Episperm Separation from Walnut Kernels: Optimization of the Techniques and Impact on the Products Stability

Walnuts are considered a super food for their nutritional value and for their given health benefits. This type of nut is characterized by the presence of a cuticle that covers the kernels: the episperm. This cuticle is rich in fibers and antioxidants both important compounds for the nutritional value of the dried fruit. However, walnut episperm can negatively affect the taste, the color, and the rheology of derivative products. The aim of this study was to investigate and prove the possibility to remove the episperm of the kernels through a simple and efficient technique. This kind of process should not damage the product, and it should also guarantee a good chemical profile and rheological stability of the products that can be obtained from walnuts, in particular the paste. Different kinds of treatments have been tested to identify which combinations of temperature, time, and/or blanching solutions better work on the walnuts. Once individuated the best separation technique, this was applied on walnuts that were later processed to obtain a walnut paste. The final product was finally analyzed from a chemical, physical, ad rheological point of view to understand the effect of the episperm separation on the paste characteristics. The nuts were characterized before each treatment evaluating moisture content, water activity, ashes, fats, proteins, pH and total acidity, color, antioxidant capacity, polyphenol content, and lipid oxidation (conjugated dienes and trienes, peroxide values). After characterization, different pre-treatments were applied on the walnuts such as: roasting, blast chiller, soaking, blanching with different solutions (HCl, NaOH, NaHCO3, citric and ascorbic acid, at different concentrations), 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 characterized through the evaluation of moisture content, water activity, ashes, fats, proteins, pH and total acidity, color, antioxidant capacity, polyphenol content, and lipid oxidation (conjugated dienes and trienes, peroxide values, and Oxitest) aiming to check the impact of the peeling. The samples were analyzed also from a rheological point of view through both rotational and oscillatory modes (flow curves, amplitude and frequency sweep tests) to define the stability of the obtained paste, and the effect given on the product by the applied peeling technique. According to the obtained results, it was possible to establish that roasting and chilling treatments were fundamental as they differently impact on thermal expansion coefficients of seed and seed coat. In particular, the pre-treatment consisting only in roasting the kernels and the soaking technique with 1% of baking soda (followed by a final roasting) were those that gave the best peeling rates on the walnut kernels: both methods yield pastes that were stable from a chemical and rheological point of view. Through this kind of study, it was possible to evidence how the peeling treatment applied on the walnut kernels can really affect the chemical and rheological stability of a walnut paste. For this reason, it is fundamental to find a feasible and effective separation technique that can be useful also in the food industry. Further studies will be necessary to better investigate the stability also through an accelerated shelf-life study and a sensory analysis to obtain important information related to the impact of the peeling treatment on the product taste and on its stability in the long term.