PHYTOCHEMISTRY Phytochemistry 65 (2004) 1199–1221 www.elsevier.com/locate/phytochem Review Sorghum phytochemicals and their potential impact on human health Joseph M. Awika *, Lloyd W. Rooney Cereal Quality Laboratory, Soil & Crop Sciences Department, Texas A&M University, College Station, TX 77843-2474, USA Received 9 September 2003; received in revised form 26 February 2004 Available online 6 May 2004 Abstract Sorghum is a rich source of various phytochemicals including tannins, phenolic acids, anthocyanins, phytosterols and policosa- nols. These phytochemicals have potential to significantly impact human health. Sorghum fractions possess high antioxidant activity in vitro relative to other cereals or fruits. These fractions may offer similar health benefits commonly associated with fruits. Available epidemiological evidence suggests that sorghum consumption reduces the risk of certain types of cancer in humans compared to other cereals. The high concentration of phytochemicals in sorghum may be partly responsible. Sorghums containing tannins are widely reported to reduce caloric availability and hence weight gain in animals. This property is potentially useful in helping reduce obesity in humans. Sorghum phytochemicals also promote cardiovascular health in animals. Such properties have not been reported in humans and require investigation, since cardiovascular disease is currently the leading killer in the developed world. This paper reviews available information on sorghum phytochemicals, how the information relates to current phytonutrient research and how it has potential to combat common nutrition-related diseases including cancer, cardiovascular disease and obesity. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Sorghum bicolor; Gramineae; Phytochemicals; Tannins; Anthocyanins; Phenolic acids; Phytosterols; Policosanols; Human health; Cancer; Cardiovascular disease; Obesity Contents 1. Introduction . 1200 2. Traditional sorghum use for food . 1201 3. Tannins in sorghum. 1201 3.1. Production and genetics of tannin sorghums ........................... 1201 3.2. Chemical composition and structure of tannins from sorghum ............... 1203 3.3. Levels of tannins in sorghum ..................................... 1203 4. Phenolic acids of sorghum ........................................... 1206 5. Sorghum anthocyanins ............................................. 1207 6. Other phenolic compounds from sorghum ................................ 1208 7. Antioxidant properties of sorghum phenols and their bioavailability ............... 1209 7.1. Condensed tannins............................................ 1209 7.2. Phenolic acids ............................................... 1209 * Corresponding author. Tel.: +1-979-845-2925; fax: +1-979-845-0456. E-mail address: [email protected] (J.M. Awika). 0031-9422/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.phytochem.2004.04.001 1200 J.M. Awika, L.W. Rooney / Phytochemistry 65 (2004) 1199–1221 7.3. Anthocyanins . 1211 7.4. Comparing sorghum with fruits . 1211 8. Phytosterols . 1213 9. Policosanols . 1213 10. Sorghum phytochemicals and human health . 1214 10.1. Sorghum and cardiovascular disease . 1214 10.2. Sorghum and obesity . 1214 10.3. Sorghum and cancer . 1215 11. Perspective . 1216 Acknowledgements.................................................... 1217 References . ........................................................ 1217 1. Introduction Haslam, 1978; Gujer et al., 1986; Gous, 1989; Gu et al., 2002; Krueger et al., 2003). Sorghum phytosterols are Sorghum is the fifth most important cereal crop in the similar in composition to those from corn and contain world after wheat, rice, corn and barley. Sorghum out- mostly free sterols or stanols and their fatty acid/ferulate performs other cereals under various environmental esters (Avato et al., 1990; Singh et al., 2003). The sterols stresses and is thus generally more economical to pro- and stanols are structurally similar, except for the duce. More than 35% of sorghum is grown directly for presence of a double bond at position 5 in sterols, which human consumption. The rest is used primarily for an- is lacking in stanols. The policosanols (fatty alcohols) imal feed and alcohol and industrial products. The exist mostly as free or esterified forms with C24–C34 United States is the largest producer and exporter of atoms, and the general formula CH3–(CH2)n–CH2OH sorghum, accounting for 20% of world production and (Fig. 1). In sorghum-free forms of the C28 (octacosanol) almost 80% of world sorghum exports in 2001–2002 (1) and C30 (triacontanol) (2) are the most abundant (USDA-FAS, 2003). World sorghum production was 57 (Avato et al., 1990; Hwang et al., 2002). million metric tons during this period. Sorghums vary widely in their phenolic composition Sorghum contains various phytochemicals (including and content, with both genetics and environment af- phenolic compounds, plant sterols and policosanols) fecting the kind and level of phenolic compounds. Based that are secondary plant metabolites or integral cellular on extractable tannin content, sorghums have been components. Phenols help in the natural defense of classified as type I (no significant levels of tannins ex- plants against pests and diseases, while the plant sterols tracted by 1% acidified methanol), e.g., TX2911 (red and policosanols are mostly components of wax and perocarp), type II (tannins extractable in 1% acidified plant oils. The phytochemicals have gained increased methanol and not methanol alone), e.g., Early Hegari interest due to their antioxidant activity, cholesterol- and type III (tannins extractable in both acidified lowering properties and other potential health benefits. methanol and methanol alone), e.g., Early Sumac vari- The phenols in sorghums fall under two major catego- ety (Cummings and Axtel, 1973; Price et al., 1978). ries; phenolic acids and flavonoids. The phenolic acids However, this classification does not account for the are benzoic or cinnamic acid derivatives (Hahn et al., varying levels of other major phenolic constituents, es- 1983; Waniska et al., 1989), whereas the flavonoids in- pecially anthocyanins. Another broad way to classify clude tannins and anthocyanins as the most important sorghum is based on both appearance and total ex- constituents isolated from sorghum to date (Gupta and tractable phenols; thus, we have white sorghums (also Fig. 1. Common policosanols found in sorghum. J.M. Awika, L.W. Rooney / Phytochemistry 65 (2004) 1199–1221 1201 called food-type) with no detectable tannins or antho- the western world, however, pigmented sorghum use cyanins and very low total extractable phenol levels; red for food is almost non-existent. Innovative ways of sorghums which have no tannins but have a red pericarp incorporating these sorghums into the mainstream diet with significant levels of extractable phenols; black are necessary to realize the benefits they may offer. sorghums with a black pericarp and very high levels of anthocyanins and the brown sorghums which have a pigmented testa and contain significant levels of tannins, 3. Tannins in sorghum with varying degrees of pericarp pigmentation. Despite the high levels and diversity of phytochemi- Tannins are the most uniquely important phyto- cals in sorghum, research on this crop as a source of chemical components of sorghum since they possess valuable health promoting compounds lags behind other properties that produce obvious and significant effects in commodities (e.g., fruits and vegetables). As a result, animals, and have also been associated with various utilization of sorghum fractions in foods to improve positive and negative impacts on human health. These nutrition is very limited. Sorghum has a big potential, aspects and their relevance are discussed in later sections given its agronomic properties, as well as the emerging of the review. evidence on the biological effects of the phytochemicals present in the grain. The purpose of this paper is to 3.1. Production and genetics of tannin sorghums demonstrate that sorghums with special characteristics exist that have potential as significant sources of con- Even though tannins are commonly associated with densed tannins, anthocyanins and other phytochemicals sorghums, more than 99% of sorghum currently pro- with properties that complement the phytochemicals duced in the US is tannin-free. Decades of breeding ef- present in fruits and vegetables. forts to eliminate tannins from sorghum were motivated mostly by the reduced feed value of the tannin sorgh- ums. Tannins bind to and reduce digestibility of various 2. Traditional sorghum use for food food/feed nutrients, thus negatively affecting productiv- ity of livestock. Current non-tannin sorghums grown for Sorghum is used in a variety of foods. The white food livestock feed in the US have virtually the same energy sorghums are processed into flour and other products, profile as corn. The limited quantities of tannin sorgh- including expanded snacks, cookies and ethnic foods, ums grown in the US are mostly identity preserved seed and are gaining popularity in areas like Japan (United stock lines. However, in many other parts of the world States Grains Council, 2001; Rooney, 2001). In the US, where pests and diseases are common, tannin sorghums the white sorghum products are used to a small extent to are still grown in significant quantities since they are substitute for wheat in products for people allergic to more tolerant of such conditions than the non-tannin