Mathematical modeling of produce washing in an industrial-scale flume washer

In America, fresh fruits and vegetables play an important role in enhancing people’s health and wellbeing (FDA, 1998). However, the cross-contamination in a sanitizing chlorine washing system is considered as a potential area of risk, where the contaminated and non-contaminated produce are being washed together in the washer (Munther et al., 2015). Therefore, cross contamination of fresh produce during washing should be regarded as a critical risk factor that can lead to foodborne disease outbreaks. In addition, fresh produce and fruits are heavily handled but undergo minimal processing before consumption. Therefore, it is necessary to develop an efficient and effective process on produce washing. In this study, two factors aimed to reduce the microbial load on the produce were considered, which were shear stress and free chlorine. Although many of chemical treatments have very high efficiency in reducing microbial population in wash water, most of the chemical forces have limitation when it comes to the removal of bacteria from produce surface (Gil et al., 2009). In the removal of pathogens from produce surface, shear stress serves as the mechanical force and chlorine serves as the chemical force. During the washing process, bacteria get detached from the produce surface and go into wash water. However, some of the bacteria in wash water might re-attach on to the produce surface. The aim of this study was to mathematically model the combined role of shear stress and free chlorine on microbial attachment and adhesion in an industrial scale flume washer. COMSOL® Multiphysics was used to simulate the washing of spherical produce in an industrial-scale flume washer. The first step was to examine how the relative horizontal positioning of two spherical produce impacted their exposure to shear stress caused by the flow in flume washer. Two spheres, 1.2 inches in diameter, were placed one behind another in a flume to represent spherical produce. The distance between the two spheres was varied. The correlation between the surface shear stress and the distance between two spheres was investigated. The simulated shear stress values were used in a mathematical model that simulated bacteria adhesion. A set of ordinary differential equations (ODEs) that described the ligand-receptor binding of bacteria and produce surface was used to quantify the number of bacteria detached and attached on the produce surface as a function of time. The shear stress values from prior simulation and chlorine sanitizer concentration were introduced to the ODEs to investigate their impact on the detachment and attachment of bacteria in this industrial-scale flume washer. This study also effectively simulated the transport of free chlorine in a flume washer when chlorine was injected at selected locations. A chlorine dynamics model was used to calculate the distribution of chlorine in a flume washer. The results showed that there was no significant change in the shear stress experienced by the sphere upstream when the distance between the two spheres was changed. The downstream sphere experienced variable shear stress, with shear stress reaching steady maximum value of 275 mPa when distance between the two spheres was no less than four times as their diameter. The ODEs approximated the number of bacteria on produce surface and in wash water under different shear stress values. Low main-flow velocity with higher injection velocity gave more uniform distribution of free chlorine. The results of this study will provide guidelines for designing produce washing equipment and the flow conditions used in produce washing.M.S.Includes bibliographical reference