Abstract
Monitoring walnut oxidation is essential to control walnut quality during storage. An accelerated oxidation method for differentiating the oxidative stability index (OSI) of walnut kernels was examined and the effects of instrument operational parameters such as temperature and airflow were evaluated. Four cultivars, Chandler, Solano, Durham, and Howard were analyzed at 110, 120, and 130 °C with 15, 20, and 25 L h−1 airflow. Analysis using 110 °C with 25 L h−1 yielded the lowest coefficients of variance (4.4) than other operational parameters; analysis using the same temperature at lower airflow, 15 L h−1, yield the highest coefficient of variance (10.5). Kernel OSI values were independent of airflow, however, dependence of temperature coefficient and Q10 were demonstrated. The results from selected parameters were correlated with fat and moisture content, peroxide value, UV absorbances, oil oxidative stability, hexanal, and rancidity to establish the relationships between OSI values and quality changes during storage. Using 0.5 g of ground kernels, at 110 °C with 25 L h−1 airflow gave a lower coefficient of variance and higher correlation with kernel quality and oxidative markers comparing to other combinations of operating parameters.
Highlights
Walnuts are known to contain high levels of triglycerides, and high proportions of unsaturated fatty acids, which have long been considered as relevant health-promoting components
Since lipid oxidation occur slowly at room temperature, accelerated methods can be applied to estimate the oxidative stability of a product in a more rapid time frame, if the temperature is exponentially related to the rate of reaction [3]
The highest value was found for airflow at 15 L h−1, and the most conservative shelf-life prediction was given by airflow at 25 L h−1. These results suggest that the airflow influences the shelf-life prediction for walnut kernels using the Rancimat method
Summary
Walnuts are known to contain high levels of triglycerides, and high proportions of unsaturated fatty acids (oleic acid, linoleic, and linolenic acids), which have long been considered as relevant health-promoting components. Unsaturated fatty acids can be oxidized in the presence of oxygen, light, moisture, and heat [1]. Fatty acid oxidation is initiated by reactive oxygen species, which lead to the formation of various primary and secondary products [2]. These products result in the modification of major quality characteristics such as color, flavor, aroma, and nutritional value, affecting the suitability of a food product [1]. Common accelerated oxidation methods include the active oxygen method, Schaal Oven method, and Rancimat method [4]. Rancimat method observes the production of volatile organic compounds while the other two methods look at peroxide formation during oxidation
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