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

The enzymatic conversion of aromatic amino acids to aroma compounds plays a role in the formation of an undesirable floral aroma in Cheddar-like chees es. In lactococci, the first step of aromatic amino acid degradation is a t ransamination, catalysed by an aromatic aminotransferase (AraT). We observe d previously that in vitro, araT inactivation prevented degradation of arom atic amino acids and decreased degradation of Met and Leu. In this study we evaluated the effect of araT inactivation in Lactococcus lactis on flavour development in St. Paulin-type cheese. The degradation of amino acids was monitored by using radiolabelled amino acids and the volatile compounds for med were analysed by CC-MS. The development of cheese odour was also evalua ted by sniffing. We confirmed that the availability of an alpha-ketoacid ac ceptor for transamination is the first limiting factor for amino acid conve rsion to aroma compounds in cheese. In the presence of alpha-ketoglutarate, araT inactivation greatly prevented formation of floral aroma compounds fr om aromatic amino acids while it did not affect the formation of volatile a roma compounds from branched-chain amino acids and methionine. However, the sensory analysis by sniffing did not reveal any significant effect of the gene inactivation although the odour of cheese made with the mutant tended to be less floral than that of cheese made with the wild type strain. (C) 2 000 Elsevier Science Ltd. All rights reserved.