الفهرس 
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Abstract
Frozen dairy products are nutritious, healthful and relatively inexpensive desserts which are preferred by people of all ages. The healthfulness and nutritive properties of these frozen dairy desserts are therefore very important. On one hand, it can represent a health guard to the consumer if produced under unsanitary condition. On the other hand, most of the consumers are now health conscious and prefer to use low products which are low in cholesterol and high in its nutritive values. br Frozen yoghurt is one of these products which now gained dominance in the frozen desserts market. It combines the refreshing qualities the previous benefits. br Results obtained in this thesis are divided into parts: br In part І, growth activity, bile tolerance and adherence of bifidobacteia to epithelial cells were studied. br In part П: Frozen yoghurt was manufactured using selected of bifidobacteria in addition to yoghurt cultures. br Part І: br Growth of bifidobacteria in MRSL. br Five species of bifidobacteria were used they were. br Bifidobacterium infantis ATCC 11551 br Bifidobacterium bifidium ATCC 11547 br Bifidobacterium longum ATCC 11549 br Bifidobacterium adolescentis ATCC 11550 br Bifidobacterium breve ATCC 11548. br Growth at low pH values, bile salts tolerance and adherence to epithelial cells were used as preliminary tests to select the best species for the production of probiotic frozen yoghurt. br The best two species in terms of growth at low pH and bile salts tolerance were Bifidobacterium infantis ATCC 11551 and Bifidobacterium adolescentis ATCC 11550 these two species were mixed with br Streptococcus salivarius subsp thermophilus (EMCC 11044) br Lactobacillus delbrueckii subsp bulgaricus (EMCC 11102). br For the production of frozen yoghurt. br The effect of adding bifidobacteria on the physical, chemical and organoleptic properties of frozen yoghurt was also studied. br Survive growth of bifidobacteria ssp in MRSL: br All studied species of bifidobacteria showed similar pattern of growth up to 96 hrs. of growth in MRSL. Regarding growth at different pH values, the most acid resistant species was Bifidobacterium adolescentis ATCC 11550 this species could grow at low pH values. It could survive growth in MRSL at pH ~3. br All species reached the maximum growth after 56 hrs of inoculation and the min. growth was noticed after 96 hrs at 660 nm. br Effect of bile salts on the viability of bifidobacteria growth in MRSL: br Results obtained in this part showed that the most resistance species towards bile salts were Bifidobacterium infantis ATCC 11551; it could grow at 0.3 % of bile salts, whereas the least resistance species was Bifidobacterium adolescentis ATCC 11550. br Adherence to epithelial cells: br The adherence of the five species of bifidobacteria to rabbit’s epithelial cells was studied using gram staining. All studied species were able to adhere to the epithelial cells. However the best species in terms of intensive adherence was Bifidobacterium adolescentis ATCC 11550 and Bifidobacterium infantis ATCC 11551 showed a poor adherence. br According to the previous results, two species were selected for the production of frozen yoghurt; they were Bifidobacterium adolescentis ATCC 11550 and Bifidobacterium infantis ATCC 11551. br Part П: br Manufacturing of frozen yoghurt mixture: - br As explained in Part І, the selected two species of bifidobacteria in addition to yoghurt culture were used to manufacture five baches of frozen yoghurt. br A mixture consists of full fat milk powder + sucrose + gelatin was dissolved in full fat cow milk (part A), then was heated at 85° C for 30 min., the mixture was cooled to 45° C. and after that, it was kept at 4° C for 24 hrs as an ageing process. br The second part of the full fat cow milk was heat treated (part B) then cooling to 45 ° C was inoculated with 3% of starters until pH 5.9. The two parts (Part A and Part B) were added together and the following treatments were carried our. br Control treatment (C) br This treatment consists of yoghurt culture only, they were br Streptococcus salivarius subsp thermophilus (EMCC 11044) br Lactobacillus delbrueckii subsp bulgaricus (EMCC 11102). br Second treatment (A+B) br Consists of Bifidobacterium adolescentis ATCC 11550.and Bifidobacterium adolescentis ATCC 11550. br Third treatment (C+A) br Streptococcus salivarius subsp thermophilus (EMCC 11044) +Lactobacillus delbrueckii subsp bulgaricus (EMCC 11102) + Bifidobacterium adolescentis ATCC 11550. br Forth treatment (C+B) br Streptococcus salivarius subsp thermophilus (EMCC 11044) +Lactobacillus delbrueckii subsp bulgaricus (EMCC 11102) + Bifidobacterium infantis ATCC 11551. br Fifth treatment (C+A+B) br Streptococcus salivarius subsp thermophilus (EMCC 11044) +Lactobacillus delbrueckii subsp bulgaricus (EMCC 11102) + Bifidobacterium adolescentis ATCC 11550. + Bifidobacterium infantis ATCC 11551. br The following tests were carried out to evaluate the quality of the manufactured frozen yoghurt. br Acidity, pH, total solids and overrun br No big difference was noticed between treatments regarding to acidity, pH, total solids and overrun. There was no effect of the bifidobacterial species on the fresh product or after storage for two months. br Overrun: br Addition of bifidobacterial species let to the increase of overrun specially the fifth treatment. br Rehological tests: br The second treatment (A+B) B. adolescentis 11550 and B. infantis 11551. Showed the best rehological properties of Hardness during storage at br (- 18° C) in comparison with the other treatments. But the third treatment (C+A) Streptococcus salivarius subsp thermophilus - Lactobacillus delbrueckii subsp bulgaricus -B. adolescentis 11550 shown the best melting resistance during storage period at (- 18° C) br Bifidobacterial count: br The best treatment in terms of bifidobacterial count was the fifth treatment (C+A+B), showed the highest bacterial population. However the number of bifidobacteria started to decrease according to the period of storage at - 18° C. br Organoleptic properties: br The fifth treatment was the most preferred one according to the organoleptic scores by the partite. There fore it is recommended to manufacture frozen yoghurt using this treatment. br In conclusion, it is recommended that used of the fifth treatment (C+A+B) was best to used in production of frozen yoghurt.