Thesis Abstract

                    ABSTRACT 

Two tannin sorghums, (high-tannin, black with tannin), high anthocyanin sorghum (black), and non-tannin sorghum (white) were used to investigate the effects of sorghum phenolic compounds on in vitro starch digestibility, estimated glycemic indices (EGI) and resistant starch contents (RS) of porridges. Sorghum varieties were chosen to have a wide range of total phenols (3-23 mg/g gallic acid) and tannin contents (0-34 mg catechin eq./g). Normal corn starch, enzyme resistant high amylose corn starch, and whole sorghum grains were cooked with the aqueous fraction of sorghum bran extracts obtained with 70% aqueous acetone. Endosperms of soft and hard sorghum varieties were mixed with sorghum brans and cooked into porridges with distilled water. Hi-tannin, black and black with tannin sorghum bran extracts significantly (p<0.05) decreased starch digestibility and estimated glycemic index (EGI) while they increased resistant starch (RS) contents of normal corn starch, enzyme resistant high amylose corn starch, and whole sorghum grain porridges. The highest reduction in starch digestibility of the porridges occurred with hi-tannin sorghum bran extracts, followed by black with tannin and black sorghum bran extracts. Double cooked corn starch porridges, which were cooked with these bran extracts had EGI values of 49-67 and RS contents of 9.1-57.7%. These RS values are higher than foods such as legumes, whole iv pasta and whole grain cereals which are considered health foods with low EGI (36-71) and high RS contents (2.9-6.8). Only brans of condensed tannin-containing sorghum varieties (tannin, black with tannin sorghums) significantly (p<0.05) decreased starch digestibility, and EGI, and increased RS contents of the endosperm porridges. When tannin sorghum bran extracts were cooked with zein added to corn starch porridges, starch digestibility of the porridges significantly (p<0.05) increased, while RS significantly (p<0.05) decreased because the zein reacted preferentially with the tannins. The cooking trials indicated that sorghums with tannins significantly reduced the activity of digestive enzymes, reduced EGI, and increased RS contents of porridges. Thus, specialty sorghum varieties have a potential to lower EGI and increase RS contents of starchy foods. Their aqueous bran extracts have potential use to reduce risk factors for type II diabetes and obesity.

Thesis Overview

1.1 INTRODUCTION

1.0 BACKGROUND STUDY

Sorghum bicolor (L.) Moench is the fifth most important cereal crop after wheat, rice, maize, and barley in terms of production (FAO 2005). Drought tolerant sorghum is a very important dietary component in many parts of Africa, Asia, and the semi-arid tropics worldwide (Serna-Saldivar and Rooney 1995). Some specialty sorghum varieties are less digestible than the other cereals (Rooney and Pflugfelder 1986). They also contain substantial levels of a wide variety of phenolic compounds, such as phenolic acids, anthocyanins, and condensed tannins. A previous study by De Castro Palomino Siller (2006) showed that tannin sorghum extrudates and porridges had reduced starch digestibility and EGI, and increased resistant starch (RS) values compared to corn extrudates and porridges. Among specialty sorghum varieties, tannin sorghum digestibility has been studied the most (Taylor et al 2007, Oliveira et al 2007, De Castro Palomino Siller 2006, Mariscal-Landín et al 2004, Matuschek and Svanberg 2004, Nyamambi et al 2000); there is little published information about the other specialty sorghum varieties, such as anthocyanin-rich black sorghum. Because obesity and diabetes are among the most important medical problems in America today, investigation of starch digestibility, EGI, and RS in the presence of sorghum phenolics, specifically condensed tannins and anthocyanins, would be useful. Postprandial blood glucose changes can be used to categorize the Glycemic Index (GI). GI is a scale that ranks carbohydrate-rich foods by how much and how quickly they raise blood glucose levels compared to a standard food (glucose or white bread). GI can be estimated by in vitro rate and extent of starch digestibility, which is called Estimated Glycemic Index (EGI) (O'Dea and Holm 1985, Lund and Johnson 1991).

Starch escapes from digestion in the small intestine (may be digested in the large intestine), does not cause a glycemic response This type of starch is called resistant starch (RS). Therefore, the objectives of this study were:

 • To characterize physical and chemical characteristics of the sorghum varieties with different amounts of phenolic compounds,

 • To determine the effects of these sorghum brans on starch digestibility of soft and hard sorghum endosperm porridges, 

• To investigate the effects of these sorghum bran extracts, which contains condensed tannins and anthocyanins, on starch digestibility, and EGI, and RS of corn starch, enzyme resistant high amylose corn starch and whole sorghum porridges.