When sourcing Soy Protein Isolate, buyers must focus on the protein content (dry basis), which should be at least 90%. Evaluate the PDCAAS (Protein Digestibility Corrected Amino Acid Score), where a score of 1.0 indicates the highest quality. For functional applications, test the emulsification, gelation, and water-binding capacity, as these determine how the protein performs in meat alternatives or dairy substitutes. Additionally, ensure the particle size (mesh) aligns with your production needs—typically 100-200 mesh for smooth beverage integration.