Chemistry Reference
In-Depth Information
OH
O
S
OH
4-methylthio-2-hydroxybutyric acid
(HMBA)
Dehydrogenase/Reductase
α
-keto-glutaric acid
glutamate
CO
2
O
O
NH
2
O
O
S
Decarboxylation
(thiamine pyrophosphate)
S
S
aminotransferases
or
transaminase
(pyridoxal phosphate)
3-(methylthio)propanal
(Methional)
OH
OH
Methionine
4-methylthio-2-oxobutyrate
(KMBA)
Cooked potato
α
,
γ
-elimination
(
pyridoxal phosphate
)
Enzyme:
Cystathione
Enzymatic
and /or
Chemical conversion
O
β
-lyase
O
γ
-lyase
L-Met-
Enzyme ?
OH
CH
3
-SH
α
-Keto butyric acid
+
NH
3
[O]
Garlic-like
Dimethyl sulphide
Dimethyldisulphide
Dimethyltrisulphide
Other sulfur compounds
Onion-like
Garlic-like
Figure 14.8.
Degradation of methionine to potent sulphur-containing odorants (reproduced
with permission from Singh et al., 2003; # Blackwell Publishing, Inc.).
sulphide (Lindsay and Rippe, 1986; Urbach, 1995; Weimer et al., 1999) which
contribute to the aroma of cheese (Milo and Reineccius, 1997). These com-
pounds are known to originate from Met (Figure 14.8). Methanethiol has been
associated with desirable Cheddar-type sulfur notes in good-quality Cheddar
cheese (Manning and Price, 1977; Manning and Moore 1979; Price and Manning
1983). However, alone or in excess, methanethiol does not produce typical
Cheddar cheese flavour (Weimer et al., 1999).
Methanethiol is readily oxidized to dimethyl disulphide and dimethyl
trisulphide (Parliment et al., 1982; Chin and Lindsay, 1994). The occurrence
of these compounds is a direct result of the methanethiol content and is
modulated by the low redox potential of cheese. Methanethiol can potentially
oxidize during analysis to form these compounds, and this may account for
some reports of dimethyl disulphide and dimethyl trisulphide in cheese.
Dimethyl sulphide (Milo and Reineccius, 1997) and dimethyl trisulphide
were noted recently as important odorants in aged Cheddar cheese (Milo
and Reineccius, 1997; Suriyaphan et al., 2001b; Zehentbauer and Reineccius,
2002). Further work is needed to define the mechanism and cheese conditions
needed for the production of these compounds.
The volatile sulphur compounds from Met can also be produced non-
enzymatically. Wolle et al. (2006) reported that incubation of Met in the
