Flavor and Volatile Compounds in Tree Nuts
Walnut (Juglans sp.) cultivars of commercial importance are of the Persian walnut (Juglans regia L.), a species which is favored because of its large kernel and thin shell. Walnut is appreciated as a snack and is used extensively in baking and confectionary products. Walnut contains a high amount of lipid (∼62%–70%) [11-13] and not surprisingly, the majority of the volatile compounds identifi ed in walnut is derived via breakdown of unsaturated fatty acids (Table 7.3). The volatile composition of walnut was fi rst examined by Clark and Nursten  (Table 7.3). They identifi ed 29 compounds, including eight carbonyls, four alcohols, and two terpenes. Among 44 volatiles later identifi ed in walnut by the same authors , hexanal, pentanal, 2-methyl-2-pentenal, and 2,3-pentanedione were suggested as the most important aroma contributors (Table 7.3). These same four compounds were among 118 volatiles detected by dynamic headspace analysis (DHA)-GC-MS in walnuts from three geographical locations (Table 7.3) . Considerable differences in volatile profi les were observed among the walnut samples studied; however, all contained an abundance of volatiles originating via oxidative decomposition of linoleic acid . Likewise, the volatile components of walnut oil also is primarily products of lipid oxidation (Table 7.4) [7,17,18]. Moderate levels of lipidderived compounds maybe important for the generation of typical walnut aroma; however, excessive oxidation may have a negative impact on fl avor [13,17,19]. To date, there are no detailed reports on the characteristic aroma components of walnut.