Summary -While catabolism of amino acids is believed to play an important role in cheese f1avor development, the pathways present in cheese microflora are poorly understood. To determine the pathways of aromatic amino acid catabolism in lactococci and effects of Cheddar cheese ripening conditions on catabolic enzymes and products, eight starter lactococcal strains were screened. Cell-free extracts prepared from these strains were found to contain an oe-ketoglutarate-dependent aminotransferase activity with tryptophan, tyrosine and phenylalanine. Tryptophan, tyrosine and phenylalanine aminotransferase specifie activities (Il mol product formed/mg proteinlmin) 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 from tryptophan by a cellfree extract of Lactococcus lactis S3 were indolepyruvic acid, indoleacetic acid and indole-3-aldehyde. Indoleacetic acid and indole-3-aldehyde can form spontaneously from indolepyruvic acid under the conditions employed. A defined medium was used to determine whether the aminotransferase(s) was expressed and which metabolite(s) accumulate under conditions that simulated those of ripening Cheddar cheese in terms of pH, salt, temperature and carbohydrate starvation. The results indicated that the aminotransferase(s) was expressed and stable under these conditions. The tryptophan metabolites that accumulated were determined to be strain-specific. lactococcus / aminotransferase / aromatic amino acid / catabolism / cheese flavor Résumé -Catabolisme des acides aminés aromatiques des lactocoques. Alors que le catabolisme des acides aminés est perçu comme jouant un rôle important dans le développement de la flaveur du fromage; les voies métaboliques liées à la microflore du fromage ne sont que très partiellement éta-blies. Pour déterminer les voies du catabolisme des acides aminés aromatiques dans les lactocoques, et les effets des conditions d'affinage du Cheddar sur les enzymes cataboliques et sur les produits, huit souches de levains lactocoques on été analysées. Des extraits de cellules préparées à partir de ces souches possédaient une activité aminotransférase oe-ketoglutarate dépendante du tryptophane, de la tyrosine et de la phénylalanine. Les activités aminotransférases spécifiques du tryptophane, de la * Oral communication at the IDF Symposium 'Ripening and Quality of Cheeses',

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We investigated the effect of capsular and ropy exopolysaccharide-producing Streptococcus thermophilus starter bacteria on Mozzarella cheese functionality and whey viscosity. Mozzarella cheeses were manufactured with Lactobacillus helveticus LH100 paired with one of four S. thermophilus strains: MR-1C, a bacterium that produces a capsular exopolysaccharide; MTC360, a strain that secretes a ropy exopolysaccharide; TAO61, a nonexopolysaccharide-producing commercial cheese starter; and DM10, a nonencapsulated, exopolysaccharide-negative mutant of strain MR-1C. As expected, cheese moisture levels were significantly higher in Mozzarella cheeses made with exopolysaccharide-positive versus exopolysaccharide-negative streptococci, and melt properties were better in the higher moisture cheeses. Whey viscosity measurements showed that unconcentrated and ultrafiltered, fivefold concentrated whey from cheeses made with S. thermophilus MTC360 were significantly more viscous than whey from cheeses made with MR-1C, TAO61, or DM10. No significant differences were noted between the viscosity of unconcentrated or concentrated whey from cheeses made with S. thermophilus MR-1C versus the industrial cheese starter TAO61. These data indicate that encapsulated, but not ropy, exopolysaccharide-producing S. thermophilus strains can be utilized to increase the moisture level of cheese and to improve the melt properties of Mozzarella cheese without adversely affecting whey viscosity.

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