Porosity and hardness of long-grain Brown rice kernels in relation to their chemical compositions

Rice kernel hardness is an important characteristic to the rice industry because of the greater economic value of whole kernels over brokens. Rice hardness may be weakened by increased porosity, i.e., void spaces formed from loose interaction between chemical components like starch, proteins and lipids. The objective of this study was to elucidate the impacts of rice proteins and lipids on porosity and hardness via heat treatment and solvent extraction. Brown rice kernels of similar thickness from four cultivars with varying protein and lipid contents were subjected to protein denaturation by heat treatment and/or lipid removal by hexane extraction and then characterized for protein solubility, residual lipids, porosity and breaking force. The results show that although porosity is generally negatively correlated with kernel hardness, porosity was primarily influenced by the quantity of proteins and lipids, whereas kernel hardness was primarily affected by their compositions. The continuity of the protein-starch matrix was the dominant factor that governed kernel hardness. This matrix is proposed to be strengthened by an increase in glutelin content and a decrease in non-polar lipids. This study demonstrates the importance of chemical composition on kernel hardness and elucidates the relationship between porosity and breaking force in rice kernels.