Comparative Analysis of Bioactive Compounds and Antioxidant Stability in Khao-Mao Rice Varieties from Ubon Ratchathani: Impact of In Vitro Gastrointestinal Digestion

Abstract

This study explores the nutritional potential and bioactive compounds of Khao-Mao, a celebrated local product (OTOP) from Ubon Ratchathani, Thailand, by analyzing 9 varieties of young rice sourced from Phibun Mangsahan and Trakanpuetpon. Interestingly, our results show that a rice variety’s “ranking” as a top antioxidant depends entirely on the type of test used. While the pigmented Kam variety was the clear winner in terms of total phenolics and standard radical scavenging (DPPH, ABTS, and FRAP), it was the Dor He and E-Tei varieties that performed best when specifically challenged with hydroxyl and superoxide ions. We also observed that the PM-sourced Kam was richer in anthocyanins than the TP version, while the non-pigmented types maintained steady chlorophyll levels. To evaluate the bio accessibility and stability of these compounds, a simulated in vitro gastrointestinal digestion model was applied. The results indicated that while phenolic recovery was relatively low, varieties such as RD6, E-Tei, and Kam maintained measurable concentrations ranging from 4.56% to 6.65%. Despite this reduction in total phenolics, these varieties retained substantial antioxidant potential, with reducing power percentages recorded at 78.78%, 92.43%, and 71.85%, respectively, following intestinal simulation. These results demonstrate that while gastrointestinal digestion significantly alters phenolic structures, Khao-Mao maintains high antioxidant efficacy through the resilience of its secondary metabolites and synergistic contributions from non-phenolic constituents. Consequently, this young rice serves as a robust source of bio accessible antioxidants, retaining functional potency despite physiological transformation. Furthermore, the IC₅₀ values for ABTS radical scavenging activity increased to 84.21% (RD6), 80.95% (E-Tei), and 82.97% (Kam), suggesting that while the antioxidant capacity remains significant, the concentration required for 50% inhibition rises following digestive transformation. In conclusion, this study identifies Kam (from Phibun Mangsahan) and E-Tei as the most promising candidates for high-value food development, even for different physiological targets. While the pigmented Kam variety is superior for general radical scavenging and anthocyanin delivery, E-Tei demonstrates exceptional resilience during digestion, maintaining the highest reducing power (92.43%) and the most favorable post-digestion IC50 value.

HIGHLIGHTS

• Superior Phytochemical Resilience: Kam (PM) and Kam (TP) cultivars exhibited exceptional gastrointestinal stability and antioxidant potency compared to other tested varieties.
• Bioactive Correlation: High concentrations of anthocyanins and phenolics in Kam varieties directly correlate with sustained antioxidant capacity throughout simulated digestion.
• Cultivar-Specific Profiles: Distinct pigment profiles were identified across varieties, with Lee Nok characterized by high chlorophyll content and E-Tei demonstrating significant initial reducing power.
• Comparative Efficacy: Post-gastric phase analysis revealed that anthocyanin-rich Kam rice maintains free-radical scavenging activity comparable to the synthetic antioxidant Butylated Hydroxytoluene (BHT).

GRAPHICAL ABSTRACT