AROMATIC AMINO-ACID CATABOLISM BY LACTOCOCCI

While catabolism of amino acids is believed to play an important role in cheese flavor development, the pathways present in cheese microflor a are poorly understood. To determine the pathways of aromatic amino a cid catabolism in lactococci and effects of Cheddar cheese ripening co nditions on catabolic enzymes and products, eight starter lactococcal strains were screened. Cell-free extracts prepared from these strains were found to contain an alpha-ketoglutarate-dependent aminotransferas e activity with tryptophan, tyrosine and phenylalanine. Tryptophan, ty rosine and phenylalanine aminotransferase specific activities (mu mol product formed/mg protein/min) ranged from 0.30 to 2.8 10(-3) 0.93 to 7.3 10(-3) and 1.5 to 7.2 10(-3), respectively. Metabolites produced f rom tryptophan by a cell-free extract of Lactococcus lactis S3 were in dolepyruvic acid, indoleacetic acid and indole-3-aldehyde. Indoleaceti c acid and indole-3-aldehyde can form spontaneously from indolepyruvic acid under the conditions employed. A defined medium was used to dete rmine whether the aminotransferase(s) was expressed and which metaboli te(s) accumulate under conditions that simulated those of ripening Che ddar cheese in terms of pH, salt, temperature and carbohydrate starvat ion. The results indicated that the aminotransferase(s) was expressed and stable under these conditions. The tryptophan metabolites that acc umulated were determined to be strain specific.