In vitro digestibility of starch in sorghum differing in endosperm hardness and flour particle size

Authors: XU, XIAOLI - Kansas State University; Bean, Scott; Wu, Xiaorong; SHI, YONG-CHENG - Kansas State University

Submitted to: Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/2022
Publication Date: 7/30/2022
Citation: Xu, X., Bean, S.R., Wu, X., Shi, Y. 2022. In vitro digestibility of starch in sorghum differing in endosperm hardness and flour particle size. Food Chemistry. 383. Article 132635. https://doi.org/10.1016/j.foodchem.2022.132635.
DOI: https://doi.org/10.1016/j.foodchem.2022.132635

Interpretive Summary: As a major component in cereal grain foods, starch plays an important role in controlling the structure, texture and stability of these foods, and is the most important source of food energy. However, among cereals, starch in sorghum flour is relatively low in digestibility and sorghum may be a good source of resistant starch, which is important in reducing glycemic index and increasing feelings of fullness after eating. Many reasons have been studied to determine the reasons for decreased digestibility of sorghum starch. This project investigated the role of grain hardness, flour particle size, cooking and the role of protein in starch digestibility. Grains with an intermediate hardness and higher levels of amylose had the highest resistant starch content. Cooking also reduced starch digestibilty, likely due to protein cross-linking which physically blocked starch from being digested.

Technical Abstract: In vitro digestibility of starch in sorghum grains differing in endosperm hardness and flour particle size was investigated. The starch digestibility increased as the particle size of flour decreased, but no clear trend was observed in digestibility of starch in sorghum flours milled from grains with different hardness. Protein matrix affected the digestion of starch. Digestion of protein increased with time of pepsin treatment, and that led to an increased starch digestion. pH value (2.0 vs. 1.3) was a critical factor affecting protein digestion. Low protein digestibility at pH 1.3 led to a low measured digestion of sorghum starch. Optimum pH (pH 2.0 for pepsin) digested more protein, resulting in a greater digestion of starch. Resistant starch (RS) content was 8.5-26.3% in isolated sorghum starch but higher (10.6-29.5%) in sorghum flours. All sorghum flours had a high level of SDS (~51-59%) and the RS content was ~11-30%. The starch in sorghum flour with median hardness had a higher amylose content (~24%) and higher RS content (~15-30%). Protein digestibility decreased after cooking while starch digestibility increased compared to native sorghum flours. Sulfhydryl groups decreased after cooking, indicating the disulfide bonds formed between protein molecules and increased barrier for enzyme to digest starch. RS content of cooked sorghum flour was much higher without pepsin treatment (16.93-23.99%) than that of cooked sorghum flour with pepsin treatment (4.86-12.53%).