Poor solubility of SPI has restricted its application. Hence, SPI
was often firstly dissolved in pH 8.0 to obtain the maximum solubility
(Mendanha et al., 2009; Molina Ortiz et al., 2009). To avoid
the possible effect of extra ions added for subsequent pH adjustment,
ultrasonic treatment was used to increase the solubility of
SPI in distilled water. It was found that ultrasonic treatment resulted
in a SPI concentration of up to 10% in water. Hence, SPI were
exposed to ultrasonic before use.