NUTRITIONAL VALUE OF MILKS.
203
The rats started in October (1933) gave results quite similar to those obtained
in October, 1932. Both the males and females grew a little slower and the differ-
ences between the raw and pasteurized milk were slightly greater than in the
earlier series, but the differences cannot be considered significant. However, in
the experiments started in December and February very significant differences
are evident. In the case of the male rats there is a progressive decrease in the
daily rate of growth from 4.19 to 2.45 gms. for the raw milk and from 3.90 to
1 .14 gms. for the pasteurized milk. The females show some indication that the
winter milk is of poorer quality than the summer milk. However, the results
are not as uniform as for the males, and the females do not show the extremely
poor growth in the February experiment that the males do. This we believe is
due to the fact that the females do not grow as fast and therefore do not need
as much of the factor or factors which winter milk is deficient in. Whatever
the limiting factor is in winter milk, the deficiency is undoubtedly a relative one.
Thus it is decreased sufficiently during winter feeding to have a very drastic effect
upon the males which have a tendency to grow faster, but not enough to produce
an equal growth impairment in females. The fact is further emphasized in the
case of pasteurized milk. The milk produced in October and November, when
the cows still have a store of nutrients from the summer feeding period, contains
a fairly adequate supply of all needed nutrients. At this period, although
pasteurization may destroy a small amount of certain constituents, the amount
remaining is still sufficient to allow the animal an adequate supply. However,
in December the supply of essential factors is reduced in the original raw milk and
pasteurization will now increase the deficiency. Thus we have a decrease in the
rate of growth of the males from 3.32 to 1.96 gm. per day due to the heat treat-
ment. In February the raw milk allows a growth of only 2.45 gm. daily and
pasteurization reduces the growth rate to 1.14 gm. It should be noted that the
females on the pasteurized milk in both the second and third experiments grew
faster than the males. This is further indication that the deficiency is more readily
demonstrated in males than in females.
The female rats on raw milk during the February experiment also grew faster
than the males but two of the rats in this group grew exceedingly well and raised
the average. In this connection we might mention a test made on milk produced
by two cows in the University herd and brought directly to the laboratory. This
experiment was also started on February 6 and the average growth for the
males was 2.62 gm. which is almost identical with 2.45 found for the commercial
milk at this time of the year. The average growth for the females was 2.65,
which is lower than the 3.14 value but more in line with the other results
for the females.
Season or the kind of feed ingested by the cow, therefore, has a much greater
effect upon the nutritive value of milk than does pasteurization. Pasteurization
will aggravate the deficiency of a poor milk, but a good milk, one produced when