Germination Time and Temperature Modulate Bioactive Compound Accumulation in Black Rice (Oryza sativa L.) and Optimize Functional Water-Soluble Extract Production.

Seed germination induces profound physiological and metabolic changes that promote the synthesis and accumulation of bioactive metabolites in cereal grains. However, the combined effects of germination time and temperature on these responses in black rice (Oryza sativa L.) remain poorly understood. This study investigated how controlled germination modulates the accumulation of GABA (γ-aminobutyric acid) and phenolic metabolites and identified optimal conditions for obtaining bioactive-rich water-soluble extracts. A central composite design was applied by varying germination time (24-96 h) and temperature (14-32 °C). Germinated flours and their corresponding water-soluble extracts were analyzed for GABA, total soluble phenolics, flavonoids, anthocyanins, proanthocyanidins, and oxygen radical absorbance capacity (ORAC). Response surface methodology, desirability analysis, principal component analysis, heatmap, and partial least squares discriminant analysis were used to characterize metabolic responses and optimize germination conditions. Germination significantly altered metabolite accumulation, with higher GABA levels observed at lower temperatures, whereas phenolic metabolites showed distinct responses to the interaction between time and temperature. Multivariate analyses revealed clear metabolic differentiation among germination treatments. Multi-response optimization identified 27 h and 19.7 °C as the optimal germination conditions. These findings demonstrate that controlled germination modulates the bioactive profile of black rice, enhancing phytochemical accumulation and supporting the production of bioactive-rich water-soluble extracts.