es more pronounced as the temperature
is further raised. Milk so heated develops on its surface a pellicle or
"skin." The cause of this change in taste is not well known. Usually it
has been explained as being produced by changes in the nitrogenous
elements in the milk, particularly in the albumen. Thoerner[133] has
pointed out the coincidence that exists between the appearance of a
cooked taste and the loss of certain gases that are expelled by heating.
He finds that the milk heated in closed vessels from which the gas
cannot escape has a much less pronounced cooked flavor than if heated in
an open vessel. The so-called "skin" on the surface of heated milk is
not formed when the milk is heated in a tightly-closed receptacle. By
some[134] it is asserted that this layer is composed of albumen, but
there is evidence to show that it is modified casein due to the rapid
evaporation of the milk serum at the surface of the milk.

_3. Digestibility._ Considerable difference of opinion has existed in
the minds of medical men as to the relative digestibility of raw and
heated milks. A considerable amount of experimental work has been done
by making artificial digestion experiments with enzyms, also digestion
experiments with animals, and in a few cases with children. The results
obtained by different investigators are quite contradictory, although
the preponderance of evidence seems to be in favor of the view that
heating does impair the digestibility of milk, especially if the
temperature attains the sterilizing point.[135] It has been observed
that there is a noteworthy increase in amount of rickets,[136] scurvy
and marasmus in children where highly-heated milks are employed. These
objections do not obtain with reference to milk heated to moderate
temperatures, as in pasteurization, although even this lower temperature
lessens slightly its digestibility. The successful use of pasteurized
milks in children's hospitals is evidence of its usefulness.

_4. Fermentative changes._ The normal souring change in milk is due to
the predominance of the lactic acid bacteria, but as these organisms as
a class do not possess spores, they are readily killed when heated above
the thermal death-point of the developing cell. The destruction of the
lactic forms leaves the spore-bearing types possessors of the field, and
consequently the fermentative changes in heated milk are not those that
usually occur, but are characterized by the curdling of