Improving chemical aqueous based intervention methods for microorganism elimination from fresh produce surfaces

Abstract

Many intervention methods theoretically have the potential to eliminate microorganisms. However, they do not perform efficiently once applied to fruits and vegetables. In this study Salmonella Typhimurium LT2 and hydrogen peroxide (H2O2) were used as model systems on 6 different types of produce to determine the effect of produce surface characteristics on sanitizer effectiveness. Microbial attachment on produce surfaces was induced after 3 h of drying at 24°C and high relative humidity (RH). Afterwards, produce was stored for 3 subsequent days and washed with sterilized tap water for 5, 10 and 15 minutes to separate weakly from strongly attached microorganisms from the produce surface. The strongly attached microorganisms were then treated with 3% H2O2 for 1, 3, and 5 minutes. These results were compared to the log reduction curves obtained with a pure liquid culture and 3% H2O2. Additionally, contact angle of water and diiodomethane on each type of surface were measured and used as indicator of wettability, and for calculating surface tension characteristics of the produce surfaces. Then these surface characteristics were related to the bacterial attachment and population reduction values obtained after applying the treatments. In general, the geometric mean equation was the most useful in predicting the surface tension values of produce surface and the polar and non-polar components of produce surface tension. Our results suggest that surface properties, such as roughness and surface tension of fruits and vegetables are important factors limiting decontamination. These surface properties allow the formation of micro-air pockets within the rough surface, thus contributing to create a protective environment for microorganisms and reducing the effectiveness of the chemical aqueous based intervention methods applied. Wettable surfaces (water contact angle < 90°) allowed more bacteria to attach after the washing and H2O2 chemical treatments. Roughness and surface polarity are intrinsic characteristics of produce surfaces which affect wettability and the spreading and penetration of the sanitizer treatment on the produce surface. Rough surface and porosity, considered an extreme case of roughness, enhance a deeper cell internalization and a protective environment for bacteria.