Microbiological and chemical quality of active dry yeast

There is an increasing demand for baker's yeast to satisfy the needs of over growing population. This necessitates that efforts be made to ensure their hygienic suitability and functional quality. This study was, therefore, executed to monitor the microbial content of 9 different brands of active dry yeast (ADY) and the Egyptian compressed yeast. In this regard, the compressed yeast recorded the worst microbiological quality where all samples contained a massive amount of total and faecal coliforms as well as 50% of samples contained salmonella. On the other hand, ADY recorded better result as percentage of unacceptable samples; total coliforms (23.3%), faecal coliforms (17.8%) and Salmonella (4.4%). The leavening ability of yeast depends on its viability and chemical composition. Concerning yeast cell viability, the compressed yeast revealed the highest viability (96.9%) while the viability of ADY brands ranged from 23 to 78.3%. All samples contained fair amounts of lipids and proteins while the intracellular trehalose -which generaJly believed to be a critical parameter for its resistance to stress such as drying-- ranged from 7.89 to 28.8%. The most important role of yeast in bread making is raising the dough to produce the characteristic loaf preferred by consumers. Therefore, evaluating the rising power (RP) and the main parameters affecting the RP includes; temperature, amount of yeast, salt and sugar concentrations were considered. Findings of this study recorded far difference in RP between the local and imported brands especially those from UK and China. Most brands required a specific temperature (35 or 40ºC) to give the maximum RP while some brands gave almost the same RP value in wide range of temperatures. Results indicated a positive correlation between yeast amount and RP while a negative correlation between salt concentration and RP was occurred. Furthermore, adding sugar up to 1.5% to the dough did improve the RP of some brands by 25%. It was rather interest to investigate the most efficient conditions for baker's yeast production. Thus, 4 strains of Saccharomyces cerevisiae were isolated from different commercial ADY. The Isolates were tested for five parameters including initial yeast level, molasses concentration, urea requirements, pH-value and agitation speed. The results recommend adjusting the cultivation molasses medium at 10% sugar with 0.15% urea at pH 5. The medium was then inoculated by the yeast strain to obtain the initial count of 103 cells / mI. Then the flasks were incubated in orbital shaker (150 rpm) at 30 ºC for 24 hours.