الفهرس 
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Abstract
Objectives:
1) examine the growth, and survival of S. flexneriunder acidic conditions relevant to those applied during milk processing, 2) study the survival of S. flexneriunder the acidic conditions of yoghurt, elucidating the effect of fat content on the pathogen’s survivability in this fermented dairy product, 3) address the ”acid adaptation” phenomenon in S. flexneri, and its potential ”cross protection” effects on the pathogen’s tolerance to heat and salt stress, 4) assess the viability of S. flexneri during exposure to various heat and salting treatments of milk and dairy products, 5) elucidate the effect of fat content on the tolerance of S. flexenerito heat and salt stress in milk, and 6) study the survival of S. flexneriduring the preparation and cold storage of Domiati cheese, in which salt is used as a main preservative agent.
Methods & Results :
1) S. flexnericould grow at mildly acidic pH values of 6.0 and 5.0 adjusted by HCl, lactic acid, or acetic acid, but at slower growth rates, compared to unacidified conditions of pH 7.3. Lower pH values ranging from 3.0 to 4.5 caused declines in the initial numbers of S. flexneri, but the pathogen could survive exposure to pH 4.0 and 3.5 for variable times depending on the acidifying agent.The order of sensitivity of S. flexnerito acids at those lower pH values were acetic acid > lactic acid >HCl.
2) The survival of S. flexneriunder the acidic conditions of yoghurt prepared from full fat (6%) and half fat (3%) buffalo’s milk was also studied. The viable numbers of S. flexneriwere generally higher in full fat yoghurt than half fat yoghurt on milk coagulation and during the cold storage of yoghurt. S. flexnericould survive for at least 14 days in full fat yoghurt, compared with 7 days in half fat yoghurt. This could be attributed to a potential protective effect of fat.
3) S. flexneriexhibited an ability to adapt to acidic conditions, since its previous exposure to pH 5.5 increased its tolerance to pH 3.5. The pattern of acid tolerance in pH 3.5 by acid-adapted cells of S. flexneridepended upon the adaptation time that ranged from 30 min to 120 min. Acid-adapted cells of S. flexneriproduced by exposure to pH 5.5 for 120 min showed more heat tolerance at 50°C than non-adapted cells of the same pathogen. However, acid adaptation decreased the salt tolerance of S. flexneriat salt levels of 6%, 8%, and 10%, and this effect was increased with increasing the salt concentration.
4) The viability of S. flexneriin TSB and reconstituted skim milk during exposure to different temperatures of 37ºC, 50ºC, 55ºC, and 60ºC was studied.In TSB, S. flexnerishowed limited growth over 1 h of incubation at 37ºC, as its viable numbers increased.On exposure to 50ºC, the viable numbers of the organism progressively declined .More rapid decrease in the viable numbers of S. flexneriwas noted at 55ºC and 60ºC, where the organism could not be detected after 50 min and 10 min of exposure to these temperatures, respectively. Similar patterns of viability changes were noted when S. flexneriwas subjected to the same temperatures in reconstituted skim milk . The effect of different fat contents of 3%, 5%, and 7% on the heat tolerance of S. flexneriin milk was studied. Varying fat content between those levels did not affect the heat tolerance of S. flexneriat 50°C. However, at 55°C, increasing fat content of milk from 3% to 5% and 7% was associated with higher viable numbers and survival times of S. flexneri. This protective effect of milk fat could be also noticed at 60°C.
5) The viability of S. flexneriin TSB and reconstituted skim milk in the presence of different salt concentrations of 6%, 8%, and 10% (w/w%) was studied. In TSB, S. flexnerigradually decreased in viable numbers over 48 h in the three salting treatments. Increasing salt concentration from 6% to 8%, and 10% was associated with decreasing the viable numbers of S. flexneriat corresponding observation times. The addition of 10% salt was most detrimental to the pathogen . Similar results were noticed with reconstituted skim milk salted with 6%, 8%, and 10% (w/w%). The effect of fat content of milk on the tolerance of S. flexnerito various salt concentrations was examined. Increasing fat content of milk from 3% to 5% or 7% , increased the tolerance of S. flexnerito 10% salt, where the pathogencould not be detected in 10% salted milk containing 3% fat, but was still viable in the same salted milk with 5% or 7% fat.
6) The survival of S. flexneriduring the preparation and cold storage of Domiati cheese prepared from milk salted with 6%, 8%, and 10% (w/w%) was studied. The viable numbers of S. flexneriwere found to decline at corresponding observation times. S. flexnericould not be detected in Domiati cheese prepared from 10% salted milk after 21 d, but was still viable for 30 d in Domiati cheese made from 6%, and 8% salted milk.
Conclusion :
1) S. flexneriis sensitive to organic acids more than minerals acids.
2) The protective of S. flexneri was associated with increasing fat content in yoghurt.
3) S. flexneriexhibited an ability to adapt to acidic conditions, showed more heat tolerance at 50°C than non-adapted cells.
4) Increasing fat content of milk could improve the heat tolerance of S. flexneri at 55°C and 60°C,and 10% salt concentration.