CONTRIBUTION OF LACTIC-ACID BACTERIA TO FLAVOR COMPOUND FORMATION IN DAIRY-PRODUCTS

Cheese flavour cannot be produced without starter bacteria. Lactic aci d bacteria convert lactose to lactic acid and this together with their productions of diacetyl and acetaldehyde ar their main contributions to the flavour of cultured milks and fresh cheeses. In matured cheeses , the starter bacteria die out quickly and the rate at which they lyse and release their enzymes into the system has an influence on the rat e at which free amino acids are formed. Rennet alone is mainly respons ible for the formation of large, medium and small peptides but, withou t interaction with other enzymes, is capable of producing only methion ine, histidine, glycine, serine and glutamic acid at quantifiable acti vity. These amino acids, together with the products of glucolysis, for m substrates for secondary flora, the nature of which, in many cases, determines the cheese variety. The also for substrates for enzymes fro m the milk, e.g. the production of H2S appears to be dependent on milk enzymes. Methionine, which is released by rennet, is further metaboli zed by starter enzymes with the production of methanethiol which plays a major role in cheese flavour possibly as a potentiator for other fl avours. alpha-Dicarbonyls, particularly methyl-glyoxal and diacetyl, a nd bacteria which can produce them, appear to play a crucial role in t he formation of cheese flavour, both the desirable flavour of full-fat cheese and the meaty-brothy off-flavour of low-fat cheese. Although, theoretically, there are many compounds in cheese which could react pu rely chemically to form flavour compounds, these reactions are also me diated by enzymes in the cheese system and it seems unlikely that stra ight out chemical reactions play a major role in the production of che ese flavour. The role of the secondary flora is likely to be much more important than that good quality cheeses have a low oxidation-reducti on potential. This is more likely to be an indicator for the establish ment of the anaerobic conditions required for the flavour forming reac tions to proceed than an active causal agent of flavour formation. The function of glutathione is more likely to be ase some sort of facilit ator in enzyme reactions than as an agent for the reduction of oxidati on-reduction potential. The ability of bacteria to accumulate gluthath ione from their media is likely to be one of the indicators of flavour generating capacity. Suitable selected strains of adjunct bacteria in crease the rate and intensity of formation of Cheddar cheese flavour b ut unsuitable adjuncts can also cause off-flavours.