Bioactive Phytochemicals from Walnut (Juglans spp.) Oil Processing By-products

Persian walnut (or English walnut) growing dates back to 7000 BC in Persia, a gene center of Juglans regia L. The top leading countries in walnut production are China, the USA, and Iran accounting for ~75% of world production. Nuts are an essential component in human nutrition because their consumption provides the required amount of energy (720 kcal per 100 g of fruits), unsaturated fatty acids, carbohydrates, proteins, fibers, sterols, tocopherols, minerals (K, P, Ca, Mg, and Na), volatiles, and other bioactive constituents. In addition, walnut kernels are rich in oil (50–70%) and protein, depending on the cultivar, location, and irrigation rate. Although mostly consumed raw, walnut kernels are increasingly processed by cold pressing into light yellow edible oil used in foods as flavoring, like salad dressings or cooking. Walnut oil is especially valued for its high content of essential fatty acids (linoleic and linolenic acids) and micronutrients such as phytosterols, squalene, and other tree nut oils polyphenols, and tocopherols. As by-products, both shell and cold-pressed cake from walnut that remains after the cold pressing process of oil can be used in various ways (food, cosmetics, pharmaceutical products, or textile industry). Especially residual walnut press cake is practical when used in food and in pharmacy, mostly integrated into other products. The reason for treating residual cake as a value-added product lies in the fact that defatted cake is generally rich in polar phenolic compounds and, as a source of natural antioxidants, is expected to show significant antioxidant activity. The most abundant polyphenols found in walnut oil cake are hydrolyzable tannins. In addition, press cake is rich in dietary fiber, protein, residual oil, polyunsaturated fatty acids, and tocopherol, all considered health-enhancing components. Therefore, by using walnut oil cake as a low-cost product, many aspects connected with the valorization of food wastes are covered, such as consumers' dietary habits, economy, and environmental protection.