Influence of radio frequency treatment on in-shell walnut quality and Staphylococcus aureus ATCC 25923 survival

Abstract

Radio frequency (RF) treatment is considered as a potential method for eliminating food-borne pathogens from low moisture foods. In this study, a treatment protocol for pasteurizing in-shell walnuts (with 15.01% w.b. of moisture) was developed using a 6 kW, 27.12 MHz RF oven with the objective of maintaining product quality. The uniformity of RF heating was improved by combining with hot air and also product mixing and holding during treatment. The optimized RF treatment protocol for in-shell walnuts involved pre-heating between an electrode gap of 16.0 cm, followed by drying for 40 min between an electrode gap of 19.0 cm, the process finished with forced air cooling of the walnuts in a single layer. This RF treatment produced in excess of a 4-log reduction of Staphylococcus aureus ATCC while differences in kernel color were not significant between control and RF treated walnuts during accelerated storage. Peroxide values of RF treated samples increased by more than 1 meq/kg during storage but showed no significant differences to controls while fatty acid values of both the control and RF treated samples were below 0.6% during storage. The moisture content and water activities of walnut shells and kernels of control and RF treated samples initially decreased and then tended to stabilize during accelerated storage. During storage the population of S. aureus ATCC 25923 in RF treated samples was gradually reduced to below the detection limit. Therefore, RF treatments could be considered as an effective method to control pathogens on in-shell walnuts.