2009_Book_FoodChemistry

532 10 Milk and Dairy Products
Ripening in hard cheeses occurs uniformly
throughout the whole cheese mass. Rind formation
is the result of surface drying, so it can be
avoided by packaging the cheese mass in suitable
plastic foils before curing commences. The
duration of curing varies and lasts several days
for soft cheeses and up to several months or even
a couple of years for hard cheeses. The yield per
100 kg fluid milk is 8 kg for hard cheeses and up
to 12 kg for soft cheeses.
All cheese ingredients are degraded biochemically
to varying extents during curing.
Fig. 10.26. Cheese making (conventional or with ultrafiltration)
tency. In the production of quark, the whey is
usually separated after souring. Cottage cheese
is generally produced in continuously operated
coagulators with special temperature regulation.
After whey separation via a filter band, the curd
grain can be washed in a screw vat, cooled, and
dried via another drying band.
10.2.8.3 Ripening
The molded cheese mass is placed in a salt bath
for some time, dried, and then left to ripen in
air-conditioned rooms. Ripening or curing is dependent
on cheese mass composition, particularly
the water content, the microflora and the external
conditions, such as temperature and humidity in
the curing rooms.
The ripening of soft cheeses proceeds inwards,
so in the early stages there is a ripened rind and
an unripe inner core. This nonuniform ripening
is due to the high whey content which causes increased
formation of lactic acid and a pH drop at
the start of ripening. In the rind, special molds
that grow more favorably at higher pH values
contribute to a pH increase by decarboxylating
amino acids.
Lactose is degraded to lactic acid by homofermentation.
In cheddar cheese, for example, the
pH drops from 6.55 to 5.15 from the addition of
the starter culture to the end of mold pressing. In
the presence of propionic acid bacteria (as in the
case of Emmental cheese), lactic acid is converted
further to propionic and acetic acids and CO 2 , according
to the reaction:
3CH 3 CHOHCOOH → 2CH 3 CH 2 COOH
+CH 3 COOH + CO 2 + H 2 O (10.14)
The ratio of propionic to acetic acid is influenced
by the redox potential of the cheese, and
in the presence of nitrates, for example, the ratio
is lower. Propionic acid fermentation is shown in
Fig. 10.27. The crucial step is the reversible rearrangement
of succinyl-CoA into methylmalonyl-
CoA:
(10.15)
The catalysis is mediated by adenosyl-B 12 ,which
is a coenzyme for transformations of the general
type:
(10.16)
Based on a study of a coenzyme B 12 -analogue, it
is obvious that a nonclassical carbanion mechanism
is involved: