Yeast diversity in blue mould ripened cheeses

[摘要] English: Blue veined cheese is considered a speciality cheese and is not as popular as other cheese varieties, such as Cheddar or Gouda. Their popularity is however increasing worldwide making it important to study their microbiological, biochemical and technological properties. The aims of this study was consequently to elicit the establishment, growth and survival of yeasts during the ripening of blue veined cheese varieties and to study the interactions between yeasts and the different microbial groups which occur during the ripening process.The enumeration of yeasts in foodstuffs with a high mould concentration proves to be difficult since yeasts are easily overgrown by the rapidly spreading moulds. This makes the enumeration, isolation and identification of yeasts difficult, hence also eliciting the ecological role of yeasts during the ripening of blue veined cheese varieties. Ten different selective media, containing anti-myctotic properties, were evaluated for their ability to suppress the growth and spreading of moulds and enhance the enumeration and recovery of yeasts from blue veined cheese. Quantitatively no statistical difference between the ten different media was detected. Qualitatively, however OGGY, MEA + SP and molybdate containing media could not be recommended. All other media performed satisfactory while MEA + Ox and MEA + BP gave superior results based on ease of isolation and enumeration. The usage of MEA + NaCl gave comparable results, but inhibited the growth of some of the yeast species most frequently found in ripening blue veined cheese varieties, when evaluated individually.Yeasts are one of the main microbial groups that occur during the ripening of blue veined cheese varieties, originating as post-pasteurisation contaminants. In chapter 3 the development of yeasts derived mainly from the dairy environment and their establishment during the ripening process was examined. They continued to dominate until the end of the ripening period, in meaningful numbers. Yeast species isolated and identified corresponds with those obtained by other researchers on similar cheese varieties.A 10 to 100 fold difference in yeast numbers were observed during the ripening period between the exterior and interior. Debaryomyces hansenii once again predominated in the present study. This is due to its ability to produce extracellular proteases and lipases and the species high tolerance towards low temperatures and high salt concentrations.Although yeasts are one of the major microbial groups present in blue veined cheese other microorganisms like Penicillium roqueforti and several different bacterial genera are also frequently encountered. All of these organisms collectively contribute to the final flavour and aroma of the product based on interactive proliferation. Consequently it was necessary to study the interactions between these organisms to improve our understanding of the ripening process and the contribution each organism makes to the process. Interaction studies in Chapter 4 revealed that various interactions occurred between yeasts and Penicillium roqueforti, as well as yeasts and bacteria essential to the final outcome of the product. The majority of yeast-bacterial interactions were neutral, although some interactions were antagonistic and stimulative. Debaryomyces hansenii was the most stimulatory yeast species towards bacterial isolates while Torulospora delbrueckii was the most antagonistic. All three microbial groups studied dominated throughout the ripening period and were present in meaningful numbers at the end of the ripening period.

[发布机构] University of the Free State