A Protein's Solubility Is Not Its Digestibility · The 2026 UK Fermentation Guidance, EFSA's Validation Caveat and the Evidence Gap in Next-Generation Protein Hydrolysates

The expanding market for enzymatically modified, plant-derived and precision-fermented proteins has exposed a methodological weakness in ingredient qualification: dispersibility, peptide size, simulated gastrointestinal release and nutritional amino-acid availability are frequently discussed as though they measured the same property. They do not. The United Kingdom's March 2026 supplementary guidance for fermentation-derived novel foods recommends a tiered approach centred on digestible indispensable amino acid scoring and in-vitro digestion, whereas EFSA's applicable novel-food guidance explicitly cautions that in-vitro models have not yet achieved general validation for true ileal amino-acid digestibility. New 2026 plant-protein comparisons strengthen the case for calibrated laboratory methods without eliminating matrix-specific exceptions. Against the backdrop of the FDA's October 2026 final guidance on ingredient identity and safety evidence, this article develops a cross-jurisdictional, analytically explicit approach to distinguishing technological performance from nutritional quality, exposure and lawful claims. A proposed validation programme for hydrolysed proteins is presented as a research hypothesis, not an established advantage.