Modeling bacteria-produce interaction during washing
Creators: name: Tian, Yuyang
Publisher: name: No Publisher Supplied
Year: 2018

description: Due to increase in health awareness, there is an increase in the demand for fresh produce. The consumption of fresh produce has increased over the last few years. Most of the fresh produce is consumed raw or without any major processing steps before consumption. According to CDC report (2015 CDC annual report), more than 26% of the reported illnesses were associated with fresh produce. Safety of fresh produce is recognized as one of the significant challenges by the US Food and Drug Administration (FDA, 2015). Maintaining freshness, safety, and high-quality of fresh produce is essential for the industry to meet consumer demand.      Physical and chemical methods are commonly used for bacterial removal and inactivation during produce washing. While chemical methods are widely studied, a few studies show that shear stress has a significant effect on bacterial removal during produce washing. In this study, a numerical approach was used to understand the effect of shear stress on the attachment of bacteria to produce surface and the detachment of bacteria from a fresh produce surface.  COMSOL® Multiphysics was used to simulate turbulent flow in an experimental benchtop washing device to understand the effect of shear stress on (a) the detachment of bacteria from produce surface to wash water and (b) the attachment of bacteria to produce surface from wash water containing contaminated organic load. A 2D axisymmetric k- turbulent flow model with swirl flow was used in the numerical simulation. The geometry consisted of a downward facing rotating disk with a produce leaf attached at the center of the disk, in a tank filled with water. Single phase flow and a two-phase flow were simulated for detachment and attachment studies, respectively. The flow profiles and velocity vectors of the turbulent flow were obtained using numerical simulation. Shear stress distribution along the radius of the leaf was calculated numerically.  The results showed that the average shear stress exerted on the leaf surface was 28.3 mPa and 109 mPa, at 100 rev/min and 200 rev/min, respectively. Experimental results (from UC Davis) showed that the corresponding detachment of bacteria was about 1 log CFU/cm2 and 1.5 log CFU/cm2, respectively. In the attachment studies involving a two-phase flow of wash water and simulated organic particles, numerically predicted average shear stress values were 137 mPa and 403 mPa, at 100 rev/min and 200 rev/min, respectively. Predicted shear stress values were higher due to the suspended particles (beads). Experimental data showed that some (up to 0.4%) bacteria from the beads got transferred to the leaf surface suggesting that attachment can take place in the presence of shear stress.      The point-attachment model using ordinary differential equations to describe the ligand-receptor binding of bacteria and produce surface was used to quantify the detachment and attachment of bacteria from produce surface as a function of time. In the mathematical model that we developed, shear and sanitizer concentration were included. MATLAB® was used to numerically solve the differential equations to study of the effect of detachment rate constant and the presence of sanitizers on the removal and transfer of bacteria. The detachment rate constant is effective in mediating the detachment and attachment of bacteria, in the absence of sanitizers in water. Nevertheless, the effect of detachment rate constant is shown not to be significant in the presence of a sanitizer during washing of produce.      Understanding the effect of shear stress on microbe - produce interaction will help improve the design of wash water systems and minimize the probability of cross contamination during washing process for fresh produce.