Contribution of lactic acid bacteria to flavour compound formation in dairy products

Cheese flavour cannot be produced without starter bacteria. Lactic acid bacteria convert lactose to lactic acid and this together with their production of diacetyl and acetaldehyde are 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 rate at which free amino acids are formed. Rennet alone is mainly responsible for the formation of large, medium and small peptides but, without interaction with other enzymes, is capable of producing only methionine, histidine, glycine, serine and glutamic acid at quantifiable levels. Free amino acids in Cheddar cheese are mainly the result of microbial peptidase activity. These amino acids, together with the products of glycolysis, form substrates for secondary flora, the nature of which, in many cases, determines the cheese variety. They also form substrates for enzymes from the milk, e.g. the production of H₂S appears to be dependent on milk enzymes. Methionine, which is released by rennet, is further metabolized by starter enzymes with the production of methanethiol which plays a major role in cheese flavour possibly as a potentiator for other flavours. —Dicarbonyls, particularly methylglyoxal and diacetyl, and bacteria which can produce them, appear to play a crucial role in the 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 purely chemically to form flavour compounds, these reactions are also mediated by enzymes in the cheese system and it seems unlikely that straight out chemical reactions play a major role in the production of cheese flavour. The role of the secondary flora is likely to be much more important than that of chemical reactions. Particularly in Cheddar and Emmental it has been shown that good quality cheeses have a low oxidation-reduction potential. This is more likely to be an indicator for the establishment of the anaerobic conditions required for the flavour forming reactions to proceed than an active causal agent of flavour formation. The function of glutathione is more likely to be as some sort of facilitator in enzyme reactions than as an agent for the reduction of oxidation-reduction potential. The ability of bacteria to accumulate glutathione from their media is likely to be one of the indicators of flavour generating capacity. Suitable selected strains of adjunct bacteria increase the rate and intensity of formation of Cheddar cheese flavour but unsuitable adjuncts can also cause off-flavours.