Inactivation of Microbes on Blueberries in Recycled Water Wash Systems

Introduction: Washing fruit in water during postharvest processing is a common practice to remove harmless extraneous material. Chlorine and other sanitizers are used at various concentrations in water to inactivate spoilage and other microbes on fruit and in water. Typical systems consist of a bath or flume containing 100 s to >1000 l of water, which is often recycled.

Purpose: The purpose of this study was to examine the microbial load of wash water and fruit relative to water quality and sanitizer concentration in actual processing operations.

Methods: Water and fruit samples from six blueberry (BB) processors were collected at timed intervals and analyzed for microbes and various water quality parameters. Escherichia coli and spoilage microbes in water and on blueberries (BB) exposed to chlorine, chlorine dioxide, or peroxyacetic acid were enumerated using standard methods.

Results: Sanitizer concentrations fluctuated over time and in some cases decreased to 0-50% of the target residual. Turbidity values ranged between 0 to >200 NTU at the beginning and end of washing, respectively, and 70% of wash water samples had higher microbial counts compared to raw water. Microbial inactivation on BBs ranged from 29% to >99.9%, and 59% of washed BBs had lower microbial counts compared to raw BBs. In some cases, the microbial load of washed BBs increased by 1-3 orders of magnitude, in which levels of E. coli increased by 288%.

Significance: Water quality parameters change appreciably when wash water is recycled. Data suggests that maintaining a stable, target sanitizer residual concentration in wash water is problematic, due to oxidant demand in recycled systems. Though the microbial load is typically reduced on the fruit, microbial counts may be exacerbated on washed BBs.