الفهرس 
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Abstract
Yeasts are unicellular filamentous fungi that take different shapes; they may be oval, spherical or cylindrical in shape. They are facultative anaerobic organisms, which can breathe and survive under both aerobic and anaerobic conditions, but forced anaerobic yeasts have not been discovered so far. They are reproduction asexually by budding or diploid division, and sexually by production of ascospores.
Yeasts are widely prevalent in air, soil and foods such as milk, some cheeses, beans and wine. In addition, yeasts are widely found in dairy products and development of new products due to their nutritional benefits and the added elements, enzymes and vitamins, as well as for the healthy properties they add to the host in addition to the nutritional value, which is what is known its probiotic features or what is known as (functional foods).
Functional foods have many health benefits that exceed the effects of normal food with maintaining the general appearance of traditional foods and are consumed as part of the usual daily food. Functional dairy products have developed in recent years to a remarkable degree, as they are used to as assist factors to reduce the risks of some diseases, and also help to improve the health status of the elderly, and thus reduce the costs of health care for them, and also work to increase the immune protection of infants and adults alike.
This study aimed to produce healthy functional dairy products with high nutritional value that deliver those probiotic microorganisms to the host in sufficient quantities to produce their own beneficial effects, and the focus here will be on add strains of probiotic yeasts in some dairy products.
The most important obtained results can be summarized as follows:-
Yeast strains in the study
Obtaining some yeast strains nonpathogenic, where ten strains belonging to five different genera were collected from reliable sources from Egyptian universities and research centers. It consisted of 5 strains belonged genus (Saccharomyces), 2 strains belonged genus (Geotrichum), one strains belonged genus (Kluyveromyces), one strains belonged genus (Debaryomyces) and one strains belonged genus (Torulaspora). Then, their purity was confirmed by re-planning on the agar plats and examining colonies and cells under a microscope.
After that, some physiological and chemical properties of the strains under study were studied through their ability to ferment some sugars such as (glucose - galactose - sucrose - lactose – D-mantol), as well as determining their ability to use some organic compounds as a carbon source or nitrogen source in aerobic or semi-conditions Aerobic such as (glucose - galactose - sucrose - lactose - maltose - manthol - starch - xylose - glycerol - ethanol - methanol - nitrite - nitrate).
The results of the tests showed that, the majority of strains have the ability to fermented glucose and sucrose, except three strains. All strains were able to use glucose, sucrose and ethanol as the sole carbon source; all strains couldn’t use methanol, nitrite and nitrate, while only one strain(H) was able to use lactose as the sole carbon source.
Probiotic criteria
In order for the term ”probiotic” to be applied to yeast strains, they must be able to withstand the pressures present inside the human body, such as grow across a wide variety of temperatures, low pH, tolerance to bile salt, high NaCl concentration, simulated gastric juice, intestinal environment α- amylase, trypsin, lysozyme, assimilation of cholesterol, and antibiotic resistance.
Tolerance of acidity
It is known that the time that the stomach takes food from the moment it enters it until it leaves to be 90 min. Although, cellular stress begins in the stomach, which has pH as low as 1.5, in most in vitro assays pH 3.0 has been preferred. This is due to the fact that a substantial decrease in the viability of strains is often observed at pH 2.0 or below, so the survival of these strains was tested for low pH degrees (1.5, 2 and 3) for a period of 60 minutes to 180 min.
The obtained results showed that most, of the strains had the ability to tolerate low pH at pH 1.5, where the survival rate after 180 min ranged between 78.03 to 94.78%, and the strain (B) Saccharomyces cerevisiae was the most tolerant than other yeast strains. While the two strains (I & J) could not grow on these conditions. All strains were able to grow at pH 3 and the survival rate ranged from 96.80 to 100.42%.
Tolerance of bile salts
When yeast cells reach the small intestine, they begin to be exposed to a solution of bile salts, which works to reduce its resistance. It is known that bile salts are produced in the liver as a result of the cholesterol catabolism process, where their concentration in the intestine is in the range of 0.3%. The effect of two concentrations of bile salts (0.3 and 0.5%) on the growth of strains for 72 hours was studied.
The results showed that all yeast strains had the ability to tolerate and grow in bile salt concentrations, where the percentage increase in growth ranged between 29.95 to 39.69% for all strains and for different concentrations.
Tolerance of gastric juices
Gastric juice contains four main components: water, hydrochloric acid (Hcl), mucin (Mucin), pepsinogen, which is transformed under the influence of hydrochloric acid into active enzyme pepsin, and this, is what is done inside the stomach. All these components are important in addition to the digestion of food, killing most of the microorganisms that leak into the food, so it was important to study the extent of the effect of this enzyme on the life-supporting yeasts under study.
from the results obtained, most the yeast strains have ability to tolerate gastric juice, as the survival rate ranged between from 58.918 to 91.097% after 240 min. The highest resistant strain was the strain (F) Geotrichum candidum, while the two strains (I & J) could not tolerate and grow under the same conditions.
Tolerance of intestinal juice
The effect of exposure of the tested strains to enteric juices (pancreatin 1% and pH 8) for 240 min was studied. The results showed that, the strain (B) Saccharomyces cerevisiae is the highest resistant strains to the growth conditions in the intestinal juice, where the survival rate reached 90.96% after duration of 240 min of incubation.
Cholesterol assimilation
Found that the increase in the level of cholesterol in the blood serum is associated with an increase in the incidence of coronary heart disease. It was also found that the discipline of nutrition is one of the effective means to reduce the level of cholesterol in the blood serum, and thus reducing the risk of coronary artery disease. Also, found that the consumption of products containing probiotics can help control cholesterol levels in both humans and animals, and cholesterol assimilation is an important criterion that determines the ability of yeasts to be probiotics. The ability of the tested strains to assimilate cholesterol was studied. The results obtained showed that, all the tested strains had the ability to assimilated cholesterol and reduce its concentration, and the strain (I) Debaryomyces hansenii was the highest capable of assimilated cholesterol and lowering its concentration than the other strains, where it reduced it by 44.74%, while the strain (A) S. cerevisiae, the least strains to lower the concentration of cholesterol, where it got a percentage of 18.42%.
Study of the proteolytic and lipolytic activity of yeast strains
The ability of the studied yeasts to contain different enzymatic degradation activities was studied, and the results showed that some strains of yeast (E, F, G & H) have proteolytic and lipolytic activity and the strain (I) has lipolytic activity only. While, the rest of the strains did not possess any of the two activities.
Use of yeasts as antimicrobials
The extent to which yeast strains were used as antimicrobial against pathogenic bacterial strains belonging to the genera E. coli, Salmonella, Shigella, Pseudomonas, Bacillus and Staphlococcus was studied. from the results obtained, it was found that the degrees of yeasts ranged from being a significant inhibitor of the growth of some pathogenic bacteria to having no effect on them.
Use of yeasts as antioxidants
The ability of yeast strains to be used as natural antioxidants was studied by studying the cells in the yeast autolysates, as well as the ethanol extract, as well as the water extract from them. To extract the antioxidant active cell-components from yeast strains and determined antioxidant components in yeast cells. The results showed that the ethanol extract of the cells was the high effective for the treatment, while the water extract was the least effective in the percentage of antioxidants from the yeast autolysates.
Interaction between yeast strains and lactic acid bacteria
The interaction between yeast strains and some lactic acid bacteria strains was studed, such as Streptococcus thermophiles, and Lb. delbrueckii subsp. bulgaricus, and Lb. acidophilus, and Lb. casei. This is done through inoculation in single and mixed cultures of both, in sterilized milk and incubation at a temperature of 30 °C for at 24 h, then counting the bacterial content as well as the number of yeasts in each of the single and mixed cultures and estimating the pH. The results obtained showed that, the yeast strains are synergistic activity on the growth of lactic acid bacteria, as it was observed that the number of lactic acid bacteria increased in cultures mixed with yeast strains than in those grown single cultures.
The use of yeast strains in the manufacture of some dairy products
Through the tests and experiments conducted on the yeast strains under study, two strains were selected and included in the manufacture of some dairy products such as (stirred yogurt and ice milk), yeast strains are saccharomyces cerevisiae (B) & Geotrichum candidum (F).
Stirred yogurt industry
Whole buffalo milk was used in the manufacture of the drink, after heat treatment at a temperature of 90°C for 15 min, then cooling at a temperature of 37°C, then the freeze-dried starter (DVS) was added and then divided into 5 treatments in addition to the control, It was as follows:-
C: control
T1: Added strain (B) Saccharomyces cerevisiae before incubation
T2: Added strain (F) Geotrichum candidum before incubation
T3: Added strain (B) Saccharomyces cerevisiae after incubation
T4: Added strain (F) Geotrichum candidum after incubation
Then all treatments were whisked and packed in sterile and airtight containers and kept inside the refrigerator at 4 ° C for 21 days. Chemical, rheological, microbiological as well as sensory tests were conducted on different storage times (fresh, 3, 7, 14 and 21 days).
The obtained results can be summarized as follow:-
• The rate of total solids decreased by adding yeast strains, especially in the treatment that contained strain (B) before incubation, and the rate of decrease with increasing the storage period up to 21 days.
• The acidity rate increased at the beginning of production and reached its highest rate on the 7 day of storage, then it began to decrease again for all treatments except for the control samples, which remained almost the same.
• The total nitrogen content increased in the treatments to which the yeast strains were added before incubation, then followed by the treatments to which the yeast strains were added after incubation compared to the control samples, but there was a decrease in the total nitrogen with an increase in the storage period in all treatments.
• There was a slight decrease in the percentage of fat with an increase in the storage period in all treatments.
• Increasing the rate of whey expulsion (syneresis) in general, with an increase in the storage period in all treatments, but the treatment samples containing strain (B) were more expelled from whey compared to the rest of the treatments.
• Increased ability of lactic acid bacteria (starter culture) to survive in treatments that contained yeast strains compared to control samples at the end of the storage period.
• All treatments obtained a high degree of acceptance until the end of the storage period compared to the control, but the treatment samples that contained strain (B) before incubation obtained the highest degrees of sensory properties at the beginning of production until the age of 7 days, then the evaluation decreased after that to obtain the lowest degrees of sensory properties in end of storage period.
• The percentage of ethanol alcohol was estimated, and its presence was not observed in most treatments except for the treatment that contained strain (B), where it was noticed that there was a small percentage of 0.298%, after 21 days of storage.
Probiotic ice milk industry
The composition of the milk ice cream mixtures was adjusted to be 5% fat, 18% nonfat milk solids, 15% sugar, 0.3% stabilizer (Garjim), and 1% vanilla. The mixture was divided into four parts, two parts were added to them before The aging step and two parts to which the strains were added after the aging step, in addition to the comparison sample (without adding yeast strains). The final product was frozen at (-18°C) for a period of 28 days. The results of the analysis of the final product indicate the following:-
• The values of density and weight in gallons per pound in ice milk for the treatments were close to those of the comparison with control samples.
• The overrun values increased slightly in the samples of the treatments to which the yeast strains were added before the aging step compared to the rest of the treatments.
• The values of the melting down rate were higher in the treatments to which the yeast strains were added before the aging step, followed by the treatments to which the yeast strains were added after the aging step, compared to the control samples.
• The rate of total solids decreased in the treatments compared to the control samples.
• The values of total nitrogen increased in all treatments compared to the comparison samples, and the treatments that contained yeast strains before the aging step got the highest values, then followed by the treatments that contained yeast strains after the aging step.
• The values of acidity and fat in the ice milk for all treatments were close to the control samples.
• The percentage of the ability of yeast strains to survive was not less than 81.77%, It should be also noted that following the 28 days of storage time, viable counts of yeast strains ranged between 6.35 and 6.79 log CFU/gm for all treatments, which is the appropriate percentage for during their shelf life for better functionality.
• The treatment containing yeast strain (B) and added before the aging step obtained the highest levels of sensory evaluation at the beginning of production and the first periods of storage, while all treatments obtained the highest degrees of sensory evaluation at the end of the storage period compared to the control samples.
• The percentage of ethanol alcohol was estimated, and its presence was not observed in all treatments except for the treatment that contained strain (B), where a very small percentage of 0.062% in T1 and 0.001 in T3 was observed.
In general from the results obtained, dairy foods serve as the ideal system for delivery of probiotics to the human gastrointestinal tract, due to provision of a favorable environment that promotes the growth and enhances the viability of these microorganisms. The probiotic yeast strains can be well incorporated into dairy based foods to develop functional and