Effect of cooling step on starch digestibility and other properties of parboiled rice.

Authors: Patindol, James; Guraya, Harmeet; Champagne, Elaine

Submitted to: United States Japan Natural Resources Protein Panel
Publication Type: Proceedings
Publication Acceptance Date: 8/24/2010
Publication Date: 8/26/2010
Citation: Patindol, J.A., Guraya, H.S., Champagne, E.T. 2010. Effect of cooling step on starch digestibility and other properties of parboiled rice. United States Japan Natural Resources Protein Panel.

Interpretive Summary: Rice with reduced starch digestibility is desired by a niche market; health-conscious consumers and dietary programs for the management of health problems like diabetes and obesity. This work looked into the possibility of modifying rice parboiling process in order to reduce the starch digestibility of the final product. A low-temperature cooling was included as a pre-drying step in the parboiling process. Presoaked, long-grain rough rice, was subjected to mild (100oC, 0 psi, 20 min) and severe (125oC, 20 psi, and 20 min) heat-moisture treatment; cooled, and divided into 6 lots for refrigerated and/or frozen storage. Lots 1, 2, 3, 4, and 5 were refrigerated (4oC) for 0, 1, 4, 7, and 14 days, respectively, in self-sealing plastic bags. As a check, lot 6 was subjected to frozen storage (-20oC) for 14 days. After holding, samples were taken out of the refrigerator, spread evenly on meshed trays, allowed to dry gently at ambient temperature overnight and then in a convection oven (30oC) to ~12% MC. The effect of holding on starch digestibility and other parboiled rice properties was minor; it was overshadowed by the effect of the severity of the heat-moisture treatment. Slowly digestible starch increased in the severely parboiled samples, whereas, resistant starch essentially remained unchanged. Cooling the grains at -20oC for 14 days increased resistant starch but unfavorably decreased head rice yield. Low-temperature holding may not be necessary; it appeared that retrogradation (staling) through gentle drying was enough to instigate optimum reduction in parboiled rice starch digestibility.

Technical Abstract: Retrogradation and the formation of amylose-lipid complex have been reported to contribute to reduced digestibility of starch in parboiled rice. This study looked at the prospect of including a low-temperature holding step in the parboiling process to enhance retrogradation, and subsequently reduce the starch digestibility of the final product. Long-grain rough rice samples were subjected to mild (100oC, 0.0 kg/cm2, 20 min) and severe (125oC, 1.5 kg/cm2, 20 min) parboiling, and then held for 0, 1, 4, 7, and 14 days at 4oC before drying. Another lot was held at -20oC for 14 days. Except for the latter, the effect of holding on starch digestibility and other parboiled rice properties was minor; it was overshadowed by the effect of the severity of the heat-moisture treatment (autoclaving). Cooked rice in vitro starch digestibility index was 83.2.6-85.2% and 75.3-81.2% for mildly and severely-parboiled samples, respectively. Slowly digestible starch increased in the severely parboiled samples, whereas, resistant starch essentially remained unchanged. Holding the grains at -20oC for 14 days increased resistant starch but unfavorably decreased head rice yield. Low-temperature holding may not be necessary; it appeared that retrogradation through gentle drying was enough to instigate optimum reduction in parboiled rice starch digestibility.