Abstract: In this study, lignin was extracted from apple juice ultrafiltration retentate using a deep eutectic solvent method and separated into fractions with different molecular masses. The structural and physicochemical properties of the lignin were systematically characterized by high performance gel permeation chromatography (HPGPC), two-dimensional nuclear magnetic resonance (2D NMR), infrared (IR) spectroscopy, and rheological analysis. The effects of the lignin on the gelatinization behavior of waxy rice starch were investigated using a rapid viscosity analyzer (RVA). A simulated digestion model was employed to evaluate the inhibitory effects of lignin fractions with different molecular masses on starch digestibility and their effects on digestive enzyme activity. The results indicated that apple lignin exhibited a narrower molecular mass distribution and superior structural homogeneity compared to commercial lignin. The high-molecular-mass fraction (LG-H) contained more β-O-4 linkages and exhibited a more intact structure. Apple lignin significantly increased the resistant starch content in a molecular mass-dependent manner. Addition of 20% LG-H increased the resistant starch content to 20.1%. Lignin regulated starch digestion through multiple pathways, including inhibition of α-glucosidase activity, static fluorescence quenching of the enzyme, and alteration of starch gelatinization properties. In summary, this study demonstrates that apple lignin effectively delays starch digestion through multiple mechanisms, providing theoretical evidence and data support for the application of apple processing byproducts in low-glycemic-index foods.

Key words: apple lignin; molecular mass; waxy rice starch; digestion resistance; enzyme inhibition