Structural changes and components' interactions alter the digestion property of in-kernel starch from thermally processed Tibetan Qingke

Abstract

Thermal processing has been applied to the property modification of the pure Tibetan Qingke (TQ) starch, but few studies attempt to elucidate the changes in physicochemical properties of in-kernel starches under whole TQ kernels thermal processing. In this study, the molecular, structural, and digestive characteristics of SUTQ, SHTQ, SMTQ, and SBTQ (starches isolated from unprocessed, heat fluidized, microwave irradiated, and baked TQ, respectively) were measured to elucidate the effects of different thermal treatments on the in-kernel starch. After TQ thermal treatments, the amylose/amylopectin ratio increased to 0.71, 0.64, and 0.68 for SHTQ, SMTQ, and SBTQ, respectively, while the molecular weight of in-kernel starches decreased by 39.29 %-82.14 %. Furthermore, the damage degree of starch surface morphology ranked as: SHTQ > SBTQ > SMTQ. The entanglement action between protein network resulting from protein polymerization and gelatinized starch aggregations exhibited most obvious in SHTQ. Moreover, the increased fluorescence intensity, newly formed dark color rings, and disrupted Maltese cross were observed in SHTQ, SMTQ, and SBTQ, and their variation trends were consistent with the damage degree of starch surface morphology. SHTQ, SMTQ, and SBTQ showed the decreased relative crystallinity (1.79 %, 15.98 %, and 14.64 %), short-range order (0.64, 0.81, and 0.80), and double helix structures (0.48, 0.61, and 0.58) and increased degree of gelatinization (100 %, 38 %, and 45.51 %), which partially transformed slowly digested starch into rapidly digested starch. In particularly, the formation of more ordered structures (V-type crystalline and interaction between protein and starch) induced by heat fluidization treatment provided the greatest thermal stability and highest resistant starch content (42.1 %) for SHTQ. Heat fluidization was a promising thermal processing for the development of TQ or TQ starch-based functional foods.

Keywords: Crystalline structure; In vitro digestion; Microstructural analysis; Thermal treatment; Tibetan Qingke starch.