Nutritional potential and functional properties of sweet and bitter lupin seed protein isolates

Protein isolates were prepared from both sweet and bitter lupin seed flours by two different methods, i.e. by alkaline water extraction/isoclectric precipitation (P1) and by micellisation (MI), and studied with regard to nutritional quality and functional properties. Protein solubility of both lupin seed flours was increased as sodium chloride concentration increased up to 1.0 M, then decreased. The minimum protein solubility of bitter lupin seed flour was quite sharp at pH 4.5, while it exhibited a broad pH range of 4.3-4.9 for sweet lupin seed flour. No significant (P < 0.05) differences were found between any isolates in their dry matter, fiber, lipids and moisture contents. Isolates-PI from both sources had significantly (P < 0.05) higher crude protein and ash contents than their isolates-MI. Bitter and sweet isolates-PI had lower values of total essential amino acids and higher values of tyrosine, phenylalanine, threonine, tryptophan and valine than isolates-MI. There was no significant (P > 0.05) difference between bitter isolates in their alkaloid contents, while both sweet lupin isolates were free of total alkaloids. Bitter lupin isolate-PI had significantly (P < 0.05) lower tannins, but sweet lupin isolate-MI had a significantly (P < 0.05) higher phytic, acid content than other isolates. Isolates-MI from both sources had higher chemical scores than their isolates-PI. The first and second limiting amino acids were total sulfur amino acids and valine, respectively, for all types of isolates. Sweet lupin isolate-MI had a higher essential amino acid index and protein efficiency ratio than other isolates. The protein solubility index, fat absorption and emulsification capacities of both isolates-MI were significantly (P < 0.05) higher than their isolates-Pl. Both sweet lupin isolates had significantly (P < 0.05) higher water absorption capacity than bitter lupin isolates. Sweet lupin isolate-MI had significantly (P < 0.05) higher foam capacity and foam stability than other lupin isolates. (C) 2001 Elsevier Science Ltd. All rights reserved.