Advanced Instruments 4250, Руководство пользователя

Страница: 18 / 121

xviii
Advanced
®
Cryoscope Model 4250 User’s Guide
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 distin-
guish 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 4250.
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 colder than
these norms may be suspected to have soured or become contaminated.
Combinations of both effects are quite possible but seldom unde-
tectable.
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.