2nd Edition of Agriculture, Forestry, and Horticulture World Conference 2026

Abstract

Rice (Oryza sativa L.) remains a staple food in the Philippines; however, the increasing prevalence of health-related concerns has driven interest toward pigmented rice varieties such as black rice, known for their high nutritional value. Despite this, limited information exists on the postharvest behavior of black rice, particularly in terms of its moisture sorption characteristics. Understanding moisture sorption behavior is essential for modeling moisture dynamics and optimizing drying, storage, and processing systems, thereby improving grain quality and reducing postharvest losses. This study aimed to model the moisture sorption isotherms of black rice paddy by determining its equilibrium moisture content (EMC) at varying temperatures and evaluating the performance of selected isotherm models. A dynamic gravimetric approach was employed to determine EMC at controlled temperatures of 30°C, 45°C, and 60°C across a wide range of relative humidity conditions (water activity, aw = 0.03–0.98) established using saturated salt solutions. The resulting sorption data were fitted to commonly used three-parameter models, including Modified Henderson, Chung-Pfost, Halsey, and Oswin equations. Model performance was evaluated using statistical parameters such as coefficient of determination (R²), root mean square error (RMSE), and reduced chi-square (χ²). In addition, selected physical properties of black rice paddy were determined to support engineering applications in postharvest systems. Results showed that the equilibrium moisture content (EMC) of black rice paddy ranged within the experimental conditions and increased with increasing water activity while decreasing with increasing temperature, consistent with the hygroscopic behavior of cereal grains. The sorption isotherms exhibited a sigmoidal (Type II) pattern, characteristic of starchy materials. Among the evaluated models, the Modified Henderson model provided the best fit across all temperatures, with high accuracy indicated by R² values of 0.983–0.994, low RMSE values of 0.003–0.006, and minimal χ² values. Physical property analysis further showed that black rice paddy has long, slender grains with a mean length of 9.57 mm and an aspect ratio of 0.34, aligning with preferred market traits. Frictional properties revealed a higher static coefficient of friction on plywood (μ = 0.53) and lower on galvanized iron (μ = 0.49), while color parameters (L* = 47.12, a* = 7.69, b* = 24.38) reflected moderate pigmentation. The findings of this study provide essential sorption modeling parameters and supporting physical property data for black rice paddy, which can be applied in the design and optimization of drying, storage, and processing systems. These results contribute to improved postharvest management and support the broader utilization of pigmented rice in the Philippines.