Abstract
Walnuts are appreciated all around the world by consumers and food industries because of their different and positive properties including flavouring, texturizing and nutritional qualities. Walnut paste production and use are commonly associated to confectionery or traditional products but may find space in different innovative foods, as fat replacer and functional component. The aim of this work was to increase the nutritional value of walnut paste and, at the same time, limit its oxidative degradation by developing a freeze-drying process with addition of polysaccharide matrices including dietary fiber. The effects of different formulations and technological treatments on product stability were evaluated. Shelled walnuts were roasted at 165 °C for 15 minutes, grinded, and refined; the obtained walnut paste was mixed with betaglucan, inulin and pectin, singly or combined with tragacanth gum and DE12 maltodextrin. The different formulations were added to water, emulsified and dehydrated by freeze-drying. Lyophilized samples were finally stored at 60 °C for 15 days. Walnut paste, samples just after lyophilization, and lyophilized samples after storage were evaluated in terms of moisture, peroxide number, acidity and pH, conjugated dienes and trienes, total phenols and tocopherols. Further analyses (color, 410 and 420 nm absorption, and total phenols content after ethanol precipitation) were carried out on freeze dried samples before and after storage. The results showed that the freeze-drying process affected the nutritional profile of the walnuts, limiting the onset of oxidative phenomena. After storage the content of total phenols and tocopherols was significantly higher already in the freshly lyophilized walnut paste, compared to the untreated one. The addition of polysaccharide compounds aided to preserve total phenols. However, when betaglucan, inulin, or pectin were individually included, a significant depletion of tocopherols was observed. Maltodextrin and tragacanth gum played a fundamental role in maintaining high levels of both total phenols and tocopherols. Nevertheless, formulations including also dietary fiber were slightly more oxidized. Among them, the inclusion of betaglucan provided the overall best results.
