Advanced Instruments 4250, Руководство пользователя

Страница: 19 / 121

xix
Foreword
However, any added water reduces productivity. The confirmation level
recommended above is based on national averages. If you find that your
local herd averages allow the establishment of a colder local confirma-
tion level (even with natural variations taken into account), you can
thereby reduce added water accordingly.
Under routine conditions where close comparisons are made between
authentic herd samples and delivered milk, less than 1% added water
generally can be detected. Close field and plant follow-up can assure an
actual added water level under 0.5% in market milk; some authorities
claim 0.2%.
For an excellent summary of the role of milk regulatory agencies in the
United States, please refer to E. H. Marth’s article, Assuring the Quality
of Milk
4
.
Percent deviation from base (% DFB)
Early milk freezing point investigators often thought the solute concen-
tration of milk to be more constant than it actually is. Contemporary
milk cryoscopy literature often referred to a concept called “ % added
water ” which, it was thought, could be calculated using a fixed base,
without actually measuring the sample with no added water. This con-
cept assumed that milk samples with freezing points equal to or colder
than the local average (usually called the “ base ” ) to contain 0% added
water, and pure water to contain 100% added water. Using this assump-
tion (with the -540.4 m°H (-521.8 m°C) national average as the base),
the current -525 m°H (-507 m°C) confirmation level would represent
2.8% added water:
100 (540 - 525) / 540 = 2.8%
and would approximately correspond to the often-used 3% “ cut-off ”
level, which thus allows for a small amount of natural variation and
measurement error.
Although we now know that the actual
“ percent added water ” of a sam-
ple cannot be determined without measuring the same sample with no
added water, it can be seen that the percent deviation from some