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 to150C (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 150C)Thermal Property Food Component Thermal Property ModelThermal conductivity, W/(m K) Protein k = 1.7881 101+ 1.1958 103t 2.7178 106t2Fat k = 1.8071 101 2.7604 104t 1.7749 107t2Carbohydrate k = 2.0141 101+ 1.3874 103t 4.3312 106t2Fiber k = 1.
12、8331 101+ 1.2497 103t 3.1683 106t2Ash k = 3.2962 101+ 1.4011 103t 2.9069 106t2Thermal diffusivity, m2/s Protein = 6.8714 108+ 4.7578 1010t 1.4646 1012t2Fat = 9.8777 108 1.2569 1011t 3.8286 1014t2Carbohydrate = 8.0842 108+ 5.3052 1010t 2.3218 1012t2Fiber = 7.3976 108+ 5.1902 1010t 2.2202 1012t2Ash =
13、1.2461 107+ 3.7321 1010t 1.2244 1012t2Density, kg/m3Protein = 1.3299 103 5.1840 101tFat = 9.2559 102 4.1757 101tCarbohydrate = 1.5991 103 3.1046 101tFiber = 1.3115 103 3.6589 101tAsh = 2.4238 103 2.8063 101tSpecific heat, kJ/(kgK) Protein cp= 2.0082 + 1.2089 103t 1.3129 106t2Fat cp= 1.9842 + 1.4733
14、103t 4.8008 106t2Carbohydrate cp= 1.5488 + 1.9625 103t 5.9399 106t2Fiber cp= 1.8459 + 1.8306 103t 4.6509 106t2Ash cp= 1.0926 + 1.8896 103t 3.6817 106t2Source: Choi and Okos (1986)19.2 2010 ASHRAE HandbookRefrigeration (SI)The initial freezing point of a food is somewhat lower than thefreezing point
15、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 con
16、tinues 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,thermophysi
17、cal 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 ther
18、mophysical 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 harvest.
19、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 FR
20、EEZING 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 not
21、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 stora
22、ge 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 model
23、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 wate
24、r 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-lize
25、s, 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) proposed t
26、he followingmodel for predicting the mass fraction of ice xice:xice= (1)wherexs= mass fraction of solids in foodMs= relative molecular mass of soluble solids, kg/kmolR=universal gas constant = 8.314 kJ/(kg molK)To= freezing point of water = 273.2 KLo= latent heat of fusion of water at 273.2 K = 333.
27、6 kJ/kgtf= initial freezing point of food, Ct = food temperature, CThe 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). Bo
28、und 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 simpl
29、e 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 150C)Thermal Property Thermal Property ModelWaterThermal conductivity, W/(mK) kw= 5.7109 101+ 1.7625 103t 6.7036 106t2
31、Thermal diffusivity, m2/s = 1.3168 107+ 6.2477 1010t 2.4022 1012t2Density, kg/m3w= 9.9718 102+ 3.1439 103t 3.7574 103t2Specific heat, kJ/(kgK) (For temperature range of 40 to 0C) cw= 4.1289 5.3062 103t + 9.9516 104t2Specific heat, kJ/(kgK) (For temperature range of 0 to 150C) cw= 4.1289 9.0864 105t
32、+ 5.4731 106t2IceThermal conductivity, W/(mK) kice= 2.2196 6.2489 103t + 1.0154 104t2Thermal diffusivity, m2/s = 1.1756 106 6.0833 109t + 9.5037 1011t2Density, kg/m3ice= 9.1689 102 1.3071 101tSpecific heat, kJ/(kgK) cice= 2.0623 + 6.0769 103tSource: Choi and Okos (1986)xsRTo2tftMsLotft- xsRTo2xwoxbL
33、otf-1tft-1.105xwo10.7138tft 1+ln-+-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,CSpecific Heat AboveFreezing, kJ/(kgK)Specific Heat Bel
34、owFreezing,kJ/(kgK)LatentHeat of Fusion,kJ/kgTotal, %xcFiber, %xfbVegetablesArtichokes, globe 84.94 3.27 0.15 10.51 5.40 1.13 1.2 3.90 2.02 284Jerusalem 78.01 2.00 0.01 17.44 1.60 2.54 2.5 3.63 2.25 261Asparagus 92.40 2.28 0.20 4.54 2.10 0.57 0.6 4.03 1.79 309Beans, snap 90.27 1.82 0.12 7.14 3.40 0.
35、66 0.7 3.99 1.85 302lima 70.24 6.84 0.86 20.16 4.90 1.89 0.6 3.52 2.07 235Beets 87.58 1.61 0.17 9.56 2.80 1.08 1.1 3.91 1.94 293Broccoli 90.69 2.98 0.35 5.24 3.00 0.92 0.6 4.01 1.82 303Brussels sprouts 86.00 3.38 0.30 8.96 3.80 1.37 0.8 3.90 1.91 287Cabbage 92.15 1.44 0.27 5.43 2.30 0.71 0.9 4.02 1.
36、85 308Carrots 87.79 1.03 0.19 10.14 3.00 0.87 1.4 3.92 2.00 293Cauliflower 91.91 1.98 0.21 5.20 2.50 0.71 0.8 4.02 1.84 307Celeriac 88.00 1.50 0.30 9.20 1.80 1.00 0.9 3.90 1.89 294Celery 94.64 0.75 0.14 3.65 1.70 0.82 0.5 4.07 1.74 316Collards 90.55 1.57 0.22 7.11 3.60 0.55 0.8 4.01 1.86 302Corn, sw
37、eet, yellow 75.96 3.22 1.18 19.02 2.70 0.62 0.6 3.62 1.98 254Cucumbers 96.01 0.69 0.13 2.76 0.80 0.41 0.5 4.09 1.71 321Eggplant 92.03 1.02 0.18 6.07 2.50 0.71 0.8 4.02 1.83 307Endive 93.79 1.25 0.20 3.35 3.10 1.41 0.1 4.07 1.69 313Garlic 58.58 6.36 0.50 33.07 2.10 1.50 0.8 3.17 2.19 196Ginger, root
38、81.67 1.74 0.73 15.09 2.00 0.77 3.75 1.94 273Horseradish 78.66 9.40 1.40 8.28 2.00 2.26 1.8 3.70 2.12 263Kale 84.46 3.30 0.70 10.01 2.00 1.53 0.5 3.82 1.86 282Kohlrabi 91.00 1.70 0.10 6.20 3.60 1.00 1.0 4.02 1.90 304Leeks 83.00 1.50 0.30 14.15 1.80 1.05 0.7 3.77 1.91 277Lettuce, iceberg 95.89 1.01 0
39、.19 2.09 1.40 0.48 0.2 4.09 1.65 320Mushrooms 91.81 2.09 0.42 4.65 1.20 0.89 0.9 3.99 1.84 307Okra 89.58 2.00 0.10 7.63 3.20 0.70 1.8 3.97 2.05 299Onions 89.68 1.16 0.16 8.63 1.80 0.37 0.9 3.95 1.87 300dehydrated flakes 3.93 8.95 0.46 83.28 9.20 3.38 13Parsley 87.71 2.97 0.79 6.33 3.30 2.20 1.1 3.93
40、 1.94 293Parsnips 79.53 1.20 0.30 17.99 4.90 0.98 0.9 3.74 2.02 266Peas, green 78.86 5.42 0.40 14.46 5.10 0.87 0.6 3.75 1.98 263Peppers, freeze-dried 2.00 17.90 3.00 68.70 21.30 8.40 7sweet, green 92.19 0.89 0.19 6.43 1.80 0.30 0.7 4.01 1.80 308Potatoes, main crop 78.96 2.07 0.10 17.98 1.60 0.89 0.6
41、 3.67 1.93 264sweet 72.84 1.65 0.30 24.28 3.00 0.95 1.3 3.48 2.09 243Pumpkins 91.60 1.00 0.10 6.50 0.50 0.80 0.8 3.97 1.81 306Radishes 94.84 0.60 0.54 3.59 1.60 0.54 0.7 4.08 1.77 317Rhubarb 93.61 0.90 0.20 4.54 1.80 0.76 0.9 4.05 1.83 313Rutabaga 89.66 1.20 0.20 8.13 2.50 0.81 1.1 3.96 1.92 299Sals
42、ify (vegetable oyster) 77.00 3.30 0.20 18.60 3.30 0.90 1.1 3.65 2.05 257Spinach 91.58 2.86 0.35 3.50 2.70 1.72 0.3 4.02 1.75 306Squash, summer 94.20 0.94 0.24 4.04 1.90 0.58 0.5 4.07 1.74 315winter 87.78 0.80 0.10 10.42 1.50 0.90 0.8 3.89 1.87 293Tomatoes, mature green 93.00 1.20 0.20 5.10 1.10 0.50
43、 0.6 4.02 1.77 311ripe 93.76 0.85 0.33 4.64 1.10 0.42 0.5 4.08 1.79 313Turnip 91.87 0.90 0.10 6.23 1.80 0.70 1.1 4.00 1.88 307greens 91.07 1.50 0.30 5.73 3.20 1.40 0.2 4.01 1.74 304Watercress 95.11 2.30 0.10 1.29 1.50 1.20 0.3 4.08 1.69 318Yams 69.60 1.53 0.17 27.89 4.10 0.82 3.47 2.06 232FruitsAppl
44、es, fresh 83.93 0.19 0.36 15.25 2.70 0.26 1.1 3.81 1.98 280dried 31.76 0.93 0.32 65.89 8.70 1.10 2.57 2.84 106Apricots 86.35 1.40 0.39 11.12 2.40 0.75 1.1 3.87 1.95 288Avocados 74.27 1.98 15.32 7.39 5.00 1.04 0.3 3.67 1.98 248Bananas 74.26 1.03 0.48 23.43 2.40 0.80 0.8 3.56 2.03 248Blackberries 85.6
45、4 0.72 0.39 12.76 5.30 0.48 0.8 3.91 1.94 286Blueberries 84.61 0.67 0.38 14.13 2.70 0.21 1.6 3.83 2.06 283Cantaloupes 89.78 0.88 0.28 8.36 0.80 0.71 1.2 3.93 1.91 300Cherries, sour 86.13 1.00 0.30 12.18 1.60 0.40 1.7 3.85 2.05 288sweet 80.76 1.20 0.96 16.55 2.30 0.53 1.8 3.73 2.12 270Cranberries 86.
46、54 0.39 0.20 12.68 4.20 0.19 0.9 3.91 1.93 28919.4 2010 ASHRAE HandbookRefrigeration (SI)Currants, European black 81.96 1.40 0.41 15.38 0.00 0.86 1.0 3.71 1.95 274red and white 83.95 1.40 0.20 13.80 4.30 0.66 1.0 3.85 1.98 280Dates, cured 22.50 1.97 0.45 73.51 7.50 1.58 15.7 2.31 2.30 75Figs, fresh
47、79.11 0.75 0.30 19.18 3.30 0.66 2.4 3.70 2.25 264dried 28.43 3.05 1.17 65.35 9.30 2.01 2.51 4.13 95Gooseberries 87.87 0.88 0.58 10.18 4.30 0.49 1.1 3.95 1.96 293Grapefruit 90.89 0.63 0.10 8.08 1.10 0.31 1.1 3.96 1.89 304Grapes, American 81.30 0.63 0.35 17.15 1.00 0.57 1.6 3.71 2.07 272European type
48、80.56 0.66 0.58 17.77 1.00 0.44 2.1 3.70 2.16 269Lemons 87.40 1.20 0.30 10.70 4.70 0.40 1.4 3.94 2.02 292Limes 88.26 0.70 0.20 10.54 2.80 0.30 1.6 3.93 2.03 295Mangos 81.71 0.51 0.27 17.00 1.80 0.50 0.9 3.74 1.95 273Melons, casaba 92.00 0.90 0.10 6.20 0.80 0.80 1.1 3.99 1.87 307honeydew 89.66 0.46 0.10 9.18 0.60 0.60 0.9 3.92 1.8
