Advanced Instruments 4C3, User Manual

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It is only appropriate to correlate freezing points determined using the
same plateau reading mode. Determinations made using different mea-
surement modes will not yield identical results.
Hortvet, Celsius and the freezing point of milk
The two calibration standards selected to bracket the milk freezing
point range were originally thought by Julius Hortvet to freeze at -422
m°C and -621 m°C. Later, they were more correctly found to freeze at
-407.4 m°C and -599.6 m°C, respectively, a factor of 1.0356 warmer.
Both milk freezing point scales derived from these two sets of numbers
are still in use today. Modern milk freezing point publications often
distinguish between the two scale units by referring to the former scale
as m°H (Hortvet) and the latter as m°C (Celsius). Either scale may be
selected in the 4C3.
R. W. Henningson found that, though varying slightly with breed and
diet, the freezing points of authentic samples of pure, fresh milk from
healthy cows in the U.S. and Canada average -540.4 m°H (-521.8
m°C)
2
. The freezing point average was -543 m°H (-524 m°C) in
England and Wales
3
.
As soon as it leaves the cow, however, milk becomes susceptible to
both dilution and contamination. Cows’ milk found to have a freezing
point warmer than established regional norms may be suspected to have
been somehow diluted. Cows’ milk found to have a freezing point cold-
er than these norms may be suspected to have soured or become con-
taminated. Combinations of both effects are quite possible but seldom
undetectable.
The AOAC, APHA and other regulatory organizations recommend -525
m°H (-507 m°C) as a confirmation level; that is, observations of milk
freezing points warmer than -525 m°H (-507 m°C) should be presumed
to indicate added water, but be confirmed by additional observations on
authentic samples from the same herd. The British Standards Institution
recommends that observations of milk freezing points warmer than -529
m°H (-511 m°C) be confirmed.
xviii
The Advanced
®
Model 4C3 User’s Guide