Tiempos de congelación para alimentos

CHAPTER 9

COOLING AND FREEZING TIMES OF FOODS
Thermodynamics of Cooling and Freezing .................................................................................... 9.1 Cooling Times of Foods and Beverages ......................................................................................... 9.1 Sample Problems for Estimating Cooling Time............................................................................. 9.6 Freezing Times of Foods and Beverages ........................................................................................ 9.7 Sample Problems for Estimating Freezing Time .......................................................................... 9.13 Nomenclature............................................................................................................................... 9.15

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RESERVATION of food is one of the most significant applications of refrigeration. Cooling and freezing of food effectively reduces the activity of microorganisms and enzymes, thus retarding deterioration. In addition, crystallization of water reduces the amount of liquid water in food items and inhibits microbial growth (Heldman 1975). In order forcooling and freezing operations to be cost-effective, refrigeration equipment should fit the specific requirements of the particular cooling or freezing application. The design of such refrigeration equipment requires estimation of the cooling and freezing times of foods and beverages, as well as the corresponding refrigeration loads. Numerous methods for predicting the cooling and freezing times offoods and beverages have been proposed, including those based on numerical, analytical, and empirical analysis. The designer is faced with the challenge of selecting an appropriate estimation method from the many available methods. This chapter reviews selected procedures available for estimating the cooling and freezing times of foods and beverages, and presents examples of these procedures.exact analytical solutions for Equation (1) exist. However, for practical cooling and freezing processes, foods are generally irregularly shaped with temperature-dependent thermophysical properties. Therefore, exact analytical solutions cannot be derived for the cooling and freezing times of foods and beverages. Accurate numerical estimations of the cooling and freezing times of foods can beobtained using appropriate finite element or finite difference computer programs. However, the effort required to perform this task makes it impractical for the design engineer. In addition, two-dimensional and three-dimensional simulations require time-consuming data preparation and significant computing time. Hence, most of the research effort has been in the development of semianalytical/empiricalcooling and freezing time prediction methods that make use of simplifying assumptions.

COOLING TIMES OF FOODS AND BEVERAGES
Before a food item can be frozen, the temperature of the food must be reduced to its initial freezing point. This cooling process, also known as precooling or chilling, removes only sensible heat and thus, no phase change occurs. The cooling time of foods and beverages isinfluenced by the ratio of the external heat-transfer resistance to the internal heat transfer resistance. This ratio, called the Biot number, is defined as Bi Z hL ⁄ k (2)

THERMODYNAMICS OF COOLING AND FREEZING
The cooling and freezing of food is a complex process. Prior to freezing, sensible heat must be removed from the food to decrease its temperature from the initial temperature to theinitial freezing point of the food. This initial freezing point is somewhat lower than the freezing point of pure water due to dissolved substances in the moisture within 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 unfrozen portion of the food is further reduced. As...