Yeast diversity in white mould-ripened cheeses
Khoury, Alice Rosal
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Surface mould-ripened cheeses represent a small proportion of world cheese production. However, these cheeses are becoming increasingly popular with the consumer, as there is an increasing demand for it (Gripon, 1987). In Chapter 1 the integrated roles of moulds, yeasts and bacteria involved in the production and maturation of white surface mould-ripened cheeses, and the assistance of the microflora in the development of the characteristic flavour and texture of mould-ripened cheeses, were thoroughly discussed. Due to the fact that yeast colonies are often overgrown by mould spreaders, we intended to develop a medium that inhibits moulds and bacteria, but supports the growth of all yeasts present in white-mould cheeses. Ten selective mycological media were evaluated and compared statistically for their suitability to enumerate yeasts and simultaneously suppress moulds in white-moulds cheeses. Malt extract agar (MEA) supplemented with 0.5% sodium propionate (SP), proved to be the only medium that totally suppressed mould growth. However, the medium also restricted yeast development, as the mean yeast counts obtained on this medium differed significantly (p<0.5) compared to the other media. Dichloran Rose Bengal agar (DRBC) and Oxytetracyline Gentamycin Glucose Yeast Extract agar (OGGYA) retarded mould growth and supported the growth of the six most dominant yeast species recovered from white-mould cheeses, making these media suitable for the isolation and enumeration of yeasts in the presence of moulds. The yeasts present in Camembert and Brie cheeses during processing were monitored in a single cheese factory during summer and winter, to determine the seasonal diversity of yeasts over a ripening period of 56 days. Despite the predominance of lactic acid bacteria during the making of Camembert and Brie, yeasts playa significant role in the ripening process reaching counts as high as 106 cfu.q' at the later stages of ripening. A diverse variety of 20 yeast species representing 10 genera were present during the winter period associated with the factory environment, during processing and ripening, whereas only seven yeast species representing six genera were isolated during summer. Although a broad spectrum of yeasts were isolated from Camembert and Brie cheeses, Debaryomyces hansenii was the most abundant yeast isolated. Other species encountered were Yarrowia lipolytica, Torulaspora delbrueckii, Rhodotorula mucilaginosa, Rhodotorula minuta, and various species of Candida. The yeasts, moulds, lactic acid and coliform bacteria present in Camembert and Brie cheeses during processing and maturation were monitored in a single cheese factory during a 56 day ripening period. The sources of microbial contamination that may lead to contamination of the curd were also determined. The whey, brine and equipment surfaces were responsible for the highest yield of contaminating yeasts and coliforms. Samples were taken at critical control points during the manufacturing process and the microbial populations enumerated after incubation at 25°C for 96h. Samples taken during manufacturing and ripening were also analyzed for organic acid (lactic, acetic, succinic and iso-butyric acids) and sugar (glucose, fructose, galactose and lactose) content using HPLC.