Sorghum Biochemistry: An Industrial Perspective

By CV Ratnavathi, Jagannath Vishnu Patil and UD Chavan

Sorghum Biochemistry: An Industrial Perspective explores the many uses for sorghum in industry and biofuels. Not only does it offer a detailed understanding of the physical and biochemical qualities of the grain, it also takes an in-depth look at the role sorghum plays in such industries as brewing and ethanol production and the mechanics of post-harvest processing and value addition.

Sorghum has long been an important staple in Africa and Asia, but its value goes far beyond its uses in human and animal consumption. Sorghum is also used in many industries, including waxes, packing material, wall board, ethanol, beverages, and brewing, and one variety called sweet sorghum has also been used as a bioenergy crop. Sorghum Biochemistry: An Industrial Perspective offers a closer look at how the grain is used in such a variety of ways, and how we can continue to optimize its potential.

• Provides detailed biochemical studies on grain sorghum to inform researchers grappling with similar issues
• Offers foundational information on the quality and composition of sorghum as a grain
• Covers a variety of uses for sorghum in many industries, including food and beverage, energy, and brewing
• Includes photos and illustrations to enhance the understanding of processes and sorghum biochemistry

• Language: English
• Publisher: Academic Press
• Release date: Jun 1, 2016
• ISBN: 9780128031827

CV Ratnavathi

Dr. Ratnavathi has a PhD in Biochemistry from Osmani University, and currently serves as the Principal Investigator on the NFBSFARA project on “Studies on sucrose accumulation in sweet sorgum for efficient ethanol production,” while also leading a research project from the DBT on the therapeutic properties of sorghum. He has led several other externally funded projects and was instrumental in establishing a food safety laboratory under the NAIP Millet Value chain project. He has developed 30 sorghum recipes and 10 semi-processed products, and has published numerous journal articles, books, and book chapters.

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Chapter 1

Sorghum Grain Quality

C.V. Ratnavathi and V.V. Komala,    ICAR-Indian Institute of Millets Research, Rajendranagar, Hyderabad, India

Abstract

Sorghum has been an important staple in the semiarid tropics of Africa and Asia for centuries. Sorghum is also a rich source of various phytochemicals including tannins, phenolic acids, anthocyanins, phytosterols, and policosanols apart from proximate composition, vitamins, and minerals. These phytochemicals have significant impact on human health. Sorghum is consumed as food in various forms as roti, tortilla, injera, kisra, tuwo, ugali, bogobe, sankati, ambali, edi, couscous, wowoto, noodles, soru, burkutu, busa, ting, and obhshera in different parts of the world. Since sorghum flour is a gluten-free food, it is a safe alternative for those with celiac disease. Apart from the traditional products like (bhakri, bhatwadi, papad) non traditional foods like popped gains, kurdai, biscuits, flakes, upma, idli, dosa, utappa, chiwada, chakli, ambali, shankarpale, and cakes are also prepared from sorghum. Alternative uses of sorghum include in nonfood sectors for the production of commercially valued products.

Keywords

Endosperm texture; grain quality; antioxidant activity; roti quality; traditional foods; animal feed

Outline

1.1 Introduction 3

1.1.1 Sorghum Species 3

1.1.2 Grain Quality 6

1.1.3 Sorghum Kernel Structure 6

1.1.4 Physical Characters 9

1.1.5 Biochemical Parameters 10

1.1.6 Protein Estimation 10

1.1.7 Protein Body and Protein Matrix Characteristics of Sorghum 12

1.1.8 Digestibility of Uncooked Sorghum Proteins 12

1.1.9 Determination of Protein Digestibility 13

1.1.10 Variation in Protein Digestibility 13

1.1.11 Starch 14

1.1.12 Amylose 14

1.1.13 Amylose Estimation 15

1.1.14 Amylopectin 15

1.1.15 Starch Granule Structure 16

1.1.16 Starch Estimation 17

1.1.17 Starch Digestibility of Raw Sorghum Grain 18

1.1.18 Starch Digestibility 18

1.1.19 Starch Digestibility With and Without Protease Pretreatment 21

1.1.20 Influence of Protein Predigestion on Starch Digestibility of Sorghum Genotypes 22

1.1.21 Fat Estimation 23

1.1.22 Tannins and Phenols of Sorghum Grain 23

1.1.23 Extraction of Polyphenols 25

1.1.24 Prussian Blue Assay 25

1.1.25 Phenolic Compounds and Antioxidant Activity of Sorghum Grains of Varying Genotypes 25

1.1.26 Phytic Acid Estimation 26

1.1.27 Phytochemical Agents 26

1.1.28 Sorghum Phytochemicals and Their Impact on Human Health 27

1.2 Comparison With Other Cereals 28

1.2.1 Alternative Uses of Sorghum 31

1.2.2 Human Food 32

1.2.3 Animal Feed 33

1.2.4 India 35

1.2.5 China 36

1.2.6 West Africa 36

1.2.7 Eastern and Southern Africa 37

1.3 Dough and Roti Making Quality of Sorghum 38

1.3.1 Dough Quality 38

1.3.2 Roti Quality 42

1.3.3 Traditional Food Products of Sorghum and Their Commercialization 45

1.3.4 Noodles 55

1.3.5 Low Calorie-Low Fat Cookies 55

1.3.6 Sorghum Health Benefits 55

References 56

Further Reading 61

1.1 Introduction

Sorghum (Sorghum bicolor (L.) Moench) is a genus of numerous species of grasses, one of which is grown for grain and many of which are used as fodder plants, either cultivated or as part of pasture. The plants are cultivated in warmer climates worldwide. Species are native to tropical and subtropical regions of all continents in addition to the southwest Pacific and Australasia. Sorghum is in the subfamily Panicoideae and the tribe of Andropogoneae (the tribe of big bluestem and sugar cane). Sorghum is divided in 29 species and two hybrids as follows:

1.1.1 Sorghum Species

Sorghum almum; Sorghum amplum; Sorghum angustum; Sorghum arundinaceum; S. bicolor—cultivated sorghum, often individually called sorghum. Also known as durra, jowari, or milo; S. bicolor subsp. drummondii—Sudan grass; Sorghum brachypodum; Sorghum bulbosum; Sorghum burmahicum; Sorghum ecarinatum; Sorghum exstans; Sorghum grande; Sorghum halepense—Johnson grass; Sorghum interjectum; Sorghum intrans; Sorghum laxiflorum; Sorghum leiocladum; Sorghum macrospermum; Sorghum matarankense; Sorghum nitidum; Sorghum plumosum; Sorghum propinquum; Sorghum purpureosericeum; Sorghum stipoideum; Sorghum timorense; Sorghum trichocladum; Sorghum versicolor; Sorghum verticiliflorum; Sorghum vulgare var. technicum—broomcorn and two hybrids Sorghum×almum; Sorghum×drummondii.

One species, S. bicolor, is an important world crop, used for food (as grain and in sorghum syrup or sorghum molasses), fodder, the production of alcoholic beverages, and biofuels. Most varieties are drought- and heat-tolerant and are especially important in arid regions, where the grain is one of the staples for poor and rural people. These varieties form important components of pastures in many tropical regions. Sorghum bicolor is an important food crop in Africa, Central America, and South Asia and is the fifth most important cereal crop grown in the world.

Sorghum (S. bicolor (L.) Moench) is one of the major cereal crops among all the millets grown mostly in arid land. It is grown as a rainfed crop in both Kharif (mansoon) and Rabi (winter). It is mainly grown in the Deccan plateau, Central and Western India apart from a few patches in northern India. Almost the entire grain produced is used for human consumption in India. It is nutritionally superior to other fine cereals such as rice and wheat, and hence, it is known as a nutritious cereal. Nutritionally sorghum grain contains 4.4 to 21.1% protein, 2.1 to 7.6% fat, 1.0 to 3.4% crude fiber, 57.0 to 80.6% total carbohydrates, 55.6 to 75.2% starch, and 1.3 to 3.5% total minerals (ash). Sorghum also provides 350 Kcal energy, calcium, phosphorus, potassium, carotene, and thiamin as well as antioxidants through phenolics and various types of tannins. It is mainly consumed as Bhakri (roti), that is, unleavened pancake in various states of India. Apart from the traditional products like bhakari, bhatwadi, papad, popped grains, kurdai, high fiber cookies, biscuits, flakes, thalipeeth, upama, rawa idali, dosa, uttappa, chiwada, chakali, papadi, ambil, shankarpale, cookies, and cakes are also prepared from sorghum and consumed as snack food items. Sorghum grains are not only a good source of nutrients but also contain special constituents such as phyto-chemicals and dietary fiber, as well as resistant starch, which are essential to human nutrition. However, the grain sorghum consumption has remained restricted to the poorer sections of society due to poor nutritional quality of grain and inferior quality of the products, such as Bhakari, as well as the very low price of this food item as compared to the other cereal grains such as wheat, rice, and other millets.

Sorghum is of African origin. A large variety of wild and cultivated sorghum are grown in the tropics and subtropics of the world. In India, sorghum constitutes an important article of food, after rice and wheat. The sorghum grain is small and rounded, varying in color from off-white to white to varying shades of red, yellow, or brown. The grain size varies, the weight ranging from 7.0 to 61 g/1000 grains, with most sorghums weighing 20–30 g/1000 grains. The chemical composition of grain sorghum is similar to that of maize. Generally, sorghum has more protein than maize, a lower fat content, and about the same amount and proportions of carbohydrate components. The proximate analysis of Indian sorghum grain indicates the percentages of moisture 11.9, protein 10.4, fat 1.9, fiber 1.6, carbohydrates 72.6, and minerals 1.6 respectively; minerals present in the grain are calcium, magnesium, potassium, and iron.

In comparison with maize, sorghum grain contains approximately the same quantities of riboflavin and pyridoxine but more pantothenic acid, nicotinic acid, and biotin. Nicotinic acid occurs in the grain in an available form. Starch is the major carbohydrate of the grain. The other carbohydrates present are simple sugars, cellulose, and hemicelluloses. The amylose content of starch varies from 21 to 28%. Starch from waxy varieties contains little amylose. Both waxy and regular starches contain free sugars up to 1.2%—sucrose being the major constituent (0.85%) followed by glucose (0.09%), fructose (0.09%), maltose, and stachyose. Sorghum grain contains no detectable amount of glucoside, but on germination a three day-old seedling gives 3.5% dhurrin, which leads to the poisoning of animals consuming such sorghum seedlings.

The percentage of different protein fractions to the total protein of sorghum grown in India is albumin 5%, globulin 6.3%, prolamin 46.4%, and glutelin 30.4%. Prolamin and glutelin are principally present in the endosperm. Amino acid analysis of various protein fractions shows that there is better distribution of all essential amino acids in globulins than in prolamins. Sorghum protein is superior to wheat protein in biological value and digestibility. A vegetarian diet based on some varieties of sorghum is somewhat better than a rice-based diet. Sorghum lipids mostly consist of triglycerides, which are rich in the unsaturated fatty acids, oleic and linoleic, their percentage being 33 and 47%, respectively.

Currently, the demand of sorghum for animal feed is the driving force for sorghum production around the world (FAO, 1995). Sorghum grain has been regarded as a nutritionally inferior grain compared with other cereals, which may be one reason why its demand as human food has declined over the years; other factors that contribute to lower human consumption include cheap imports of corn to countries where sorghum is the staple food, complex processing techniques, and long preparation time for sorghum products. Even in the feed industry, sorghum is confronted with the same dilemma: somewhat lower feed value compared with other feed grains, primarily corn. Although slightly less nutritious, sorghum is still widely used as an animal feed due to its lower cost.

Sorghum is an important crop for food and fodder in the semiarid tropic regions (14–24°N latitude and 70–82° longitude) in India. It is mainly cultivated in the states of Maharashtra, Karnataka, Madhya Pradesh, Andhra Pradesh, Rajasthan, Gujarat, Tamil Nadu, and Uttar Pradesh and is mainly grown in Kharif (rainy), Rabi (post rainy), and summer seasons. India is the second largest producer of sorghum worldwide and has the largest area of sorghum under cultivation. Sorghum is grown both as a rainy season (June–October) and post rainy season (September–January) crop. The rainy season sorghum is often damaged due to deterioration and fetches lower prices and hence is less profitable. This molded grain has promoted its use in the nonfood and Industrial sector.

The superiority of sorghum over other crops lies in its ability to produce grain with a relatively limited supply of water. The crop is unique in that it can remain dormant during stress periods and renew growth when conditions are more favorable. Currently sorghum is a dietary staple for more than 500 million people in over 30 countries. On a worldwide basis, an average of 50% of the sorghum grown is used for human consumption. In addition to human consumption, sorghum is a popular component in animal feed, for both roughage and grain, because it is much less expensive to produce than other crops such as corn. Sorghum is also used in industrial waxes, packing material, wallboard, ethanol, beverages, and brewing. Despite its worldwide economic importance, sorghum is less well characterized at the genetic and molecular levels than many other cereal crops. Sorghum is, however, considered a good model species for drought tolerance, especially in grasses, and many researchers are beginning to realize its importance to dry-land agriculture. Today’s agriculturalists are faced with a unique challenge of how to cope with the loss of arable land and plateaus in yield increases in the face of a growing world population. For at least the past century, the population increase has been exponential. From the present population of approximately 6.5 billion, the world population is projected to reach 8.3 billion by 2025; to meet the projected food demand for this increasing population, researchers and producers agree on the necessity of an increase in the average yield of cereal crops.

At present most of the sorghum produced in India is consumed as a human food in the form of Roti or Chapatti (unleavened flat bread). On the other hand, a different sorghum called sweet sorghum with its juicy sweet stalk proved its potential as a bioenergy crop. Sorghum is known to have a highest productivity rate of about 50 g dry matter/m²/day in a number of locations around the world, and also it has been reported to surpass sugarcane, another C4 plant. Sweet sorghum also produces panicle with good quality grain. Grain sorghum and sweet sorghum offer themselves as a very good raw material for alternate uses.

1.1.2 Grain Quality

Grain quality is a nebulous term that means different things to different people. Grain quality largely depends on the grain type and its end use. It includes a range of properties that can be defined in terms of physical (moisture content, kernel size), sanitary (fungi and mycotoxin count), and intrinsic (fat content, protein content, hardness, starch content) quality characteristics. The quality properties of a grain are affected by its genetic traits, the growing period, timing of harvest, grain harvesting and handling equipment, drying system, storage management practices, and transportation procedures.

1.1.3 Sorghum Kernel Structure

Sorghum kernels are generally spherical in shape and come in different sizes and colors. Typical sorghum seeds are usually 4 mm long, 2 mm wide, and 2.5 mm thick with color ranging from black, red, purple, brown, yellow to white. Fig. 1.1 illustrates the sorghum grain structure (Rooney and Miller, 1982). The basic anatomical components are pericarp (outer layer), germ (embryo), and endosperm (storage tissue). The distribution of these components differs among varieties and environment, with an average of 8, 82, and 10% for pericarp, endosperm, and germ, respectively (Hubbard et al., 1950). The pericarp has a thickness from 8 to 160 μm and is divided into three layers: epicarp, mesocarp, and endocarp (Earp and Rooney, 1982). Immediately underneath the pericarp is a layer known as the seed coat or testa. Like the pericarp, the thickness of the testa also varies from 8 to 40 μm depending on sorghum genotype (Earp and Rooney, 1982). This layer is responsible for storing tannins and pigments in some of the sorghum genotypes. Tannins are polyphenolic compounds that negatively affect the nutritional quality of sorghum by binding and precipitating proteins, thus rendering the protein indigestible (Butler et al., 1984). In addition, tannins also can interact with various digestive enzymes and reduce the activity of these enzymes (Maxson et al., 1973; Griffiths and Mosely, 1980; Butler et al., 1984).

Figure 1.1 Structure of the grain sorghum showing the pericarp, endosperm (aleurone layer, corneous and floury), and germ (scutellum (S) and embryonic axis (EA)).

Endosperm, the major storage tissue, is composed of the aleurone layer, peripheral endosperm, vitreous (hard) endosperm, and floury (soft) endosperm. The aleurone layer is a single layer of cells that is located right under the seed coat. This layer is rich in proteins and enzymes, oils, vitamin B complexes, and minerals. Rooney and Miller (1982) provided a detailed picture and description of the sorghum endosperm. The location of peripheral endosperm is not well defined but is usually located right under the aleurone layer with thickness between two to six block cells. Right under the peripheral endosperm is the vitreous endosperm followed by the floury endosperm. Vitreous and floury endosperm can be distinguished from one another based on the packing and shape of the starch granules, and the distribution of the protein matrix. Vitreous endosperm contains polygonal-shaped starch granules surrounded by a continuous protein matrix, while floury endosperm contains spherical starch granules with a discontinuous protein matrix.

Indian sorghum genotypes include 160 germplasm lines, 200 elite SPV genotypes, and 60 released parents, hybrids, and many other varieties. These genotypes were analyzed for both physical, and biochemical characters. The physical characters include (1) grain size, (2) 100 grains weight, and (3) endosperm texture. The proposed biochemical characters to be evaluated are (1) %starch, (2) %protein, (3) %fat, (4) %in vitro protein digestibility, (5) phytic acid content, (6) amylose, (7) starch digestibility, and (8) dietary fiber. The progress of analysis of the lines and distribution of the genotypes analyzed for each physical and chemical parameter is presented in Table 1.1.

Table 1.1

1.1.4 Physical Characters

The physical characters include (1) grain size, (2) 100 grains weight, (3) endosperm texture, and (4) grain