Purpose: This study quantifies and models the effect of key weather attributes on the fate of E. coli introduced onto the surface of grove oranges through foliar spraying.
Methods: Orange trees were sprayed with low microbial quality water (~106 CFU E. coli /ml) in 19 separate monthly trials over 3 harvest seasons. Three replicates of 10 oranges each were harvested from each of three trees at appropriate intervals until E. coli could not be detected by enrichment. E. coli were enumerated by plate count and most probable number techniques. Solar radiation (SR), temperature, relative humidity (RH) and rainfall data were also obtained from the Florida automated weather network and plotted against log change in E. coli concentration between sampling points for each month.
Results: E. coli populations declined more rapidly during dry weather compared to humid weather. Change in RH, SR, rainfall, temperature and time interval between sampling points did not show strong linear correlations with log change in E. coli between sampling points (R2 < 0.4). However, SR and temperature were significantly (P < 0.001) negatively correlated with log change in E. coli/h and RH was significantly (P < 0.001) positively correlated with log change in E. coli/h. Logistic regression analysis showed that log reductions in E. coli concentration were higher and more likely to occur due to dry weather (high SR and low RH) compared to log increases due to wet weather (low SR and high RH).
Significance: The assumptions in the FDA produce rule regarding a 0.5 log CFU/day reduction of E. coli on the surface of fruit may be an oversimplification.