Chemistry Reference
In-Depth Information
Citrate
Threonine
Citrate lyase
Threonine
aldolase
Glycine
Acetate
Acetaldehyde
Oxaloacetate
decarboxylase
CO
2
TPP
CO
2
TPP
Glycolysis
Lactose
Pyruvate
[
Acetaldehyde-TPP
]
Acetyl CoA
Acetolactate
synthase
CENTRAL METABOLISM
CoASH
Acetolactate
Diacetyl
synthase
Acetolactate
decarboxylase
TPP
-CO
2
Acetoin reductase
Diacetyl reductase
Acetoin
Diacetyl
2,3-Butanediol
NAD+
NADH
NAD+
NADH
Figure 14.5.
Metabolism of citrate by lactic acid bacteria (modified from Singh et al., 2003).
The principal flavour compounds produced from the metabolism of
citrate are acetate, diacetyl (2,3-butanedione), acetoin (3-hydroxy-2-buta-
none) and 2,3-butandiol (Cogan, 1995) (Figure 14.5). Diacetyl is usually
produced in small amounts but acetoin is generally produced at much higher
concentration (10-50 fold higher than diacetyl).
14.2.3.3.
Lipolysis and Related Reactions
Like all types of food with a high fat content, lipolytic (enzymatic
hydrolysis by lipases and esterases) and oxidative (chemical) changes are
likely to occur in dairy products. Lipases and esterases in cheese originate
from milk, starter, secondary starter and non-starter bacteria. A number of
psychrotrophic organisms, which can dominate the microflora of refriger-
ated milk, produce heat-stable lipases. The hydrolysis of triglycerides,
which constitute more than 98% of milk fat, is the principal biochemical
transformation of fat, which leads to the production of free fatty acids
(FFAs), di- and mono-glycerides and possibly glycerol (Figure 14.6).
FFAs contribute to the aroma of cheese. Individual FFAs, particularly
