Yeast diversity in white mould-ripened cheeses

[摘要] Surface mould-ripened cheeses represent a small proportion of world cheeseproduction. However, these cheeses are becoming increasingly popular withthe consumer, as there is an increasing demand for it (Gripon, 1987). InChapter 1 the integrated roles of moulds, yeasts and bacteria involved in theproduction and maturation of white surface mould-ripened cheeses, and theassistance of the microflora in the development of the characteristic flavourand 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, butsupports the growth of all yeasts present in white-mould cheeses. Tenselective mycological media were evaluated and compared statistically fortheir suitability to enumerate yeasts and simultaneously suppress moulds inwhite-moulds cheeses. Malt extract agar (MEA) supplemented with 0.5%sodium propionate (SP), proved to be the only medium that totally suppressedmould growth. However, the medium also restricted yeast development, asthe mean yeast counts obtained on this medium differed significantly (p<0.5)compared to the other media. Dichloran Rose Bengal agar (DRBC) andOxytetracyline Gentamycin Glucose Yeast Extract agar (OGGYA) retardedmould growth and supported the growth of the six most dominant yeastspecies recovered from white-mould cheeses, making these media suitablefor the isolation and enumeration of yeasts in the presence of moulds. The yeasts present in Camembert and Brie cheeses during processing weremonitored in a single cheese factory during summer and winter, to determinethe seasonal diversity of yeasts over a ripening period of 56 days. Despitethe predominance of lactic acid bacteria during the making of Camembert andBrie, yeasts playa significant role in the ripening process reaching counts ashigh as 106 cfu.q' at the later stages of ripening. A diverse variety of 20 yeastspecies representing 10 genera were present during the winter periodassociated with the factory environment, during processing and ripening,whereas only seven yeast species representing six genera were isolatedduring summer. Although a broad spectrum of yeasts were isolated fromCamembert and Brie cheeses, Debaryomyces hansenii was the most abundant yeast isolated. Other species encountered were Yarrowia lipolytica,Torulaspora delbrueckii, Rhodotorula mucilaginosa, Rhodotorula minuta, andvarious species of Candida.The yeasts, moulds, lactic acid and coliform bacteria present in Camembertand Brie cheeses during processing and maturation were monitored in asingle cheese factory during a 56 day ripening period. The sources ofmicrobial contamination that may lead to contamination of the curd were alsodetermined. The whey, brine and equipment surfaces were responsible forthe highest yield of contaminating yeasts and coliforms. Samples were takenat critical control points during the manufacturing process and the microbialpopulations enumerated after incubation at 25°C for 96h. Samples takenduring manufacturing and ripening were also analyzed for organic acid (lactic,acetic, succinic and iso-butyric acids) and sugar (glucose, fructose, galactoseand lactose) content using HPLC.

[发布机构] University of the Free State