Chemistry Reference
In-Depth Information
CASEINS
Chymosin
Plasmin
Cell-Envelope Proteinase
Peptidases
Small Peptides
+
Amino acids
Cheese matrix
Oligopeptide transport systems
Amino acid transport systems
Bacterial cell
Peptidases
Endopeptidases
Aminopeptidases
Di-/tri-peptidases
Proline-specific peptidases
Deaminases
AMINO ACID POOL
Amines
DMS Cabbage
DMDS Onion
DMTS Garlic
Transaminases
(Aminotransferases)
[O] / Enzyme
Dehydrogenases
S-compounds
Methanethiol
Hydroxy-Acids
α-Keto Acids
Lyases
Decarboxylases
Dark chocolate, spicy aroma
Acyltransferase
/
esterase
2-Methylpropanal
2-Methylbutanal
3-Methylbuanal
Dehydrogenases
Carboxylic Acids
Aldehydes
Thio-esters
Cooked potato aroma
Dehydrogenases
3-(Methylthio) propanal
Acyltransferase
/
esterase
Slightly sweet, fresh aroma
2-Methylpropan-1-ol
3-Methylbutan-1-ol
2-Methylbutan-1-ol
Alcohols
Esters
Figure 14.7.
Generation of flavour compounds from milk protein degradation. DMS,
dimethyl sulphide; DMDS, dimethyl disulphide; DMTS, dimethyl trisulphide (modified from
Singh et al., 2003).
lactis (Gao and Steele, 1998). These enzymes initiate the degradation of Val,
Leu, Ile, Phe, Tyr, Trp and Met, all of which are known precursors of cheese
flavour compounds. Inactivation of aminotransferases involved in the break-
down of amino acids by lactococci has been shown to reduce aroma forma-
tion during cheese ripening (Rijnen et al., 1999b).
-KAs are central intermediates and can be converted to hydroxyl
acids, aldehydes and CoA-esters (Figure 14.7). These reactions are mostly
enzymatic, but some chemical conversion steps have also been described, like
the formation of benzaldehyde from phenylpyruvic acid (Smit et al., 2004).
The aldehydes formed can generally be dehydrogenated or hydrogenated to
their corresponding alcohols or organic acids, which are, in their turn, sub-
strates for esterases and acyltransferases leading to (thio)esters (Figure 14.7).
Other enzymatic routes for the conversion of amino acids involve lyases
(e.g., cystathionine
-lyase, threonine aldolase) and deamination/decarboxylation
(resulting in the formation of amines). For further details see Smit et al. (2005).
The volatile fraction of cheese contains several sulfur-containing com-
pounds such as methanethiol, methional, dimethyl sulphide, dimethyldisulphide,
dimethyltrisulphide, dimethyltetrasulphide, carbonyl sulphide and hydrogen
