2017 ASHS Annual Conference

California grows more than 99% of the walnuts consumed in the United States and exports $1.4 billion worth of product globally. The monounsaturated fats and omega-3 fatty acids present in walnuts make them prone to quality loss during storage. Understanding oxidation and rancidity of walnuts, determining optimal storage conditions, and creating a predictive model for walnut quality degradation is crucial to improve the shelf life of walnuts to meet an increasing demand. Water activity, the ratio of the vapor pressure of water contained within a substance to the vapor pressure of pure water, has been shown to dictate important physiological processes pertinent to walnut quality deterioration including lipid oxidation, browning, enzymatic activity, and microbial growth, but information on water activity pertaining to walnut storage is lacking. Surveys of major walnut processors in California showed a limited understanding and utilization of water activity in their storage operations, but a desire to better understand rancidity in their products. To understand water activity in regards to optimizing storage conditions for improved quality, a two-year shelf life study of California walnuts was conducted. Four varieties of walnuts (Chandler, Howard, Tulare, and Vina) were stored at 5°C, 10°C, and 15°C as both cracked kernels and in-shell full walnuts. For each temperature, walnuts were stored in metal bins at three different humidities: 20%, 40%, and 60% for the first year of the experiment and 40%, 60%, and 80% for the second year of the experiment. Walnuts were analyzed every three months for color, water activity, moisture content, and mold development. Walnut oil was extracted and analyzed chemically for oxidation products; hexanal, peroxides, and free fatty acids. A trained sensory panel was used to determine the organoleptic effects of storage, especially textural changes and detectable rancidity. All of these parameters were evaluated in conjunction with water activity to evaluate how water activity relates to quality degradation in stored walnuts. Temperature was shown to have the greatest effect on quality degradation, but walnuts stored at relative humidities of 20% were found to have a higher amount of oxidation products. A range of water activities correlating to the greatest levels of quality degradation was discovered and optimal storage conditions for individual varieties were learned.