Thermal processing modulates starch and non-starch macromolecules digestibility in chestnut: Structural reconfiguration and molecular interactions

The thermal process is the predominant method in chestnuts industry. However, it presents nutritional paradox due to high glycemic index (GI) conflicting with low-GI dietary guidelines. This study evaluated four conventional cooking methods systematically, revealing their striking differences in expected glycemic index (eGI). eGI values of moist-heat processed chestnuts (boiled: 81.40; steamed: 86.67) were higher than dry-heat chestnuts (baked: 69.47; fried: 69.85). Through structure analysis, texture profile analysis and ¹H low-field nuclear magnetic resonance (LF-NMR), the mechanisms of eGI differences were elucidated. Structural and texture in baked/fried samples demonstrated superior textural integrity, correlating with restricted starch accessibility. Furthermore, LF-NMR results shown that immobilized water (A21) in fresh chestnut (4.23) decreased after cooked, and A21 in moist-heat processed chestnuts (boiled: 2.25; steamed: 1.11) were higher than dry-heat counterparts (fried: 1.07; baked: 0.89), while free water (A22) increased inversely (boiled: 32.36; steamed: 27.34; fried: 22.02; baked: 20.18) when compared with fresh chestnut (14.15), indicating that the presence of immobilized water could block the binding of enzymes and starch, thereby slowing down the digestion. These results indicated that water phase transitions would be the predominant determination of chestnut GI. These findings provide valuable insights for designing low-GI chestnut products through targeted processing optimization.