*4.3. Preparation of OPs*

OPs were prepared by enzymatic hydrolysis from the oyster (*Crassostrea hongkongensis*) meat as described previously [81,90]. Hong Kong oyster meat (~3 kg) was homogenized in distilled water (1:3 (*w*/*w*) at 8000 rpm for 5 min. Homogenized oysters were hydrolysed using neutral protease (2 × 105 U/g, Pangbo Biotech, Nanning, China) at a protease/substrate ratio of 2.0% (*w*/*v*) (pH 7.0). The neutral protease was incubated for 4 h at 50 ◦C and then inactivated at 100 ◦C for 15 min. The hydrolysate was centrifuged at 15,000 rpm for 20 min at 4 ◦C to obtain the supernatant. The supernatant was collected and ultrafiltered using a membrane bioreactor system with a molecular mass cut-off value of 3 kDa to collect the fractions (<3 kDa). The samples were collected and freeze-dried for further analysis.