الفهرس 
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Abstract
Gomaa Nour El-Dien Abd El-Rahman. “Production of Baker’s Yeast from Saccharomyces cerevisiae Strains Resistant to Some Contaminants”. Unpublished Ph.D. Thesis, Department of Food Science, Faculty of Agriculture, Ain Shams University, 2015
The aim of this study was to determine the heavy metals content of crude molasses and treated molasses samples collected during 2013 seasons from El-Hawmdia for Chemicals Factory. Nine S. cerevisiae strains were tested to select the best strains able to resist lead, cadmium, copper and nickel as compared to factory strain. The ten strains were screened on the basis of their resistance to heavy metals at different concentrations (0, 2, 4, 8, 12, 16, 20 and 24 ppm) for lead or copper and 0, 0.5, 1, 2, 4, 6, 8 and 10 ppm for cadmium or nickel.
The slope values of yield, total viable cells and gassing power of produced baker’s yeast as well as concentrations of lead, cadmium, copper and nickel in produced baker’s yeast were investigated as indicators of strains resistance to heavy metals. Also, the combined effect of nickel and copper on selected S. cerevisiae strains was studied.
In addition, the effect of molasses treatment by different methods on decreasing of heavy metals was investigated. The treated molasses by cation exchange resin then sulfuric acid (the best treatment) was used as growth medium of selected strains and compared with synthetic medium for selection the best medium for heavy metals resistance.
Moreover, the effect of storage on quality of produced baker’s yeast through determination of total viable cells, gassing power, trehalose and glycogen content of selected S. cerevisiae strains was investigated. Finally, application of the selected strains in production of balady and pan bread was evaluated.
The molasses collected at autumn season recorded the highest values of studied heavy metals. The strains S. cerevisiae FH-620 (S8), S.cerevisiae FC-620 (S1) and S. cerevisiae FAT-12 (S9) showed the highest resistance against lead, cadmium, copper and nickel and had the minimum levels of both four investigated metals in produced baker’s yeast as compared to factory strain. The optimum concentrations of nickel or copper as separate treatment was 4 ppm, while the optimum concentrations of combined nickel and copper were 6 and 4 ppm, respectively
The molasses treated by cation exchange resin then sulfuric acid had the lowest concentrations of heavy metal compared with other treatments. The molasses medium had a protective effect on properties of selected yeast strains against all concentrations of toxic metals (lead and cadmium) as well as high concentrations of essential metals (copper and nickel). Meanwhile, the molasses medium had a negative effect at low concentrations of copper and nickel. The total viable cells, gassing power, trehalose and glycogen of all selected S. cerevisiae strains were decreased by increasing the storage period. The balady and pan bread produced by S. cerevisiae FH-620 (S8) strain as a fermentative agent had the best properties of bread quality.
Key words: Baker’s yeast, Saccharomyces cerevisiae, molasses, resistance, lead, cadmium, copper, nickel, yield, total viable cells and gassing power.