1、19.1CHAPTER 19THERMAL PROPERTIES OF FOODSThermal Properties of Food Constituents 19.1Thermal Properties of Foods 19.1Water Content 19.2Initial Freezing Point 19.2Ice Fraction 19.2Density . 19.6Specific Heat 19.6Enthalpy . 19.7Thermal Conductivity. 19.9Thermal Diffusivity. 19.17Heat of Respiration 19
2、.17Transpiration of Fresh Fruits and Vegetables 19.19Surface Heat Transfer Coefficient 19.24Symbols 19.27HERMAL properties of foods and beverages must be knownTto perform the various heat transfer calculations involved in de-signing storage and refrigeration equipment and estimating processtimes for
3、 refrigerating, freezing, heating, or drying of foods and bev-erages. Because the thermal properties of foods and beveragesstrongly depend on chemical composition and temperature, and be-cause many types of food are available, it is nearly impossible to ex-perimentally determine and tabulate the the
4、rmal properties of foodsand beverages for all possible conditions and compositions. How-ever, composition data for foods and beverages are readily availablefrom sources such as Holland et al. (1991) and USDA (1975). Thesedata consist of the mass fractions of the major components found infoods. Therm
5、al properties of foods can be predicted by using thesecomposition data in conjunction with temperature-dependent math-ematical models of thermal properties of the individual food constit-uents.Thermophysical properties often required for heat transfer calcu-lations include density, specific heat, en
6、thalpy, thermal conductivity,and thermal diffusivity. In addition, if the food is a living organism,such as a fresh fruit or vegetable, it generates heat through respirationand loses moisture through transpiration. Both of these processesshould be included in heat transfer calculations. This chapter
7、 summa-rizes prediction methods for estimating these thermophysical proper-ties and includes examples on the use of these prediction methods.Tables of measured thermophysical property data for various foodsand beverages are also provided.THERMAL PROPERTIES OF FOOD CONSTITUENTSConstituents commonly f
8、ound in foods include water, protein,fat, carbohydrate, fiber, and ash. Choi and Okos (1986) developedmathematical models for predicting the thermal properties of thesecomponents as functions of temperature in the range of 40 to300F (Table 1); they also developed models for predicting thethermal pro
9、perties of water and ice (Table 2). Table 3 lists the com-position of various foods, including the mass percentage of mois-ture, protein, fat, carbohydrate, fiber, and ash (USDA 1996).THERMAL PROPERTIES OF FOODSIn general, thermophysical properties of a food or beverage arewell behaved when its temp
10、erature is above its initial freezing point.However, below the initial freezing point, the thermophysical prop-erties vary greatly because of the complex processes involved dur-ing freezing.The preparation of this chapter is assigned to TC 10.9, Refrigeration Appli-cation for Foods and Beverages.Tab
11、le 1 Thermal Property Models for Food Components (40 t 300F)Thermal Property Food Component Thermal Property ModelThermal conductivity, Btu/hftF Protein k = 9.0535 102+ 4.1486 104t 4.8467 107t2Fat k = 1.0722 101 8.6581 105t 3.1652 108t2Carbohydrate k = 1.0133 101+ 4.9478 104t 7.7238 107t2Fiber k = 9
12、.2499 102+ 4.3731 104t 5.6500 107t2Ash k = 1.7553 101+ 4.8292 104t 5.1839 107t2Thermal diffusivity, ft2/h Protein = 2.3170 103+ 1.1364 105t 1.7516 108t2Fat = 3.8358 103 2.4128 107t 4.5790 1010t2Carbohydrate = 2.7387 103+ 1.3198 105t 2.7769 108t2Fiber = 2.4818 103+ 1.2873 105t 2.6553 108t2Ash = 4.556
13、5 103+ 8.9716 106t 1.4644 108t2Density, lb/ft3Protein = 8.3599 101 1.7979 102tFat = 5.8246 101 1.4482 102tCarbohydrate = 1.0017 102 1.0767 102tFiber = 8.2280 101 1.2690 102tAsh = 1.5162 102 9.7329 103tSpecific heat, Btu/lbF Protein cp= 4.7442 101+ 1.6661 104t 9.6784 108t2Fat cp= 4.6730 101+ 2.1815 1
14、04t 3.5391 107t2Carbohydrate cp= 3.6114 101+ 2.8843 104t 4.3788 107t2Fiber cp= 4.3276 101+ 2.6485 104t 3.4285 107t2Ash cp= 2.5266 101+ 2.6810 104t 2.7141 107t2Source: Choi and Okos (1986)19.2 2010 ASHRAE HandbookRefrigerationThe initial freezing point of a food is somewhat lower than thefreezing poi
15、nt of pure water because of dissolved substances in themoisture in the food. At the initial freezing point, some of the waterin the food crystallizes, and the remaining solution becomes moreconcentrated. Thus, the freezing point of the unfrozen portion of thefood is further reduced. The temperature
16、continues to decrease asseparation of ice crystals increases the concentration of solutes insolution and depresses the freezing point further. Thus, the ice andwater fractions in the frozen food depend on temperature. Becausethe thermophysical properties of ice and water are quite different,thermoph
17、ysical properties of frozen foods vary dramatically withtemperature. In addition, the thermophysical properties of the foodabove and below the freezing point are drastically different.WATER CONTENTBecause water is the predominant constituent in most foods,water content significantly influences the t
18、hermophysical propertiesof foods. Average values of moisture content (percent by mass) aregiven in Table 3. For fruits and vegetables, water content varies withthe cultivar as well as with the stage of development or maturitywhen harvested, growing conditions, and amount of moisture lostafter harves
19、t. In general, values given in Table 3 apply to matureproducts shortly after harvest. For fresh meat, the water contentvalues in Table 3 are at the time of slaughter or after the usual agingperiod. For cured or processed products, the water content dependson the particular process or product.INITIAL
20、 FREEZING POINTFoods and beverages do not freeze completely at a single tem-perature, but rather over a range of temperatures. In fact, foods highin sugar content or packed in high syrup concentrations may neverbe completely frozen, even at typical frozen food storage tempera-tures. Thus, there is n
21、ot a distinct freezing point for foods and bev-erages, but an initial freezing point at which crystallization begins.The initial freezing point of a food or beverage is important notonly for determining the foods proper storage conditions, but alsofor calculating thermophysical properties. During st
22、orage of freshfruits and vegetables, for example, the commodity temperature mustbe kept above its initial freezing point to avoid freezing damage. Inaddition, because there are drastic changes in the thermophysicalproperties of foods as they freeze, a foods initial freezing point mustbe known to mod
23、el its thermophysical properties accurately. Exper-imentally determined values of the initial freezing point of foods andbeverages are given in Table 3.ICE FRACTIONTo predict the thermophysical properties of frozen foods, whichdepend strongly on the fraction of ice in the food, the mass fractionof w
24、ater that has crystallized must be determined. Below the initialfreezing point, the mass fraction of water that has crystallized in afood is a function of temperature.In general, foods consist of water, dissolved solids, and undis-solved solids. During freezing, as some of the liquid water crystal-l
25、izes, the solids dissolved in the remaining liquid water becomeincreasingly more concentrated, thus lowering the freezing temper-ature. This unfrozen solution can be assumed to obey the freezingpoint depression equation given by Raoults law (Pham 1987).Thus, based on Raoults law, Chen (1985) propose
26、d the followingmodel for predicting the mass fraction of ice xice:xice= (1)wherexs= mass fraction of solids in foodMs= relative molecular mass of soluble solids, lbm/molR=universal gas constant = 1.986 Btu/lb molRTo= freezing point of water = 491.7RLo= latent heat of fusion of water at 491.7R = 143.
27、4 Btu/lbtf= initial freezing point of food, Ft = food temperature, FThe relative molecular mass of the soluble solids in the food maybe estimated as follows: Ms= (2)where xwois the mass fraction of water in the unfrozen food and xbis the mass fraction of bound water in the food (Schwartzberg1976). B
28、ound water is the portion of water in a food that is boundto solids in the food, and thus is unavailable for freezing.The mass fraction of bound water may be estimated as follows:xb= 0.4xp(3)where xpis the mass fraction of protein in the food.Substituting Equation (2) into Equation (1) yields a simp
29、le wayto predict the ice fraction (Miles 1974):xice= (two xb)(4)Because Equation (4) underestimates the ice fraction at tem-peratures near the initial freezing point and overestimates the icefraction at lower temperatures, Tchigeov (1979) proposed anempirical relationship to estimate the mass fracti
30、on of ice:xice= (5)Fikiin (1996) notes that Equation (5) applies to a wide variety offoods and provides satisfactory accuracy.Table 2 Thermal Property Models for Water and Ice (40 t 300F)Thermal Property Thermal Property ModelWaterThermal conductivity, Btu/hftF kw= 3.1064 101+ 6.4226 104t 1.1955 106
31、t2Thermal diffusivity, ft2/h w= 4.6428 103+ 1.5289 105t 2.8730 108t2Density, lb/ft3w= 6.2174 101+ 4.7425 103t 7.2397 108t2Specific heat, Btu/lbF (For temperature range of 40 to 32F) cw= 1.0725 5.3992 103t + 7.3361 105t2Specific heat, Btu/lbF (For temperature range of 32 to 300F) cw= 9.9827 101 3.787
32、9 105t + 4.0347 107t2IceThermal conductivity, Btu/hftF kice= 1.3652 3.1648 103t + 1.8108 105t2Thermal diffusivity, ft2/h ice= 5.0909 102 2.0371 104t + 1.1366 106t2Density, lb/ft3ice= 5.7385 101 4.5333 103tSpecific heat, Btu/lbF cice= 4.6677 101+ 8.0636 104tSource: Choi and Okos (1986)xsRTo2tftMsLotf
33、32t 32-xsRTo2Loxwoxbtf32-1tf32t 32-1.105xwo10.71381 tft+ 1.8ln-+-Thermal Properties of Foods 19.3Table 3 Unfrozen Composition Data, Initial Freezing Point, and Specific Heats of Foods*Food ItemMoisture Content,%xwoProtein,%xpFat, %xfCarbohydrateAsh, %xaInitialFreezing Point,FSpecific Heat AboveFreez
34、ing,Btu/lbFSpecific Heat BelowFreezing,Btu/lbFLatentHeat of Fusion,Btu/lbTotal, %xcFiber, %xfbVegetablesArtichokes, globe 84.94 3.27 0.15 10.51 5.40 1.13 29.8 0.93 0.48 122Jerusalem 78.01 2.00 0.01 17.44 1.60 2.54 27.5 0.87 0.54 112Asparagus 92.40 2.28 0.20 4.54 2.10 0.57 30.9 0.96 0.43 133Beans, sn
35、ap 90.27 1.82 0.12 7.14 3.40 0.66 30.7 0.95 0.44 130lima 70.24 6.84 0.86 20.16 4.90 1.89 30.9 0.84 0.49 101Beets 87.58 1.61 0.17 9.56 2.80 1.08 30.0 0.93 0.46 126Broccoli 90.69 2.98 0.35 5.24 3.00 0.92 30.9 0.96 0.43 130Brussels sprouts 86.00 3.38 0.30 8.96 3.80 1.37 30.6 0.93 0.46 123Cabbage 92.15
36、1.44 0.27 5.43 2.30 0.71 30.4 0.96 0.44 132Carrots 87.79 1.03 0.19 10.14 3.00 0.87 29.5 0.94 0.48 126Cauliflower 91.91 1.98 0.21 5.20 2.50 0.71 30.6 0.96 0.44 132Celeriac 88.00 1.50 0.30 9.20 1.80 1.00 30.4 0.93 0.45 126Celery 94.64 0.75 0.14 3.65 1.70 0.82 31.1 0.97 0.42 136Collards 90.55 1.57 0.22
37、 7.11 3.60 0.55 30.6 0.96 0.44 130Corn, sweet, yellow 75.96 3.22 1.18 19.02 2.70 0.62 30.9 0.86 0.47 109Cucumbers 96.01 0.69 0.13 2.76 0.80 0.41 31.1 0.98 0.41 138Eggplant 92.03 1.02 0.18 6.07 2.50 0.71 30.6 0.96 0.44 132Endive 93.79 1.25 0.20 3.35 3.10 1.41 31.8 0.97 0.40 135Garlic 58.58 6.36 0.50
38、33.07 2.10 1.50 30.6 0.76 0.52 84Ginger, root 81.67 1.74 0.73 15.09 2.00 0.77 0.90 0.46 117Horseradish 78.66 9.40 1.40 8.28 2.00 2.26 28.8 0.88 0.51 113Kale 84.46 3.30 0.70 10.01 2.00 1.53 31.1 0.91 0.44 121Kohlrabi 91.00 1.70 0.10 6.20 3.60 1.00 30.2 0.96 0.45 131Leeks 83.00 1.50 0.30 14.15 1.80 1.
39、05 30.7 0.90 0.46 119Lettuce, iceberg 95.89 1.01 0.19 2.09 1.40 0.48 31.6 0.98 0.39 138Mushrooms 91.81 2.09 0.42 4.65 1.20 0.89 30.4 0.95 0.44 132Okra 89.58 2.00 0.10 7.63 3.20 0.70 28.8 0.95 0.49 129Onions 89.68 1.16 0.16 8.63 1.80 0.37 30.4 0.94 0.45 129dehydrated flakes 3.93 8.95 0.46 83.28 9.20
40、3.38 6Parsley 87.71 2.97 0.79 6.33 3.30 2.20 30.0 0.94 0.46 126Parsnips 79.53 1.20 0.30 17.99 4.90 0.98 30.4 0.89 0.48 114Peas, green 78.86 5.42 0.40 14.46 5.10 0.87 30.9 0.90 0.47 113Peppers, freeze-dried 2.00 17.90 3.00 68.70 21.30 8.40 3sweet, green 92.19 0.89 0.19 6.43 1.80 0.30 30.7 0.96 0.43 1
41、32Potatoes, main crop 78.96 2.07 0.10 17.98 1.60 0.89 30.9 0.88 0.46 113sweet 72.84 1.65 0.30 24.28 3.00 0.95 29.7 0.83 0.50 104Pumpkins 91.60 1.00 0.10 6.50 0.50 0.80 30.6 0.95 0.43 132Radishes 94.84 0.60 0.54 3.59 1.60 0.54 30.7 0.97 0.42 136Rhubarb 93.61 0.90 0.20 4.54 1.80 0.76 30.4 0.97 0.44 13
42、5Rutabaga 89.66 1.20 0.20 8.13 2.50 0.81 30.0 0.94 0.46 129Salsify (vegetable oyster) 77.00 3.30 0.20 18.60 3.30 0.90 30.0 0.87 0.49 110Spinach 91.58 2.86 0.35 3.50 2.70 1.72 31.5 0.96 0.42 132Squash, summer 94.20 0.94 0.24 4.04 1.90 0.58 31.1 0.97 0.42 135winter 87.78 0.80 0.10 10.42 1.50 0.90 30.6
43、 0.93 0.45 126Tomatoes, mature green 93.00 1.20 0.20 5.10 1.10 0.50 30.9 0.96 0.42 134ripe 93.76 0.85 0.33 4.64 1.10 0.42 31.1 0.97 0.43 135Turnip 91.87 0.90 0.10 6.23 1.80 0.70 30.0 0.96 0.45 132greens 91.07 1.50 0.30 5.73 3.20 1.40 31.6 0.96 0.42 131Watercress 95.11 2.30 0.10 1.29 1.50 1.20 31.5 0
44、.97 0.40 137Yams 69.60 1.53 0.17 27.89 4.10 0.82 0.83 0.49 100FruitsApples, fresh 83.93 0.19 0.36 15.25 2.70 0.26 30.0 0.91 0.47 120dried 31.76 0.93 0.32 65.89 8.70 1.10 0.61 0.68 46Apricots 86.35 1.40 0.39 11.12 2.40 0.75 30.0 0.92 0.47 124Avocados 74.27 1.98 15.32 7.39 5.00 1.04 31.5 0.88 0.47 107
45、Bananas 74.26 1.03 0.48 23.43 2.40 0.80 30.6 0.85 0.48 107Blackberries 85.64 0.72 0.39 12.76 5.30 0.48 30.6 0.93 0.46 123Blueberries 84.61 0.67 0.38 14.13 2.70 0.21 29.1 0.91 0.49 122Cantaloupes 89.78 0.88 0.28 8.36 0.80 0.71 29.8 0.94 0.46 129Cherries, sour 86.13 1.00 0.30 12.18 1.60 0.40 28.9 0.92
46、 0.49 124sweet 80.76 1.20 0.96 16.55 2.30 0.53 28.8 0.89 0.51 116Cranberries 86.54 0.39 0.20 12.68 4.20 0.19 30.4 0.93 0.46 12419.4 2010 ASHRAE HandbookRefrigerationCurrants, European black 81.96 1.40 0.41 15.38 0.00 0.86 30.2 0.89 0.47 118red and white 83.95 1.40 0.20 13.80 4.30 0.66 30.2 0.92 0.47
47、 120Dates, cured 22.50 1.97 0.45 73.51 7.50 1.58 3.7 0.55 0.55 32Figs, fresh 79.11 0.75 0.30 19.18 3.30 0.66 27.7 0.88 0.54 113dried 28.43 3.05 1.17 65.35 9.30 2.01 0.60 0.98 41Gooseberries 87.87 0.88 0.58 10.18 4.30 0.49 30.0 0.94 0.47 126Grapefruit 90.89 0.63 0.10 8.08 1.10 0.31 30.0 0.95 0.45 131Grapes, American 81.30 0.63 0.35 17.15 1.00 0.57 29.1 0.89 0.49 117European type 80.56 0.66 0.58 17.77 1.00 0.44 28.2 0.88 0.52 116Lemons 87.40 1.20 0.30 10.70 4.70 0.40 29.5 0.94 0.48 126Limes 88.26 0.70 0.20 10.54 2.80 0.30 29.1 0.94 0.48 127Mangos 81.71 0.51 0.27 17
