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  • 7/30/2019 flat design for FAC

    1/4qAprilJune 1999 9(2)202

    RESEARCH REPORTS

    Optimizing FlatDesign forForced-airCooling ofBlueberriesPackaged inPlasticClamshells

    Jerry C. Leyte1 and

    Charles F. Forney2

    ADDITIONAL INDEX WORDS. precooling,

    pressure cooling, vent holes, contain-ers, pallet, Vaccinium corymbosum

    SUMMARY. Forced-air cooling rates ofhighbush blueberries (Vaccinium

    corym bosum L.) packaged in 6-oz(177-mL) or 1-pt (473-mL) clamshellcontainers were affected by positionsof vent ho les in corrugated flats. Mostrapid cooling occurred in flats withvents across the top of the flat.Additional vents aligned in front ofclamshells resulted in more rapid anduniform coo ling than vents placedbetween clamshells. Vent ho les in the

    bottom of flats had no effect oncooling rates. Clamshells cooled moreslowly in the front of the pallet wherecold air entered than in the back ofthe pallet where cold air exited. Fruitin 6 -oz clamshells cooled faster thanfruit in 1 -pt clamshells.

    R apid removal of field heatfrom fresh highbush blue-berries following harvest iscritical to maintain q uality and extendstorage life. Blueberries store best at32 F (0 C) and t herefore need to be

    cooled to this temperature as quicklyas possible following harvest(H ardenburg et al., 198 6). D elays in

    cooling result in decreased fruit qual-ity and increased decay (Hudson andTietjen, 1981). Room cooling ofpalletized, packaged fruit is not ad-equate to prevent quality loss becauseof slow coo ling rates and un even cool-ing within the pallet of berries (Boyette,1996). Forced air cooling enables pal-lets of fruit to be sufficiently cooledwithin a few hours with even t empera-ture distribution throughout the pal-let improving fruit storage life (Boyette,1996).

    Recently, plastic clamshells havebegun to replace the traditional 1-pt(473-mL) fiber cup containers as thepreferred container for packaging b er-ries (Singh, 1991). These containersare available in numero us designs withvarious arrangemen ts and sizes of ventho les. Also, smaller clamshells are en-tering the marketplace resulting in

    many flats of berries holding 24 con-tainers instead of the traditional 12one-pint containers. The closed na-ture of the clamshell causes greaterrestrictions in the contact of cold airwith the fruit compared t o t he tradi-tional open pint b asket. Th is has raisedquestions regarding the optimum flatdesign for efficient cooling of thesecontainers. Flat design has a stronginfluence on rates of fruit cooling(Arifin and Chau, 1988; mond et al.,1994; Talbot et al., 1995). The po si-tion o f vents on the flat, in relation to

    the clamshells they ho ld, may stronglyinfluence the cooling efficiency of thecontained product. The objective of

    1Agricultural engineer, N ova Agri Associates Ltd ., PortWilliams, N. S. B0P 1T0, Canada.2Storage physiologist, Agriculture and Agri-FoodCanada, Atlantic Food and Horticulture ResearchCentre, 3 2 Main St., Kentville, N.S., B4N 1J5, Canada.

    Contribution 2186. We thank Sherry Fillmore andBrad Walker for assistance in statistical analysis. Thecost of pub lishing this paper was defrayed in part by thepayment of page charges. Under postal regulations,this paper therefore must be h ereby marked advertise-mentsolely to indicate this fact.

    Fig. 1. Patterns of air vents oncorrugated cardboard flats used inforced-air cooling study. Drawingsrepresent a side view of the flats endpanel and the shaded areas representvents. Terminology referring to ventpatterns is listed to the right of eachdesign.

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    this study was to determine the opti-mum combination of vent holes in acorrugated cardboard flat that will pro-

    vide rapid and uniform cooling of freshblueberries throughout a pallet.

    Materials and methodsForced-air cooling experiments

    were conducted t o compare the cool-ing rates of berry flats using variousvent designs. The flats outside dimen-sions were 19 13 4 inches (48 33 10 cm) which held 24 6-oz (177-mL)clamshells, and 20 15 3/ 43 1/ 4 inches(51 40 8 cm) which held 12 1-pt(473-mL) clamshells. Four inlet de-signs were studied for each flat size,

    with either a solid or vented bottom(Fig. 1). Vented bottoms consisted offour 1-inch (2.5-cm) diameter holeslocated 4 inches (10 cm) from eachside. Air vents in th e sides of flats weredesignated as being in front when theyaligned with the vents of the clamshells,or between when they were locatedbetween the clamshells. Vents locatedalong the top edge of the inlet andoutlet sides were designated as topopen or top closed. For each flat de-sign, the percent surface area attrib-

    uted to vents is listed (Table 1). Theorientation of clamshells to vents inthe flats is shown in Fig. 2. The flatswere stacked on a standard 40 48inch (102 122 cm) pallet in a 2 3grid, 4 levels high for a tot al of 24 flatsof product (Fig. 3). A commercialpallet of blueberries would have a simi-lar 2 3 grid with 16 layers of flats.Height was scaled down to 1/ 4 while

    other dimensions remained full scalefor the experiment al trials performed .The plastic clamshells used were mod-els 3535-125 (6-oz) and 5040-275(1-pt ), designed and supplied by UltraPac Inc. (Rogers, Minn.) . Theclamshells had air inlets on all sides ofthe container as well as on top of the lid(Fig. 4). The clamshells were stackedin a 4 3 grid with the 6-oz clamshellstwo layers high and the 1-pt clamshellsa single layer within each cardboardflat.

    Cooling experiments were per-

    formed in a 32 F (0 C) cold roomusing a portable precooling fan andplenum constructed at the KentvilleResearch Centre (Fig. 3). The plenumcovered the full width and height of thepallet. The plenum opening had a foambumper pad on all sides to create a tightseal against thepallet of fruitand to eliminateshort-circuitingof air around thepallet. Air wasdrawn through

    the pallet by a20-inch (51-cm) fan drivenby a variablespeed 1/ 3 horsepower motorwith a capacityof 1500 ft3/ min(42.5 m3min1)at 0.5 inches (13mm) of staticpressure (P20TB r e e z e w a y

    Panel Fan; Pennsylvania Ventilator Co.,Philadelphia).

    All tests were cond ucted betweenMay and July 1996, which is beforethe Nova Scotia highbush blueberryseason. Blueberries from the southernU.S. were used for measuring the fruittemperatu re at various locations withinthe pallet. Copper-constantan ther-mocouples, monitored by a data log-ger (CR7; Campbell Scientific, Lo-gan, Utah), were inserted in the cen-ters of berries to measure the internalfruit temperature during the coolingexperiments. A thermocouple andblueberry were placed in the center of48 selected clamshells filled with dryyellow-eyed beans (Phaseolus vulgarisL.) as a product substitute to create thenecessary air resistance and heat sourcebefore cooling. Yellow-eyed beans werean economical substitute for blueber-

    ries because of their similarity in size.Differences in air flow resistance be-tween beans and blueberries may af-fect absolute cooling rates, but theseeffects would be relative to treatm enteffects.

    Clamshells without a blueberryand thermocouple also were filled withbeans. Temperatures were measuredin each flat and the position of the flat

    Table 1. To tal percent vent area of corrugated flats that held 24 6-oz (17 7-mL)or 12 1-pt (4 73-mL) clamshells. Vents were located across the top of the endpanel (closed or open) and/or in front or between clamshells in the flat.

    Flat vent design % Vented

    Top Side 6-Ounce 1-Pint

    Closed Front 8.1 7.7Between 4.6 5

    Open Front 18.2 21.8Between 14.7 19.1

    Fig. 3 . Arrangement of flats on thepallet and their orientation to the fanand plenum for forced air cooling.

    The top, sides, and bottom of theflats were covered wi th a plastic tarp.A bumper pad around the plenumopening provided a seal that pre-vented air from bypassing the flatsthrough the pallet end. The pallet offlats and plenum were located in acold room held at 32 F (0 C) .

    Fig. 2. Orientation of 6 -oz ( 177-mL)and 1-pt (473-mL) clamshells tovents in end panels of cardboardflats. Thick lines represent theoutline of flats.

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    RESEARCH REPORTS

    in the 2 3 grid was designated as thein, middle, or out position in relationto the direction of air flow (Fig. 5).Each flat containing 6-oz clamshellshad t wo layers of clamshells. Two ther-mocouples were placed in each flatwith one thermocou ple located in eachlayer (Fig. 5). In the 1-pt flats thethermocouples were placed in the topor bottom third of each clamshell tokeep the th ermocouple locations simi-lar to that of the 6-oz. clamshells. Twoflat designs were tested simultaneouslywith each flat type stacked on one side

    of the pallet (Fig. 3). The blueberrieswere replaced after 2 t o 3 d to ensurethe fruit remained turgid.

    Before cooling, the pallet was heldat room temperature to allow the en-tire pallet to reach an equilibrium t em-perature of 59 to 68 F (15 to 20 C).Thermocouples were checked to en-sure uniform fruit temperatures priorto b eing placed in th e cold room forcooling. The pallet was placed againstthe precooler and a plastic tarp wasused to cover the t op, sides, and bot-

    tom of the flats resting on thepallet. Cooling air was drawnby the fan throu gh th e flats tothe fan plenum where it reen-tered the room. When theprecooler was started, a staticpressure of 0.5 inches (1.3 cm)was quickly established usingthe variable speed fan and aninclined manometer. The pres-

    sure differential was measured betweenthe room air and the center of theplenum. The cooling start time wasrecorded and the temperatures fromeach thermo couple were logged every5 min for 3 h which was sufficient forcalculating the time required to re-duce the initial temperature d ifferencebetween the b lueberries and the roomair by 7/ 8 (7/ 8-cooling time).

    The experiment was conductedas a 2 2 2 3 factorial for the 6 -oz

    and 1-pt clamshell flats. There weretwo levels of top vents (open or closed),two levels of side vents (front or be-tween), two levels of bottom vents(solid or vented), and three levels ofposition within the pallet (in, middle,or out). The factorial experiment wasconducted as a balanced incompletedesign over eight runs. Each run in-cluded two of the eight flat types withone flat type comprising half of thepallet (Fig. 3) . Each flat type was testedtwo t imes. Temperature data from eachthermocouple were fitted by regres-

    sion to a logarithmic function usingGenstat 5 version 3.2 and the 7/ 8-cooling times were calculated for eachblueberry in which temperature wasmonitored (Payne et al., 1993). The7/ 8-cooling times from each flat de-sign and position within the palletwere analyzed using ANO VA and dif-ferences between mean values weredetermined using least significant dif-ference (P < 0.05). Experiments using6-oz and 1-pt clamshells were ana-lyzed separately.

    Results and discussionTh e 7/ 8-cooling times for all flat

    designs and for the in, middle, and o utlocations are shown (Figs. 6 and 7).Fruit in flats with open tops containingthe 6-oz (177-mL) clamshells cooled10%to 40%faster than those in closedtopped flats (Fig. 6) . Similarly, fruit inflats with open tops containing 1-ptclamshells cooled 15 % to 55% fasterthan those in closed topped flats (Fig.7). The vent on the top of the flatsallowed cold air to be drawn into the

    vent ho les in the top of the clamshellsincreasing cooling rates. In addition,the vent area of open topped flats was2.2 to 3 .8 times greater than flats withclosed to ps (Table 1) , which may haveallowed greater air flow across the fruitresulting in more rapid cooling. How-ever, Talbot et al. (199 5) found straw-berries in pint clamshells cooled fasterin flats with slotted sides and closedtops compared to traditional flats withthe open tops. They suggested thatflats with top vents channeled air pastthe fruit and d id not cool it as rapidlyas when air was drawn through thefruit with side vents. Similarly, Arifinand Chau (1987) found strawberriesin 1-pt open baskets cooled 10% to20%faster in flats with side vents andclosed tops than in flats with solid sidesand open tops when vent area wasmaintained at a constant value (14%to

    18%). In our study the rapid and evencooling ob served in the flats with opentops and vented sides may have re-sulted from greater vent area and en-hanced mixing of cold air throughoutthe flat.

    Position of the vents in the sidesof the flats had no significant effect onthe cooling rates of the 6-oz clamshells,however, in the 1-pt clamshell flats,front vents resulted in 10% to 40%faster cooling rates than between ventsin comparable flats. Vents located inthe front position aligned with those

    of the clamshells may be more effectivein drawing air through the fruit thanthe vents between the clamshells. Ventsbetween the clamshells may result inchanneling of air past the fruit result-ing in slower cooling rates. Faster cool-ing of fruit in flats with front ventingcould also be a result of the one add i-tional vent hole compared t o t he be-tween flats. This additional vent pro-vided 2.7% more vent area. In o therstudies, side vents that align with ventsin the fruit containers resulted in faster,more even cooling than top-vented

    flats (mond et al., 1996; Talbot et al.,1995).

    Venting the bottom of the flathad no effect on cooling rates of eitherthe 6-oz clamshell flats or the 1-ptclamshell flats (Figs. 6 and 7). Talbotet al. (1995) also ob served little differ-ence in the rate of cooling betweenflats with vented or unvented bot-toms. While venting the bottom offlats does not improve cooling rates,these vents may be beneficial to im-prove air circulation and maintain fruit

    Fig. 4. Design of 6-oz ( 177-mL) and1-pt (4 73-m L) plastic clamshellcontainers.

    Fig. 5. Position of thermocouples inclamshell containers within each levelof flats on the pallet. Thermocoupleswere located in the center of 6-oz(177-mL) clamshells in the top orbottom layer within the flat or in themiddle of the upper (top layer) orlower (bottom layer) third of 1 -pt(473-mL) clamshells.

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    Fig. 7. The 7/8-cooling times ofblueberries in 1-pt (473 -mL)clamshells at different locationswithin the pallet and in cardboardflats with different vent hole pat-

    terns. Points represent the means of16 thermocouples. The error barrepresents the least significantdifference (P < 0.05) between means.

    Fig. 6. The 7/8-cooling times ofblueberries in 6-oz (177-mL)clamshells at different locationswithin the pallet and in cardboardflats with different vent hole pat-

    terns. Points represent the means of16 thermocouples. The error barrepresents the least significantdifference (P < 0.05) between means.

    temperature during cold storage.In addition to the flat design, th e

    position of the flat in the pallet affectedcooling rates. Fruit cooled slowest inflats where the cold air entered thepallet and fastest in flats where the coldair left the pallet. Differences in cool-ing rates from the in to the ou t posi-tions in t he 6 -oz clamshell flats ranged

    from 0 to 22 min depending on flatdesign (Fig. 6). In the 1-pt clamshellflats differences were substantiallygreater. Flats in th e out position coo led20 t o 70 min faster than flats in the inposition (Fig. 7). This slower rate ofcooling in the inlet side of the palletmay be the result of air channeling pastthe clamshells. As the air penetratesdeeper into the pallet, turbulent mix-ing may increase contact of the cold airwith the fruit in the clamshell contain-ers resulting in increased coo ling rates.A similar effect was observed by mondet al. (1996), when strawberry fruit inopen baskets was cooled in flats withopen tops. Fruit cooled more slowly inthe front o f the flat than in the middleand they suggested that air flow pat-terns created a wake which reducedthe efficiency of cooling in the firstbasket of fruit. In other studies using

    open baskets of strawberries, fruitcooled fastest in th e front of the palletwere the air entered and slowest in th eback (Arifin and C hau, 19 87; mondet al., 1996).

    In general, fruit in 6-oz clamshellscooled more rapidly and more evenlythrough the pallet than fruit in 1-ptclamshells. Differences in cooling rateswere as great as 65%(80 min) whencomparing 6-oz and 1-pt clamshells inflats with closed t ops and between sidevents placed in the front of the palletwhere the cold air entered. Th ere were

    minimal differences in coo ling rates ofthe two clamshell types when theywere held in flats with open tops and infront side vents. Talbot et al. (1995)found that strawberries cooled fasterin pint than in quart clamshells. Thefast coo ling rates observed in the smallclamshell may be the result of thegreater surface area to volume ratiofound in the 6-oz clamshells com-pared to the 1 -pt clamshells. This wouldfacilitate faster heat exchange betweenthe warm fruit and the cold air. Inaddition, the greater surface irregu-

    larities of the stacked 6-oz clamshellscompared to the smooth sides of the 1-pt clamshell may have caused greaterturbulence, resulting in better mixingof the cold air with the fruit.

    An ideal flat design would allowrapid, even coo ling o f all fruit th rough-out the pallet. From the results of thisstudy, when cooling blueberries pack-aged in plastic clamshells, the mostrapid, even cooling occurred in flatswith open tops and vents positioned infront o f the clamshells on the sides of

    the flats. Flats of this design resulted in7/ 8-cooling times of