Yeast diversity in white mould-ripened cheeses
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Date
2002-01
Authors
Khoury, Alice Rosal
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
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.
Description
Keywords
Cheese -- Microbiology, Yeast, Cheesemaking, Dissertation (M.Sc. (Microbiology and Biochemistry))--University of the Free State, 2002