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

Authors: SIAW, MICHELLE - University Of Arkansas; YANG, YA-JANE - University Of Arkansas; McClung, Anna; MAUROMOUSTAKOS, ANDY - University Of Arkansas

Submitted to: LWT - Food Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/4/2021
Publication Date: 3/6/2021
Citation: Siaw, M.O., Yang, Y., McClung, A.M., Mauromoustakos, A.S. 2021. Porosity and hardness of long-grain brown rice kernels in relation to their chemical compositions. LWT - Food Science and Technology. http://doi.org/10.1016/j.lwt.2021.111243.
DOI: https://doi.org/10.1016/j.lwt.2021.111243

Interpretive Summary: Harvested rice from the field must go through a milling process to produce the white milled rice that is most commonly consumed. The amount of whole milled grains produced, minus the amount of broken grains, largely determines the economic value of the crop. Thus, having rice varieties that produce high milling yields is important to growers and rice millers. Rice kernel hardness is an important factor influencing grain breakage during milling. Grain hardness is influenced by the internal porosity of the grain, where the starch granules are not densely packed. The basis for these loosely packed starch cells is due to the interaction of grain chemical components like starch, proteins and lipids. This study evaluated the impact of rice proteins and lipids on grain porosity and hardness. Brown rice kernels of similar thickness from four cultivars were subjected to protein denaturation by heat treatment and/or lipid removal by chemical extraction and then were characterized for kernel porosity and hardness. In general, as kernel porosity increased, kernel hardness decreased. In addition, factors that influenced porosity were primarily the quantity of proteins and lipids in the grain. In contrast, the continuity of the protein-starch matrix within the kernel was the main factor controlling kernel hardness. These results show the importance of chemical composition on kernel hardness and demonstrate ways to improve rice and its processing to increase economic value.

Technical Abstract: 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.