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Abstract
A systematic scientific investigation of starch digestibility and glucose response of the most commonly consumed Vietnamese rice varieties, rice and wheat products have not been studied yet. Starchy staples were investigated for their physicochemical properties, in vitro starch digestibility and ex-vivo glucose response using a Caco-2 cell model. The potential of using black rice as an alternative to white rice was also studied. Eleven rice starches were analysed for amylose content, swelling power, gelatinisation (DSC) and pasting (RVA) properties, degree of crystallisation (XRD), and short range order structure (FTIR). Raw and cooked rice (white and brown) were analysed for their in vitro starch digestibilities to explore the relationship with physicochemical properties. Cooked rice grains and twelve rice and wheat products were studied for their RDS, SDS and RS. Anthocyanins from black rice were identified, quantified and purified using HPLC-PDA/mass spectrometry before being subjected to measuring α-glucosidase, glucose uptake and transport. Digestibility of rice starch with amylose contents ranging from 0.2-28.4% was independent of physicochemical properties when cooked in limited water. Vietnamese cooked white rice was high in RDS (82.5%) but low in SDS (10.5) and RS (7%). Cooked brown rice was of slightly lower starch digestibility, though statistically significant (p < 0.05), compared to white rice. A typical rice-based meal with a high proportion of carbohydrate (67.5%) resulted in 20% higher starch digestibility compared to a 55% carbohydrate diet. Rice and wheat products had significantly lower starch digestibility (p < 0.001) compared to white rice. Rice and wheat product hydrolysates had significantly lower (p < 0.001) glucose response and GI (67.2 vs. 82.3) compared to white rice. Black rice contained anthocyanins (191 mg/100g). Cya-3-Glu and Pn-3-Glu were major anthocyanins which significantly inhibited α-glucosidase (32% maltase, 57% sucrase, at a concentration of 1000 µg/mL), glucose uptake (IC50 = 2300 and 2328 µM) and transport (IC50 = 2639 and 2413 µM). Anthocyanin mixture (93% Cya-3-Glu, 7% Pn-3-Glu), showed high sucrase inhibition (66% at a concentration of 1000 µg/mL), glucose uptake (IC50 = 1662 µM) and glucose transport (IC50= 1781 µM) measured by using Caco-2 cell models and ᴅ-[U-14C]glucose as a tracer.