Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and western blot analysis

For SDS-PAGE, the milk protein samples were separated on 12% Tris-glycine polyacrylamide gels under denaturing and reducing conditions, and the protein content was quantified with bicinchoninic acid (BCA) assay. For the western blot analysis, the diluted milk samples were separated on 10% Tris-glycine polyacrylamide gels under denaturing and reducing conditions and then transferred to polyvinylidene fluoride (PVDF) membranes (Invitrogen Corporation, Carlsbad, CA, USA), which were incubated with a polyclonal anti-human hBSSL antibody (dilution, 1:1,000; Catalog Number sc-34878) and a horseradish peroxidase-conjugated secondary anti-goat immunoglobulin (Ig)G antibody (dilution, 1:20,000; Sino-American Co., Beijing, China).