Effect of Technological Treatments and Polysaccharide Ingredients on Oxidative Stability of Innovative Freeze-Dried Walnut Products

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Many confectionery companies employ semi-finished lipid products, such as nut pastes, as an ingredient for the preparation of several foods. Nevertheless, during the storage, nut pastes have some drawbacks due both to the onset of rancidity, both to the separation between oil and semi-solid phase. The development of stable semi-finished walnut products in form of powder would allow to extend the walnut use in various preparations by increasing their convenience. As is well known, walnuts are highly nutritious fruits with well evidenced health benefits. In spite of natural antioxidant content walnut kernels are rich in polyunsaturated fatty acids and vulnerable to oxidation. In order to preserve thermo sensitive substances, a freeze-drying (lyophilizing) technique was developed by addition of polysaccharide matrices (including dietary fiber) in a walnut paste obtained by roasted kernels. This study aims to provide information on the impact of technological treatments and ingredient combinations on oxidative stability of freeze-dried walnut products. In preliminary tests, kernels were roasted at 110 °C for 5 min, 10 min, and 15 min. Residual moisture, pH and total acidity, peroxide value, and polyphenol contents were measured, as quality evaluation, in order to select the optimum operating conditions for the paste preparation. Optimal paste was obtained by milling 10 min roasted kernels, as it ensured a low residual humidity without affecting peroxides and polyphenols. Water and different combinations of maltodextrins DE12, tragacanth gum, and barley betaglucan were added to optimal walnut paste by creating emulsions which were then lyophilized. Quality evaluation was carried out on samples before and after freeze-drying treatment, and on freeze-dried products after 7 day, 60 °C storage. The formulation, containing maltodextrin and tragacanth gum, supplemented with betaglucans (at a ratio of 1:100 with the optimal walnut paste), showed no differences in the values of pH, acidity and polyphenols measured throughout treatment. All of the samples kept peroxide values < 10 meqO2/Kg expressed on dry matter. In particular, the addition of betaglucans gave the lyophilized samples a firm texture, without altering the oxidative chemical-physical parameters.