2.5.2. Flavours in dairy products

The flavour of dairy products has been extensively studied. Despite many results, which give a satisfactory understanding of the mechanisms involved, we are often not yet able to differentiate analytically between samples which are perceived as different on tasting, nor are we able to control the formation of flavour. The role of lactic acid itself raises some questions as the acidification step seems to determine the final flavour quality of ripened cheeses. Besides lactic acid, the free fatty acids appear to be important as precursors of flavour compounds such as methyl ketones, essential for blue cheese characteristics or as precursors of compounds arising from oxidation, which may have a negative or positive effect. One example of this, 1-octen-3-ol, which is objectionable in butter, is of importance for the flavour of mould-ripened cheeses. Whatever the importance of these, the main rout to flavour formation, at least in cheese, is protein degradation, which leads to essential volatile compounds such as sulfur compounds but also to not-volatile compounds which have been much less studied. One of the main questions which is yet to be answered, is how can initial conditions later have so profound influence on the evolution of the protein fraction (Adda, 1986).

• lactic homo-fermentation, produces lactic acids,
• lactic hetero-fermentation, produces lactic acid, ethanol, acetic acid and CO₂,
• alcoholic fermentation, produces ethanol, CO₂, acetic acid and acethaldehyde
• propionic fermentation, produces propionic acid, acetic acid and CO₂,
• ABE fermentation, produces butyric acid, acetic acid, butanol, acetone, ethanol, CO₂ and H₂.

The lactic flora activity leads to a series of compounds which are of importance to most dairy products. Lactic acid is of special importance, for the acidity it creates in the various cultured products and also because the pH drop, which follows lactic acid production, determines the basic structure, characteristic of each cheese variety.

The next metabolite of importance is acetaldehyde. It is recognized as giving the characteristic flavour to yogurt which is obtained by the growth of mixed cultures of Streptococcus thermophilus and Lactobacillus bulgaricus, but it also appears as a by-product in cultured cream and butter as a metabolite of Str.cremoris diacetylactis and Leuconostoc citrovorum.

Another important flavour constituent of cultured products diacetyl.It can be formed from the citrate present in milk by Str. diatylactis, Leuconostoc citrovorum and other organisms that can utilize citrate. It strongly contributes to acid cream butter flavour.

The role of milk fat as a precursor of flavour compounds is also of importance. Triglycerides account for about 98% of milk fat. The importance of free fatty acids, which differ considerably in chain length and flavour intensity, varies between products.

Proteins seem to play a important role in the development of flavour in cheese since proteolysis leads to the formation of important volatile and non-volatile compounds, but probably also because they interact, through active sulphydryl groups, which have been shown to relate closely to flavour formation, with oxidation-reduction reactions which could occur during enzymatic ripening of milk or as a consequence of lactic acid production.

In cheese, bitterness results from the presence of low molecular weight hidrophobic peptides arising mainly from casein. Several factors such as manufacturing conditions, rennet but, above all, cheese microflora have been shown influence the formation of this peptides. Another reason is that some micro-organisms, such as Penicillium caseicolum of mould-ripened cheese, are themselves able to cause bitterness if their growth is significant.