*2.2. Extraction of HPI*

HPI was extracted according to Tang's method, with some changes [16]. Degreased powder was dissolved in 20 volumes of distilled water, and the extraction solution was adjusted to a pH of 9.0 generally with 2 mol/L of NaOH, stirred at 50 ◦C for 1 h, and the centrifugal force was set to 10,000× *g* at 4 ◦C. The centrifugal time was 20 min to yield a clear liquid. The pH of the clear liquid was adjusted to 4.8 with 2 mol/L of HCl, the centrifugal force was set to 8000× *g* at 4 ◦C, the centrifugal time was 20 min, and the liquid was cooled overnight. The supernatant was discarded and the remaining precipitate was washed with distilled water three times to remove the salt. The resulting precipitate was then dissolved in distilled water, adjusted to a pH of 7.0, placed in a low-temperature refrigerator for pre-freezing for 24 h, and freeze-dried. The protein content of the freeze-dried powder, as determined by the Kjeldahl method, was 93.7% (N × 6.25).
