UC Davis

AbstractCalifornia walnut growers and processors sell product all across the globe, and like with any raw food, safety assurance is critically important. The walnut industry of California utilizes either a chemical method of pasteurization or heat pasteurization to provide an effective kill step on any surface bacteria that might be present on the walnuts. In this study I only focus on two heat pasteurization methods: continuous and batch. The impact of the heat pasteurization methods on walnut storage quality is unknown. My thesis research analyzed the impact each pasteurization method has on walnut quality during storage, and the effect low oxygen storage may have in mitigating potential negative impacts caused by each heat pasteurization method. Quality was analyzed in the lab by measuring lipid degradation compound accumulation (peroxide value, free-fatty acid content), kernel color change, volatile compound accumulation (hexanal), and vitamin E degradation (tocopherol). Additionally, a trained sensory panel created a lexicon of descriptors in order to analyze the walnuts for changes in aroma, taste, and texture. Results showed that continuous walnut pasteurization accelerated lipid degradation, degraded vitamin E content, and increased negative sensory characteristics such as rancidity. Low oxygen storage of continuous pasteurized walnuts significantly reduced the rate of lipid and tocopherol degradation, and resulted in a lower sensory scores for rancidity. However, low oxygen storage of continuous pasteurized walnuts had significantly lower nut quality in comparison to either unpasteurized or batch pasteurized walnuts. Batch pasteurized walnuts had reduced lipid degradation and an improved sensory profile compared to unpasteurized walnuts, while vitamin E content was unaffected. Low oxygen storage of batch pasteurized walnuts further reduced lipid degradation, and caused an even greater improvement on sensory characteristics especially compared to continuous pasteurized walnuts. The best method of pasteurization and storage was found to be batch pasteurization stored in low oxygen conditions.