Evaluation of Microbial Loads and The Effects of Antimicrobial Sprays in Postharvest Handling of California Walnuts

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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 hardshell cultivars. Microbial populations were 1.6e2.2 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.

John C. Frelka
Linda J. Harris
Food Microbiology