Atmospheric cold plasma alters surface morphology, hydration behavior, and phenolic release in Philippine pigmented rice

Atmospheric cold plasma (ACP) treatment was investigated as a non-thermal surface-engineering approach to enhance the hydration and bioactive profile of Philippine rice varieties—brown (NSIC RC 222), black (97-SDS07041), and red (97-CAT006111). Grains were exposed to ACP for 1 and 2 min and analyzed using SEM, ATR-FTIR, total phenolic content (TPC), water absorption kinetics, and contact angle measurements. SEM micrographs revealed surface etching, with 1–5 μm fissures and microcavities forming after 2-min exposure, increasing surface area and permeability. ACP-treated grains exhibited significantly greater hydration, with 60-min water uptake rising by 31.85% in black and 59.37% in red rice, and cooking water uptake rising by 45.16%, 25.00%, and 22.22% in brown, black, and red rice, respectively. The contact angle decreased from 101.9° to 93.9° in brown rice and from 100.0° to 91.2° in black rice, confirming enhanced surface wettability and improved capillary diffusion. FTIR spectra (4000–400 cm⁻¹) indicated the preservation of starch and protein backbones, although reduced –OH (3300 cm⁻¹) and C–O–C (~ 1000 cm⁻¹) intensities suggested oxidation of polysaccharides and phenolic compounds. TPC exhibited a two-stage mechanistic response, decreasing after 1 min and then increasing at 2 min, which was attributed to the plasma-induced diffusion of bound phenolics. These findings establish ACP as a tunable, eco-efficient postharvest technology for modulating rice grain surface chemistry to improve hydration dynamics and phenolic bioaccessibility without compromising structural integrity.