Inactivation of lactococcal aromatic aminotransferase prevents the formation of floral aroma compounds from aromatic amino acids in semi-hard cheese

摘要

The enzymic conversion of aromatic amino acids to aroma compounds plays a role in the formation of an undesirable floral aroma in Cheddar-like cheeses. In lactococci, the first step of aromatic amino acid degradation is transamination, catalysed byan aromatic aminotransferase (AraT). It has been observed previously that, in vitro, araT inactivation prevented degradation of aromatic amino acids and decreased degradation of Met and Leu. In this study the effect of araT inactivation in Lactococcus lactis on flavour development in St. Paulin-type cheese was evaluated. The degradation of amino acids was monitored by using radiolabelled amino acids and the volatile compounds formed were analysed using GC-MS. The development of cheese aroma was also evaluated by sniffing. It was confirmed that the availability of an alpha-ketoacid acceptor for transamination is the first limiting factor for amino acid conversion to aroma compounds in cheese. In the presence of alpha-ketoglutarate, araT inactivation prevented formation of floral aroma compounds from aromatic amino acids while not affecting the formation of volatile aroma compounds from branched-chain amino acids and methionine. However, sensory analysis (sniffing) did not reveal any significant effectof the gene inactivation, although the aroma of cheese made with the mutant tended to be less floral than that of cheese made with the wild-type strain.