is he seeks to
reconcile the discrepancies that appear in the experiments of other
investigators.

~Digestive milk enzym theory.~ In 1897 Babcock and the writer[202] showed
that milk underwent digestive changes spontaneously when bacterial
activity was suspended by the addition of such anaesthetics as ether,
chloroform and benzol. The chemical nature of the by-products produced
by this auto-digestion of milk resembles quite closely those found in
ripened cheese, except that ammonia is not produced as is the case in
old cheese. The cause of the decomposition of the casein, they found to
be due to the action of a milk enzym which is inherent to the milk
itself. This digestive ferment may be separated from fresh milk by
concentrating centrifuge slime extracts by the usual physiological
reagents. This ferment, called by them _galactase_, on account of its
origin in milk, is a proteolytic enzym of the tryptic type. Its activity
is destroyed by strong chemicals such as formaldehyde, corrosive
sublimate, also when heated to 175 deg. F. or above. When such extracts
are added to boiled milk, the digestive process is started anew, and the
by-products produced are very similar to those noted in a normal cheese.

Jensen[203] has also shown that the addition of pancreatic extracts to
cheese accelerated the formation of soluble nitrogenous products.

The action of galactase in milk and cheese has been confirmed by
Freudenreich[204] and Jensen,[205] as well as by American investigators,
and this enzym is now generally accepted as one of the factors concerned
in the decomposition of the casein. Freudenreich believes it is able to
change casein into albumose and peptones, but that the lactic-acid
bacteria are chiefly responsible for the further decomposition of the
nitrogen to amid form.

Failure before to recognize the presence of galactase in milk is
attributable to the fact that all attempts to secure sterile milk had
been made by heating the same, in which case galactase was necessarily
destroyed. A brief exposure at 176 deg. F. is sufficient to destroy its
activity, and even an exposure at lower temperatures weakens its action
considerably, especially if the reaction of the medium is acid. This
undoubtedly explains the contradictory results obtained in the ripening
of cheese from pasteurized milk, such cheese occasionally breaking down
in an abnormal manner.

The results mentioned on page 172, in which cheese failed to ripen w