Physico-chemical Properties of Milk - Advanced Dairy Chemistry - page 724

Thus, the specific heat capacity at a given temperature is equal to the rate of
change of enthalpy with temperature, at that temperature (Smith and Van
Ness, 1987; Figura and Teixeira, 2007).
Specific heat is sensible heat, the addition or removal of which results in
temperature change but no phase change. Latent heat, on the other hand, has
to do with phase change but not temperature change; it is defined as the
quantity of heat absorbed or released per unit mass when a pure material
changes phase (solid $ liquid or liquid $ vapour) at constant temperature.
For the conditions (apart from the absence of phase changes) under which
Equation 52 is valid, the latent heat of fusion, L, is equal to the enthalpy
change accompanying the phase change solid $ liquid:

L ¼ h (54)

When a mixture of substances with different melting points (e.g., milk fat, a
mixture of triacylglycerols) is heated or cooled within its melting point range,
both sensible and latent heat must be added or removed to effect changes in
temperature. A measured specific heat capacity thus includes both latent and
sensible heat requirements and must, therefore, be termed an apparent spe-
cific heat capacity. In this case, Equation 52 is written as

h ¼ Z

2

ð Jkg 1 Þ

c app ðÞ d

(55)

1

where c app ( ) is apparent specific heat capacity expressed as some function of
temperature (J kg 1 K 1 ). This equation is valid for any temperature interval
lying above the freezing point and any interval whose lower limit is the
freezing point.
The apparent specific heat capacity of milk fat is markedly tempera-
ture dependent at temperatures < 408C because of latent heat of fusion
effects. A pronounced maximum is exhibited in the range 15-208C, and a
secondary maximum or inflection at about 308C. The apparent specific
heat capacity decreases to relatively low values at temperatures < 0and
> 408C (Riedel, 1955); Phipps, 1957). The precise shape of the apparent
specific heat capacity-temperature curve depends on the milk fat solid-li-
quid ratio that existed at each temperature during measurement. This, in
turn, depends on fat composition (proportions of different triacylglycer-
ols) and on the thermal history of the fat sample (Riedel, 1955; Phipps,
1957; Sherbon, 1988). Representative values of c app forsummermilkfat
are shown in Table 15.6.