Relationships between flavour and microbiological profiles in Serra da Estrela cheese throughout ripening

Cheeses manufactured in certified dairies in the Portuguese region of Serra da Estrela, using refrigerated and non-refrigerated raw sheep milk, were quantitatively evaluated in terms of indigenous microflora and volatile compounds during a ripening period of up to 6 mo. Viable counts were obtained for lactococci, lactobacilli, leuconostoc, enterococci, yeasts, Enterobacteriaceae and staphylococci; analyses of volatiles were performed by chromatography after solid-phase micro-extraction. Treatment of all analytical data produced by principal component analysis revealed correlations between the major microbial groups present in cheese and patterns of volatiles generated. End products resulting from the degradation of sugars, free amino acids and glycerides constituted the predominant volatiles of Serra da Estrela cheeses. Among volatile, short-chain carboxylic acids detected were acetic, propionic, iso-butyric and iso-valeric acids; these compounds are known to be breakdown products of Gly, Ala and Ser, of Thr, of Val, and of Ile, respectively, following oxidative deamination. Semi-volatile fatty acids and their corresponding ethyl esters appeared in the cheese, probably as a result of the activity of lipases produced by yeasts and Enterobacteriaceae. These ethyl esters, which are responsible for fruity flavours, were especially pronounced in cheeses manufactured from refrigerated milk.