Note that, while small negative % DFB values may indicate unwatered
milk, a larger negative % DFB may indicate sour milk, and a very large
negative % DFB could indicate a false reading-that perhaps the sample
did not actually freeze at all.
While the regulatory agencies have been active in recommending upper
limits and confirmation levels, no lower limit has yet been officially
recommended. It would seem equally important to establish a lower
limit (because it can help preclude contamination). Also, as stored milk
ages, increasing acidity affects its freezing point as well as its flavor and
consistency. The AOAC METHODS appropriately warn that titratable
acidity over 0.18% will appreciably lower the freezing point.
Definitions
Supercooling: The tendency of a substance to remain in the liquid state
when cooled below its freezing point.
Crystallization Temperature: Aqueous solutions can be induced to
freeze (i.e., crystallize) most reliably when supercooled. The crystalliza-
tion temperature is the temperature at which crystallization is induced.
During crystallization, the heat of fusion raises the temperature of the
sample to an ice/water freezing point plateau.
Heat of Fusion: The heat released when the mobile molecules of a liq-
uid are frozen into rigid crystals.
Freezing Point: The temperature at which the liquid and solid phases
of a substance will coexist in equilibrium.
Freezing Point Plateau: The constant temperature maintained during
the time that ice and liquid exist in equilibrium after crystallization is
initiated (see Figure 1).
Freezing Point Depression: The freezing point of pure water is 0°C.
Solutes in a water-based solution generally depress (lower) the freezing
point in proportion to the concentration.
% Added Water: Water added to a solution raises its freezing point in
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