Phytochemical constituents and antioxidant capacity of different pecan [Carya illinoinensis (Wangenh.) K. Koch] cultivars

Abstract

Six pecan cultivars were analyzed for their antioxidant capacity (AC), total phenolics (TP), condensed tannin (CT), HPLC phenolic profile, tocopherol and fatty acid composition. Kernels which included the outer brown testa or pellicle, and shells which is the hard cover that surrounds the kernel, were evaluated for each cultivar. Strong correlations were found in kernels between AC and TP for both DPPH (r² = 0.98) and AC_ORAC (r² = 0.75) antioxidant assays. AC_ORAC values ranged from 372 to 817 μmol trolox equivalents/g defatted kernel, corresponding to Desirable and Kanza cultivars, respectively. CT ranged from 23 to 47 mg catechin equivalents/g defatted kernel and TP from 62 to 106 mg of chlorogenic acid equivalents/g defatted kernel. After a consecutive basic-acid hydrolysis, gallic acid, ellagic acid, catechin and epicatechin were identified by HPLC. The TP, AC and CT were 6, 4.5 and 18 times higher, respectively, for shells compared to kernels. The presence of phenolic compounds with high antioxidant capacity in kernels and shells indicates pecans can be considered an important dietary source of antioxidants.

Introduction

Pecans [Carya illinoinensis (Wangenh.) K. Koch], native from North America, belong to the Juglandaceae family which also includes walnuts (Juglans sp.) (Hall, 2000). Pecan is distributed over an area of geographic and climatic variation extending from northern Illinois and southeastern Iowa to the gulf coast of United States (Grauke, 1991). Recent studies have shown that pecan kernels may improve human serum lipid profile and lower low density lipoprotein levels, due to their high monounsaturated fatty acid content (Rajaram, Burke, Connell, Myint, & Sabate, 2001). Wu et al. (2004) screened common foods and vegetables across the US and reported pecan kernels have the highest antioxidant capacity (AC) and total extractable phenolic content (TP) within the nut group and ranked pecans among the foods with highest phenolic content. According to several studies, phenolics have antiradical activity and are thought to reduce incidence of chronic diseases, including Alzheimer, Parkinson, some types of cancer, and other degenerative diseases (Awika and Rooney, 2004, Li et al., 2005, Mertens-Talcott and Percival, 2005, Zhao et al., 1999).

Senter, Horvat, and Forbus (1980) screened kernels from Stuart and Schley cultivars for phenolic acid content using a gas chromatographic method revealing the presence of hydroxybenzoic acid derivatives. Gallic acid constituted 138 μg/g of defatted kernel and accounted for 78% of the phenolic acid constituents.

Proanthocyanidins or condensed tannins (CT) have also been reported in pecan kernels (Polles, Hanny, & Harvey, 1981). These type of compounds have biological activities such as antioxidant and antimutagenic properties, which are affected by the degree of polymerization, the monomer structure, and the bond type between monomers (Grimmer, Parbhoo, & McGrath, 1992). Gu et al. (2002) reported the presence of B type proanthocyanidins (C4–C8 and C4–C6 bonds) in pecan kernels. Prodelphinidins (3-O-gallates) were also found including epigallocatechin, epicatechin-3-O-gallate, and the more common flavan-3-ols, catechin and epicatechin. However, no reference was made to cultivar type, growth location, storage or processing conditions. In another study Gu et al. (2004) characterized the degree of polymerization of CT and reported a total proanthocyanidin content of 494.1 ± 86.2 mg/100 g FW in pecan kernels (∼0.5% w/w). Monomers, dimers, trimers, tetramers through hexamers, heptamers through decamers, and polymers above 10 subunits were present in amounts of 17.2, 42.1, 26, 101, 84, and 223 mg/100 g, respectively.

Despite the studies mentioned above, the influence of pecan cultivars on phytochemical content and antioxidant capacity has not been characterized. This information is key for breeding programs as well as for the food industry in the selection of cultivars with enhanced nutraceutical properties. The objective of the present study was to characterize six different pecan cultivars for their nutraceutical constituents, including phenolic compounds, antioxidant capacity, vitamin E content and fatty acid profiles.