Chemistry Reference
In-Depth Information
Table 15.6.
Effect of temperature on the apparent specific heat
of summer milk fat
a
c
app
(J kg
1
K
1
)
Temperature (8C)
20
2090
0
3340
15
5430
25
2930
30
3760
40
2090
a
Data from Riedel (1955).
In contrast to that of milk fat, the specific heat capacity of the non-fat
part of whole milk and cream at temperatures greater than the product's
freezing point includes no latent heat requirement. The composition of this
non-fat part is approximately that of skim milk, the specific heat capacity of
which increases essentially linearly but slowly with temperature over a wide
range (Phipps, 1957; Fern´ ndez-Martin, 1972a). Representative values for
skim milk are 3899 and 3988 J kg
1
K
1
at 0 and 508C, respectively (Phipps,
1957).
It follows from the above that the specific heat capacity of whole milk
and cream must be termed apparent specific heat capacity at temperatures
<
408C. The fat content and thermal history of the product, and fat composi-
tion, determine precisely how the apparent specific heat capacity varies with
temperature. The apparent specific heat capacity of whole milk exhibits a
maximum in the temperature range 15-208C (Phipps, 1957; Fern´ ndez-Martin,
1972a), a minimum at
408C and a gradual increase with temperature above
408C. The picture is similar for creams, but the maximum for high-fat creams is
higher than that for milk because of the large latent heat contribution from the
milk fat. At temperatures away from the temperature of the maximum, appar-
ent specific heat capacity is inversely related to fat content, and, for concen-
trated milks and creams, to %TS (Phipps, 1957; Rambke and Konrad, 1970;
Fern´ ndez-Martin, 1972a).
Phipps (1957) showed that the apparent specific heat capacity of cream
measured under otherwise standardized conditions depends on the degree of
dispersion of the milk fat (as resulting from homogenization treatments), as
this influences the crystallization/melting behaviour of the fat.
At temperatures
>
408C, milk fat is completely molten and no latent
heat effects occur during heating or cooling of milk and cream. Fern´ ndez-
Martin (1972a) measured the specific heat capacity of skim milk, half-skim
milk and whole milk, and concentrates (
30% TS) of these, over the
