PRODUCTION Of NUTRIENT SOURCES FOR RHIZOBIUM

specific cases, but Brown (1978) proposed the term “xerotolerant” to cover this entire group of
microorganisms.
The natural habitats of the xerotolerant yeasts include floral nectaries; the yeasts are
commonly associated with bees and honey. If honey is fermented, xerotolerant yeasts are usually
responsible. Their commercial significance lies primarily in the food industry since they can cause
spoilage of wine must, syrups and conserves, fruit juices, dessert wines, dried fruits, molasses and
malt extract. They are also used in the preparation of various oriental fermented foods including soy
sauce and miso paste. Their habitats therefore normally place them in contact with high
concentrations of non-electrolytes and sometimes, in special circumstances, with moderately high
concentrations of salt (Brown, 1978). The majority of yeasts display a moderate tolerance to growth at
low water potentials, but some of those with the capacity to grow well at very low water potentials
(i.e. the osmotolerants) are very important economically as food spoilage yeasts. Examples of
osmotolerant yeasts included Candida mogii, Debaryomyces hansenii, Metschnikowia bicuspidata,
Schizosaccharomyces octospoms and Zygosaccharomyces rouxii (Walker, 1998).
1.5 Production of sugar alcohols by yeasts
Different strains of S. cerevisiae were found to form amounts of glycerol varying between 4.2
to 10.4 g/l (Radler and Schutz, 1981), and two diluted honeys (napunyah, Eucalyptus ochrophloia,
and medium amber) were fermented at 20°C and 30°C using a wine yeast S. cerevisiae (AWRI 729)
and mead yeast S. bisporus (AWRI 366). Fermentations with wine yeast proceeded faster to give
higher alcohol and lower residual sugar products than those with mead yeast which utilised glucose
more slowly than fructose. The fermentation ceased after similar times at both temperatures although
alcohol production and sugar utilization were more rapid during early stages of fermentation at 30°C
than at 20°C (Wootton et al., 1983).
The production of glycerol by Hansenula anomala in molasses corn steep liquor based
media was studied. The accumulation and yield of glycerol was dependent on the medium
composition and aeration rate. The control of pH did not affect the yield, but the intermittent addition
��