Refrigeracion y congelacion

CHAPTER 8

THERMAL PROPERTIES OF FOODS
Thermal Properties of Food Constituents ................................ Thermal Properties of Food ..................................................... Water Content ........................................................................... Initial Freezing Point ................................................................ Ice Fraction............................................................................... Density ...................................................................................... Specific Heat ............................................................................. 8.1 8.1 8.1 8.1 8.1 8.7 8.7 Enthalpy .................................................................................... 8.8 ThermalConductivity ................................................................ 8.9 Thermal Diffusivity .................................................................. 8.17 Heat of Respiration ................................................................. 8.17 Transpiration of Fresh Fruits and Vegetables ........................ 8.19 Surface Heat Transfer Coefficient.......................................... 8.24 Nomenclature .......................................................................... 8.27

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HERMAL properties of foods and beverages must be known to perform the various heat transfer calculations involved in designing storage and refrigeration equipment and estimating process times for refrigerating, freezing, heating, or drying of foods and beverages. Because the thermalproperties of foods and beverages strongly depend on chemical composition and temperature, and because many food items are available, it is nearly impossible to experimentally determine and tabulate the thermal properties of foods and beverages for all possible conditions and compositions. However, composition data for foods and beverages are readily available from sources such as Holland et al.(1991) and USDA (1975). These data consist of the mass fractions of the major components found in food items. The thermal properties of food items can be predicted by using this composition data in conjunction with temperature-dependent mathematical models of the thermal properties of the individual food constituents. qÜÉêãçéÜóëáÅ~ä=éêçéÉêíáÉë=çÑ=ÑççÇë=~åÇ=ÄÉîÉê~ÖÉë=íÜ~í=~êÉ=çÑíÉåêÉèìáêÉÇ=Ñçê=ÜÉ~í=íê~åëÑÉê=Å~äÅìä~íáçåë=áåÅäìÇÉ=ÇÉåëáíóI=ëéÉÅáÑáÅ=ÜÉ~íI ÉåíÜ~äéóI=íÜÉêã~ä=ÅçåÇìÅíáîáíóI=~åÇ=íÜÉêã~ä=ÇáÑÑìëáîáíóK=få=~ÇÇáíáçåI=áÑ íÜÉ=ÑççÇ=áíÉã=áë=~=äáîáåÖ=çêÖ~åáëãI=ëìÅÜ=~ë=~=ÑêÉëÜ=Ñêìáí=çê=îÉÖÉí~ÄäÉI áí=ÖÉåÉê~íÉë=ÜÉ~í=îá~=êÉëéáê~íáçå=~åÇ=äçëÉë=ãçáëíìêÉ=îá~=íê~åëéáê~íáçåK _çíÜ=çÑ=íÜÉëÉ=éêçÅÉëëÉë=ëÜçìäÇ=ÄÉ=áåÅäìÇÉÇ=áå=ÜÉ~í=íê~åëÑÉê=Å~äÅìä~J íáçåëK= qÜáë= ÅÜ~éíÉê= ëìãã~êáòÉë= éêÉÇáÅíáçå= ãÉíÜçÇë= Ñçê= Éëíáã~íáåÖíÜÉëÉ=íÜÉêãçéÜóëáÅ~ä=éêçéÉêíáÉë=~åÇ=áåÅäìÇÉë=Éñ~ãéäÉë=çå=íÜÉ=ìëÉ=çÑ íÜÉëÉ= éêÉÇáÅíáçå= ãÉíÜçÇëK= få= ~ÇÇáíáçåI= í~ÄäÉë= çÑ= ãÉ~ëìêÉÇ= íÜÉêãçJ éÜóëáÅ~ä=éêçéÉêíó=Ç~í~=Ñçê=î~êáçìë=ÑççÇë=~åÇ=ÄÉîÉê~ÖÉë=~êÉ=éêçîáÇÉÇK

ture in the food. At the initial freezing point, a portion of the water within the food crystallizes and the remaining solution becomes more concentrated. Thus, the freezing point of the unfrozenportion of the food is further reduced. The temperature continues to decrease as the separation of ice crystals increases the concentration of the solutes in solution and depresses the freezing point further. Thus, the ice and water fractions in the frozen food depend on temperature. Because the thermophysical properties of ice and water are quite different, the thermophysical properties of frozen foodsvary dramatically with temperature. In addition, the thermophysical properties of the food above and below the freezing point are drastically different.

WATER CONTENT
Because water is the predominant constituent in most foods, water content significantly influences the thermophysical properties of foods. Average values of moisture content (percent by mass) are given...