Abstract
Abstract The purpose of this study was to evaluate the behavior of E. coli O157:H7 during lactose hydrolysis and fermentation of traditional and low lactose yogurt. It also aimed to verify E. coli O157:H7 survival after 12 h of storage at 4 ºC ±1 ºC. Two different types of yogurts were prepared, two with whole milk and two with pre-hydrolyzed whole milk; in both groups one yogurt was inoculated with E. coli O157:H7 and the other one was not inoculated. The survival of E. coli and pH of yogurt were determined during fermentation and after 12-h refrigeration. The results showed that E. coli O157:H7 was able to grow during the fermentation period (from 4.34 log CFU.mL-1 to 6.13 log CFU.mL-1 in traditional yogurt and 4.34 log CFU.mL-1 to 6.16 log CFU.mL-1 in low lactose yogurt). The samples with E. coli O157:H7 showed gas formation and syneresis. Thus, E. coli O157:H7 was able to survive and grow during fermentation of traditional and low lactose yogurts affecting the manufacture technology. Moreover, milk contamination by E. coli before LAB addition reduces the growth of L. bulgaricus and S. thermophilus especially when associated with reduction of lactose content.
Highlights
Escherichia coli is one of the most prolific microorganisms in human intestinal tract and it is normally harmless; certain strains can be pathogenic, like enterohemorrhagic E. coli (EHEC)
Milk contamination by E. coli before LAB addition reduces the growth of L. bulgaricus and S. thermophilus especially when associated with reduction of lactose content Keywords: β-galactosidase; EHEC; fermented milk; lactic acid bacteria; lactose hydrolysis
At the third hour the yogurt inoculated with E. coli O157:H7 began to show a formation of small bubbles of gas due to E. coli metabolism, forming CO2 gas (Fig. 1A)
Summary
Escherichia coli is one of the most prolific microorganisms in human intestinal tract and it is normally harmless; certain strains can be pathogenic, like enterohemorrhagic E. coli (EHEC). These strains carry genetic determinants for attaching-effacing lesions that cause hemorrhagic colitis with severe abdominal pain and cramps followed by bloody diarrhea. Many other foodborne outbreaks have been reported involving different products such as ground beef[3] and yogurt[4]. Yogurt is considered safe due to low acidity, some authors have reported E. coli O157:H7 survival during the storage period[5,6]. It has been shown that E coli O157:H7 cells have an effective mechanism to resist to extreme acid stress situations, and its resistance depends on the interaction with environmental compounds[7]
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