Amino acid catabolism is the final step in the conversion of caseins to flavour compounds and a part of a complex combination of biochemical pathways in cheese flavour formation.
Lactobacillus helveticus is a thermophilic lactic acid bacterium that is used in cheese
manufacture as a primary starter culture or as an adjunct culture. It has shown high proteolytic activities in conversion of caseins to peptides and further to amino acids and flavour compounds. Better understanding of the enzyme activity properties and the influence of different properties on final cheese flavour is favourable for developing new cheese products with enhanced flavour.
The aim of this Ph.D. study was to investigate the importance of strain variation of Lb. helveticus in relation flavour formation in cheese related to amino acid catabolism. Aspects of using Lb. helveticus as starter as well as adjunct culture in cheese manufacture were studied. Amino acid catabolism related enzyme activities were studied in vitro from eight out of 39 Lb. helveticus strains selected based on different pulsed field gel electrophoresis profiles. Amino acids can be initially converted into a-keto acids by transamination reaction. Lb helveticus strains demonstrated strain dependent aminotransferase activities and the substrate specificities varied between the strains. In general, activities were lowest towards Met and Asp among the substrate tested, and the strains made up three groups depending on their activities towards branched chain and aromatic amino acids.
After transamination of amino acids the formed a-keto acids may be dehydrogenased to hydroxy acids or converted to flavour compounds such as aldehydes, alcohols and carboxylic acids. The highest hydroxy acid dehydrogenase activities were detected towards central metabolites oxaloacetate (from Asp) and a-keto butyrate (from Met or Thr) followed by activities towards aromatic a-keto acids. The branched chain a-keto acids were dehydrogenated only to a limited extent or not at all, which may leave more substrate available for flavour compound formation.One of the strains showed exceptionally high activities and therefore flavour formation in cheese is expected to be different using this strain. The a-keto acid decarboxylase activity was investigated as enzyme activity related to the formation of flavour compounds. No activity was
detected using a-keto acids from Phe and Leu as a substrate, which indicates that Lb. helveticus is using another pathway in conversion of a-keto acids to flavour compounds.
Two Lb. helveticus strains with different amino acid catabolism enzyme activities were tested as a primary starter culture in Finnish Emmental cheese manufacture. Higher release of free amino acids was detected in the cheeses made with one of the strains, and as a consequence propionic acid formation was accelerated. Contribution of Lb. helveticus on flavour formation of Finnish Emmental was shown to be mainly due to their stimulation influence on propionic acid bacteria growth, which caused sweeter flavour with less bitterness. In addition, three Lb. helveticus strains were tested as adjunct culture in semi-hard cheese manufacture. Increased formation of
branched chain aldehydes were detected in cheeses made with Lb. helveticus strain, in which low or no hydroxy acid dehydrogenase activities were detected towards branched chain a-keto acids. However, impact of Lb. helveticus was mainly detected as a higher level of the total free amino acids and a changed distribution of the individual amino acids. Due to these properties sweeter flavour and less bitterness was obtained in cheeses by addition of Lb. helveticus.
The presented results increased the understanding strain variation of Lb. helveticus on amino acids catabolism in vitro and its impact on flavour formation used as starter culture or as adjunct culture. This knowledge is useful for selecting strains for cheese manufacture and for developing new cheese products.