الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
The increasingly technical demand for improved flavour and br keeping quality of yoghurt gained through the addition of some natural br preservatives is inevitable for expanding their utilization scale in food br and dairy products. Therefore, the present study was carried out to find br the effect of some natural preservatives on some pathogenic and some br spoilage microorganisms, available in the field of food and to evaluate br the experimental application of their preferring on the chemical, br microbiological and organoleptical properties of one of most popular br dairy products namely flavoured yoghurt. These agents were applied br for the milk used for yoghurt making. Studying their effect on flavour br and keeping quality of the products was conducted. br The importance of this study was denoted to the short shelf life of br yoghurt and growth of mold and yeasts on the surface of yoghurt in br most time especially in absence of the optimal cooling for this product br in the sailing centers which may be followed by the secretion of br aflatoxin which causing harmful effect for the general health. br The study was divided into two parts br Part 1: br The antimicrobial activity of some natural preservatives against some br pathogenic and spoilage bacteria compared with the antimicrobial br activity of some chemical preservatives used in the field of dairy. br At first: the inhibitory effect measured by the inhibition zone and the br obtained results could be summarized as follows: br Most tested essential oils and natural agents (Kombucha - br propolis) have antibacterial activity against growth of B. subtilis, E. br coli, Str. pyogenes, and Staph. aureus, the inhibition zone diameter br produced by each preservative was as follows:- br Summary br 119 br · Rosemary inhibition zones diameter were 15, 21, 15 and 20 mm br for the tested microorganisms respectively. br · Garlic inhibition zones diameter were 14, NI, 22, 11 mm for the br tested microorganisms respectively. br · Onion inhibition zones diameter were 12, NI, NI, 11 mm for the br tested microorganisms in the same order. br · Black cumin inhibition zones diameter were NI, NI, 14 and 15 br mm for the tested microorganisms in sequence. br · Thyme inhibition zones diameter were 22, 22, 16 and 24 mm br for the tested microorganisms in the same order. br · Parsley inhibition zones diameter were 13, NI, NI and NI for the br tested microorganisms respectively. br · Celery inhibition zones diameter were 21, 16, 21 and 21 mm for br the tested microorganisms respectively. br · Kombucha inhibition zones diameter were 18, 21, NI and 16 br mm for the tested microorganisms respectively. br · Ethanolic extraction of propolis (20%) inhibition zones br diameter were 15, NI, 20 and 11 mm. br · Water extraction of propolis (20%) inhibition zones diameter br were 11, NI, NI and 8 mm br · Hydrogen peroxide (250 ppm) inhibition zones diameter were 8, br 10, NI and NI mm for the tested microorganisms respectively. br · Sodium nitrate, sodium nitrite, potassium nitrate and potassium br nitrite (200 ppm) had no inhibition effect on the tested strains. br At second: The minimum inhibitory concentration (MIC) for the br natural preservatives gained positive results for inhibition in the last br test with B. subtilius, E. coli, Str. pyogenes, and Staph. aureus. br It could be concluded that most natural preservatives used in the br present study had antimicrobial activity against the last mentioned br strains of bacteria under study with different rates. The highest value of br MIC was recorded for water extract of propolis 20% with Staph. br Summary br 120 br aureus and it was 78000 ppm, while the lowest value was recorded for br garlic with B. subtilis and it was 20 ppm. br Part: II br Producing yoghurt contains some natural preservatives and br flavouring agents and studying the chemical, microbial and sensory br qualities along the refrigeration storage period. br Preliminary experiment were applied to select the suitable br preservatives (desirable flavour) and the optimum concentrations to be br used. This part was devoted to the manufacture of yoghurt by using br mixed milk (cows´ and buffalos´ 1:1, 3.2% fat) with added the last br mentioned selected natural preservatives with desirable flavour as br following br - Cont.1 yoghurt without any additives br - Cont.2 yoghurt with adding alcohol at the same concentration br used in the essential oils. br - T1 yoghurt with 3% Kombucha br - T2 yoghurt with 4% Kombucha br - T3 yoghurt with 0.1% ethanolic extraction of propolis br - T4 yoghurt with 0.1% black cumin essential oil br - T5 yoghurt with 0.05% parsley essential oil br - T6 yoghurt with 0.1% parsley essential oil br - T7 yoghurt with 0.1% garlic essential oil br The control and treatments were stored at refrigerator (5°C) for br 28 days. The resultant yoghurt was chemically, microbiologically and br organoleptically analyzed when fresh and at 7, 14, 21 and 28 days. br The results could be summarized as follows: br 1- Addition of natural preservatives to the milk for yoghurt making br prolonged the coagulation time with different rates in comparison br to the control. br Summary br 121 br 2- Treated yoghurt recorded a slight decrease in acidity with a br different rate among the treatments in comparison to the control, br while pH took an opposite trends in all treatments. br 3- All the treatments used had no significant effect among the br treatments and among the control for TS, fat, protein and ash br content within the same stage of storage period. br 4- Addition of natural preservatives to the milk prepared for yoghurt br making, prolonged the shelf life of the product with 100% to be br 28 days, with the exception of garlic oil, which had the same br storage period of the control (14 days). br 5- Addition of natural preservatives to the yoghurt significantly br increased the TVFA comparing to the control when fresh and br along the storage period, with different rates among the br treatments due to the volatile oils content. br 6- All the treatments of yoghurt recorded a pronounced increase in br acetaldehyde content with different rates than the control when br fresh and during storage period. Acetaldehyde was increased at br the beginning of the storage then followed by a decrease up to the br end of storage. br 7- The panelist scoring revealed that the T1 and T2 treated yoghurt br with Kombucha resulted in better and highest sensory acceptance br than the control and other treatments when fresh and during the br storage period up to 28 days followed by T3 and T4. br 8- On the other hand, the garlic oil (T7) came in the last -#111;-#114;-#100;-#101;-#114; when br fresh and along the storage up to 14 days of storage. br 9- Among the different treatments, there was no pronounced br difference between the treatments of yoghurt firmness when br fresh. By advancing storage up to 14 days, there were an increase br for the curd firmness and it was more pronounced for T1 and T2, br followed by slight decrease after 21 days due to the casein br hydrolysis. br Summary br 122 br 10- The treated yoghurt recorded a little decrease in the total viable br bacterial counts (TVBC) than the control when fresh and at the br different stage of storage period. Generally, the TVBC were little br increased during the first stage of storage followed by a little br decrease in counts up to the end of storage period. br 11- The lactic acid bacterial counts followed the same trend of the br total bacterial count with a different rate in counts and among br treatments. br 12- Coliforms were not detected along the period. br 13- Mold and yeasts were not detected in all treatments along the br storage period with an exception in some samples at the end of br storage due to the sample contamination. br -#102;-#114;-#111;-#109; such study, it can be concluded that, the addition of some br natural preservatives (i.e. Kombucha 3 - 4%, ethanolic extraction of br propolis 0.1%, black cumin essential oil 0.1% and parsley essential oil br 0.05 - 0.1%) can produce yoghurt with good organoleptic, rheological br properties and with prolonged shelf life. br References br 123 br REFERENCES br Abd