Dairy Chemistry And Biochemistry

406 DAIRY CHEMISTRY AND BIOCHEMISTRY

CH,.CH,-CH,-COOH

P. Bulyrate

0.4 Acslyl-CoA

Aceiyl-P

ACCIY~~

ATP CH,-CH~-CH~-CO-CoA

t)utyryl.coA

NAI)Hi NAD 1

CHI-CH Cmtonyl-CoA =CH-CO-CoA

2p, 7 NAD*

~CHI-CO-COOH 2CHyHCOH

Pymvale Lactate

t

ZCHKO-CoA AcetyI-CoA

+

-COOH

NAD' NADH?

Figure 10.16 Metabolism of glucose or lactic acid by Clostridium tyrobutyricurn with the
production of butyric acid, CO, and hydrogen gas.

source, producing propionic acid, acetic acid and CO, (Figure 10.15):

3CH3CHOHCOOH + 2CH,CH,COOH + CH3COOH + CO, + H,O

Lactic acid Propionic acid Acetic acid

Propionic and acetic acids probably contribute to the flavour of Swiss-
type cheeses, while the CO, is responsible for their large characteristic eyes.
Lactic acid may be metabolized by Clostridium tyrobutyricum to butyric
acid, CO, and hydrogen (Figure 10.16); butyric acid is responsible for
off-flavours and the CO, and H, for late gas blowing. Clostridia are
controlled by good hygienic practices, addition of nitrate or lysozyme,
bactofugation or microfiltration. The principal sources of clostridia are soil
and silage.
In surface mould-ripened cheeses, e.g. Camembert and Brie, Penicillium
camemberti, growing on the surface, metabolizes lactic acid as an energy
source, causing the pH to increase. Lactic acid diffuses from the centre to
the surface, where it is catabolized. Ammonia produced by deamination of
amino acids contributes to the increase in pH which reaches about 7.5 at
the surface and 6.5 at the centre of the cheese. Ripening of Camembert and
Brie is characterized by softening (liquefaction) of the texture from the
surface towards the centre. Softening is due to the increase in pH, proteolysis
and diffusion of calcium phosphate to the surface, where it precipitates due
to the high pH. These events are summarized in Figure 10.17.