Yeasts of the surface microflora of Feta cheese

Feta cheese samples deriving from three factories of North and Central Greece were examined for their surface yeast microflora. Yeasts were enumerated after 4, 20, 60, 180 days and 1 year of ripening and isolates were obtained from 4-, 20- and 60-day-old cheese. High counts of yeasts were recorded at day 4 and the general trend was a gradual decrease of yeast numbers throughout ageing up to 1 year. The 188 isolates obtained were, for the most part, classified as Saccharomyces cerevisiae (47.9% of the isolates), Debaryomyces hansenii (30.9%) and Pichia farinosa (11.2%). Less frequently isolated species were Candida intermedia, C. tropicalis, C. versatilis, C. catenulata, Pichia membranaefaciens, Saccharomyces kluyveri and Kluyveromyces marxianus. Most strains grew in the presence of 7.5% and 15% (w/v) NaCl. Moreover, proteolytic activity on milk agar was detected only in one strain of Pichia farinosa. Nevertheless, API-ZYM tests applied on strains of predominant species revealed that Sacch. cerevisiae, D. hansenii and P, farinosa exhibit mainly activities of leucine aminopeptidase, valine and cystine aminopeptidase, esterase, and esterase-lipase. Thus, salt-resistant yeasts growing on the surface of Feta cheese during ripening may contribute to ripening changes through enzymes hydrolysing peptides and milkfat.