Purpose: To evaluate the efficacy of a peroxyacetic acid (PAA)-based sanitizer against native microbiota in float tank water and on walnuts collected during commercial walnut hulling.
Methods: Walnuts were commercially hulled with either water (control) or PAA (25 ppm) in the float tank. Duplicate individual samples of water and walnuts from each of three trials (n = 6) were neutralized with Dey/Engley broth at 0, 5, 10, 15, 20, and 25 min of hulling, and plated onto tryptic soy agar (aerobic plate count [APC]) and onto CHROMagar ECC (coliforms and E. coli). Total dried solids, PAA and peroxide concentrations, pH, temperature, and oxidation/reduction potential of the float tank water were determined throughout hulling.
Results: Populations of APC, coliforms, and E. coli were 5.6, 5.0, and 2.8 log CFU/ml, respectively, in the unused float tank water and 6.7, 5.5, and 4.3 log CFU/g, respectively, on the pre-hulled inshell walnuts. Populations of coliforms (4.3 - 5.7 log CFU/ml) and E. coli (1.8 - 3.1 log CFU/ml) were significantly lower (P < 0.05) in the PAA-containing float tank water than in the control water (5.0 - 6.7 log CFU/ml and 3.4–5.0 log CFU/ml, respectively) at all sampling times but PAA did not have a significant impact on the microbiota of the inshell walnuts or the APC of the float tank water. Within minutes, significant increases in total solids and decreases in PAA and hydrogen peroxide concentrations were observed (15, 10, and 25 min after initiation of hulling, respectively).
Significance: Addition of a PAA-based sanitizer reduced some microbial counts in the float tank but not on the corresponding walnuts; addition of PAA to the float tank should be balanced with an assessment of cost and potential negative impact on walnut quality.