Is there variation in resistant starch among high amylose rice varieties?

Authors: Chen, Ming Hsuan; McClung, Anna

Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 2/25/2016
Publication Date: 9/15/2017
Citation: Chen, M., McClung, A.M. 2017. Is there variation in resistant starch among high amylose rice varieties?. Proc. 36th Rice Technical Working Group, Galveston, TX. p. 169-170. March 1-4,2016. CDROM

Interpretive Summary:

Technical Abstract: Resistant starch (RS) is the fraction of the starch and the products of starch degradation that resist digestion in the small intestines of healthy humans and is partially or entirely fermented in the colon by the microbiota. RS in food lowers postprandial glucose concentration and has potential in prevention of colon cancer and inflammatory bowel disease. Increasing RS in rice may improve or lower its glycemic index and promote colon health. Rice is consumed after cooking and the majority of RS in rice comes from retrograded amylose forming ordered crystalline regions during cooling of gelatinized starch. Previous studies have utilized rice varieties with amylose concentrations ranging from waxy (0% amylose) to high amylose and showed that RS concentration was positively correlated with amylose concentration. The objective of this research was to determine the variation in RS concentration among just high amylose rice varieties that were collected from diverse origins. Forty high amylose rice varieties were selected from two rice germplasm collections. One is the Rice Diversity Panel 1 consisting 421 varieties collected from 10 geographic regions where rice is grown. The other one is the USDA Core collection consisting 1790 entries from 114 countries. Because amylose concentration is affected by the growing environment, for the first round of selection high amylose varieties were identified based on genetics using alleles associated with the Waxy gene including the RM190 SSR marker and alleles of the intron 1 and exon 6 SNP markers that predict high amylose rice. Combined with previous phenotypic data, forty varieties that had the highest amylose concentration were selected and grown in Stuttgart, AR, in 2014 and 2015. Three US cultivars, Bengal (low amylose), Presidio (intermediate amylose) and Dixiebelle (high amylose) were also grown for comparison. The milled rice was cooked at 1:2 (w/v) rice to water ratio for a total of 20 min (3 min for water inside the beaker to start boiling and then additional 17 min to cook the rice). The 17 min cooking time was the minimum cooking time necessary for both Presidio and commercially purchased long grain from AR. The percent fully cooked rice kernels, which was defined as having fully cooked centers with no opaque area, was determined by compressing 10 cooked kernels under glass and observed visually. The RS was determined based on the in vitro method of AACC 32.40. The RS concentration obtained using this method has been shown to be very similar to RS collected from an ileostomy, in vivo. A 2.3-fold difference in RS concentration was found among the 40 varieties. Thirty-four out of the 40 varieties had higher RS concentration than Dixiebelle. There was no correlation between % fully cooked kernels and RS concentration suggesting the high RS concentration was not due to under- cooked rice that contained non-gelatinized starch, which is not digestible. In both years, RS concentration was positively correlated with amylose concentration. It is well known that there is a difference in cooked rice texture among high amylose rice varieties that can be distinguished by the RVA parameter, Setback from Trough (SB) viscosity, and is genetically controlled by a functional SNP in exon 10 of the Waxy gene. Among these 40 varieties plus Dixiebelle, 18 varieties were high-SB genotypes and 23 were low-SB genotypes. Mean comparison showed that the SB phenotype was significantly different between the high-SB and low-SB genotypes; while there was no significant difference in RS concentration between the two. Therefore, high RS rice can be selected for the cooked rice texture that best suits consumer’s preference. In conclusion, we identified rice varieties with higher RS than typical US high amylose cultivars that might improve health of the consumer.