الفهرس | Only 14 pages are availabe for public view |
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. |