Soy protein isolate (SPI) was obtained from Yuwang Group Ltd.
(Shandong Province, China). The approximate composition of raw
material were as follows: water content 8.15%, fat content 0.06%
(dry base), ash content 4.81% (dry base), total protein content
(N 6.25) 92.68% (dry base), nitrogen solubility index 63.92%.