Date of Award

Spring 1-1-2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy Degree

Department

Chemical And Biomedical Engineering

First Advisor

Kothapalli, Chandrasekhar

Second Advisor

Jorge E. Gatica

Third Advisor

Joanne M. Belovich

Abstract

In this study, we developed a comprehensive mathematical model to predict the free chlorine (FC) concentration and bacterial cross-contamination during produce wash processes. A second-order chemical reaction model for FC decay which utilizes a proportion of chemical oxygen demand (COD) as an indicator of organic content in the wash water was employed, yielding an apparent reaction rate of 9.45 ± 0.22 x 10-4 ^M-1.min-1. Using a proportion of successive changes in COD in the wash water due to produce washing, typically ranging from 6 to 11% across produce types, the model was able to consistently predict experimental FC levels, however, we note that while the FC level drops, the COD level stays constant. Therefore, we established the total amino acids concentration as an alternative indicator of organic load, and modified our model based on modeling the reaction kinetics of chlorine and amino acids. Apparent reaction rate between FC and amino acids was in the range of 15.3 - 16.6 M-1.s-1 and an amplification factor in the range of 11.52 - 11.94. This study also presents a modified disinfection kinetics model to evaluate the potential effect of organic content on the chlorine inactivation coefficient of Escherichia coli O157:H7 in produce wash process. While the chlorine inactivation coefficient of E. coli was 70.39 ± 3.19 L.mg-1.min-1 in the absence of organic content, it dropped by 73% for a COD level of 600 - 800 mg.L-1. Finally, the mechanisms by which FC inactivates E. coli was studied. Results showed that at low levels of FC and shorter exposure times, cell surface became rough and plicate; however, holes and wrinkles formed on the cell surface at higher FC concentrations or at longer exposure times, causing significant damage to the cell membrane. The cellular permeability changed due to chlorination, resulting in a significant decrease in the number iii of viable cells. Besides, around 3.45% ± 0.62 of cells lost their culturability and transform to viable but not culturable (VBCN) state during the disinfection process in low CT values (> 0.2 mg.min.L-1). Our studies have implications in the design of disinfection protocols relevant for produce wash industry.

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