15D-20 |
Longevity of Escherichia coli on apple fruit in an orchard |
S. Y. Lee, P. M. Gray, R. H. Dougherty, and D. H. KANG. Department of Food Science and Human Nutrition, Washington State University, PO Box 646376, Pullman, WA 99164-6376 The consumption of fresh apple juice and apple cider has been linked to increasing numbers of outbreaks associated with Escherichia coli O157:H7. Apples can become contaminated with E. coli O157:H7 not only during processing but also while still on the tree. This study was undertaken to ascertain if E. coli can survive and grow on apples in an orchard environment during the course of a growing season using three different E. coli strains. Four apple cultivars, Red Delicious, Golden Delicious, Gala, and Fuji, were sprayed with aqueous bacterial suspensions (ca. 105 CFU/ml) to compare their ability to promote or inhibit the survival of E. coli due to differences or similarities in structure or genetics. Spraying was performed at different fruit developmental stages from July through October 2001. Three apples of each cultivar were collected at 0, 1, and 3 days after spraying, shaken and rubbed for 1 min in 50 ml 0.2% peptone, then enumerated using Petrifilm (3M Co.) and Purple Base Broth containing 0.5% bile salts and 4-methylumbelliferyl-beta-D-glucuronide (MUG). When E. coli was enumerated immediately after spraying, the populations on apple skin were around 104-5 CFU/apple. However, these populations were reduced to undetectable levels (0.3 log10 CFU/apple) in apples collected 1 day after spraying. The same results occurred in both immature and mature apples sprayed at different times throughout the growing season. There were no significant differences relative to cultivar. Also, when E. coli was stored for a week at 4°C to produce more environmentally-resistant cells, they followed the same pattern of actively growing cells. These results show it is difficult for E. coli to survive and grow on apple fruit surfaces in an orchard environment, regardless of the fruit developmental stage.
Session 15D, Food Microbiology: Fruits and vegetables
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