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"Water contents of fruits and vegetables are from Lutz and Hardenburg (1968) except for Jerusalem artichokes; dried beans; and peas, yams, dried apples, figs, peaches, prunes, and raisins; the latter are from Watt and Merrill (1963). Water contents of meats, dairy, and poultry products, miscellaneous candy, and nuts are also from Watt and Merrill; water contents of eggs (yolks, salted) and fish are from ASHRAE (1972,1974, and 1978). 'Freezing points of fruits and vegetables are from Whiteman (1957), and average freezing points of other foods are from ASHRAE (1972,1974, and 1978). "Specific heat was calculated from Siebel's formulas (1892).

^Latent heat of fusion was obtained by multiplying water content expressed in decimal form by 144, the heat of fusion of water in Btu/lb.

Source: Reprinted with permission of the American Society of Heating; Refrigerating and Air-Conditioning Engineers from the 1989 ASHRAE Handbook—

Fundamentals. Ref. 11.

"Water contents of fruits and vegetables are from Lutz and Hardenburg (1968) except for Jerusalem artichokes; dried beans; and peas, yams, dried apples, figs, peaches, prunes, and raisins; the latter are from Watt and Merrill (1963). Water contents of meats, dairy, and poultry products, miscellaneous candy, and nuts are also from Watt and Merrill; water contents of eggs (yolks, salted) and fish are from ASHRAE (1972,1974, and 1978). 'Freezing points of fruits and vegetables are from Whiteman (1957), and average freezing points of other foods are from ASHRAE (1972,1974, and 1978). "Specific heat was calculated from Siebel's formulas (1892).

^Latent heat of fusion was obtained by multiplying water content expressed in decimal form by 144, the heat of fusion of water in Btu/lb.

Source: Reprinted with permission of the American Society of Heating; Refrigerating and Air-Conditioning Engineers from the 1989 ASHRAE Handbook—

Fundamentals. Ref. 11.

varies with the stage of development or maturity when harvested and also with type of species, growing conditions, and the storage conditions after harvest. The values given in the table apply to mature product shortly after harvest. For meat the water content values are for the time of slaughter or after the aging period. In reality the water content varies considerably, not only between different animals but also between different muscles from the same animal. For processed products the water content depends on the specific process used.

The freezing points given in Table 1 are based on experiments in which the product has been cooled slowly until freezing occurred. For fruits and vegetables the highest temperature at which the product freeze are given and for other foods average freezing temperature is shown.

With reference to specific heat it should be observed that this is a function of temperature. The value given in Table 1 are from 0°C. In a unfrozen product the specific heat will be slightly lower as temperature rise, and in frozen foods there is a large change in specific heat as temperature decreases. The latter is, of course, related to the changes in composition, in particular the water content. When calculating specific heat of a frozen product it is assumed that the water is frozen to ice and that the specific heat involved is that of ice. This assumption is not totally correct. The freezing of most foods that is transferring from liquid water to ice is a gradual process that occurs over a wide temperature range.

With reference to the latent heat of fusion given in Table 1, these values are also subject to error because they do not consider the chemical composition other than water content. They are, in other words, the product of the heat of fusional water and the water content.

The variations that occur in values given on thermal properties should be taken into account in practical applications. Today a comprehensive work is carried out at several research institutes and universities, eg, at Campden Food and Drink Research Association, UK (12) to compare different data. Besides Table 1, a compilation of thermal properties presented by Polley and co-workers is of interest (13).

The definitions of the thermal properties normally listed are

Water content—the mass of water in the product divided by the total mass expressed in percentage. Average freezing point—the temperature at which the liquid and solid state of a product are in equilibrium expressed in Celsius.

Latent heat—the quantity of heat necessary to change 1 kilogram of liquid to solid without change of temperature measured as kJ/kg.

Specific heat—the amount of heat needed to rise the temperature of 1 kg of a food product 1° measured as kJ/kg, °C.

Heat of respiration—the quality of heat generated per 24 h measured as kJ/24 h, kg.

The large variations that occur for thermal properties of food products explain why theoretical calculations on freezing time are difficult and should not be used to determine the requirements for a specific freezer. Those requirements should be determined by practical tests.

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