Walnut (Juglans regia L.) characterization: Protein composition, lipid and tannin content, and sensory quality.

摘要

Walnuts contained about 16.66% protein and 66.90% lipids. Linoleic (61.21%) and linolenic (13.81%) acids accounted for up to 75% of the total fatty acids in walnut lipids. The majority of walnut proteins were soluble in 0.1 M NaOH with a minimum solubility at pH 4. Albumins, globulins, prolamins, and glutelins respectively accounted for 6.81, 17.57, 5.33, and 70.11% of the total protein. Both protein solubility and electrophoretic analyses revealed glutelins to be the major walnut proteins. The total protein contained at least seven major polypeptides with estimated molecular weight range 19,590-21,830 and 33,340-37,150. Lysine was the first limiting essential amino acid in total proteins, globulins, and glutelins. Leucine and methionine plus cysteine were the limiting amino acids, respectively, for prolamins and albumins. Hydrophobic and acidic amino acids dominated the amino acid composition in all protein fractions. Minimum extractability of nitrogen at 0.5-1 M TCA represented the nonprotein nitrogen compounds in defatted walnuts. The nonprotein nitrogen values in walnuts ranged from 6.24 to 6.50% of the total nitrogen. Native or heat denatured walnut glutelins were easily hydrolyzed in vitro by the common digestive proteases. The tannin content of inshell and shelled walnuts was in the range of 889-1095 and 574.4-667.2 mg catechin equiv./100 g walnut, respectively. After three weeks of storage, walnut assayable tannins were reduced by about 20%. Blanched walnuts (BW) retained about 2% of assayable tannins, had a buttery-sweet taste, and rated less astringent then unblanched walnuts (UW). The astringency scores and tannin content of BW were correlated (r = 0.9249).