Masters Theses

Date of Award

12-1997

Degree Type

Thesis

Degree Name

Master of Science

Major

Food Science and Technology

Major Professor

David Golden

Committee Members

F. Ann Draughon, Sam Beattie

Abstract

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).

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