Cheese flavour formation by amino acid catabolism

The degradation of amino acids, rather than their release, is the rate-limiting factor in the formation of cheese flavour.

“Recent results, however, showed that the content of free amino acids in cheese could be increased, either by adding amino acids or by enhancing peptidolysis, without any concomitant increase in flavour, suggesting that the rate-limiting factor in the formation of cheese flavour is the degradation of amino acids rather than their release [67].”

Amino acids can be catabolised by several reactions: deamination, decarboxylation, oxidative desamination, transamination and elimination.

“Amino acids can be catabolised by several reactions: deamination, decarboxylation, oxidative desamination, transamination and elimination [14,55,67].”

Only transamination and elimination initiate the conversion of branched-chain amino acids, aromatic amino acids and Met by cheese-related bacteria.

“However, only transamination and elimination initiate the conversion of branched-chain amino acids, aromatic amino acids and Met by cheeserelated bacteria [67].”

The content of α-ketoacid in cheese was recently demonstrated to be the rate-limiting factor of amino acid conversion by lactococci.

“The content of α-ketoacid in cheese was recently demonstrated to be the rate-limiting factor of amino acid conversion by lactococci [53,65,67].”

The proportion of end products differed from that observed in lactic acid bacteria, where α-hydroxy acids were generally the main products found.

“The proportion of end products differed from that observed in lactic acid bacteria, where α-hydroxy acids were generally the main products found (Fig. 1) [67].”

Cystathionine β-lyase or cystathionine γ-lyase were found in Lactococcus lactis and several lactobacillus species: Lb. fermentum, Lb. helveticus and Lb. casei.

“These enzymes were found in Lactococcus lactis and several lactobacillus species: Lb. fermentum, Lb. helveticus and Lb.casei [67].”

Leu aminotransferase activity of PAB is enhanced at pH 8.0.

“Leu aminotransferase activity of PAB was enhanced at pH 8.0, as generally observed for these enzymes [67].”

α-ketoacid dehydrogenase complexes have been found in numerous organisms, including L. lactis.

“α-ketoacid dehydrogenase complexes have been found in numerous organisms, including L. lactis [67].”

Phenylacetic acid and benzaldehyde can originate from non-enzymatic degradation of phenylpyruvic acid, whereas benzoic acid can result from further oxidisation of benzaldehyde.

“Phenylacetic acid and benzaldehyde can originate from non-enzymatic degradation of phenylpyru-vic acid, whereas benzoic acid can result from further oxidisation of benzaldehyde [67].”

Phenyllactic acid is probably formed by reduction of phenylpyruvic acid by hydroxyacid dehydrogenase, as observed in lactic acid bacteria.

“Phenyllactic acid is probably formed by reduction of phenylpyruvic acid by hydroxyacid dehydrogenase, as observed in lactic acid bacteria [67].”

These reactions lead to the corresponding hydroxy acids, carboxylic acids with one carbon atom less, and aldehydes with one or two carbon atoms less.

“These reactions lead to the corresponding hydroxy acids, carboxylic acids with one carbon atom less, and aldehydes with one or two carbon atoms less [67].”

The ability to convert amino acid to flavour compounds in PAB seems to be very strain-dependent.

“As it is the case with many bacteria [67], the ability to convert amino acid to flavour compounds in PAB seems to be very strain-dependent [13].”