الفهرس 
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Abstract
The objective of this study was to investigate the possibility to make a high-quality probiotic Ras cheese using some strains of probiotic Lactobacilli as adjunct cultures and to study the effect of this on the chemical, microbiological, and organoleptic properties of Ras cheese. the treatments were divided into:
C: (Control) (Streptococcus thermophiles: Lb. delbrueckiisubsp. Bulgaricus, 1:1).
T1: Lb. acidophilus + (Streptococcus thermophiles: Lb. delbrueckii sub.sp. bulgaricus, 1:1).
T2: Lb. helveticus + (Streptococcus thermophiles: Lb. delbrueckii subsp. Bulgaricus, 1:1).
T3: Lb. casei + (Streptococcus thermophiles: Lb. delbrueckii subsp. Bulgaricus, 1:1).
Then the spoiled fungal species of these cheeses were isolated and identified. Also, the natural presence of mycotoxins in fresh and stored cheese samples was determined. As well as the ability of the highest isolated fungi to produce mycotoxins, proteolytic and lipolytic enzymes was examined.
5.1. Improving the Properties of Ras cheese with adding some probiotic Lactobacillus spp. as adjunct cultures.
The aim of studying this part is to improve the quality of the Egyptian hard cheese (Ras type) by m adding some Lactobacillus spp. as probiotic adjunct cultures. Strains of Lb. acidophilus, Lb. helveticus and Lb. casei were used and the chemical, microbiological and sensory characteristics of the cheese during 90 days of ripening were evaluated.
1- pH values and moisture content were decreased, while the total nitrogen (TN), fat, fat/dry matter, titratable acidity and salt contents were increased significantly during the ripening period in all treatments.
2- For the ripening indices, significantly higher content of soluble nitrogen (SN), SN/TN, soluble tyrosine, soluble tryptophan and total volatile fatty acids were found in the experimental cheese compared to the Control cheese.
3- Higher Lactobacilli count was observed in the adjunct treated cheeses, which reflects the positive retention of Lactobacilli in the experimental cheeses.
4- Also, the use of probiotic adjunct cultures reduced the growth of yeasts, moulds and prevented the coliform bacteria on Ras cheesecompared to Control.
5- There was higher acceptability for the experimental cheeses than for the Control.
6- Ras cheese was considered a good carrier for Lactobacillus probiotic strains since they were found to survive during cheese manufacture, ripening and storage.
5.2. Isolation and identification of fungi from Ras cheese made by adding some Lactobacillus spp. and its effect on the production of toxins and their own enzymes.
The aim of studying this part is to isolate and identify the spoiled fungal species from Ras cheese made with adding some probiotic Lactobacillus spp.and to determine the natural presence of mycotoxins in the fresh and the storage cheese samples, as well as examine the ability of the isolated fungi to produce mycotoxins, proteolytic and lipolytic enzymes.
1- Fifteen species and one species variety belonging to 6 fungal genera were isolated from Ras cheese samples at fresh and 90 days of storage using two isolation media at 28°C.
2- On Czapek’s medium, there were 14 species and one variety belonging to 6 genera. On Potato Dextrose Agar medium, there were 12 species and one variety belonging to 5 genera.
3- However,Ras cheese samples were completely free of mycotoxins, 7 out of 18 tested A. flavus isolated from cheese samples had the ability to produce aflatoxin B1 and two isolates had the ability to produce aflatoxin B1 and G1.
4- Also, 3 isolates out of 10 tested A. niger had the ability to produce Ochratoxin A.
5- All tested A. flavus and A. niger isolates had the ability to produce each of lipases and proteases; from which 17 and 9 isolates possessed high lipase and protease activities, respectively.
6- The total number of fungi isolated from Ras cheese samples processed with Lactobacillus bacteria was within the appropriate limits for the standard specifications (102-103 cfu/gm).
7- However, the samples were contaminated with mycotoxin-producing fungi such as A. flavus and A. niger, the use of milk free of mycotoxins and the use of a controlled manufacturing method led to the production of Ras cheese completely free of mycotoxins.
8- On the other hand, the isolated fungi were positively to the production of the proteolytic and lipolytic enzymes and thus could affect the sensory properties and the nutritional value of Ras cheese.
9- However, the isolated fungi are dangerous and produce toxins and many enzymes, the final specifications of the cheese are excellent and free of toxins, this result could be attributed to the use of Lactobacillus bacteria that improved the product.
10- Therefore, we can recommend that we do not rely in the standard specifications on the number of fungi only because the numbers may be within the permissible limits, but there are species that have the ability to produce the toxins in proportions affecting the product.