CAPILLARY GAS CHROMATOGRAPHY ANALYSIS OF LIPID COMPOSITION AND EVALUATION OF PHENOLIC COMPOUNDS BY MICELLAR ELECTROKINETIC CHROMATOGRAPHY IN ITALIAN WALNUT (JUGLANS REGIA L.): IRRIGATION AND FERTILIZATION INFLUENCE

ABSTRACT

Rapid analytical techniques were used to analyze the lipid fraction and to characterize phenolic compounds in Italian walnuts (Chandler cv.) grown in the same experimental orchard under different agronomical conditions. Lipid components such as fatty acids, sterols and tocopherols were determined by capillary gas chromatography using a flame ionization detector. Linoleic and linolenic acids were the most representative ω‐6 and ω‐3 essential dietary fatty acids present. A total lipid chromatogram was used to simultaneously determine free sterols and tocopherols. Walnuts contained substantial amounts of γ‐ and δ‐tocopherol, which explains their antioxidant properties. Sitosterol, Δ⁵‐avenasterol and campesterol were the major free sterols found. Micellar electrokinetic chromatography was used to analyze simple and complex phenols (tannin profile), while the total phenol amount and the antioxidant activity of phenolic extracts were assessed by spectrophotometric assays with Folin‐Ciocalteu and ABTS^•+ reagents, respectively. The total phenol content was higher than most foodstuffs. Moreover, more than 96% of the phenolic fraction belonged to the tannin class. Statistical analysis of the data showed that higher levels of nitrogen fertilization led to a significant increase in linoleic acid. This effect can induce a faster oxidation of lipid fraction of walnuts. Nevertheless, major nitrogen fertilization and irrigation were able to enrich walnuts in phenolic compounds.

PRACTICAL APPLICATIONS

Walnuts (Juglans regia L.) are a good source of essential fatty acids, tocopherols and phenolic compounds that contribute to reducing the risk of coronary heart disease, and also inhibit the oxidation of human plasma and low density lipoproteins. In this work, the applicability of fast analytical determinations was utilized to discriminate different walnut samples. Total lipid chromatogram was used for the simultaneous determination of the profile of sterols and tocopherols; micellar electrokinetic chromatography was used for the phenolic separation, which takes less than 10 min. The proposed methods are ideal for routine analysis of the lipidic and phenolic fractions in order to control the quality of walnuts.