alpha-Keto acids. 3

Although it is not fully clear, the rate-limiting step in amino acid catabolism appears to be the action of aminotransferases on amino acids. Hence, there has been attention paid to accelerating this step by increasing in cheese the concentration of the co-substrate for aminotransferase action, alpha-ketoglutarate.

alpha-Ketoglutarate may be produced from glutamic acid by the action of glutamate dehydrogenase (essentially a reversal of aminotransferase action) or, in citrate-positive lactococci, by the citrate-oxalate pathway. Another possible pathway involving citrate metabolism, the citrate-isocitrate pathway, does not appear to be operational in lactic acid bacteria.

A number of authors (e.g., Yvon et al., 1998; Banks et al., 2001; Shakeel-Ur-Rehman and Fox, 2002) have added alpha-ketoglutarate to cheese and seen its effect on the products of amino acid catabolism. For example, Banks et al. (2001) added alpha-ketoglutarate to Cheddar curd at salting and saw a statistically significant increase in the concentration of some volatile flavour compounds derived from amino acid catabolism (see table).

Another strategy (e.g., Rijnen et al., 2000) involves cloning the gene for glutamate dehydrogenase into a strain of Lactococcus; likewise, these authors also found an effect on the production of volatile flavour compounds in model cheeses.

Further reading

Banks, J.M., Yvon, M., Gripon, J.C., de la Fuente, M.A., Brechany, E.Y., Williams, A.G. and Muir, D.D. (2001). Enhancement of amino acid catabolism in Cheddar cheese using a-ketoglutarate: amino acid degradation in relation to volatile compounds and aroma character. Int. Dairy J. 11, 215-243

Curtin, Á.C. and P.L.H. McSweeney (2004). Catabolism of amino acids in cheese during ripening. In Cheese: Chemistry, Physics and Microbiology, Volume 1, General Aspects, 3^rd edition, P.F. Fox, P.L.H. McSweeney, T.M. Cogan and T.P. Guinee (eds), Elsevier Applied Science, Amsterdam. pp. 436-454.

Rijnen, L., Courtin, P., Gripon, J-C. and Yvon, M. (2000). Expression of a heterologous glutamate dehydrogenase gene in Lactococcus lactis highly improves the conversion of amino acids to aroma compounds. Appl. Environ. Microbiol. 66, 1354-1359

Shakeel Ur-Rehman and Fox, P.F. (2002). Effect of added a-ketoglutaratic acid, pyruvic acid or pyridoxal phosphate on proteolysis and quality of Cheddar cheese. Food Chem. 76, 21-26.