asein in solutions containing salt, causing it to dissolve to some
extent, thus forming the initial compounds of digestion.[184] This
solution of the casein is expressed physically by the "stringing" of the
curds on a hot iron. This causes the curds to mat, producing a close,
solid body, free from mechanical holes. Still further, the development
of this acid is necessary for the digestive activity of the pepsin in
the rennet extract.

In some varieties of cheese, as the Swiss, acid is not developed and the
character of the cheese is much different from that of cheddar. In all
such varieties, a great deal more trouble is experienced from the
production of "gassy" curds, because the development of the
gas-producing bacteria is held in check by the rapid growth of the
lactic acid-producing species.

~Bacteria in green cheese.~ The conditions under which cheese is made
permit of the development of bacteria throughout the entire process. The
cooking or heating of curds to expel the excessive moisture is never so
high as to be fatal to germ life; on the contrary, the acidity of the
curd and whey is continually increased by the development of bacteria in
the same.

The body of green cheese fresh from the press is, to a considerable
extent, dependent upon the acid produced in the curds. If the curds are
put to press in a relatively sweet condition the texture is open and
porous. The curd particles do not mat closely together and "mechanical
holes," rough and irregular in outline, occur. Very often, at relatively
high temperatures, such cheese begin to "huff," soon after being taken
from the press, a condition due to the development of gas, produced by
gas-generating bacteria acting on the sugar in the curd. This gas finds
its way readily into these ragged holes, greatly distending them, as in
Fig. 30.

[Illustration: FIG. 30. _L_, a sweet curd cheese direct from the press.
"Mechanical" holes due to lack of acid development; _P_, same cheese
four days later, mechanical holes distended by development of gas.]

~Physical changes in ripening cheese.~ When a green cheese is taken from
the press, the curd is tough, firm, but elastic. It has no value as a
food product for immediate use, because it lacks a desirable flavor and
is not readily digestible. It is nothing but precipitated casein and
fat. In a short time, a deep-seated change occurs. Physically this
change is demonstrated in the modification that the curd undergoes.
Gr