Doctor of Philosophy (PhD)


Nutrition and Food Sciences

Document Type



A comprehensive understanding of the behavior of microbial contaminants on produce surfaces in agricultural farms and during post-harvest activities is essential for the development of produce safety risk management strategies. Our studies investigated the survival and the attachment of E. coli on watermelon surfaces in agricultural settings with different levels of vegetation, and the effect of residual sanitizers on the survival and the attachment of E. coli O157: H7 and L. monocytogenes on spinach leaves. The attachment strength (SR) of the E. coli cells on watermelon surfaces significantly increased (P<0.05) from 0.04 to 0.99 in the first 24 h, which was primarily due to the decrease in loosely attached population, given that the population of strongly attached cells was constant. The daily die-off rate of E. coli ranged from -0.12 to 1.3 log CFU/cm2. Chlorine treatment reduced the E. coli level by 4.2 log CFU/cm2 (initial level 5.6 log CFU/cm2) and 0.62 log CFU/cm2 (initial level 1.8 log CFU/cm2), on the watermelons that had an attachment time of 30 min and 120 h respectively. The survival rate of generic E. coli inoculated on watermelon rind discs was variable with the level of vegetation. The discs placed at low vegetation level had the highest die‐off (3 log CFU/cm2) compared to medium and high vegetation levels. On spinach leaves treated with chlorine, significant reductions (P < 0.05) in E. coli O157:H7 and L. monocytogenes populations were observed within 15 min, with total reductions of 2.64 and 3.15 log CFU/cm2 respectively after 48 h. On 0.5% lactic acid treated leaves, the reductions in E. coli O157:H7 and L. monocytogenes populations were 3.07 log CFU/cm2 (24 h) and 1.40 log CFU/cm2 (48 h), respectively. The effect of residual sanitizers was significantly greater on loosely attached populations compared to strongly attached populations. The results reported in these studies may be useful while developing pre-harvest and post-harvest risk management strategies.



Committee Chair

Adhikari, Achyut