Chemical Composition of Select Pecan [Carya Illinoinensis (Wangenh.) K. Koch] Varieties and Antigenic Stability of Pecan Proteins

Chemical composition of 25 pecan varieties revealed considerable differences in moisture (2.1-6.4%), protein (6.0-11.3%), lipid (65.9-78.0%), total soluble sugars (3.3-5.3%), ash (1.2-1.8%) and tannins (0.7-2.7%) when analyzed on an edible portion basis. Pecan varieties had similar protein polypeptide profiles as revealed by SDS-PAGE, IEF, and pecan pAb based Western blotting analysis. Both pH and ionic strength were important for pecan protein solubilization. Borate Saline Buffer (pH 8.45) was an optimum solvent for extraction of pecan proteins among the mild buffers tested. Protein solubility was minimal in pH 3-7 range and increased significantly on either side of this pH range. Increasing ionic strength from 0 to 4 M NaCl significantly improved (~8 fold) protein solubilization. Glutelin fraction (63.6%) accounted for the major portion of the total solubilized pecan proteins followed by globulin (31.5%), prolamin (3.4%) and albumin (1.5%) respectively. The majority of the pecan polypeptides were in the MW and pI range of 12,000-66,000 Da and pH 4.0-8.3 respectively. Pecan globulins contained the most glycoprotein polypeptides. Lysine was the first limiting essential amino acid in the defatted flour, globulin, prolamin and alkaline glutelin fractions. Leucine and tryptophan were the first limiting essential amino acid in albumin and acid glutelin fractions respectively. The minimum nitrogen solubility (5.9-7.5%) at 0.25-0.75 M TCA represented the non-protein nitrogen of pecan meal. Pecan pAb-based inhibition ELISA could detect pecan proteins as low as 32 ng/ml. The assay, however, was not suitable for specific detection of pecan in foods as it showed cross-reactivity to various tree nut and seed proteins. Pecan contained major allergenic polypeptides in the 50-66 kDa and 16-20 kDa range when tested with human sera IgE from 15 pecan allergic subjects. Pecan globulins contributed to the majority of 50-66 kDa allergens. ELISAs and Western blotting assays indicated that pecan proteins subjected to various thermal treatments remained antigenically stable. Complete proteolysis and loss of antigenicity was not observed in SGF and SIF in vitro digestion studies. Western blotting of SGF digested proteins displayed several low molecular weight antigenic peptides (16-20 kDa) that were either originally present in the pecan extract or were generated by pepsin under the digestion conditions.