Food & Nutrition

Arab Journal of Food & Nutrition
103. Survival of Escherichia coli 0157:117 during the Manufacture and Storage
of Fruit Yoghurt (2009)
Mahmoud Khalil A1-Ta’ani\ Jordan University of Science & Technology
Supervisor: Dr. Reyad Shaker
Escherichia coli 0157:H7 has emerged as a major food-borne pathogen of
significant public health concern; it has been implicated in various outbreaks involving
milk and different dairy products. The objectives of this study were to assess the
behavior of E. coli 0157 :H7 during the manufacture of fruit yoghurt at different
fermentation and storage temperatures and the use of different type of starter cultures.
Reconstituted milk was fermented at 37, 40 or 45°C, using thermophilic starter culture
and the resultant product was stored at 4, 10 or 15°C for seven days. Also fruit yoghurt
was made using mesophilic starter culture. Samples of milk and yoghurt were
analyzed by plating on MacConkey agar for E. coil 0157:H7 count and on MRS for
LAB count; pH of samples was recorded.
During fermentation period, the populations of E. coli 0157:H7 grow in presence
or absence of LAB regardless to fermentation temperature. E. coil 0157:H7 increased
in presence of thermophilic LAB more than 2.75 log, while the pathogen in presence
of the mesophilic LAB increased 1.67 log. The growth of LAB during fermentation for
all experiments showed the same trend. The pH values of milk containing E. coli
0157:H7 and LAB decreased gradually to reach 4.6 ± 0.1 at the end of fermentation
period, while the pH value of milk containing E. coli 0157:H7 was 6.4 ± 0.1.
During cooling the acidified milk, there was no growth but also no death of E.
coil 0157:H7 at 4 and 10°C with slight increase at 15°C. In the absence of LAB, E.
coil 0157:H7 survived with slight decrease at 4°C, while it grew well at 10 and 15°C
to reach more than 8.26 log. During cooling, it was found in all experiments that
significant differences were observed in the population of E. coli 0l57:H7 in the
presence or absence of LAB.
During storage of fruit yoghurt, the number of E. coli 0l57:H7 was declined for
all experiments and sometimes was approaching undetectable levels at the end of 7
days storage at different temperatures. The sensitivity of E. coil 0157:H7 to acidity and
lower temperature was noticed during storage.
The overall results indicate that E. coil 0157:H7 did not tolerate the acidity,
cooling and storage temperature in fruit yoghurt after seven days storage. The
pathogen clearly does not survive in yoghurt commercially prepared with good
manufacturing practices, and the rate of inactivation is most rapid at which
commercially processed fruit yoghurt is fermented, cooled and stored.
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