Title: Effects of sorghum (Sorghum bicolor (L.) Moench) tannins on alpha-amylase activity and in vitro digestibility of starch in raw and processed flours Authors
Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 12, 2013
Publication Date: April 12, 2013
Citation: Mkandawire, N.L., Kaufman, R.C., Bean, S.R., Weller, C.L., Jackson, D.S. and Rose, D. 2013. Effects of sorghum (Sorghum bicolor (L.) Moench) tannins on alpha-amylase activity and in vitro digestibility of starch in raw and processed flours. Journal of Agricultural and Food Chemistry. 61:4448-4454. Interpretive Summary: Recently, the effects of sorghum tannins on starch digestibility in humans have been of interest, since they may inhibit starch digestion and contribute to reducing glycemic index and increasing resistant starch. Sorghum tannins could inhibit starch digestion by binding to starch or by binding to starch degrading enzymes. Tannin containing sorghum lines have been reported to vary widely not only in tannin content, but also in molecular weight and antioxidant activity. Thus, the objectives of this study were 1) to determine the effect of tannins on starch digestion and 2) to investigate the mode, if present, in which tannins inhibit digestion of cooked starches. Significant variability was seen in the impact on starch digestibility in cooked sorghum samples from a diverse set of tannin-containing sorghum varieties. Data collected in this study indicate that sorghum tannins reduce starch digestibility by binding starch digestive enzymes rather than starch. The degree of amylase inhibition depended on tannin molecular weight, with high molecular weight tannins being more inhibitory. This research points out the importance of considering tannin chemistry in addition to simply tannin content when developing sorghum lines with improved human health characteristics.
Technical Abstract: The effect of condensed tannins (CT) on in vitro starch digestibility in cooked, wholegrain sorghum flours and on corn starch was investigated. CT extracts were also tested for their inhibitory effect on alpha-amylases. Rapidly digestible starch, slowly digestible starch, and resistant starch were not correlated with CT content in wholegrain flours. Resistant starch increased from 7% to 23% when purified CTs were added to starch up to 37.5% (starch weight). There was a significant reduction in alpha-amylase activity when the enzyme was incubated with CT extracts prior to cooked starch addition, but not when CT extracts were incubated with cooked starch prior to a-amylase addition. A CT extract containing a higher ratio of high:low molecular weight CTs had a greater inhibitory effect on alpha-amylases than an extract with a lower ratio. Thus, CT composition may be more relevant to reducing starch digestibility in CT-containing sorghum varieties than CT content. Under the conditions tested in this work, CTs inhibited cooked starch digestion through interactions with a-amylase (and not by interacting with starch).