الفهرس 
Acknowledgement
YoghurtOnly 14 pages are availabe for public view
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Abstract
This work was carried out to evaluate the chemical composition and microbiological quality of fermented milks produced by big dairy factories and small dairies in addition to fermented milks manufactured in the laboratory.
This study was conducted to determine the following points:-
1- Chemical and microbiological quality of Plain yoghurt, Low-Fat yoghurt, Probiotic yoghurt and stirred yoghurt, yoghurt manufactured by flavoring ingredient such as fruits and miscellaneous fruit products and Laban Rayeb produced by four big dairy factories.
2- Chemical and microbiological quality of fermented milk (zabady) produced by small 6 dairies in Assiut town.
3- Chemical and microbiological quality of fermented products manufactured in the laboratory from the reconstituted milk (NIDO, 13%W/V) by using the products of big dairy factories and small dairies as a starter.
4- Chemical and microbiological changes occurred in all fermented products produced by big dairy factories, small dairies and in the laboratory occurred during storage at 5±2ºC.
Samples of fermented dairy products were obtained from 3 different sources i .e:
- The first group of samples was collected from the main store of four big dairy factories, i.e., Nestlé, Juhayna, Enjoy and Lactel in Assiut city, samples were collected 4 times from each factory. At each time of sampling 31 cups were taken (valid for two weeks at 5±2ºC).
The second group of samples was collected from the products of 6 small dairies, in Assiut city, namely Khair-baladna (A), Alban EL-Asiuty (B), EL-Sayda Zeinab (C), Kased Karim (D), Albain El-reif (E) and Wade Ibrahim (F), samples were collected four times from each factory at each time of sampling 15 cups were collected (valid for 4-5 days at 5± 2ºC). Regarding samples of this group the surface layer of one cup was aseptically removed, then the surface portion and the bottom portion were microbiologically analysed separately.
The third group of samples was manufactured in the laboratory as follows. As soon as the first and second group of samples were arrived to the laboratory, one cup from each was taken and used as a starter for manufacturing of the third group of samples from the reconstituted milk (NIDO 13% W/V).
The results obtained could be summarized as follows:
Plain Yoghurt:
1. Total Solids Content:
Results show that the total solids percentage of the examined plain yoghurt samples obtained from big dairy factories, i.e., Juhayna, Nestlé, Enjoy and Lactel were 12.29, 12.92, 13.30 and 13.25%, respectively in the first day of storage.
Results showed that the total solids content after 15 days of storage gradually increased reaching 12.85, 13.79, 13.82 and 13.71% in samples produced by Juhayna, Nestlé, Lactel and Enjoy, respectively.
2. Fat contents:
The data showed that the fat percentage in the examined plain yoghurt samples obtained from big dairy factories were 3.5, 3.5, 3.5 and 2.8% in the first day of storage in yoghurt produced by Juhayna, Nestlé, Enjoy and Lactel, respectively. It reached 3.7, 3.8, 3.9, and 3.3% respectively at the 15th day of storage.
3. Protein content:
The obtained results indicated that there was big difference in the percentage of total protein content in the product of each big dairy factory, where the percentage of T.P. in Juhayna, Nestlé, Enjoy and Lactel of samples in the first day of storage, was 3.841, 3.510, 3.390 and 2.320%, respectively.
4. Soluble Nitrogen Content:
The soluble nitrogen percentage of the examined plain yoghurt samples ranged from 0.080 to 1.090% in the first day of storage in the product of Enjoy and Nestlé, respectively. It reached 0.098 to 1.790% at the 15th day of storage.
5. Titratable Acidity:
The values of titratable acidity of the examined plain yoghurt samples ranged from 0.76% to 1.18% in the first day of storage in the product of Nestlé and Enjoy, respectively. It reached 0.90% to 1.57% at the 15th day of storage.
6. pH:
The pH values of the examined plain yoghurt samples ranged from 4.14 to 4.25 in the first day of storage in the product of Enjoy and Juhayna, respectively. It reached 3.86 to 3.93 at the 15th day of storage.
Fruit Yoghurt:
1. Total Solids Content:
The total solids percentage in the examined fruit yoghurt samples were 24.13, 25.63 and 26.02% in fruit yoghurt Juhayna strawberry, peach, and mango, and it was 26.15, 26.46 and 26.43% in fruit yoghurt Nestlé strawberry, peach, and mango, and it was 21.01% in fruit yoghurt Lactel strawberry in the first day of storage. It reached after 15 days of storage 27.66, 27.98 and 27.87%, and 30.16, 29.83 and 28.83%, and 21.57%, respectively.
2. Fat contents:
The fat percentage in the examined fruit yoghurt samples was 3.5, 3.5 and 3.6% in fruit yoghurt Juhayna strawberry, peach, and mango, and it was 3.7, 3.4 and 3.8% in fruit yoghurt Nestlé strawberry, peach and mango, and it was 3.0% in fruit yoghurt Lactel strawberry, respectively, in the first day of storage, and it reached 3.8, 4.0 and 4.3%, and 4.0, 3.9 and 4.6%, and 3.6% at the 15th day of storage.
3. Protein contents:
Results obtained for the total protein content in the examined samples of fruit yoghurt were 3.180, 3.466 and 3.573% in fruit yoghurt Juhayna strawberry, peach, and mango, respectively, and it were 3.394, 3.840 and 3.662% in fruit yoghurt Nestlé strawberry, peach, and mango; and it were 3.054% in fruit yoghurt Lactel strawberry, in the first day of storage and it was increased to 4.431, 4.555 and 4.359%, and 4.019, 4.109 and 4.030% , and 3.590% at the 15th day of storage.
4. Soluble Nitrogen Content:
The percentage of the soluble nitrogen content in the examined fruit yoghurt samples were 0.112, 0.084 and 0.084% in fruit yoghurt Juhayna strawberry, peach, and mango, and it was 0.084, 0.126 and 0.070% in fruit yoghurt Nestlé strawberry, peach, and mango, and it was 0.070% in fruit yoghurt Lactel strawberry, respectively, in the first day of storage, and it was increased to 0.126, 0.126 and 0.126%, and 0.098, 0.140 and 0.126 %, and 0.140% at the 15th day of storage.
5. Titratable Acidity:
The percentage of titratable acidity values were 1.24, 1.26 and 1.26% in fruit yoghurt Juhayna strawberry, peach, and mango, and it was 0.84, 0.84 and 1.00% in fruit yoghurt Nestlé strawberry, peach, and mango, and it was 0.81% in fruit yoghurt Lactel strawberry, respectively, in the first day of storage. It reached 1.50, 1.46 and 1.52%, and 1.18, 1.10 and 1.17%, and 0.96% at the 15th day of storage.
6. pH:
Values of pH in the examined fruit yoghurt samples were 4.10, 3.89 and 3.92 in fruit yoghurt Juhayna strawberry, peach, and mango, and it was 4.22, 4.33 and 4.22 in fruit yoghurt Nestlé strawberry, peach, and mango, and it was 3.92 in fruit yoghurt Lactel strawberry, in the first day of storage, respectively. It was decreased to 4.00, 3.77 and 3.77, and 3.96, 3.98 and 4.03, and 3.63 at the 15th day of storage.
Rayeb Milk:
1. Total Solids Content:
The percentage of total solids in the examined rayeb milk samples ranged from 09.07 to 14.02% in the first day of storage in the product of Juhayna and Nestlé, it reached 09.67 to 15.13% at the 15th day of storage, respectively.
2. Fat Contents:
The fat percentage in the examined rayeb milk samples were 1.5, 1.7 and 1.7% in the first day of storage in the product of Juhayna, Nestlé and Lactel, respectively. It reached 1.8, 2.0 and 2.0%, after 15 days of storage.
3. Protein Contents:
The percentage of T.P. in rayeb milk produced by Juhayna, Nestlé and Lactel of samples varied in the first day of storage and it was 3.037, 3.573 and 2.769%, respectively, and it reached 3.394, 4.734 and 3.394%, after 15 days of storage.
4. Soluble Nitrogen Content:
Contents of soluble nitrogen in examined rayeb milk samples were 0.084, 0.056 and 0.056% in the first day of storage in the products of Juhayna, Nestlé and Lactel, respectively, and it reached 0.126, 0.084 and 0.94% after 15 days of storage.
5. Titratable Acidity:
The values of titratable acidity in the examined rayeb milk samples ranged from 0.76 to 1.01% in the first day of storage in the product of Juhayna and Nestlé, respectively, and it was increased to 0.99 to 1.48% after 15 days of storage.
6. pH:
The values of pH in the examined rayeb milk samples were 4.16, 4.17 and 3.86 in the first day of storage in the product of Juhayna, Nestlé and Lactel, respectively, and it was decreased to 4.00, 3.98 and 3.60 after 15 days of storage.
Low-Fat yoghurt:
1. Total Solids Content:
The percentage of total solids content in the examined Low-Fat yoghurt samples ranged from 10.87 to 13.87% in the first day of storage in the products of Lactel and Juhayna, respectively, and it reached 12.42 to 13.27% after 15 days of storage.
2. Fat Contents:
The obtained results showed that the fat percentage in the examined Low-Fat yoghurt samples were 0.8, 0.0 and 0.0% in the first day of storage in the product of Juhayna, Nestlé and Lactel, respectively, and it was 1.2, 0.0 and 0.0 %, after 15 days of storage.
3. Protein Contents:
The percentage of T.P. in different of samples varied in the first day of storage, and it was 4.198, 4.913 and 4.466% in the product of Juhayna, Nestlé and Lactel, respectively, and it reached 4.823, 5.449 and 4.823%, after 15 days of storage.
4. Soluble Nitrogen Content:
The percentage of the soluble nitrogen content in the three examined Low-Fat yoghurt samples were 0.070, 0.098 and 0.084% in the first day of storage in the product of Juhayna, Nestlé and Lactel, respectively. It reached 0.126, 0.154 and 0.132% after 15 days of storage.
5. Titratable Acidity:
The values of titratable acidity of the examined Low-Fat yoghurt samples ranged from 0.89 to 1.10% in the first day of storage in the product of Lactel and Juhayna, respectively, and it reached from 1.07% to 1.25% after 15 days of storage.
6. pH:
The pH values in the examined Low-Fat yoghurt samples ranged from 4.03 to 4.56 in the first day of storage in the product of Lactel and Juhayna, respectively. It reached 3.80 to 4.39 after 15 days of storage.
Probiotic yoghurt:
1. Total Solids Content:
Total solids content in the examined probiotic yoghurt samples ranged from 11.91 to 12.63% in the first day of storage in the product of Lactel and Juhayna, respectively, and it reached 14.61 to 14.70% after 15 days of storage.
2. Fat Contents:
The fat percentage in the examined probiotic yoghurt samples ranged from 3.0 to 3.5% in the first day of storage in the product of Lactel and Juhayna, respectively. It reached 3.4 to 3.9% after 15 days of storage.
3. Protein Contents:
The percentage of T.P. in the first day of storage was 3.930 and 3.662% in the product of Lactel and Juhayna, respectively, and it was increased to 4.466 and 4.173% after 15 days of storage.
4. Soluble Nitrogen Content:
The percentage of soluble nitrogen in the examined probiotic yoghurt samples was 0.084 and 0.056% in the first day of storage in the product of Juhayna and Lactel, respectively. It reached 0.154 and 0.070% after 15 days of storage.
5. Titratable Acidity:
The values obtained for the percentage of titratable acidity in the examined probiotic yoghurt samples ranged from 1.00 to 0.80% in the first day of storage in the product of Juhayna and Lactel, respectively. It reached 1.30 to 1.30% after 15 days of storage.
6. pH:
The pH values of the examined probiotic yoghurt samples ranged from 3.66 to 4.73 in the first day of storage in the product of Lactel and Juhayna, respectively, and it reached 3.50 to 4.28 after 15 days of storage.
Stirred yoghurt:
1. Total Solids Content:
The percentage of total solids content in the examined stirred yoghurt samples were 10.20 in Zabado Juhayna strawberry, and it was 9.67, 9.21, and 9.41% in Zabado Nestlé strawberry, peach, and mango, and it was 9.90 and 9.01% in Zabado Lactel strawberry and peach, respectively in the first day of storage. It was increased to11.29% after 15 days of storage in Zabado Juhayna strawberry, and it reached 10.95, 10.71 and 10.27% in Zabado Nestlé strawberry, peach, and mango, respectively, and were 11.90 and 10.36 % in Zabado Lactel strawberry and peach.
2. Fat Contents:
The fat percentage in the examined stirred yoghurt samples were 1.7% in Zabado Juhayna strawberry, and it was 1.6, 1.5 and 1.7% in Zabado Nestlé strawberry, peach, and mango, and it was 1.6 and 1.5% in Zabado Lactel strawberry and peach, in the first day of storage, respectively. It was increased to 1.9%, and 1.7, 1.6 and 1.9%, and 1.8 and 1.7%, after 15 days of storage.
3. Protein Contents:
The percentage of T.P. in different samples of stirred yoghurt was different. The percentage of T.P. in the product of Juhayna was 2.841% in Zabado strawberry, however, it was 2.712, 2.465 and 2.573% in Zabado Nestlé strawberry, peach, and mango, and it was 2.812 and 2.301% in Zabado Lactel strawberry and peach, respectively in the first day of storage. It reached 2.966%, and 2.779, 2.501 and 2.662%, and 2.948 and 2.679% after 15 days of storage.
4. Soluble Nitrogen Content:
The soluble nitrogen content in the examined stirred yoghurt samples was 0.084% in Zabado Juhayna strawberry, and it was 0.028, 0.056 and 0.143% in Zabado Nestlé strawberry, peach, and mango, and it was 0.084 and 0.056% in Zabado Lactel strawberry and peach in the first day of storage. It reached to 0.140%, and 0.084, 0.098 and 0.280%, and 0.140 and 0.098% after 15 days of storage, respectively.
5. Titratable Acidity:
The values of titratable acidity of the examined stirred yoghurt samples were 0.88% in Zabado Juhayna strawberry, and it was 0.73, 0.60 and 0.93% in Zabado Nestlé strawberry, peach, and mango, and it was 0.73 and 0.76% in Zabado Lactel strawberry and peach, in the first day of storage, respectively. It reached 1.50%, and 1.10, 0.78 and 1.84%, and 0.86 and 0.87% after 15 days of storage.
6. pH:
The pH values of the examined stirred yoghurt samples were 4.04 in Zabado Juhayna strawberry, and it was 4.14, 4.32 and 3.99 in Zabado Nestlé strawberry, peach, and mango, and it was 3.98 and 3.94 in Zabado Lactel strawberry and peach, in the first day of storage, respectively. It was decreased to 3.74, and 3.96, 4.16 and 3.66, and 3.78 and 3.75 after 15 days of storage.
Plain Yoghurt:
1. Total Bacterial Counts (T.B.C.):
Total bacterial counts in plain yoghurt were 15.6x106, 31.0x105, 7.0x106, and 33.0x106 cfu/ml in the first day of storage in the products of Juhayna, Nestlé, Enjoy and Lactel, respectively. It reached 13.0×106, 2.5x105, 67.0x106 and 47.0x105 cfu/ml, after 15 days of storage.
2. Count of Lactic Acid Bacteria (L.A.B.):
Count of lactic acid bacteria in plain yoghurt were 17.6x107, 11.5x106, 18.0x107, and 21.0x106 cfu/ml in the first day of storage in the product of Juhayna, Nestlé, Enjoy and Lactel, respectively. It reached 15.25x106, 20.35x106, 22.5x107and 30.0x105 cfu/ml at the 15th day of storage.
Fruit Yoghurt:
1. Total Bacterial Counts (T.B.C.):
Total bacterial counts in fruit yoghurt were 45.5x106, 15.5x105 and 97.0x105cfu/ml in fruit yoghurt Juhayna strawberry, peach, and mango, and it was 45.0x105, 32.5x105 and 41.0x105 cfu/ml in fruit yoghurt Nestlé strawberry, peach, and mango, and it was 43.0x105 cfu/ml in fruit yoghurt Lactel strawberry, respectively in the first day of storage, after 15 days of storage it was 17.0x105, 20.0x105 and 27.0x105 cfu/ml in fruit yoghurt Juhayna strawberry, peach, and mango, and it reached 47.0x106, 29.0x105 and 18.0x105 cfu/ml in fruit yoghurt Nestlé strawberry, peach, and mango. It reached to 44.0x105 cfu/ml in fruit yoghurt Lactel strawberry.
2. Count of Lactic Acid Bacteria (L.A.B.):
Counts of lactic acid bacteria in fruit yoghurt were 42.5x106, 20.5x106 and 85.5x106 cfu/ml in fruit yoghurt Juhayna strawberry, peach, and mango, and it was 37.5x105, 64.0x105 and 54.5x106 cfu/ml in fruit yoghurt Nestlé strawberry, peach, and mango, and it was 34.5x106 cfu/ml in fruit yoghurt Lactel strawberry, respectively in the first day of storage. It reached after 15 days of storage 53.0x105, 11.5x105, and 81.0x105 cfu/ml, and 33.5x105, 95.5x105 and 53.5x105 cfu/ml, and it was 89.0x105 cfu/ml.
Rayeb Milk:
1. Total Bacterial Counts (T.B.C.):
Total bacterial counts in rayeb milk were 25.0x105, 23.0x105 and 17.0x106 cfu/ml in the first day of storage in the products of Juhayna, Nestlé and Lactel, and it reached 18.0x105, 26.5x105 and 12.3x105 cfu/ml respectively at the 15th day of storage.
2. Count of Lactic Acid Bacteria (L.A.B.):
The values of L.A.B were 33.0x105, 53.0x105 and 1.00x105 cfu/ml in the first day of storage in the products of Juhayna, Nestlé and Lactel, respectively, and it reached 26.0x107, 63.0x107and 48.0x105 cfu/ml after 5 days of storage.
Low-Fat yoghurt:
1. Total Bacterial Counts (T.B.C.):
The T.B.C. were 55.5x105, 43.5x105 and 40.0x105 cfu/ml in the first day of storage in the products of Juhayna, Nestlé and Lactel, respectively, and it was 16.5x106, 55.0x106 and 63.0x106 cfu/ml after 15 days of storage.
2. Count of Lactic Acid Bacteria (L.A.B.):
The values of L.A.B were 52.5x106, 65.0x106 and 12.15x106 cfu/ml in the first day of storage in the products of Juhayna, Nestlé and Lactel, respectively, and it reached 30.0x105, 64.0x105 and 76.0x105 cfu/ml after 15 days of storage.
Probiotic yoghurt:
1. Total Bacterial Counts (T.B.C.):
Total bacterial counts in probiotic yoghurt were 44.0 ×104 and 18.0×102 cfu/ml in the first day of storage in the products of Juhayna and Lactel, respectively. It reached 50.0x103 and 50.5x103 cfu/ml after 15 days of storage.
2. Count of Lactic Acid Bacteria (L.A.B.):
Counts of lactic acid bacteria in probiotic yoghurt were 43.0x104 and 46.5x104 cfu/ml in the first day of storage in the products of Juhayna and Lactel, respectively. It reached 20.0x105 and 32.0x105 cfu/ml after 15 days of storage.
Stirred yoghurt:
1. Total Bacterial Counts (T.B.C.):
The counts of T.B.C. were 16.0x105 cfu/ml in Zabado Juhayna strawberry, and it was 25.5x105, 42.0x105, and 45.0x105 cfu/ml in Zabado Nestlé strawberry, peach, and mango, and it was 24.5x106 and 38.5x105 cfu/ml in Zabado Lactel strawberry and peach in the first day of storage, respectively. It reached after 15 days of storage to 10.0x105 cfu/ml in Zabado Juhayna strawberry, and it reached 43.5x105, 34.5x105 and 23.0x105cfu/ml in Zabado Nestlé strawberry, peach, and mango, and 18.0x105 and 23.5x106 cfu/ml in Zabado Lactel strawberry and peach.
2. Count of Lactic Acid Bacteria (L.A.B.):
Counts of L.A.B were 35.0x105 cfu/ml in Zabado Juhayna strawberry, and it was 35.0x105, 32.5x105, and 17.5x104 cfu/ml in Zabado Nestlé strawberry, peach, and mango, and it was 41.5x105 and 51.5x105 cfu/ml in Zabado Lactel strawberry and peach, respectively in the first day of storage. It reached after 15 days of storage to 75.0x105 cfu/ml, and 45.5x105, 41.5x105 and 38.5x104 cfu/ml, and 56.5x104 and 37.5x105 cfu/ml.
Microbiological Analysis:-
Count of Yeast and Moulds (Y. &M.) and Coliform Bacteria:
Microbiological analysis of all commercially samples by big dairy factory and experimental yoghurt samples after 15 days of storage revealed the absence of coliforms, yeasts and moulds, which might be due to the addition of anti fungi chemical to milk prior to fermentation by the manufacturer.
Organoleptic Examination:-
1. Plain Yoghurt:-
The scores of all sensory attributes of yoghurt samples manufactured by Enjoy and Lactel were higher than those produced by Nestle and Juhayna. Yoghurt produced by lactel was evaluated as more acid followed by the product of Enjoy.
2. Fruit Yoghurt
The obtained results indicated that mango as fruit flavor yoghurt preferred over peach and strawberry. The highest scores for taste, flavor, body, texture and appearance and overall acceptability were given to fruit yoghurt mango.
3. Rayeb Milk:-
The scores of rayeb milk produced by Nestle were higher than those produced by Juhayna and Lactel.
4. Low-Fat Yoghurt:-
The average value of overall acceptability of Low-Fat yoghurt produced by Juhayna was higher than those produced by Nestle and Lactel.
5. Probiotic Yoghurt:-
Among probiotic and plain yoghurt samples, the average overall acceptability score was highest for the probiotic yoghurt as compared to plain yoghurt.
6. Stirred Yoghurt
The average values of overall acceptability of stirred yoghurt was higher than those of plain yoghurt.
Products of Small Dairies:
• The obtained results showed that at the beginning of incubation period their was no big difference of the 2 portions of samples produced by small dairies concerning the total bacterial count, however at the end of the incubation period some times the surface layer contained higher bacterial counts comparing with the bottom layer.
• On the other hand the total L.A.B counts showed that the differences among different samples were not great.
• Regarding yeast and mould, the minimum and maximum count reached 63x10² and 110.5x10² after 4 days of storage in samples produced by small dairies.
• Only 2 samples from the samples produced by small dairies were positive for (coliform bacteria) M.P.N overall the storage period.
• Concerning chemical determinations, the pH of all samples produced by small dairies was lower comparing to samples manufactured in the laboratory; on the other hand the acidity was higher.
• Only one sample from samples produced by small dairies contained highest T.S when compared to all other samples.
• Concerning fat content, there were great differences, between samples manufactured by small dairies with minimum value of 2.7% and a maximum value of 4.8%.
• Percentage of total protein showed the same trend with a minimum value of 2.676% and 3.662% as a maximum.
• The obtained results showed great differences concerning the S.N content among samples produced by small dairies either at the beginning or at the end of storage period.
• The organoleptic properties examination indicated that their was big difference between the different samples produced by small dairies, which may be due to the difference in the raw milk used by each dairies.
This study recommends that all fermented milk should be made from heated milk in big dairy factories to ensure the safety of these products and they must comply with the Egyptian Standards.