Chemistry Reference
In-Depth Information
constituents. Fat globules, casein micelles and whey proteins make negligible
contributions to the freezing point depression (Sherbon, 1988). Lactose, chloride
salts and other water-soluble constituents (such as calcium, potassium, magne-
sium, lactate, phosphate, citrate, etc.) contribute about 55, 25 and 20%, respec-
tively, to the freezing point depression of milk (Jenness and Patton, 1959).
The vast majority of individual milks have a freezing point in the range
0.512 to
0.5508Candfewmilksfalloutsidetherange
0.520 to
0.5308C
(Davis and MacDonald, 1953; Shipe, 1959; Henningson, 1963; Eisses and Zee,
1980). Although the concentration of individual solute constituents in milk may
vary, the total molality of the soluble constituents remains fairly constant; since
the milk secretion process dictates that the osmotic pressure of milk is kept in
equilibrium with that of blood, a constant osmolality is maintained in milk by the
passage of blood constituents into the mammary gland. Any decrease or increase
in the concentration of lactose is compensated for by a proportional change in
the concentrations of sodium and chloride. The constant osmolality of milk is
reflected in the relative constancy of the freezing point depression.
Environmental and nutritional factors, such as season, climate, feed,
stage of lactation, breed of cow, time of day, water intake and clinical
mastitis, have only relatively small effects on the freezing point of milk,
although substantial variations are observed in extreme circumstances (see
reviews by Shipe, 1959; Brathen, 1983).
The effect of processing on the freezing point of milk has been reviewed
by Shipe (1959) and Harding (1983). Heat treatment of milk causes the
transfer of soluble salts to the colloidal state and changes in lactose that
include its interaction with proteins and its conversion into formic acid.
Such effects would be expected to increase the freezing point of milk. How-
ever, reports on the effects of heat treatment of milk on its freezing point are
inconclusive; some workers reported increases in the freezing point while
others reported no effect. Kessler (1984) reported that pasteurization, UHT
heat treatment or preheating at 958C for 303 s did not affect the freezing point
of milk. However, the freezing point increased by almost 0.0108C when milk
was UHT treated by direct steam injection, apparently owing to the degassing
caused by flash cooling. Similarly, vacuum treatment of milk has been shown
to increase its freezing point, presumably due to the removal of dissolved
gases from the milk (Shipe, 1964; Demott, 1967). Freezing and subsequent
thawing of milk does not affect its freezing point provided the thawed milk is
well mixed before samples are taken. Any processing that involves fermenta-
tion of lactose to lactic acid (e.g., in the production of yoghurt) results in a
lowering of the freezing point, as one mole of lactose is converted into four
moles of lactic acid.
The determination of freezing point is used extensively to assess
whether milk has been adulterated by the addition of water and to quantify
