P2-51 The Impact of Shell Damage and Paracetic Acid on Microbial Loads of Harvested and Hulled Walnuts

Tuesday, July 24, 2012
Exhibit Hall (Rhode Island Convention Center)
John Frelka, University of California - Davis, Davis, CA
Tyann Blessington, University of California-Davis, Davis, CA
Linda Harris, University of California-Davis, Davis, CA
Introduction: At harvest, walnuts are mechanically shaken to the orchard floor, collected and transported to a hulling facility where the outer hull is removed by abrasion and water rinse. Shells can be damaged in this process potentially exposing the kernel to various degrees of microbial risk. 

Purpose: To determine the impact of shell damage and peracetic acid (PAA) treatment on the microbial loads of walnuts collected at a commercial hulling facility.

Methods: Inshell Chandler variety walnuts were collected from the tree canopy, before (at receipt), during (from the float tank), and after (from the sort line) hulling, after drying, and at monthly intervals during ambient storage. Immediately before the sort line, walnuts were rinsed with either water or 200 ppm PAA. Inshell walnuts were shaken for 2 min in buffer; kernels extracted from walnuts with visibly intact or damaged shells were stomached for 30 s in buffer. Samples were plated onto tryptic soy agar with cyclohexamide or Chromagar ECC to determine aerobic plate (APC) or coliform (CC) counts, respectively.

Results: Levels of APC and CC on inshell walnuts were 5 and 4 log CFU/nut, respectively, from the tree samples. These levels increased and then decreased approximately 1 log CFU/nut during hulling and drying, respectively; levels further declined during storage. Microbial levels on kernels extracted from preharvest walnuts were at the limit of detection (20 CFU/nut), increased by 1 to 2 log CFU/nut during hulling, and declined after drying and during storage. Consistently, but not always significantly, higher counts were observed on kernels extracted from walnuts with broken shells. Counts on PAA-treated walnuts (both inshell and kernels) were not significantly lower than those treated with water.

Significance: Microbial levels on walnut kernels increase during hulling regardless of shell damage; higher populations are associated with shell breakage.