2019 ASHS Annual Conference

California walnut production is increasing and consumers recognize the health benefits of including walnuts in their diets. With an increase in supply and demand, the walnut industry must enhance the marketability of their product and better understand factors contributing to oxidation, as prolonged storage of walnut kernels can lead to rancidity and off flavor development. To measure rancidity in their product, the walnut industry currently analyzes peroxide values and free fatty acids of oil pressed from the walnut kernels. These measurements are difficult to assess and don’t necessarily correspond to the amount of rancidity nor sensory perception of rancidity, thus a better indicator of walnut rancidity development is necessary for the success of the walnut industry. To understand the role of storage in lipid oxidation, we completed a shelf-life study of California walnuts. Walnuts of the Chandler variety were stored at 5°C and 15°C and at 40% and 60% relative humidity for 12 months. Every three months, walnuts were cracked and analyzed for color, water activity, and moisture content. Walnut oil was extracted and analyzed for peroxide values and free fatty acids. A consumer sensory analysis was completed to determine if off flavors were discernable to untrained consumers. All of these parameters were compared to hexanal, a volatile byproduct of lipid oxidation, that was quantified using gas chromatography and solid phase micro extraction. Storage at 5°C was able to retard hexanal formation for 12 months. Peroxide values were higher at 15°C at each time point and increased at a faster rate over time than walnuts stored at 5°C. Relative humidity of storage did not impact peroxide values, as it is a measure of oxidative rancidity. However, free fatty acid content, a measure of hydrolytic rancidity, was higher at 60% relative humidity compared to 40% relative humidity for both temperatures. Walnuts stored at 5°C had lower amounts of hexanal, free fatty acids, and peroxide values than walnuts stored at 15°C after just three months. Storage using oxygen absorbing sachets also retarded formation of these oxidation byproducts. Consumers disliked or were indifferent to walnuts stored at higher temperatures after 12 months of storage. These findings were communicated to walnut storage operators to stress the importance of temperature management in walnut storage for improved quality.