-products found in maturing
cheese indicates that the general character of the ripening change is a
peptonization or digestion of the casein.

Until recently the most widely accepted views relating to the cause of
this change have been those which ascribed the transformation to the
activity of micro-organisms, although concerning the nature of these
organisms there has been no unanimity of opinion. The overwhelming
development of bacteria in all cheeses naturally gave support to this
view; and such experiments as detailed above strengthened the idea that
the casein transformation could not occur where these ferment organisms
were destroyed.

The very nature of the changes produced in the casein signified that to
take part in this process any organism must possess the property of
dissolving the proteid molecule, casein, and forming therefrom
by-products that are most generally found in other digestive or
peptonizing changes of this class.

~Digestive bacterial theory.~ The first theory propounded was that of
Duclaux,[197] who in 1887 advanced the idea that this change was due to
that type of bacteria which is able to liquefy gelatin, peptonize milk,
and cause a hydrolytic change in proteids. To this widely-spread group
that he found in cheese, he gave the generic name _Tyrothrix_ (cheese
hairs). According to him, these organisms do not function directly as
ripening agents, but they secrete an enzym or unorganized ferment to
which he applies the name _casease_. This ferment acts upon the casein
of milk, converting it into a soluble product known as _caseone_. These
organisms are found in normal milk, and if they function as casein
transformers, one would naturally expect them to be present, at least
frequently, if not predominating in the ripening cheese; but such is not
the case. In typical cheddar or Swiss cheese, they rapidly disappear (p.
168), although in the moister, softer varieties, they persist for
considerable periods of time. According to Freudenreich, even where
these organisms are added in large numbers to the curd, they soon
perish, an observation that is not regarded as correct by the later
adherents to the digestive bacterial theory, as Adametz and Winkler.

Duclaux's experiments were made with liquid media for isolation
purposes, and his work, therefore, cannot be regarded as satisfactory as
that carried out with more modern technical methods. Recently this
theory has been revived by Adametz,[198] who