Walnut, the fruit of the walnut tree (Juglans regia L.), has been used in human nutrition since ancient times. Walnut trees were cultivated in Europe as early as 1000 BC (Martinez et al., 2010). The walnut tree is a deciduous medium-sized to tall tree that can reach up to a height of 45 m (Ecocrop, 2012). The fruit is a spherical drupe with a green fleshy husk that opens at maturity to reveal a nut. The nut consists of a hard lignified shell containing a wrinkled kernel made up of two halves separated by a partition. Depending on the ecotype, shells can be large or small, light brown or dark brown in colour, smooth or knobby (Chukhina, 2012; Ecocrop, 2012; Molnar et al., 2011).
Walnut is a crop of economic importance to the food industry: the kernel is consumed, fresh or toasted, alone or in other edible products, such as confectioneries, pastries or sauces. It is globally popular and valued for its nutritional, health and sensory attributes. Kernels are a nutrient-dense food, largely because of their high fat content and protein, vitamin and mineral profiles. Walnut kernels are also a good source of a wide variety of flavonoids, phenolic acids and related polyphenols (Martinez et al., 2010; Ecocrop, 2012). Walnut kernels contain about 50-70% of an oil rich in unsaturated fatty acids, particularly linoleic acid (49-63%) (Shahidi et al., 2005; Ecocrop, 2012).
Walnut leaves contain large amounts of tannins and are a potential source of antioxidants for the food, cosmetics and pharmaceutical industries (Ghasemi et al., 2011). In Spain, they are used in traditional veterinary medicine for the cure of colic in horses (Akerreta et al., 2010), and, in Italy, as a vermifuge (Guarrera, 1999), but not as a feed. The green husk contains a lot of phenols and flavonoids and is a source of dye (Ghasemi et al., 2011; Cosmolescu et al., 2010; Ecocrop, 2012). In France, the vin de noix (walnut wine) is obtained by macerating whole immature fruits (including husks) in wine or alcohol. Walnut sap is sweet and the trees are tapped during spring to harvest it (Ecocrop, 2012).
Walnut processing yields several by-products. Walnut oil meal (or walnut cake) is the product of oil extraction and is often called walnut meal. In California, small pieces of kernel accumulated during the manufacturing process are also called walnut meal (Federal Register, 2010). A fibrous product called muller meal is marketed in the United States, possibly a mixture of kernel particles and ground shells (McGregor, 2000). All these products contain highly variable amounts of protein, fat and fibre depending on the technology and it is difficult to categorize them accurately. Most of the research and observations concerning those products is extremely old and often dates from the 19th century.