Fermen ted and Microbial Foods
9.1 INTRODUCTION
So far in this book we have been almost exclusively concerned with the
negative roles that micro-organisms play in food. There is however a
huge diversity of foods where microbial activity is an essential feature of
their production. Some are listed in Table 9.1 and in this chapter we will
describe a few of these in more detail and discuss some general features
of food fermentation.
Almost without exception, fermented foods were discovered before
mankind had any knowledge of micro-organisms other than as witness
to the effects of their activity. It was simply an empirical observation that
certain ways of storing food effected desirable changes in its character-istics (Table 9.2). Originally the most important of these changes must
have been an improvement in the shelf-life and safety of a product,
although these became less important in the industrialized world with the
advent of alternative preservation methods such as canning, chilling and
freezing. Modern technologies have in no way diminished the sensory
appeal of fermented products however. This is clear from the way people
rarely enthuse over grape juice or milk as some are prone to do over the
vast array of wines and cheeses.
We now know that, in food fermentation, conditions of treatment and
storage produce an environment in which certain types of organism can
flourish and these have a benign effect on the food rather than spoiling it.
The overwhelming majority of fermented foods is produced by the
activity of lactic acid bacteria and fungi, principally yeasts but also, to
a lesser extent, moulds. Both groups of organisms share a common
ecological niche, being able to grow under conditions of low pH and
reduced a
w
, although only lactic acid bacteria and facultative yeasts will
prosper under anaerobic conditions. As a consequence, they frequently
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occur together in fermented foods; in some cases members of both
groups act in concert to produce a product while in others, one group
plays the role of spoilage organisms. Some examples of these are
presented in Table 9.3.
Table 9.1 Some ferment ed foods
Food Ingredients Geographical Distribution
Busa Rice, millet, sugar Turkey
Beer Barley Widespread
Cheese Milk Widespread
Chicha Maize and others S. America
Dawadawa Locust beans W. Africa
Gari Cassava Nigeria
Idli/dosa Rice and black gram India
Injera Tef Ethiopia
I-sushi Fish Japan
Kefir Milk Eastern Europe
Kenkey Maize, sorghum Ghana
Kimchi Vegetables Korea
Koko Maize, sorghum Ghana
Leavened bread Wheat Europe, N. America
Lambic beer Barley Belgium
Mahewu Maize S. Africa
Nam Meat Thailand
Ogi Maize, sorghum, millet Nigeria
Olives Mediterranean Area
Palm wine Palm sap Widespread
Poi Taro Hawaii
Puto Rice Philippines
Salami Meat Widespread
Salt stock, cucumbers Cucumbers Europe, N. America
Sauerkraut Cabbage Europe, N. America
Sorghum beer Sorghum S. Africa
Sourdough bread Wheat, rye Europe, N. America
Soy sauce, miso Soy beans S.E. Asia
Tempeh Soy beans Indonesia
Tibi Fruit Mexico
Yoghurt Milk Widespread
Table 9.2 Effect s of food ferment ation
Raw material Stability Safety Nutritive Value Acceptability
Meats þþ þ (þ )
Fish þþ þ (þ )
Milks þþ þ (þ )(þ )
Vegetables þ ( þ ) (þ )
Fruits þ þþ Legumes ( þ )(þ ) þ
Cereals (þ ) þ
þþ Definite improvement
þ Usually some improvement
( þ) Some cases of improvement
No improvement
311 Chap ter 9
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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-me nt s o n t hem .
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
Table 9.3 Yeas ts a nd lactic acid bacter ia in ferment ed foods
Yeasts
1
Lactic acid bacteria
2
Yeasts and lactic acid bacteria
Modern