SPI employed in this study contents mainly acidic amino acids, glutamic acid (19.2% by weight) and aspartic acid
(11.5% by weight), and approximately 1% by weight of cystine, so thus isoelectric pH is acid, 4.6 for pure SPI. The
FTIR spectrum of pure SPI is shown in Fig. 1a. The main absorption peaks are related to C=O stretching at 1630 cm-
1
(amide I), N–H bending at 1530 cm-1 (amide II) and C–N stretching(amide III) at 1230 cm
-1[21,22]. The broad band
observed in the 3500–3000 cm-1 range is attributable to free and bound O–H and N–H groups, which are able to
form hydrogen bonding with the carbonyl group of the peptide linkage in the protein [23].