Background and objectives: Paddy rice was dried in a fixed-bed dryer at 20, 40, 60, 80, and 100°C, with air speed set at 0.5 m/s. The drying kinetics, moisture diffusivity, and morphologic characteristics of black rice grains from IAC 600 genotype were determined. Findings: Black rice grains dried at 60, 80, and 100°C exhibited a reduction in the head rice percentage after dehusking (HRD) as the temperature increased. This behavior was confirmed by the occurrence of cracks in the endosperm of black rice when observed using an SEM. There was a reduction in cooking time, hardness, adhesiveness, and in the extractability of water-soluble proteins as well as in the extractability of proteins in 2% sodium dodecyl sulfate media in black rice subjected to 100°C of drying temperature, as compared to 20°C. Conversely, increases in the rehydration ratio of grains dried at 100°C were observed. Increases in the peak and breakdown viscosities and reductions in pasting temperature, final viscosity, and setback viscosity in Rapid Visco Analyzer profile of 100°C-dried black rice were also observed.
Conclusions:The increase in drying temperature promoted the increase in drying rate (k) and D eff . When black rice grains were dried at 60, 80, and 100°C, there was a decrease in HRD because of the evident increase in the intensity of endosperm fissures. The fissure formation is attributed to drying at temperatures of 60, 80, and 100°C occurring above the glass transition temperature (rubbery region) coupled with a non-efficient tempering. At 100°C, modifications were observed in the flour pasting properties, indicating the higher extent of protein-starch interactions, confirmed by the reduction of protein extractability. Significance and novelty: Few reports exist on black rice structure and cooking quality as a function of post-harvest conditions. The present work provides scientific support on how drying temperature impacts black rice quality.
K E Y W O R D Scooking quality, glass transition temperature, moisture diffusivity, pasting properties, protein solubility