الفهرس 
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Abstract
This study was planned to produce extracellular bioemulsifier
that could potentially be used in food and dairy industries and many
other applications from different yeasts using dairy by-products milk
permeate or cheese whey. Five different single yeast cultures namly
Candida kefyr EMCC ٦٨, Candida tropicalis EMCC ٢, Candida
utilis EMCC ١٢٠, Candida utilis EMCC ١٠٢ and Kluyveromyces
marxianus EMCC ٧٦ and ١١ mixed combination contains pair of
them, in addition to one mixed of the five strains were grown in three
different media namly modified Czapek’s yeast broth, milk permeate
and Ras cheese whey. The bioemulsifier was extracted from each
culture at the end of fermentation after ٧٢ h and characterizatied for
its yield % and emulsification activity % . Out of the sixteen tested
yeast cultures, Cand. utilis EMCC١٠٢, Kluy. marxianus EMCC٧٦
and Cand. kefyr EMCC ٦٨ + Cand. tropicalis EMCC ٢ (١:١)
produced the higher yield and activity of extracellular bioemulsifier
and therefore were chosen for further invesyigations.
In second part of study it was interesting to study the effect of
initial pH values (٦ and ٧), fermentation temperature (٢٥ and ٣٠°C) ,
inoculum level (٢ and ٥ %) and agitation rate (٢٠٠ and ٢٥٠) on the
fermentation parameters of selected yeast cultures. The highest
production of extracellular bioemulsifier was achieved by
fermentation in Ras cheese whey of Cand. utilis ١٠٢, Kluy.
marxianus ٧٦ and Cand. kefyr + Cand. tropicalis (١:١) cultures with
agjusting the initial pH to ٧, inoculums level of ٥٪, agitation rate of
٢٥٠ rpm , fermentation temperature at ٣٠◦C after ٧٢ h. Scale-up
fermentation using ١٤ L fermentor enhanced the yield of
exteracellular bioemulsifier by some yeast cultures. This means high
yield of exteracellular bioemulsifier with low production costs.
In the third part of study the extracellular bioemulsifier of
yeast cultures grown on Ras cheese whey as fermentation medium
was examined for its chemical composition and stability under
different conditions. Fat, ash, protein and carbohydrate contents of
bioemulsifier were determined. All emulsifiers composed mainly of
protein (١٧-٢٠ %) and carbohydrate (٧٥-٨٢ %). The pH had little
effect on the emulsion stability by Cand. utilis ١٠٢ , Kluy. marxianus
٧٦ and Cand. kefyr + Cand. tropicalis (١:١) between pH values (٢ -
١١). Emulsion stability of yeast cultures emulsifiers showed slight
decrease with increasing pH values. There were no loss of emulsion
stability occurred in the presence of ١ to ٣% (w/v) sodium
chloride,but a loss in emulsion stability was noticed with higher Nacl
ratios. Stable emulsion were formed in the presence of sucrose up to
١٥ % (w/v). Emulsion with bioemulsifier of Cand. utilis ١٠٢ , Kluy.
marxianus ٧٦ and Cand. kefyr + Cand. tropicalis (١:١) were not
disrupted by different heat treatments with no changes during storage
at ٤◦C /٣٠ days. Much loss in emulsion stability during storage at
room temperature ٢٥◦C /١٥ days was observed.
The application of bioemulsifier produced by Kluy.
marxianus ٧٦ in production of frozen yoghurt was investigated in
fourth part of study. High quality frozen yoghurt with preferable
texture and consistence, were produced with adding ٠٫٢ - ٠٫٣ % of
bioemulsifiers compared to commercial emulsifier.
Key words: Yeast, emulsifier, emulsification activity, emulsification
stability, frozen yoghurt.