9.2 Yeasts
The yeasts are true fungi which have adopted an essentially single celled
morphology reproducing asexually by budding or, in the case ofSchizo-
saccharomyces, by fission. Although they have a simple morphology, it is
probable that they are highly evolved specialists rather than primitive
fungi. Their natural habitat is frequently in nutritionally rich environ-
ments such as the nectaries of plants, plant exudates, decaying fruits and
the body fluids of animals. The yeasts frequently show complex nutri-
tional requirements for vitamins and amino acids.
The yeast morphology has undoubtedly evolved several times for there
are species with Ascomycete or Basidiomycete affinities and quite a
number with no known sexual stage. Although a number of yeasts
almost always occur as single celled organisms, quite a few can develop
the filamentous structure of a typical mould. Indeed, there are a number
of moulds which can take on a yeast morphology under certain condi-
tions, usually in the presence of high nutrient, low oxygen and enhanced
carbon dioxide concentrations.
A major taxonomic study of the yeasts by Kreger–van Rij (1984)
describes about 500 species divided into 60 genera of which 33 are
considered to be Ascomycetes, 10 Basidiomycetes and 17 Deuteromycetes.
A number of yeasts, though certainly not all, are able to grow anaerobi-
cally using a fermentative metabolism to generate energy. The majority, if
not all, of these fermentative yeasts grow more effectively aerobically and
anaerobic growth usually imposes more fastidious nutritional require-
ments on them.
Although there is a large diversity of yeasts and yeast-like fungi, only a
relatively small number are commonly associated with the production of
fermented and microbial foods. They are all either ascomycetous yeasts
or members of the imperfect genusCandida.Saccharomyces cerevisiaeis
the most frequently encountered yeast in fermented beverages and foods
based on fruits and vegetables, an observation which is reflected in the
existence of more than eighty synonyms and varieties for the species. All
strains ferment glucose and many ferment other plant-associated
Table 9.3 Yeasts and lactic acid bacteria in fermented foods
Yeasts^1 Lactic acid bacteria^2 Yeasts and lactic acid bacteria
Modern European beers Yoghurt Sourdough bread
Bread Sauerkraut Kefir
Wine Salami Soy sauce
Cider Cheese African beers
Lambic beer
(^1) The presence of lactic acid bacteria in these foods is often associated with spoilage
(^2) The presence of yeasts in these foods is often associated with spoilage
312 Fermented and Microbial Foods