PRODUCTION Of NUTRIENT SOURCES FOR RHIZOBIUM

1.2�Classification of yeasts
Mankind has been concerned with fermentations for at least 8,000 years. The ancient people
of Sumaria, Babylon, and Ur documented beer making in their artwork, and somewhat later (~ 4,000
B. C.), the Egyptians depicted the baking of leavened bread. Although these processes were
eventually used world wide, it was not until 1837 that independent studies by Cagniard de la Tour,
Schwann, and Kutzing show yeasts to be responsible for fermentation. S. cerevisiae was the species
generally used in baking and brewing, but there were about 600 known yeast species and certain
others are also important commercially.
The yeasts are a phylogenetically diverse group of fungi whose teleomorphs or sexual states
are found among two major taxonomic classes, the Ascomycotina and the Basidiomycotina.
Consequently, the term “yeast” is one of convenience that describes predominantly unicellular
organisms that undergo vegetative cell division either by budding or by fission. Furthermore, in
contrast to higher fungi, the sexual state of a yeast is not enclosed in a fruiting body (Verachtert et al.,
1990).
1.2.1 Ascomycetous yeasts
Historically, ascomycetes have been placed in two taxonomic classes, or as subclasses
in some treatments, the Hemiascomycetes and the Euascomycetes. Hemiascomycetes are
characterized by asci that are not enclosed within a fruiting body (ascocarp), whereas Euascomycetes
usually form asci within or upon fruiting bodies. Budding yeasts, fission yeasts and yeastlike genera
such as Ascoidea, Cephaloascus, and Taphrina are assigned to the Hemiascomycetes, but taxonomic
treatment of yeasts typically exclude the yeastlike taxa.
Earlier proposals for the ascomycetous yeast have been summarized by Lodder (1970)
and Kreger-van Rij (1984). Those systems were mostly based on type of vegetative cell division,
ploidy and morphology of ascospores. Another aspect of classification is whether the ascomycetous
yeasts represent primitive organisms, as is suggested by their appearance, or whether they are reduced
forms derived from various more advanced fungal lineages. Cain (1972) proposed that yeasts
represent reduced forms, citing the hat–shaped (galeate) ascospores common to Pichia (Hansenula),
Cephaloascus, and Ceratocystis as evidence of an adaptation that would have occurred just once in
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