Expression of a heterologous glutamate dehydrogenase gene in Lactococcus lactis highly improves the conversion of amino acids to aroma compounds

The first step of amino acid degradation in lactococci is a transamination, which requires an alpha-keto acid as the amino group acceptor. We have pre viously shown that the level of available alpha-keto acid in semihard chees e is the first limiting factor for conversion of amino acids to aroma compo unds, since aroma formation is greatly enhanced by adding alpha-ketoglutara te to cheese curd. In this study we introduced a heterologous catabolic glu tamate dehydrogenase (GDH) gene into Lactacoccus lactis so that this organi sm could produce alpha-ketoglutarate from glutamate, which is present at hi gh levels in cheese, Then we evaluated the impact of CDH activity on amino acid conversion in in vitro tests and in a cheese model by using radiolabel ed amino acids as tracers. The GDH-producing lactococcal strain degraded am ino acids without added alpha-ketoglutarate to the same extent that the wil d-type strain degraded amino acids with added alpha-ketoglutarate. Interest ingly, the GDR-producing lactococcal strain produced a higher proportion of carboxylic acids, which are major aroma compounds. Our results demonstrate d that a GDH-producing lactococcal strain could be used instead of adding a lpha-ketoglutarate to improve aroma development in cheese.