carbohydrates such as sucrose, maltose and raffinose but none can
ferment the animal sugar lactose. In the tropicsSchizosaccharomyces
pombeis 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.Kluy-
veromyces marxianusis 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. marxianusvar.marxianus(¼K. fragilis) is the perfect state
ofCandida kefirand has been isolated from eastern European fermented
milks such as koumiss and kefir.K. marxianusvar.bulgaricushas been
isolated from yoghurt and K. marxianusvar. lactisfrom 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 rouxiiis espe-
cially associated with the fermentation of plant products in which the
addition of salt is an integral part of the process. Many strains of
Hansenula anomalaandDebaryomyces hanseniican 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 asPichia guil-
liermondiiandSaccharomycopsis fibuligeragrow 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. 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 activ-
ities are undesirable.
One of the most important yeasts associated with spoilage isZygo-
saccharomyces bailii. It has the ability to grow at relatively low water
activities and low pH, as well as being remarkably resistant to preserv-
atives, such as sorbic, benzoic and ethanoic acids, sulfur dioxide and
ethanol, commonly used to prevent microbial spoilage of fruit juices,
fruit juice concentrates, fermented beverages, pickles and sauces.Z. bailii
is strongly fermentative and spoilage of products stored in plastic packs
and glass bottles can lead to explosion of the containers. The survival of
a single cell in a product containing an appropriate nitrogen source and
fermentable carbohydrate can result in spoilage, so pasteurization or
membrane filtration before filling, followed by stringent hygiene to
prevent post-treatment contamination, are essential.
Chapter 9 313