الفهرس Only 14 pages are availabe for public view
 |  | from | 160 |  |  |
| | | from | 160 | | |
Abstract
Salted whey produced from the Egyptian Domiati cheese after supplementing of its carbohydrate and nitrogen sources was used for production of exopolysaccharides (EPSs) by Halomonas eurihalina F2-7 and Xanthomonas campestris pv. campestris. The effect of whey pH, salt percent, sugars and non-protein nitrogen contents and the microorganism inoculum size and fermentation temperature and period on microorganism growth and EPS yield and chemical composition were studied in part I of this study. In part II, low-fat yoghurt (LFY) made with the obtained Halomonas and Xanthomonas EPSs (part I) at different ratios (35, 75 & 100 mg/L) compared with those corresponding ratios of commercial xanthan and with control full fat yoghurt (FFY) and LFY were evaluated for chemical composition, rheological properties, viable microbial counts and sensory attributes when fresh and throughout 10 days cold (5°C) storage.
The obtained results can be summarized as follows:
Part I: Production of Exopolysaccharides (EPS) by Halomonas eurihalina F2-7 and Xanthomonas campestris pv. campestris from salted whey.
Fermentation medium components which produces maximum cells growth and EPS yield varied between both organisms. Halomonas eurihalina F2-7 produced maximum EPS yield (2.8 g/L) when whey contained 7.0% salt, supplemented with mannitol (10 g/L) and yeast extract (6 g/L) and the pH was 7.2 at 32°C for 10 days. While Xanthomonas campestris pv. campestris produced 13.6 g/l EPS (~ 5 times of Halomonas EPS) when grow in whey which its protein was hydrolyzed (? 67.15% hydrolysis ratio), salted with 3.0% NaCl, and supplemented with 3.0% sucrose at pH 7.5 for 5 days under shaking (200 rpm/min) at 25°C. Sugars content and pyruvic acid and sulphats percentage of the EPS was also affected by the growth medium and the produced organisms. Glucose and mannose appeared to be the principal sugars but with varying ratios.
The EPS yield followed the trend of its cells growth and the yield reached the maximum at the microorganism stationary phase. Both microorganisms differed in their EPS yield and their effect on fermentation pH. While Halomonas eurihalina F2-7 increased the pH from 7.2 to 7.95, Xanthomonas campestris pv. campestris reduced the pH from 7.5 to 6.65.
Part II: Utilization of EPS produced by Halomonas eurihalina F2-7 and Xanthomonas campestris pv. campestris from salted whey in low fat yoghurt making.
Laboratory EPS gave similar influence of commercial xanthan for shortening the LFY curdling period compared to that of control LFY (2.42 vs. 3.10 h) . the increase in TA values and the decrease in pH values throughout coagulum formation were significantly faster by fermentation progress and the presence of EPS but were insignificant with EPS type and concentration. During 10 days cold (5°C) storage, these trends were significantly continued with storage period and EPS concentration and insignificant with EPS type. Yoghurt strength was markedly influenced by reducing the fat content (17.0% increase), the presence of polymer (29.0- 48.0%) decrease0, the polymer concentration (4.0-23.0% increase) and the cold storage (2.8-12.6% decrease). Not only the presence of the polymer in LFY coagulum but also its type also significantly reduced the levels of syneresis up to 29.7% with maximum reduction for yoghurt containing 100 mg/L of H-EPS. That was confirmed by the significant increase in WHC of their coagulum particularly with higher EPS ratio. Coagula of higher fat percentage or EPS exhibited high significant increase in apparent viscosity values (29.0 and 32.0- 70.5%, in order). In spit of the close acetaldehyde values of all fresh yoghurts (118-123 ppm.) significant differences were obtained with EPS type and concentration and storage period. Viable lactic acid bacteria was higher in yoghurt contained polymers compared to those of control LFY with maximum for yoghurt contained 100 mg comm.-X. These counts were decreased faster by storage in the presence of polymers. Moulds, yeasts and coliform were not detected in all yoghurt samples when fresh and at the end of storage. The presence of EPS in LFY coagula improved thier individual sensory attributes and overall acceptability scores with maximum for LFY made with H-EPS.