ns very few bacteria. A preservative, boric acid, is added by
the manufacturer to restrain the bacteria, otherwise the extract
would soon be unfit for use. The bacteria in the commercial rennet
extract are too few to be of any importance whatever in the ripening
process.

Rennet extract contains an enzyme, rennin, that causes the milk to
curdle; also another enzyme, pepsin, that exerts a digestive action
on the curdled casein. Pepsin is always found in the stomach juices
of all animals, but no digestive action takes place, unless the
reaction is distinctly acid, as is the ease under normal conditions,
since hydrochloric acid is excreted by the walls of the stomach.
Outside of the stomach, the same conditions must obtain with
reference to the presence of acid, if pepsin is to exert a digestive
effect. In the cheese curd, the milk sugar is rapidly changed into
lactic acid by the action of the bacteria. This gives the proper
chemical reaction for peptic action, and the enzyme is then able to
act on the paracasein, the nitrogenous part of the cheese. If milk
contains no acid-forming bacteria, conditions will not permit of
peptic action, and as a consequence, the ripening processes do not
take place. If the sugar is fermented by some organism that does
not form acid, as the lactose-fermenting yeasts, the cheese does not
ripen. The lactic bacteria are therefore an essential factor in
inaugurating the ripening changes in all types of rennet cheese.

=Preservative action of acid.= In a previous chapter it was shown that
raw milk does not undergo putrefaction because of the restraining
effect of the acid formed by the lactic bacteria on the putrefactive
organisms. This same phenomenon is noted in cheese. Milk always
contains putrefactive bacteria which pass into the cheese, but they
cannot grow therein because of the high acidity. In the absence of
the acid-forming organisms in the cheese, the cheese may remain
tough and rubbery, on account of the lack of suitable conditions for
the action of the pepsin of the rennet extract, or when the milk
contains large numbers of digesting organisms, the cheese may
develop a putrefactive condition, as noted by the offensive odor and
soft pasty texture.

=Other factors concerned in cheese ripening.= There are other factors
that are also concerned in the complex series of ripening changes
noted in cheddar cheese. All animal fluids and tissues, if kept
under perfectly sterile conditions at