Abstract
Walnuts are characterized by the presence of a cuticle (episperm) that covers the kernels: although rich in fibers and antioxidants, it can negatively affect the derived products’ taste, color, and rheology. The aim of this study was to investigate its removal through a simple and efficient technique: by applying a process that does not damage the product and simultaneously ensuring good chemical profile and rheological stability of its derivatives, in particular of the paste. Different treatments were applied on the walnuts such as: roasting, blast chilling, soaking with different solutions (HCl, NaOH, NaHCO3, citric and ascorbic acid, at different concentrations), blanching (with pure water and NaHCO3 solutions), and steam blanching. The walnuts were then blown with compressed air to be peeled. Once individuated two optimal techniques, the treated walnuts were processed to obtain a paste that was chemically characterized and analyzed from a rheological point of view through both rotational and oscillatory modes (flow curves, amplitude and frequency sweep tests). The roasting and chilling treatments exhibited a positive impact on thermal expansion coefficients of seed and seed coat. The pre-treatments consisting only in the roasting and the soaking technique with 1% of baking soda (followed by a final roasting) gave the best peeling rates on kernels: both methods yield pastes that were chemically and rheologically stable. Further studies are needed to investigate how the peeling treatment may affect product taste and shelf-life.
