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In this article we will discuss about the fermentation process of yeasts.
The yeasts are true fungi which have adopted an essentially single celled morphology reproducing asexually by budding or, in the case of Schizosaccharomyces, 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 environments such as the nectaries of plants, plant exudates, decaying fruits and the body fluids of animals. The yeasts frequently show complex nutritional requirements for vitamins and amino acids.
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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 conditions, 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 anaerobically 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 requirements on them.
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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 genus Candida.
Saccharomyces cerevisiae is 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 carbohydrates such as sucrose, maltose and raffinose but none can ferment the animal sugar lactose. In the tropics Schizosaccharomyces pombe is frequently the dominant yeast in the production of traditional fermented beverages where a natural fermentation is allowed to occur, especially those produced from cereals such as maize and millet.
Kluyveromyces marxianus is able to hydrolyse lactose and ferment galactose. There are a number of varieties which had previously been recognized as separate species associated with a range of different fermented milk products.
K. marxianus var. marxianus (= K. fragilis) is the perfect state of Candida kefir and has been isolated from eastern European fermented milks such as koumiss and kefir. K. marxianus var. bulgaricus has been isolated from yoghurt and K. marxianus var. lactis from buttermilk, Italian cheese and fermented milks from Manchuria.
Because of its ability to grow at low water activities in the presence of high concentrations of sugar or salt, Zygosaccharomyces rouxii is especially associated with the fermentation of plant products in which the addition of salt is an integral part of the process.
Many strains of Hansenula anomala and Debaryomyces hansenii can also grow in fairly concentrated salt solutions and the latter is frequently isolated from brined meat products and fermented sausages.
Although able to ferment carbohydrates, yeasts such as Pichia guilliermondii and Saccharomycopsis fibuligera grow best as surface pellicles and have been isolated from a number of tropical fermented products. The latter is able to break down starch and is associated with ‘chalky bread’.
Geotrichum candidum is usually considered as a filamentous mould but it has a strong affinity with the ascomycetous yeasts and is frequently isolated as part of the surface flora of fermented milk products such as cheeses.
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It is important to realize that, although all these species of yeasts and yeast like fungi are thought to play a positive role in the production of a diverse range of fermented foods, they also occur as spoilage organisms in other commodities where their biochemical activities are undesirable.
The following key indicates how the genera discussed above differ from each other:
1. Vegetative reproduction by cross-wall formation followed by fission -Schizosaccharomyces
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1. Vegetative reproduction by budding – 2
2. Ascospores not formed – Candida
2. Ascospores formed – 3
3. Nitrate assimilated – Hansenula
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3. Nitrate not assimilated – 4
4. Abundant true mycelium as well as budding – 5
4. True mycelium scarce or absent – 6
5. Asci formed exclusively on the true hyphae – Saccharomycopsis
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5. Asci not formed exclusively on the true hyphae – Pichia
6. Asci dehiscent – Kluyveromyces
6. Asci persistent – 7
7. No conjugation preceding ascus formation – Saccharomyces
7. Conjugation preceding ascus formation – 8
8. Ascospores warty or with ridges – Debaryomyces
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8. Ascospores spherical and smooth – Zygosaccharomyces
