We want a good, clean fight
Keeping post-harvest water clean is one of the quiet challenges of commercial apple packing. Flumes and dump tanks help move fruit quickly and efficiently but they can also become highways for harmful microbes if water quality slips.
That balancing act was the focus of a presentation from Laura K. Strawn of Virginia Tech, Phil Tocco and Teresa Bergholz of Michigan State University and Claire M. Murphy of Washington State University. Their in-field validation study examined how commercial water management practices performed in three Michigan apple packinghouses during normal operations.
The team focused on a distinction that is often overlooked but matters greatly in food safety: validation and verification. Validation determines whether a process is capable of maintaining product and water quality under real operating conditions. Verification confirms that the established process is being followed correctly. Validation asks whether the right process is in place while verification checks whether that process is being carried out as intended.
For this project, successful validation meant microbial populations needed to remain steady or decline over time. Researchers compared microbial levels at baseline and tracked changes between incoming and outgoing apples within each discrete water system. The goal was to determine whether the flume systems prevented microbes from spreading from one piece of fruit to another during routine operations.
They monitored both microbial and physicochemical measurements over water hold times lasting as long as 80 hours. They measured aerobic plate counts, Enterobacteriaceae, total coliforms and generic E. coli in both post-harvest water and on apples. They also tracked turbidity, pH, temperature and sanitizer concentration. Statistical analysis evaluated differences across locations and time as well as relationships between water quality and microbial levels.
One of the clearest findings involved sanitizer management. Peroxyacetic acid concentrations ranged from 15 to 180 ppm while free chlorine levels varied from 7 to 50 ppm. Those numbers fluctuated considerably throughout normal operations and highlighted the dynamic nature of commercial water systems.
Despite those swings, the overall results were encouraging. Researchers found no statistically significant differences in microbial indicator levels on apples before entering and after leaving the flume systems. That outcome suggests active sanitizer programs successfully prevented cross-contamination during post-harvest washing even as fruit moved continuously through shared water.
At the same time, the study reinforced an important limitation. While sanitizers stopped microbes from spreading, they did not significantly reduce microbial populations already present on apples. Washing protected against contamination but it did not function as a microbial kill or reduction step. That distinction carries practical importance for packinghouses because it reinforces that food safety depends on multiple preventive measures rather than relying on post-harvest washing alone.
Another notable finding involved generic E. coli. The organism appeared in fewer than 2% of all samples collected from water and apples. Because it was detected so infrequently, researchers concluded it offers limited value as an indicator organism for routine post-harvest water monitoring in commercial apple operations. Instead, aerobic plate counts, Enterobacteriaceae and total coliforms proved to be more practical and informative indicators of water quality. Monitoring these organisms alongside sanitizer levels provides packinghouses with better tools for evaluating system performance and identifying potential risks before problems develop.
This is a practical reminder that careful monitoring remains the cornerstone of post-harvest water management. Maintaining sanitizer concentrations at validated thresholds allows packinghouses to limit cross-contamination even as operating conditions shift throughout the workday.
Together, validation and verification ensure safety is consistent. They create a stronger system that protects fruit, supports consumer confidence and helps commercial apple packinghouses keep clean water working as one of their strongest food safety defenses.
by Enrico Villamaino
