1、28.1CHAPTER 28METHODS OF PRECOOLING FRUITS, VEGETABLES, AND CUT FLOWERSProduct Requirements 28.1Calculation Methods 28.1COOLING METHODS 28.3Hydrocooling . 28.3Forced-Air Cooling 28.6Forced-Air Evaporative Cooling 28.8Package Icing . 28.8Vacuum Cooling . 28.9Selecting a Cooling Method 28.11Cooling Cu
2、t Flowers 28.11Symbols 28.11RECOOLING is the rapid removal of field heat from freshlyPharvested fruits and vegetables before shipping, storage, or pro-cessing. Prompt precooling inhibits growth of microorganisms thatcause decay, reduces enzymatic and respiratory activity, and reducesmoisture loss. T
3、hus, proper precooling reduces spoilage and retardsloss of preharvest freshness and quality (Becker and Fricke 2002).Precooling requires greater refrigeration capacity and coolingmedium movement than do storage rooms, which hold commoditiesat a constant temperature. Thus, precooling is typically a s
4、eparateoperation from refrigerated storage and requires specially designedequipment (Fricke and Becker 2003). Precooling can be done byvarious methods, including hydrocooling, vacuum cooling, air cool-ing, and contact icing. These methods rapidly transfer heat from thecommodity to a cooling medium s
5、uch as water, air, or ice. Coolingtimes from several minutes to over 24 hours may be required.PRODUCT REQUIREMENTSDuring postharvest handling and storage, fresh fruits and vege-tables lose moisture through their skins through transpiration. Com-modity deterioration, such as shriveling or impaired fl
6、avor, mayresult if moisture loss is high. To minimize losses through transpi-ration and increase market quality and shelf life, commodities mustbe stored in a low-temperature, high-humidity environment. Vari-ous skin coatings and moisture-proof films can also be used duringpackaging to significantly
7、 reduce transpiration and extend storagelife (Becker and Fricke 1996a).Metabolic activity in fresh fruits and vegetables continues for ashort period after harvest. The energy required to sustain this activ-ity comes from respiration, which involves oxidation of sugars toproduce carbon dioxide, water
8、, and heat. A commoditys storagelife is influenced by its respiratory activity. By storing a commodityat low temperature, respiration is reduced and senescence is de-layed, thus extending storage life. Proper control of oxygen andcarbon dioxide concentrations surrounding a commodity is alsoeffective
9、 in reducing the respiration rate (Becker and Fricke 1996a).Product physiology, in relation to harvest maturity and ambienttemperature at harvest time, largely determines precooling require-ments and methods. Some products are highly perishable and mustbegin cooling as soon as possible after harvest
10、; examples includeasparagus, snap beans, broccoli, cauliflower, sweet corn, canta-loupes, summer squash, vine-ripened tomatoes, leafy vegetables,globe artichokes, brussels sprouts, cabbage, celery, carrots, snowpeas, and radishes. Less perishable produce, such as white potatoes,sweet potatoes, winte
11、r squash, pumpkins, and mature green toma-toes, may need to be cured at a higher temperature. Cooling of theseproducts is not as important; however, some cooling is necessary ifambient temperature is high during harvest.Commercially important fruits that need immediate precoolinginclude apricots; av
12、ocados; all berries except cranberries; tart cher-ries; peaches and nectarines; plums and prunes; and tropical andsubtropical fruits such as guavas, mangos, papayas, and pineapples.Tropical and subtropical fruits of this group are susceptible to chill-ing injury and thus need to be cooled according
13、to individual tem-perature requirements. Sweet cherries, grapes, pears, and citrus fruithave a longer postharvest life, but prompt cooling is essential tomaintain high quality during holding. Bananas require special rip-ening treatment and therefore are not precooled. Chapter 21 listsrecommended sto
14、rage temperatures for many products.CALCULATION METHODSHeat LoadThe refrigeration capacity needed for precooling is muchgreater than that for holding a product at a constant temperature orfor slow cooling. Although it is imperative to have enough refriger-ation for effective precooling, it is unecon
15、omical to have more thanis normally needed. Therefore, heat load on a precooling systemshould be determined as accurately as possible.Total heat load comes from product, surroundings, air infiltra-tion, containers, and heat-producing devices such as motors, lights,fans, and pumps. Product heat accou
16、nts for the major portion oftotal heat load, and depends on product temperature, cooling rate,amount of product cooled in a given time, and specific heat of theproduct. Heat from respiration is part of the product heat load, butit is generally small. Chapter 24 discusses how to calculate therefriger
17、ation load in more detail.Product temperature must be determined accurately to calculateheat load accurately. During rapid heat transfer, a temperature gra-dient develops in the product, with faster cooling causing larger gra-dients. This gradient is a function of product properties, surface heattra
18、nsfer parameters, and cooling rate. Initially, for example, hydro-cooling rapidly reduces the temperature of the exterior of a product,but may not change the center temperature at all. Most of the prod-uct mass is in the outer portion. Thus, calculations based on centertemperature would show little
19、heat removal, though, in fact, sub-stantial heat has been extracted. For this reason, the product mass-average temperature must be used for product heat load calculations(Smith and Bennett 1965).The product cooling load can then be calculated asQ = mcp(ti tma)(1)where m is the mass of product being
20、cooled, cpis the products spe-cific heat, tiis the products initial temperature, and tmais the prod-ucts final mass average temperature. Specific heats of various fruitsand vegetables can be found in Chapter 19.The preparation of this chapter is assigned to TC 10.9, Refrigeration Appli-cation for Fo
21、od and Beverages.28.2 2010 ASHRAE HandbookRefrigerationPrecooling Time Estimation MethodsEfficient precooler operation involves (1) proper sizing of refrig-eration equipment to maintain a constant cooling medium tempera-ture, (2) adequate flow of the cooling medium, and (3) proper productresidence t
22、ime in the cooling medium. Thus, to properly design aprecooler, it is necessary to estimate the time required to cool thecommodities from their initial temperature (usually the ambient tem-perature at harvest) to the final temperature, just before shipping and/or storage. For a specified cooling med
23、ium temperature and flow rate,this cooling time dictates the residence time in the precooler that is re-quired for proper cooling (Fricke and Becker 2003).Accurate estimations of precooling times can be obtained byusing finite-element or finite-difference computer programs, but theeffort required ma
24、kes this impractical for the design or process engi-neer. In addition, two- and three-dimensional simulations requiretime-consuming data preparation and significant computing time.Most research to date has been in the development of semianalytical/empirical precooling time estimation methods that us
25、e simplifyingassumptions, but nevertheless produce accurate results.Fractional Unaccomplished Temperature DifferenceAll cooling processes exhibit similar behavior. After an initiallag, the temperature at the foods thermal center decreases exponen-tially (see Chapter 20). As shown in Figure 1, a cool
26、ing curvedepicting this behavior can be obtained by plotting, on semilogarith-mic axes, the fractional unaccomplished temperature difference YEquation (2) versus time (Fricke and Becker 2004).(2)where tmis the cooling medium temperature, tiis the initial com-modity temperature, and t is the commodit
27、y final mass average tem-perature. This semilogarithmic temperature history curve consistsof an initial curvilinear portion, followed by a linear portion. Simpleempirical formulas that model this cooling behavior, such as half-cooling time and cooling coefficient, have been proposed for esti-mating
28、the cooling time of fruits and vegetables.Half-Cooling TimeA common concept used to characterize the cooling process is thehalf-cooling time, which is the time required to reduce the tempera-ture difference between the commodity and the cooling medium byhalf (Becker and Fricke 2002). This is also eq
29、uivalent to the timerequired to reduce the fractional unaccomplished temperature differ-ence Y by half.The half-cooling time is independent of initial temperature andremains constant throughout the cooling period as long as the cool-ing medium temperature remains constant (Becker and Fricke 2002).Th
30、erefore, once the half-cooling time has been determined for agiven commodity, cooling time can be predicted regardless of thecommoditys initial temperature or cooling medium temperature.Product-specific nomographs have been developed, which, whenused in conjunction with half-cooling times, can provi
31、de estimatesof cooling times for fruits and vegetables (Stewart and Couey 1963).In addition, a general nomograph (Figure 2) was constructed tocalculate hydrocooling times of commodities based on their half-cooling times (Stewart and Couey 1963). In Figure 2, product tem-perature is plotted along the
32、 vertical axis versus time measured inhalf-cooling periods along the horizontal axis. At zero time, theproduct temperature is the initial commodity temperature; at infinitetime, product temperature equals water temperature. To use Figure2, draw a straight line from the initial commodity temperature
33、atzero time (left axis) to the commodity temperature at infinite timei.e., the water temperature (right axis). Then draw a horizontal lineat the final commodity temperature (left and right axes). The inter-section of these two lines determines the number of half-coolingperiods required (bottom axis)
34、. Multiply the half-cooling time forthe particular commodity by the number of half-cooling periods toobtain the hydrocooling time.The following example illustrates the use of the general nomo-graph for determining hydrocooling time.Example 1. Assume that topped radishes with a half-cooling time of 2
35、.2min are to be hydrocooled using 32F water. How long would it take tohydrocool the radishes from 80F to 50F?Solution. Using the general nomograph in Figure 2, draw a straight linefrom 80F on the left to 32F on the right. Then draw a horizontal lineat the final commodity temperature, 50F. These line
36、s intersect at 1.4half-cooling periods. Multiply this by the half-cooling time (2.2 min) toobtain the total hydrocooling time of 3.1 min.Using nomographs can be time consuming and cumbersome,however. Cooling time of fruits and vegetables may be determinedwithout the use of nomographs by using the ha
37、lf-cooling time Z:(3)Values of half-cooling times for the hydrocooling of numerouscommodities have been reported (Bennett 1963; Dincer 1995;Dincer and Genceli 1994, 1995; Guillou 1958; Nicholas et al. 1964;OBrien and Gentry 1967; Stewart and Couey 1963). Tables 1 to 3summarize half-cooling time data
38、 for a variety of commodities. Fig. 1 Typical Cooling CurveFig. 1 Typical Cooling CurveYtmttmti-ttmtitm-=Fig. 2 General Nomograph to Determine Half-Cooling Peri-odsFig. 2 General Nomograph to Determine Half-Cooling Periods(Stewart and Couey 1963)ZYln2ln-=Methods of Precooling Fruits, Vegetables, and
39、 Cut Flowers 28.3Cooling CoefficientCooling time may also be predicted using the cooling coefficientC. As shown in Figure 1, the cooling coefficient is minus the slopeof the ln(Y ) versus time curve, constructed on a semilogarithmicaxis from experimental observations of time and temperature(Becker a
40、nd Fricke 2002). The cooling coefficient indicates thechange in the fractional unaccomplished temperature difference perunit cooling time (Dincer and Genceli 1994). The cooling coeffi-cient depends on the commoditys specific heat and thermal con-ductance to the surroundings (Guillou 1958). Using the
41、 coolingcoefficient for a particular cooling process, cooling time may beestimated as(4)The lag factor j is a measure of the time between the onset ofcooling and the point at which the slope of the ln(Y ) versus curvebecomes constant i.e., the time required for the ln(Y ) versus curveto become linea
42、r. The lag factor j can be found by extending thelinear portion of the semilogarithmic cooling curve to the ln(Y ) axis;the intersection is the lag factor j.By substituting Y = 0.5 into Equation (4), which corresponds tothe half-cooling time, cooling coefficient C can be related to half-cooling time
43、 Z as follows:(5)Cooling coefficients have been reported by Dincer (1995, 1996),Dincer and Genceli (1994, 1995), Henry and Bennett (1973), andHenry et al. (1976) for hydrocooling and hydraircooling (see theCooling Methods section for discussion of these methods) variouscommodities, as summarized in
44、Tables 2 to 4.Other Semianalytical/Empirical Precooling Time Estimation MethodsChapter 20 discusses various semianalytical/empirical methodsfor predicting cooling times of regularly and irregularly shapedfoods. These cooling time estimation methods are grouped into twomain categories: those based on
45、 (1) f and j factors (for either regularor irregular shapes), and (2) equivalent heat transfer dimensionality.Numerical TechniquesBecker and Fricke (1996b, 2001) and Becker et al. (1996a,1996b) developed a numerical technique for determining coolingrates as well as latent and sensible heat loads cau
46、sed by bulk refrig-eration of fruits and vegetables. This computer model can predictcommodity moisture loss during refrigerated storage and the tem-perature distribution within the refrigerated commodity, using aporous media approach to simulate the combined phenomena oftranspiration, respiration, a
47、irflow, and convective heat and masstransfer. Using this numerical model, Becker et al. (1996b) foundthat increased airflow decreases moisture loss by reducing coolingtime, which quickly reduces the vapor pressure deficit between thecommodity and surrounding air, thus lowering the transpirationrate.
48、 They also found that bulk mass and airflow rate were of pri-mary importance to cooling time, whereas relative humidity had lit-tle effect on cooling time.COOLING METHODSThe principal methods of precooling are hydrocooling, forced-air cooling, forced-air evaporative cooling, package icing, and vac-u
49、um cooling. Precooling may be done in the field, in central coolingfacilities, or at the packinghouse.HYDROCOOLINGIn hydrocooling, commodities are sprayed with chilled water, orimmersed in an agitated bath of chilled water. Hydrocooling is effec-tive and economical; however, it tends to produce physiological andpathological effects on certain commodities; therefore, its use is lim-ited (Bennett 1970). In addition, proper sanitation of the hydrocool-ing water is necessary to prevent bacterial infection of c
