Relationship between in vitro digestibility and multi-structures of four unconventional starches from Chinese tropical fruits (sweetsop, avocado, chempedak, and Pouteria campechiana) extracted using an ultrasound method

Abstract

The development of unconventional starch resources has gained interest in recent years. Herein, we comprehensively characterize avocado seed (ASS), chempedak seed (CPSS), Pouteria campechiana seed (PCSS), and sweetsop pulp (STS) starches extracted using a high-intensity ultrasound method. High starch yields (≥ 23.14 %) and starch purity (≥ 99.17 %) were observed for all samples. ASS, CPSS, and PCSS had an A-type crystal structure, while STS showed a B-type crystal structure. ASS showed the highest weight-average molecular weight, long chain content, average polymerization degree, conformation index, molecular density index, α-1,4-glycosidic bond content, double helix content, amylose content, relative crystallinity, semi-crystalline order, and V-type polymorph content while also exhibiting the lowest radius of gyration and fractal dimension. Consequently, ASS featured a smaller blocklet size than that of CPSS and STS, implying a higher ability of re-association of ASS molecular chains during retrogradation. This resulted in that ASS exhibited a lower breakdown viscosity, higher pasting temperature, and a greater final viscosity than those of CPSS and STS, thereby featuring the lowest contents of rapidly digestible starch (RDS: 1.86%) and slowly digestible starch (SDS: 12.99 %), decreased equilibrium concentration (C∞: 28.48 %), reduced SDS enzymolysis rate (kSDS: 0.48 × 10−2 min−1), and highest resistant starch content (RS) than the others samples. Moreover, compared with STS (11.44 %, 24.63 %, 1.43 × 10−2 min−1, 48.34 %, and 63.93 %), CPSS and PCSS had lower RDS (6.18–7.39 %), SDS (19.68–22.77%), kSDS (0.73–1.16 × 10−2 min−1), and C∞ (35.61–44.22%), while showing higher RS (69.84–74.14 %). These results suggest that STS is a high-GI food, whereas ASS, PCSS, and CPSS are middle-GI foods. Based on statistical analysis, all these results indicated that the lamellar structure ordering, molecular network density, and crystallite size have a significantly negative effect on in vitro digestibility. Therefore, our study provides a theoretical basis for processing unconventional starch-based products.