Influence of heat-stress, apple variety and preservatives in cider on viability of Escherichia coli O157:H7

This research was composed of three investigations. The first investigation was undertaken to evaluate the performance of tryptone soya agar (TSA; control medium) and three selective media, BCM™ 0157:H7(+) agar (BCM), modified eosin methylene blue agar (MEMB), and sorbitol MacConkey agar (SMAC), for recovery of 2 strains off E. coli 0157:H7 heated at 56, 58, and 60 °C for 60 min from tryptone soya broth (TSB). TSA and MEMB were equally effective at recovery of heat stressed E. coli 0157:H7 and superior to SMAC and BCM at all temperatures (P>0.05). At 56 and 58 °C, BCM provided the poorest recovery of heat-stressed E. coli 0157:H7. However, there was no significant difference in recovery on BCM and SMAC at 60 °C (P>0.05). The salami strain was more sensitive to heat stress than the cider strain (P<0.05). MEMB was satisfactory as a selective medium used for the enumeration of heat-stressed E. coli 0157:H7.

The second investigation evaluated the survival of Escherichia coli 0157:H7 in ground Golden Delicious, Red Delicious, Rome, and Winesap apples stored at 4, 10, and 25 °C. E. coli 0157:H7 populations were monitored over 18 (4 °C), 12 (10 °C) and 5 days (25 °C), at which time, mold contamination was visibly apparent. At 25 °C, Red Delicious apples supported survival of E. coli 0157;H7 better (P<0.05) than the other varieties. followed by Golden Delicious and Rome apples, which were not statistically different (P> 0.05). Winesap apples were the least favorable (P<0.05) for survival of E. coli 0157:H7 at 25 °C. Although recovery was not significantly different (P>0.05), E. coli 0157:H7 populations increased during storage in ground Golden Delicious and Red Delicious apples. At 10 °C, survival of E. coli 0157:H7 was poorest (P<0.05) in ground Red Delicious apples, while there was no significant difference in survival of E. coli 0157:H7 in ground Golden Delicious, Rome, or Winesap varieties (P>0.05). When stored at 4 °C, Golden Delicious and Rome apples were not statistically different in supporting survival of the pathogen (P> 0.05), while there was no statistical difference in the recovery oiE. coli 0157:H7 from groimd Red Delicious, Rome, and Winesap apples (P> 0.05). In general. apple pH increased during storage and was associated with mold growth. Results of this investigation indicate that there is no trend toward a particular apple variety supporting survival of E. coli 0157:H7. However, variation in apple pH dming storage can negatively or positively influence E. coli 0157:H7 survival at 25 °C.

The final investigation determined the survival of Escherichia coli 0157:H7 in apple cider containing no preservatives, 0.025% dimethyl dicarbonate (DMDC), 0.045% sodium benzoate, 0.003% sulfur dioxide, and a combination of 0.045% sodium benzoate and 0.003% sulfur dioxide stored 4, 10, and 25 °C. Regardless of storage temperatme, E. coli 0157:H7 survived well in unpreserved apple cider, with viable cells detected by direct plating for up to 18 days. When stored at 4 °C, E. coli 0157:H7 was not detectable in cider containing 0.025% DMDC after 3 days of storage, and DMDC was more efficient (P < 0.05) at inactivating E. coli 0157:H7 than any of the other treatments. E. coli 0157:H7 was not detected by direct plating after 3, 15, 9, and 12 days in cider (4 °C) containing 0.025% DMDC, 0.003% sulfur dioxide, 0.045% sodium benzoate, and 0.003% sulfur dioxide + 0.045% sodium benzoate, respectively. At 10 °C, E. coli 0157:H7 was not detectable in cider containing 0.025% DMDC, 0.003% sulfur dioxide, 0.045% sodium benzoate, and 0.003%sulfiir dioxide and 0.045% sodium benzoate after 9,15,12, and 9 days, respectively. E. coli 0157:H7 populations were reduced to undetectable levels at 2, 2, 1, and 1 day at 25°C in ciders containing 0.025% DMDC, 0.003% sulfur dioxide, 0.045% sodium benzoate, and 0.003% sulfur dioxide + 0.045% sodium benzoate, respectively. E. coli 0157:H7 was more resistant to preservatives at 4°C than at 25°C. Overall, test populations were sublethally injured in cider containing preservatives, and to a lesser extent, in unpreserved cider stored for several days. Injury was evidenced by the inability of E. coli 0157:H7 to form colonies on MEMB while being capable of growth on TSA. Generally, injury was more pronounced in cider containing DMDC, followed by sulfur dioxide + sodium benzoate, sodium benzoate, and sulfur dioxide (P<0.05).