolingcan remain in the package ev n after trans-port, This excess ice can kee the productcold if the cold chain is brok n. However,this is an inefficient use of ic , and the
weight of the ice can add sig ificantly tothe freight load, sometimes li iting theamount of product hauled. A ice weightequal to 20 to 30% of the pro uct weight isneeded for initial cooling, bu liquid icingoften adds an ice weight equ I to the prod-uct weight. Also, during tran port of mixedloads, water from melting ice can damageneighboring boxes that are n t water-toler-ant, and vehicle insulation ca become wet.Ice and meltwater can be a sa ety hazard atwholesale distribution.
i-
d
Figure 11.15--~ ~~ ~ ~ ~~---
Pallet liquid-icing machine in operation. The high-volume flow of the ice-
water mix is pumped into chamber and flows through container vents to
deposit the ice throughout the package.
..
Figure 11.16
A package of liquid-iced broccoli is opened to show the penetration of
ice throughout the package.
Cut flowers are often forced-air cooled,
but if the box is handled in an unrefrigeratedenvironment, bags of ice may be added to thebox to prevent heating. This system allows ameasured amount of ice to be added, and the
melt water is contained, preventing damageto the product and fiberboard boxes.
VACUUM COOLING
Vacuum cooling takes place by evaporatingwater from the product at very low atmos-pheric pressure. Products that easily releasewater may cool in 20 to 30 minutes.Vegetables that have a high surface-to-massratio and that release water rapidly, such asleafy green vegetables (especially iceberg let-tuce), are best suited to this method. It is
COOLING I 05
also sometimes used to cool eel ry, somesweet corn, green beans, carrots and bellpeppers. It is used with carrots nd peppersprimarily to dry the surface and stems,respectively, and to inhibit post arvest decay.Even boxes of film-wrapped pro ucts cancool quickly provided the film a lows easymovement of water vapor.
Moisture loss and consequen cooling isachieved by pumping air out of large steelchamber containing the product (fig. 11.17).Reducing the pressure of the at ospherearound the product lowers the oiling tem-perature of its water, and as the ressurefalls, the water boils, quickly re oving heatfrom the product. Water vapor i removed bycondensing it on refrigerated co'ls locatedbetween the ports and the vacu m pump.Vacuum cooling causes about 10 productweight loss (mostly water) for e- ch 6°C(11°F) of cooling. This amount f weightloss can be objectionable for gre n onions,celery, and some leaf lettuces. S me coolersare equipped with a water spray system thatadds water to the surface of the roduct dur-
ing the cooling process. Like hy rocoolingwater, this water must be disinf cted if it is
recirculated. Water can also be s rayed onthe product before it enters the ooler. Therapid release of the vacuum at t e end of theprocess can force surface water i to somevegetables, giving them a water-. oakedappearance.
A typical vacuum tube, some imes calleda retort, holds 800 boxes of iceb rg lettuce(20 pallets). Some small vacuUl coolershold only a single pallet. Most v cuum cool-ing equipment is portable and is used in twoor more production areas each y ar, allowingthe high capital cost of vacuum oolers to beamortized over a longer operati g season.Most coolers used today have m chanicalrefrigeration and rotary vacuum pumps.
COOLING BEFORE PACKING
Cooling problems with produc s in uni-tized pallets, or liner-packed pr ducts, canbe avoided by cooling the prod cts beforepacking. However, this increas s coolingcosts if the products are cooled beforeculling and sorting operations. f 20% cul-lage occurs after cooling, the c oling costincreases 25%. If 50% is culled (for exam-
ple, diverting pears to a proces or), thecooling cost per ton of packed roduct
III
106 CHAPTER II
Figure 11.17
Vacuumcooler being loaded. Batchesof product are filled into the cham-ber,which is then closedand the vacuumis drawn.Thisunit usesapatented processthat introduceswater during the cooling cycleto reducewater evaporationfrom the product.
doubles. Another disadvantage to cooling
before packing is that cold fruit is more
subject to mechanical damage in packingthan warm fruit. For cantaloupes and cher-
ries, these problems are avoided by remov-
ing most culls before hydrocooling.
Some rewarming occurs when produce is
packed after cooling. A mild breeze canrewarm unpacked products to near ambient
temperatures within 30 minutes. Some pack-ers minimize this by only partially cooling
the product before packing, followed by
complete c~oling after packing. One packersolves the problem another way: Fruit arriv-
ing from the field is forced-air cooled in bins;the bin dump is located in the forced-air bin
cooling room. The cooled fruit moves fromthe cold room to a nearby packing area,where it is sorted, sized, and volume-filledinto containers within 3 or 4 minutes.
Packed containers are conveyed into a cold
room for palletizing within 6 or I minutes of
leaving the bin cooler. In this system, prod-
uct rewarming is minimal; the product mustbe finish cooled.
SELECTING A COOLINGMETHOD
The physiological or physical characteristicsof a product may limit the suitable coolingmethods. For example, strawberries, whichcannot tolerate free moisture because of dis-
ease and injury problems, cannot be cooledby hydrocooling or package-icing, andbecause they require fast cooling after harvest,
~
room cooling is not suitable. cuum coolingis fast but causes noticeable m isture loss in
berries. Thus, forced-air cooti g is the onlyeffective cooling method for st awberries.Other commodities, such as s me deciduous
fruits and many vegetables, ar suited to sev-eral cooling methods. Table 11.1 lists thecooling methods commonly u ed for varioustypes of fruits, vegetables, and flowers.
If a cooling facility is used for severaltypes of commodities, it mayor may not bepossible to use the same met od for allproducts. Table 11.1 shows t at vacuumcooling, package icing, and r om cooling areused for only a few products; hydrocoolingis suited to a much wider var ety; andforced-air cooling is adaptabl to most prod-ucts and is therefore ideal for operationswhere a wide variety of prod cts must becooled. This is why forced-ai and roomcooling are most often recom ended forsmall-scale operations, which typically han-dle many commodities and ay change theproducts they handle as the arket changesfrom year to year. In some ca es the productmix may require that more th n one coolingsystem be used.
PRODUCT TEMPERATURE
REQUIREMENTSA facility that must handle P
t
ducts with
very different optimal storage temperaturesusually needs separate coolin facilities.Keeping chilling-sensitive co moditiesbelow their critical threshold emperaturetoo long will cause damage.
If product temperature req irements arenot very different, careful coo er manage-ment may allow a common coler to beused. For example, summer s uash can beforced-air cooled in a O°C (32 F) room if it isremoved from the cooler at laC (45°F) flesh
temperature. It should then b stored at laC(45°F). Many chilling-sensitive commoditiescan be safely kept for short p riods belowtheir chilling threshold tempe ature.
COSTS OF OPERATING C OlERS
Capital costs vary significant! among dif-ferent types of coolers. Liqui ice coolersare the most expensive to pu hase, fol-lowed by vacuum coolers (in luding unitsequipped with water spray ca ability),forced-air coolers, and hydro oolers. Figure11.18 shows the capital cost, xpressed in
cost per daily cooling capacity, of four typesof coolers, based on 1998 data. The wide
cost range for liquid icing reflects the varia-tion in the amount of ice that is put in thecarton. If just enough ice for product cool-ing is used, much less refrigerating capacityis needed and capital cost is lower. However,many broccoli shippers add extra ice to han-dle refrigerating needs in transport, andthey also add an extra 4.5 kg (10 Ib) of iceso that the box arrives at the market withunmelted ice.
The capital cost per unit cooled can beminimized by using the equipment as muchas possible. Vacuum-cooling equipment isvery compact and is often portable. In Cali-fornia, vacuum coolers are moved as har-vest locations change during the year. It iscommon in the western United States for
portable vacuum coolers to be used morethan 10 months per year. Forced-air coolingfacilities can be used for short-term storageof product during the harvest season andfor long-term storage of product after theseason ends.
Energy costsThe energy cost of cooling varies greatlyamong coolers (fig. 11.19). Energy use isexpressed in terms of an energy coefficient(EC), defined as
cooling work done
EC = (expressed in kilowatt-hours)electricity purchased (kWh)
FAR
When packaged, only use FA
-
-~-, -~ ---
~--~- ----
~---
High EC numbers indicate
j
n energy-effi-cient operation. The range f EC for eachtype of cooler reflects differ nces in designand operation procedures b tween coolers ofthe same type.
Actual energy costs for 0
1
' erating a coolercan be calculated using the ormula below(assuming a value for EC). nergy costs canbe less than 5% of total cos s in efficient
cooling systems.
Wx DxRxC
Electricity cost = 3,413 x EC
in product (OF)
El .. WxectnClty cost =
R= Room cooling
VC = Vacuum cooling
WVC = Water spray vacuum coolin
In English units:
where
W = weight cooled (lb)TD = temperature reductioR = electricity rate ($/kWh)EC = energy coefficientCp = 1 Btu/lb-oF3,413 Btu/kWh
In 51 (metric) units:
where
W = weight cooled (kg)TD = temperature reductioR = electricity rate ($/kWh)EC = energy coefficientCp = 4,184 ]/kg-OC3.6 jlkWh
Figure 11.18
Capital cost of commonlyusedcooling systems(in 1998 dollars).
gin HC>;re-c::S 3~~'0~ 2 ,-re~t::.:>u
4
8.0>;re-c.;,.,
6.0 ~~~'0
4.0 g.;;~u
2.0
0 0Liquid ice Forcedair HydroVacuum
Figure 11.19
Energy use of commonly used cooling systems.
1-
h
gnrs of "E:
:~ 2::::Q)0u>-0>1:;;<::u.J
oler
w
can
)F)
C)
3
0Vacuum Hydro Water spray
vacuumPackageice Forcedair
labor and other equipment costs must be
included in calculating total operating costs.Although no specific data are available for
these costs, they can vary significantly. For
example, a hydrocooler built into a packingline requires very little labor and other
equipment, but stand-alone coolers used infield packing operations require operators
and lift trucks for moving product in andout of the cooler.
If a cooling method requires that the
product be packaged in a special carton, theextra cost of the carton should be included
in a comparison of cooler types. For exam-ple, package icing, hydrocooling, and water-
spray vacuum cooling need water-resistantpackaging. This can increase the cost of an
individual box by 25 cents to $1 depending
on the design, size, and quantity of boxespurchased.
COOLING 109
Other considerations
Marketing tradition may dicta e the choice ofa cooling method. For example, some mar-kets require that broccoli box s arrive withice in them. Shippers selling i this marketmust select a package-ice cool ng system.
Existing facilities may dete mine the typeof cooler to be used. An exist"ng cold-storageroom can often be used for fo ced-air coolingof small amounts of product y installing asmall portable fan. larger am unts of prod-uct usually require installatio of morerefrigeration capacity and a p rmanent airhandling system.
A short harvesting season i a particularlocation may cause an ope rat r to considerusing portable cooling equip ent. The cool-er can be moved to a grower's other produc-tion areas or leased to shipper in otherareas, eliminating the cost of uying separatepermanent coolers for each 10 ation. Someportable coolers can be leased or jointlyowned by shippers and cooler manufactur-ers, eliminating or reducing t e need for cap-ital expenditure.
Some growers contract wit commercialcompanies to do their cooling This requiresno direct capital investment a d no operat-ing or management costs. But the growerloses some control over the pr duct, oftencannot control when product' s cooled, andloses the chance to make a pr fit from thecooling operation. Cooling co peratives cangive a grower some of the adv ntages ofowning a cooler while reducinr: individualinvestment costs.
ESTIMATING REFRIGERATIONCAPACITY
After deciding which cooling ethod ormethods to use, the operator ust estimatethe amount of refrigeration ca acity needed.This will help determine how arge a cooleris needed. In general, 2.3 kW f refrigerationrequires about 1 kW of compr ssor capacity,or 1 ton requires a little less t an 2 horse-power of compressor capacity. Coolersrequiring less than about 40 k (11.4 tons)of refrigeration capacity can of en be built bythe grower.
The refrigeration capacity n
teded for large
systems must be determined b T a refrigera-tion engineer. The engineer wi 1 consider anumber of factors, such as
pi
I 10 CHAPTER II
Figure 11.20
. amount of product cooled
. temperature of incoming product
. rate at which the product is received atthe cooler
. required speed of cooling
. variety of products cooled and theirunique cooling requirements
. building design and how it affects heatgain to the refrigerated volume
. heat input from lights, fan motors, fork-lifts, people, etc.
An estimate of the amount of refrigera-tion capacity needed for small-scale facili-ties does not require detailed calculations.Figure 11.20 can be used to estimate therefrigeration capacity needed for a coolerhandling up to 450 kg/hr (1,000 Ib/hr). Forexample, if a product is cooled from 23.9°C(l5°F) to 1. lOC (35°F) (a temperature dropof about noc [40°F]), and the cooler musthandle a maximum of 400 kg/hr (900 lb/hr) ,then 16 kW (4.5 tons) of refrigerationcapacity is necessary. Estimates from thisfigure are based on reasonably fast cooling;slow cooling, as is achieved by room cool-ing, requires slightly less refrigerationcapacity. The figure is also based on theassumption that heat input to the coolerfrom sources other than the product areless than 25% of the total.
---
Approximate mechanicalrefrigeration requirementsfor small scalecoolersbasedonmaximum hourly product input and product temperaturedrop.
06
(kg/hr)200 400300100
5
4
2
20
18
v;-c0~ 4~OJ
~OJ
~ 3c.210t 2.g>"Qja:
16 ~::.
14 ~~
12 ~c
10 .~10
8 t.E'
6 ~a:
0
0 200 400 600 800
Maximum rate of product entering cooler (Ib/hr)
0
1,000
Some small-scale cooli g operationspurchase ice for cooling. igure 11.21 canbe used to estimate the da ly amount of iceneeded to operate a small ooler. For exam-ple, if 2,000 kg/day (4,40 Ib/day) of productare cooled by about noc 40°F), a little morethan 1,000 kg (1.1 tons) f ice would bemelted. The figure is base on 50% of the icebeing used for product co ling and the restof the cooling potential 10 t to outside heatgain. This efficiency level s common foruninsulated hydrocoolers. For more detailson designing and operatin coolers, consultthe UC ANR publication ommercial Coolingof Fruits, Vegetables, and Fowers (Thompsonet al. 1998).
EFFECTIVE COOLER MANAGEMENT
Proper management of a cooler involveseffective product cooling t minimum cost.Records of cooler operation are vital toenable a manager to evalu te the cooler.Good records should incl de
. a sampling of incoming and outgoingproduct temperature fot each lot and thetype of product cooled
. the temperature of the
/
OOling mediumduring each cooling cy le
. the length of cooling c cles
. the quantity of product cooled in eachcycle
. operating conditions ofi refrigeration sys-tem, such as suction a~d head pressures
. monthly energy use
Knowledge of incomin product tempera-ture is helpful in estimati g the cooling timerequired; outgoing product temperature isessential for determining he quality of thecooling process. The aver ge product tem-perature should be within acceptable toler-ances, and, just as import nt, the warmestproduct temperatures sho ld be withinacceptable tolerances. A g od operator checksoutgoing product tempera ures in variousparts of the load to deter ine where thewarmest product tends to e, and then con-trols the operation to get roduct in this areabelow required temperatu e. For example, intunnel-type forced-air coo ers, the warmestproduct is usually next to the return air tun-nel in the pallet farthest fr m the fan. Hydro-coolers tend to cool prod ct fairly uniformly,
....
In
ice
!m-
,ductmore
e iceest~at
ilsult
,lingson
;t.
he
s-s
ra-ne
ks
eaIn
l-
0-
y,
Figure 11.21
Approximateamount of ice neededto operate small-scalecoolersbasedon amount ofproductcooledper day and product temperaturedrop.
3.001,000
(kg/day)2,000 4,0003,000
2.5
2,000
';ii"2.0<=0
~>-~ 1.5
C;...1,500 -;:ro~
:vCo
1,000 ~:vCo
~ 1.0
0.5 500
00 2,000 4,000 6,000 8,000
Product cooled per day (Ib/day)
010,000
and the warmest product is in areas withrestricted water flow, perhaps caused by mis-aligned box vents. Vacuum coolers tend tocool very uniformly. The performance of liq-uid icing systems is determined by the uni-formity of ice added to each box.
Other factors are useful in determiningthe long-term performance of a cooler. Forexample, if cooling times begin to increaseand the temperature of the cooling mediumdoes not change, then there is a good chancethat flow of the cooling medium through theproduct is being restricted (assuming thatthe type of product and its incoming temper-ature remain constant). If the temperature ofthe cooling medium shows a trend ofincreasing during the cooling cycle, theremay be problems in the refrigeration system,or there may be too much product in thecooler. Changes in operating conditions ofthe refrigeration system can give clues topossible problems and their solUtions.
Regular maintenance is important for alltypes of coolers. In vacuum coolers, doorseals must be checked regularly and pressuregauges must be recalibrated about once peryear. Daily cleaning is vital for proper hydro-cooler operation. Trash screens, the waterdistribution pan, and the water reservoirmust be cleaned each day and chlorine levelsmust be checked several times a day. Fluid
COOLING I I I
II
levels and other features of th~ refrigeration
system should be checked dailrCOLD ROOMS
Cooled product will quickly wfrm up unlessit is loaded directly into refrigerated trans-port vehicles or placed in cold rooms.
Rewarming wastes the benefitsiof cooling;and cooled products left in a wprm environ-ment are also subject to conderp.sation, whichmay lead to disease. To help s01ve these
problems, a cold room should ~e associatedwith the cooler. In some cases, Ithe coolermay be a part of the cold room4 as withforced-air coolers, bUt this is not recom-mended. Small cold rooms can be commer-
cially constructed, purchased if) prefabricat-
ed form and erected by grower~, constructedby growers, or purchased as used refrigerat-ed transport vehicles (rail cars, 'trailers, ormarine containers). The cost of~the coldroom should be added to the total capitalcost of a cooling facility. I
I
SUMMARY IEffective cooling and temperatu~e manage-ment requires a complete understanding ofproduct and market requiremen'ts, and of thecooling methods available. I
. Rapid thorough cooling and good producttemperature management are! essential for
successful produce marketing.
. Cooling is part of the total srlstem of han-dling perishables. Effects on <;:oolingratemust be considered wheneve~ a change ismade in packaging or handling.
. Requirements for cooling an~ cold storagediffer, and they should be coI1sidered astwo separate operations.
. Four cooling methods and variations areavailable to achieve rapid cooling. Select acooling method or methods t~at fit theneeds of your customers and ~he range ofcommodities you handle.
. Fast cooling can often be achifved throughminor modifications of existiIJ.g coolingfacilities. Design requirement~ should bedetermined by a qualified refrigerationengineer after evaluating the domplete
refrigeration system. The incr~ased costsinvolved in achieving faster cqoling maybe relatively small when the tC/Jtalcost ofthe cooling system is considertd.
I I 2 CHAPTER II
. Cooling time can often be reduced byattention to details of air or water
management, package design, packingmaterial, and pallet stacking patterns.
. Keep careful records of cooling perfor-mance. Good cooler management requiressystematic measurement and recording ofproduct temperatures.
REFERENCES
Hardenburg, R. E., A. E. Watada, and C. Y.Wang.
1986. The commercial storage of fruits, vegeta-
bles, and norist and nursery stocks. USDA
Handb. 66. 130 pp.
Isenberg, F M. R, R F Kasmire, and]. E. Parson.
1982. Vacuum cooling vegetables. Cornell Univ.
Coop. Ext. Bull. 186. 10 pp.
jeffrey,].]. 1977. Engineering principles related to
the design of systems for air cooling of fruits
and vegetables in shipping containers. Proe.
29th Int!. Conf. on Handling Perishable Agricul-
tural Commodities. East Lfnsing: Mich. StateUniv. 151-164.
Rij, R. E.,] F
.
Thompson, and
1
. s. Farnham. 1979.
Handling, precooling, and temperature manage-
ment of cut nower crops f r truck transporta-
tion. Oakland: Univ Calif. Coop. Ext. Leanet
21058.
Sargent, S. A., M. T. Talbot, and
f
' K. Brecht. 1989.
Evaluating precooling met ods for vegetable
packinghouse operations. roe. Fla. State Hort.
Soc. 101:175-182.
Thompson,]. E, and R. F Kasmfre. 1981. An evapora-
tive cooler for vegetable crpps. Calif. Agrie.35(3-4): 20-21.
Thompson,]. F, F G. Mitchell,
f
. R. Rumsey, R. E
Kasmire, and C. H. Crisos o. 1998. Commercial
cooling of fruits, vegetable and flowers. Oak-
land: Univ. Calif. Div. of A . and Nat. Res. Pub!.
