The effect of different processing parameters on the efficacy of commercial post-harvest washing of minimally processed spinach and shredded lettuce

The effect of different processing parameters on the efficacy of commercial post-harvest biocidal washes to decrease the bacterial loading on spinach and lettuce has been evaluated. Sampling was performed at two spinach processors (Facility A & B) and a shredded lettuce producer (Facility C). Aerobic colony counts (ACC) and coliform counts were determined on samples taken at pre- and post-wash. In parallel, the heterotrophic plate count (HPC) and coliform levels in wash water was also determined. Processing parameters measured were the temperature of leafy greens (pre- and post-washing) and wash water. The sanitizer levels (peroxyacetic acid, oxidation–reduction potential), pH, conductivity and turbidity were also measured. The wash process in Facility B had a residence time of 50s for the spinach, maintained a constant hypochlorite concentration and continuously re-charged the tanks with fresh water. In contrast, Facility A had a short residence time (15s) did not maintain a constant sanitizer (peroxyacetic acid) concentration or re-charge tanks with fresh water. Despite the differences in processing operations there was no statistical difference between the log count reductions (LCR) obtained in ACC and coliform counts although counts were only reduced by <0.6logcfu/g. The carriage of Escherichia coli on pre-wash spinach was 19% and 25% in Facility A and B respectively. There was a high prevalence (57% positive) of E. coli in the wash water of Facility A with none being recovered in water samples taken from Facility B. Yet, the carriage of E. coli on post-wash spinach was the same in the two facilities (7%). Lettuce harboured a lower level of both ACC and coliforms with LCR being significantly greater than spinach. In general, the LCR in ACC and coliforms could be positively correlated to bacterial counts of pre-washed leafy greens and conductivity (solids content) of the wash water. A negative correlation was found between LCR and water temperature. Interestingly, within the ranges measured the LCR was independent of the bacterial loading of the water. The results of the study confirmed the limited efficacy of biocidal washes to remove field acquired contamination. Although it is thought maintaining a low microbial loading in the wash water and maintaining sanitizer concentration is key the current study suggests high conductivity and low temperature of the wash water enhances the LCR achieved.