Evaluation of microbial loads and the effects of antimicrobial sprays in postharvest handling of California walnuts

Changes in aerobic plate count (APC) and Escherichia coli/coliform count (ECC) of inshell walnuts and walnut kernels were evaluated during commercial harvest and postharvest handling. APC and ECC for inshell walnuts collected from the tree were 6 and 4 log CFU/nut, respectively; counts increased by 1 log during harvest and hulling and decreased by 1 log during drying. Application of up to 200 ppm peracetic acid after hulling with or without a subsequent 2% lauric arginate spray reduced APC and ECC by less than 1 log CFU/nut; counts were not significantly different from the water control. A decrease in shell integrity was evident after drying: visible shell damage increased from 4 to 47% of walnuts after drying. Counts on kernels extracted from visibly intact walnuts from the tree were near the limit of detection (1.7 log CFU/nut). These counts increased by at least 1.4 log CFU/nut after hulling for both thin- and hard-shell cultivars. Microbial populations were 1.6–2.2 log CFU/nut higher for kernels from walnuts with broken shells than for kernels from walnuts with visibly intact shells before, but not after, drying. A better understanding of how microbial populations are affected by postharvest handling practices is important in the development of walnut-specific food safety programs. •Aerobic plate and Escherichia coli/coliform counts determined for inshell walnuts and kernels.•Microbial counts increased during harvest and hulling steps.•Microbial reductions during drying were small.•Application of a peracetic acid spray after hulling had minimal impacts on microbial loads on walnuts.•Kernels from walnuts with visibly damaged shells had significantly higher counts than kernels from intact shells.