Influence of postharvest drying process on walnut bioactive compounds

Walnuts (Juglans regia L.) are an important component of the Mediterranean diet and they have been long consumed as a highly nutritious food in many parts of the world. Walnut consumption was associated with lower BMI, waist circumference, systolic blood pressure and higher high density lipoprotein-cholesterol, compared with no consumption. These effects have been attributed to different bioactive compounds that have been contained in these nuts. However, walnuts are consumed after a drying process; because of that the aim of this work is to determine the influence of drying on some bioactive compounds. Three different walnut samples were obtained from a local company located in Forli (Italy). The fresh samples were drying as reported in the regional advisories for Integrated Crop Management. Fatty acids, sterols, tocopherols and phenolic compounds have been analyzed in fresh and dried walnut samples. The major fatty acids found in walnuts are oleic (18:1), linoleic (18:2), and linolenic (18:3) acids; no statistical differences were noticed among fresh and dried samples. Four tocols were also identified. Gamma-tocopherol was the principal tocol determined in walnut samples; drying process increased the tocols content. GC-MS analysis permitted the identification of six sterols. As expected, sitosterol represented the first sterol compound in all samples. Drying process negatively affected the sterols content; in fact total sterol losses ranged from 6 to 34 %. Finally, walnut phenolic compounds were determined by HPLC-MS. A total of 45 phenolic compounds were identified and quantified. As expected, gallic and ellagic derivatives represented 88.6-90.2 % of total phenolic compounds. As reported for sterols, phenolic compounds content decreased (19-44%) from fresh to dried walnut samples.