Nutritional and Rheological Properties of Sorghum

This article was downloaded by: [Canadian Research Knowledge Network] On: 22 June 2010 Access details: Access Details: [subscription number 918588849] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 3741 Mortimer Street, London W1T 3JH, UK International Journal of Food Properties Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t713597259 Nutritional and Rheological Properties of Sorghum Arun G. Kulamarvaa; Venkatesh R. Soslea; G.S. Vijaya Raghavana a Department of Bioresource Engineering, MacDonald Campus of McGill University, Ste-Anne-deBellevue, Canada To cite this Article Kulamarva, Arun G. , Sosle, Venkatesh R. and Raghavan, G.S. Vijaya(2009) 'Nutritional and Rheological Properties of Sorghum', International Journal of Food Properties, 12: 1, 55 — 69 To link to this Article: DOI: 10.1080/10942910802252148 URL: http://dx.doi.org/10.1080/10942910802252148 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. International Journal of Food Properties, 12: 55–69, 2009 Copyright © Taylor & Francis Group, LLC ISSN: 1094-2912 print / 1532-2386 online DOI: 10.1080/10942910802252148 NUTRITIONAL AND RHEOLOGICAL PROPERTIES OF SORGHUM Downloaded By: [Canadian Research Knowledge Network] At: 20:27 22 June 2010 Arun G. Kulamarva, Venkatesh R. Sosle, and G.S. Vijaya Raghavan Department of Bioresource Engineering, MacDonald Campus of McGill University, Ste-Anne-de-Bellevue, Canada Sorghum is a gluten-free cereal and forms the staple diet of a majority of the populations living in the semi-arid tropics. Sorghum contains various phenolic and antioxidant compounds that could have health benefits, which make the grain suitable for developing functional foods and other applications. It is usually consumed in the form of bread made from the grain flour. Sorghum dough has poor viscoelastic properties compared to wheat dough and mechanical methods for production of sorghum roti are scarce. This article reviews the nutritional and rheological properties of sorghum in relation to their mode of consumption. Keywords: Rheology, Dough, Viscoelastic property, Texture, Gluten intolerance, Sorghum, Tannins. INTRODUCTION Sorghum is a major cereal in the semi-arid regions of the world where it is an important food and feed crop. Sorghum species (Sorghum vulgare and Sorghum bicolor) are members of the grass family. Sorghum is known by a variety of names: great millet and guinea corn in West Africa; kafir corn in South Africa; dura in Sudan; mtama in eastern Africa; jowar in India, and kaoliang in China.[1] It is usually referred to as milo or milo-maize in North America. The USA is a major producer of sorghum, but the grain is not consumed as human food except for a very small fraction, but as animal fodder, whilst in the semi-arid tropics of Africa and India the grain forms the staple diet for large populations, where nearly all the produce is used directly as human food. Sorghum, like other cereals, is an excellent source of starch and protein. It is a gluten-free cereal, which bears significance in the present day scenario where the occurrence of Celiac Disease (CD), an immunological response to gluten intolerance is on the rise. Grain sorghum contains phenolic compounds like flavonoids,[2] which have been found to inhibit tumour development.[3] The starches and sugars in sorghum are released more slowly than in other cereals[4] and hence it could be beneficial to diabetics.[5] Sorghum is consumed in various forms around the world like baked bread, porridge, tortillas, couscous, gruel, steam-cooked products, alcoholic, and non-alcoholic beverages, Received 20 April 2007; accepted 6 June 2008. Address correspondence to Dr. G.S. Vijaya Raghavan, Department of Bioresource Engineering, MacDonald Campus of McGill University, Ste-Anne-de-Bellevue H9X 3V9, CANADA. E-mail: Vijaya.raghavan@mcgill.ca 55 Downloaded By: [Canadian Research Knowledge Network] At: 20:27 22 June 2010 56 KULAMARVA, SOSLE, AND RAGHAVAN and so on. The potential food and industrial applications of sorghum have been reported.[6,7] It has the potential to be processed into starch, flour, grits, and flakes, and it is used to produce a wide range of industrial products. It can also be malted and processed into malted foods, beverages, and beer. On account of its nutritional significance, and its easy adaptability to a wide range of growing conditions and lesser water requirements, sorghum has the potential to be incorporated in the diets of human populations around the world, more specifically to those intolerant to wheat. Processing of sorghum flour into products has faced several limitations. The traditional method of preparing sorghum bread is usually very laborious and cumbersome. Efforts to mechanize the production process of sorghum bread have been rare. Sorghum, being a gluten-free cereal behaves quite differently from wheat and has poor rheological properties in terms of its pliability, extensibility, and rollability. Mechanization of the preparation of sorghum would require elucidation of the properties of sorghum dough, which play a role in its behaviour. The rheological properties of doughs describe how they deform, flow, or rupture under applied stress and could be used as a tool in the selection and specification of appropriate raw materials. They are of importance in terms of product formulation and optimization, quality control, machining properties of the dough, scale-up of the process and automation.[8,9] The objective of this review is to highlight the nutritional benefits of sorghum and the significance of its rheological properties in terms of processing. Sorghum as Food Sorghum is an important cereal crop in Africa and Asia and is consumed in different forms like tortillas, porridges, couscous and baked goods. Earlier works have reviewed the use of sorghum as human food and reported the various forms in which sorghum is being consumed.[10] Preparation of extruded products from sorghum has also been reported.[11] The use of sorghum in pasta processing has been evaluated.[12] Sorghum products including expanded snacks, cookies, and ethnic foods are gaining popularity in areas like Japan.[13] White sorghum products are used to a small extent in the US to substitute for wheat in products for people allergic to wheat gluten. In India, 70% of the total sorghum produced in India is consumed in the form of roti, which is an unleavened flat bread.[14] Chemical Composition and Nutrient Value of Sorghum Sorghum is a gluten free cereal which bears significance in the present day scenario where the occurrence of Celiac Disease (CD), an immunological response to gluten intolerance is on the rise. It is reported that in the United States of America the prevalence of CD is 1:22 and 1:39, respectively, in first and second degree relatives of CD patients,1:56 in patients having either gastrointestinal symptoms or a disorder associated with CD, and 1:133 in non-risk individuals.[15] The worldwide prevalence of this syndrome has been reported to be in the range of 1 in 250 and 1 in 300.[16] Grain sorghum contains phenolic compounds like flavonoids which have been found to inhibit tumor development,[3] and the diversity of phenolic compounds in sorghum and their nutritional chsignificance has been reported.[17] Studies of the mature sorghum grain structure show that the embryo constitutes roughly 10%; the bran layers (pericarp) about 8%; and the endosperm more than 80% of the grain. The relative proportions may vary with genetic background, environment and degree of maturity. The composition of kernel fractions of sorghum is given NUTRITIONAL AND RHEOLOGICAL PROPERTIES OF SORGHUM 57 Table 1 Nutrient content of whole kernel and its fractionsa. Kernel fraction % of kernel weight Proteinb (%) 100 82.3 12.3 12.3 (80) 18.9 (15) 6.7 (43) Whole kernel Endosperm Germ 9.8 Bran 7.9 Downloaded By: [Canadian Research Knowledge Network] At: 20:27 22 June 2010 a Mineral (%) Lipid (%) bg Starch (%) 1.67 0.37 (20) 10.4 (69) 2.0 (11) 3.6 0.6 (13) 28.1 (76) 4.9 (11) 73.8 82.5 (94) 13.4 (20) 34.6 (4) Niacin Riboflavin Pyridoxin (mg/100 g) (mg/100 g) (mg/100 g) 4.5 4.4 (76) 8.1 (17) 4.4 (7) 0.13 0.09 (50) 0.39 (28) 0.40 (22) 0.47 0.40 (76) 0.72 (16) 0.44 (8) Values in parentheses represent percentage of whole kernel value; and bN × 6.25. Source:[105] in Table 1. The largest part of the kernel, the endosperm, is comparatively poor in mineral matter and oil content. The endosperm is what contributes mainly to the kernel’s protein (80%), starch (94%) and B-complex vitamins (50 to 75%) composition.[18] The germ fraction of sorghum is rich in minerals and B-complex vitamins and contains over 68% of the total mineral matter, 75% of the oil, and 15% protein of the whole kernel. Sorghum bran is low in ash, protein and rich in fiber. Processing removes the outer pericarp, and thus, proportionally increases the protein and reduces the cellulose, lipid, and mineral content of the grain. Carbohydrate Content Akin to other cereals, starch is the principal storage form of carbohydrate in sorghum and the average starch content is 69.5%.[19] Arabinoxylans (pentosans) in cereals play an important role in the bread-making quality and have proven to influence the water balance and rheological properties of dough and starch retrogradation.[20,21] They are complex polysaccharides with arabinose residues branching on a xylan backbone. The carbohydrate composition and structural features of arabinoxylans of sorghum with good roti making quality have been evaluated.[22,23] Sorghum has similar amounts of starch as wheat flour, but with significantly lower α –amylase (40–50%) and amylolytic (10%) activity when compared to wheat flour.[24] Proteins Proteins form the second major component of sorghum grains. The protein content of sorghum is affected by both genetic and environmental factors. The protein content of sorghum is known to vary along with the changes in its amino acid composition.[25] The protein content of sorghum is equivalent to that of wheat and maize (Table 2). High fiber content and poor digestibility of nutrients is a characteristic feature of sorghum grains, which severely influences its consumer acceptability. Sorghum cultivars have been proven to have reduced amounts of lysine, threonine and total sulphur amino acids.[26] It is reported that the leucine/isoleucine ratio was imbalanced in comparison with the FAO/WHO reference protein[27] and baking reduced the tannin levels to zero in the cultivars studied. Breads fermented for 18h had higher vitamin B12 and pantothenic acid levels but lower P levels as compared to unfermented breads. There 58 7.9 11.6 9.2 10.4 Proteina (g) N×6.25. Sources:[18,106,107] a Rice (brown) Wheat Maize Sorghum Food 2.7 2.0 4.6 3.1 Fat (g) 1.3 1.6 1.2 1.6 Ash (g) 1.0 2.0 2.8 2.0 Crude fibre (g) 76.0 71.0 73.0 70.7 Carhohydrate (g) Table 2 Nutrient composition of sorghum (per 100 g edible portion; 12% moisture). 362 348 358 329 Energy (kcal) 33 30 26 25 Ca (mg) 1.8 3.5 2.7 5.4 Fe (mg) 0.41 0.41 0.38 0.38 Thiamin (mg) Downloaded By: [Canadian Research Knowledge Network] At: 20:27 22 June 2010 0.04 0.10 0.20 0.15 Riboflavin (mg) 4.3 5.1 3.6 4.3 Niacin (mg) NUTRITIONAL AND RHEOLOGICAL PROPERTIES OF SORGHUM 59 was a slight reduction in amino acid levels in fermented bread. The nutrient composition of sorghum is at par with wheat and rice, but the protein quality is poor due to its high leucine and tannin contents,[28] and hence, it would be beneficial to incorporate other cereal or legume flours to enrich its nutritional quality. Downloaded By: [Canadian Research Knowledge Network] At: 20:27 22 June 2010 Starch and Protein Digestibility Sorghum grain has been reported to have the lowest raw starch digestibility due to restrictions in accessibility to starch caused by endosperm proteins.[29] The digestibility of the starch, dependent on hydrolysis by pancreatic enzymes, determines the available energy content of cereal grain. The chemical nature of the starch, particularly the amylose and amylopectin content, is yet another factor that affects its digestibility. The starch digestibility was reported to be higher in low-amylose, i.e., waxy, sorghum than in normal sorghum.[30] The presence of tannins in the grain contributes to the poor digestibility of starch in some varieties of sorghum.[31] Tannins isolated from sorghum grain were shown to inhibit the enzyme X-amylose, and they also bind to grain starches to varying degrees.[32] The low starch digestibility has also been attributed to a high content of dietary fiber.[33] Lower starch digestibility has been reported in case of cooked sorghum flours than normal maize flour, irrespective of the endosperm type.[34] Several authors have reported a similar reduction in digestibility of sorghum after cooking and it has been estimated that the digestibility decreases by around 24–31%.[35,36] It is indicated that starch digestibility in cooked sorghum flour has been attributed to the formation of disulphide bonds during cooking, which leads to toughening at the surface and interior of protein bodies. Both in vitro and in vivo studies have demonstrated wide variability in protein digestibility of sorghum varieties.[37] Values ranging from 49.5 to 70%[38] and from 30 to 70%[39] have been reported. These values were lower than that observed for corn protein (78.5 percent). In certain sorghum varieties, the presence of condensed polyphenols or tannins in the grains is another factor that adversely affects protein digestibility and amino acid availability.[40] A decrease in the protein digestibility of sorghum on cooking was attributed to reduced solubility of prolamin and its reduced digestibility by pepsin.[41] Processing of the grain by methods such as steaming, pressure-cooking, flaking, puffing or micronization of the starch increases the digestibility of sorghum starch. This has been attributed to a release of starch granules from the protein matrix, rendering them more susceptible to enzymatic digestion.[42,43] The in-vitro digestibilities of starch and protein in sorghum flours have been shown to be improved by cooking in the presence of reducing agents like cysteine, sodium metabisulphite, or ascorbic acid[35,44–46] as the reducing agents minimize the formation of disulphide bonds. The in-vitro digestibility of sorghum proteins has been shown to have improved by fermentation.[47] Fermentation is also reported to have lead to an increase in lysine and methionine content.[48] Protein digestibility of sorghum could also be improved by malting the grain.[49] Malt pretreatment also resulted in a reduction in phytic acid content, which is a significant antinutritional factor. Germinated sorghum extract had a very low paste viscosity, while pretreatment of sorghum flour with small amounts of papain or trypsin enzymes lead to an improvement in the in vitro protein digestibility of sorghum, without affecting the paste viscosity.[50] 60 KULAMARVA, SOSLE, AND RAGHAVAN Downloaded By: [Canadian Research Knowledge Network] At: 20:27 22 June 2010 Other Nutrients Sorghum has a higher crude fat content (3%) than wheat or rice. The germ and aleurone layers are the major sources of the fat content. The germ contributes to about 80% of the total fat.[51] The mineral composition of sorghum grains (Table 3) is highly variable. Sorghum is a rich source of B-complex vitamins. Other fat-soluble vitamins, namely D, E, and K, have also been found in sorghum grain. Sorghum is not a source of vitamin C. The concentrations of thiamin, riboflavin, and niacin in sorghum were comparable to those in maize. Sorghum does not contain vitamin A, although certain yellow endosperm varieties contain small amounts of β-carotene—a precursor of vitamin A. Cellulose, the major insoluble fibre component of sorghum varied from 1.19 to 5.23% in sorghum varieties.[52] Grain sorghum does contain phenolic compounds other than tannin that affect its sensory and nutritional quality.[53] Sorghum phenols have been shown to act as antioxidants in vitro.[54] Antioxidants have been reported to be able to decrease the risk of several diseases including cancer, atherosclerosis, rheumatoid arthritis, inflammatory bowel disease, and cataracts by lowering the amount of free radicals. They can also be used as antifungal, antibacterial, and antiviral agents.[55] The sensory and nutritional value of sorghum flakes on supplementation with wheat flour has been reported.[56] Keregero and Mtebe[57] have evaluated the acceptability of food products including bread and buns made using wheat-sorghum composite flours. Enhancement of the nutritional profile of sorghum by supplementing it with flours from other cereals or legumes has been attempted.[58] The nutritional composition and sensory characteristics of porridges made with different combinations of soy and sorghum grits have been reported.[59] Rheological Properties Rheological studies are one of the most convenient methods for measuring indicators of quality and texture of food products. The rheological properties of doughs describe how they deform, flow or rupture under applied stress and could be used as a tool in the selection and specification of appropriate raw materials. Knowledge of the fundamental rheological properties of any dough can be an indication of how the dough is going to behave under various processing conditions. They are of importance in terms of product formulation and optimization, quality control, machining properties of the dough, scale-up of the process, and automation. Dynamic Rheometry Dynamic rheometry gives information on the flow and elastic properties of materials and has been widely used to elucidate the rheological characteristics of food materials including doughs. In a dynamic measurement, the sample is usually put between two round plates or between a cone and a plate. The system is maintained at desired temperature, Table 3 Mineral composition of sorghum (mg %)a. Grain Sorghum a Number of cultivars P Mg Ca Fe Zn Cu Mn Mo Cr 6 352 171 15 4.2 2.5 0.44 1.15 0.06 0.017 Expressed on a dry-weight basis. Source: Sankararo and Deosthale.[108] Downloaded By: [Canadian Research Knowledge Network] At: 20:27 22 June 2010 NUTRITIONAL AND RHEOLOGICAL PROPERTIES OF SORGHUM 61 and a sinusoidal deformation at different frequencies is applied. As a result, we get storage and loss modulus as a function of frequency. Storage modulus, which represents the energy stored during deformation, is related to the elastic energy of the sample, while the loss modulus, which represents the energy lost during deformation, is related to the viscose energy. The measurement of the storage modulus (G’) and the loss modulus (G”), together with the phase angle (δ), could provide a good indication of the stiffness and extensibility of the dough. A high value of G’ and a low G’’ indicates a stiff dough, while a lower G’ indicates a softer and more extensible dough.[60] The loss tangent (G”/G’) represents the samples ability to dissipate energy and provides a measurement of the ratio of the viscous to elastic response of the material being tested. The complex modulus (G*) is usually calculated as a function of frequency using these parameters. It is a known fact that gluten proteins play a principal role in the rheological properties of wheat flour doughs.[61] However, the properties of dough from a gluten-free cereal like sorghum are more fluid than wheat dough. The rheological properties of sorghum dough have not favored its being utilized popularly as a source for bread-making. Hart et al.[62] have reported a lack of consistency and elasticity in sorghum doughs at lower moisture contents; while increasing the moisture content only lead to a batter like consistency. The dough broke apart easily and its properties did not improve upon kneading either. Addition of compressed yeast lead to an improvement in dough rise, but the loaved collapsed during baking. It was determined that addition of gum and starch lead to an improvement in the performance of the dough yielding breads of acceptable quality. Chandrashekar and Desikachar[63] analyzed the rolling quality of sorghum dough in relation to some of its physiochemical properties. They have correlated good rolling quality with lower gelatinization temperature, higher peak viscosities and set backs as determined by a Brabender Viscograph. Lower gelatinization temperatures lead to greater degree of gelatinization, resulting in better adhesive doughs, which would be easily rolled. Higher water uptake at 70°C was correlated to starch damage in flour and the amylose, protein, and prolamine content had no relation to the rolling quality, which was determined subjectively. Equal quantities of flour and boiling water for mixing have been recommended. In order to overcome the problems associated with a lack of gluten, sorghum-based composite flours have been evaluated in bread making over the years.[64–66] Bread baked with 0, 5, 10, and 15% wheat-sorghum composite flour showed good volume, good external and internal characteristics, and a high percentage of acceptability.[67] However, in all these studies, addition of sorghum flour to wheat flours at higher levels lead to poor dough rheological properties. The water absorption and extensibility of wheat dough decreased on addition of sorghum flour and it also resulted in lesser loaf volume and weight of bread.[68] The addition of wheat flour to sorghum flour improved the dough rheological properties.[69,70] The rheological characteristics and breadmaking quality of sorghum composite flours can also be improved by the addition of exogenous gluten proteins,[71] zein,[64] or cysteine.[72] Torres et. al.[73] have demonstrated an increase in the maximum stress peak, stress during relaxation period, and dough viscosity when sorghum flour was used in 70:30 wheat-sorghum composite dough. Sorghum flour with smaller particle size distribution had greater water absorption and stress during relaxation. The rheological properties were evaluated by uniaxial compression tests. The chemical, sensory, and rheological properties of porridges made from blends of sprouted sorghum, bambara groundnuts, and fermented sweet potatoes were examined.[74] The composite flours were found to have higher levels of lipids, protein, ash, crude fiber, and minerals compared to the traditional sorghum complementary food. The porridges from the composite flours were 62 KULAMARVA, SOSLE, AND RAGHAVAN about seven times less viscous than the porridge from the traditional sorghum complementary food and generally acceptable in the sensory evaluation. Downloaded By: [Canadian Research Knowledge Network] At: 20:27 22 June 2010 Textural Properties and Texture Profile Analysis The rheological properties of dough in relation to its texture measurements could help understand the behavior of dough during processing. The texture profile analysis (TPA) has been used for the textural evaluation of a wide range of foods. Bourne[75] has demonstrated a method to evaluate texture profile parameters from the force-deformation curves obtained by the Instron Universal Testing Machine (UTM). The food sample is compressed twice, successively, between two parallel plates and the force-time curves are plotted. Some of the textural parameters derived from these curves are defined[76] as follows: Hardness: the peak force during the first compression cycle. Cohesiveness: the ratio of the positive force area during the second compression to that during the first compression. Adhesiveness: the negative force area for the first compression, representing the work necessary to pull the compressing plunger away from the sample. Springiness or elasticity: the height that the food recovers during the time that elapses between the end of the first compression, and the start of the second compression. Gumminess: the product of hardness and cohesiveness. Chewiness: the product of gumminess and springiness. Mechanical and textural properties of foods have been determined from the stressstrain relationships obtained through uniaxial compression tests.[77] The elastic and viscous properties as functions of time and strain are necessary to characterize dough. The results of the uniaxial compression procedure have also been interpreted in terms of Apparent Biaxial Extensional Viscosity (ABEV) for application in flattening, sheeting and rolling of doughs.[78] Diehl[79] has reviewed the rheological techniques for texture and quality measurement of solid and liquid food products made from sorghum. The effect of incorporating different types of cereal flours on the viscoelastic properties of black gram dough in relation to the sensory textural attributes has been studied by using the uniaxial compression method.[80] Higher resistance to compression when sorghum flour was added to black gram flour has been reported. The Instron Universal Testing Machine was used in this study to determine the tensile properties of chapaties prepared from composite flours. Stiffness, breaking strength, and deformation of rectangular strips of chapaties were measured by a tensile test performed using an Instron.[81] Kernels with waxy endosperm had lower strength and stiffness and a greater deformation to breakage than non-waxy types. The tensile test has been used to show that sorghum varieties with a corneous endosperm texture yielded stronger tortillas than those with an intermediate endosperm.[82] A method using a back extrusion cell mounted on an Instron was used to measure textural characteristics of sorghum dough,[83] and it was found that the force and energy required for the extrusion were higher in case of good quality cohesive dough. The influence of flour-water-soluble components on cohesiveness of dough from different varieties of sorghum has also been discussed. The optimum quantity of water required to make acceptable doughs varied among the varieties studied. The varieties were rated on the basis of their rollability into roties as well. Stickiness of a food product depends on both the cohesive forces in the food and the adhesive forces between the food and with whatever it comes into contact.[84] By pulling two parallel plates apart at a constant rate, a measure of stickiness could be obtained. Grain hardness has a positive correlation with the texture and the amylase content with overall acceptability of chapattis made from sorghum flour.[85] The dough hardness was NUTRITIONAL AND RHEOLOGICAL PROPERTIES OF SORGHUM 63 Downloaded By: [Canadian Research Knowledge Network] At: 20:27 22 June 2010 recorded using the Instron Testing Machine. The peak force to compress dough samples to 75% of original thickness was considered as the hardness. The results confirmed the role of water soluble components in determining dough hardness. A higher yield of desirable sorghum couscous granules was reported when flours from hard grain sorghum were used.[86] The textural characteristics of cooked sorghum grain were also determined by texture profile analysis and rapid visco analyzer measurements.[87] The kernel size had no consistent effect on the visco analyzer measurements (peak viscosity, holding strength, and final viscosity) while a larger kernel size yielded a better texture profile in terms of higher values for hardness, less sticky and more cohesive product. The effect of raw and gelatinized sorghum flours on the structure and texture of baked corn and tortilla chips have been studied,[88] and a significant effect has been reported. Thermophysical Properties The physico-chemical properties of the starch affect the textural characteristics of the food preparations made from grain. The behavior of starch in water is temperature and concentration dependent.[89] Grain starches in general show very little uptake of water at room temperature and their swelling power is also small. At higher temperatures, water uptake increases and starch granules collapse, which leads to solubilization of amylose and amylopectin to form a colloidal solution. This is the gelatinization stage. Heat treatment of starch in a limited amount of water leads to swelling of the granules with very little loss of soluble material and partial gelatinization of the starch. On cooking, the gelatinized starch tends to return from the soluble, dispersed and amorphous state to an insoluble crystalline state. This phenomenon is known as retrogradation or setback; it is enhanced with low temperature and high concentration of starch. Amylose, the linear component of the starch, is more susceptible to retrogradation. The gelatinization temperature of isolated sorghum starch and that of finely ground flour of the corresponding endosperm has been reported to be the same. On the other hand, the pasting temperature—i.e., the temperature at which starch attains peak viscosity when heated with water to form a paste—was found to be about 10°C higher for the sorghum flour than for the isolated starch.[90] Zhang and Hamaker[91] have reported the interactions between sorghum starch, protein and free fatty acids by analyzing the paste viscosity profile in a Rapid ViscoAnalyzer. The starch gelatinization range of sorghum (68–78°C) is higher than that of wheat (58–64°C).[92,93] This factor along with a low water holding capacity have been correlated with grittiness, dry mouthfeel, and higher firming ratio of sorghum composite breads.[94] The effect of malting of sorghum and wet-heat treating the malt in order to reduce the pasting temperature and increase the water holding capacity of sorghum flour so as to yield softer bread more resistant to crumbing when used with wheat flour has been evaluated.[95] The functionality of grain sorghum components in relationship to those of wheat flour in a high ratio cake, a product dependent on starch for the main structural component has been investigated.[96] Lowering the high gelatinization temperature of the sorghum starch by replacing sucrose with dextrose was found to greatly improve cake volume and texture. Suhendro et. al.[97] have worked on the cooking characteristics of sorghum-flour noodles and reported better results when the flour water mixture was preheated. They have also stated that starch gelatinization occurred to a higher degree in finer flours than coarse Downloaded By: [Canadian Research Knowledge Network] At: 20:27 22 June 2010 64 KULAMARVA, SOSLE, AND RAGHAVAN flours and yielded better noodles. Steam pre-treatment of the flour has been suggested to induce faster gelatinization. It has been suggested that the cooking characteristics of sorghum maybe influenced by the relative proportions of amylase and amylopectin present. The quality of cooked sorghum has been strongly associated with the total and soluble amylose content grain and also the soluble protein content.[98] The availability of gelatinized starch and the amount of amylopectin and amylase affect the functionality of dough used for baked snacks. Increasing the free amylopectin content has been shown to yield softer, cohesive dough and provides film formation, sheet extensibility and better puffing when heated.[99] The swelling power of starch and its solubility significantly influenced the cooking quality of sorghum.[100] The percentage weight increase of cooked grain was negatively correlated with starch solubility at 60°C, a temperature at which most of the starch granules will have reached gelatinization stage. The swelling power of starch at 60° and 90°C and solubility at 25° and 50°C were inversely correlated with gruel solid content, which directly depended on the starch content of the grain. The starch gelatinization temperature did not show any significant effect on the cooking quality of sorghum. Plasticity of sorghum flour dough mostly arises from the gelatinization of starch when the dough is prepared in hot or boiling water. The stickiness of the cooked flour is a function of the starch gelatinization. Porridge prepared from hard endosperm of sorghum is less sticky than that prepared from grains with a larger proportion of floury endosperm.[101] Dough prepared with cold water has poor adhesiveness and is difficult to roll thin. Thus, heat modification of the starch, when the dough is prepared with hot water, determines its rolling properties.[102] The hydration capacity of the grain is an important factor that affects the cooking quality and sensory attributes of sorghum products. Higher water uptake, low gelatinization temperature, high peak paste viscosity, and high setback are the starch properties that have been shown to be associated with good quality of roti, which is the most common form in which sorghum is consumed on the Indian subcontinent. Lowamylose or waxy sorghum produced sticky dough (masa) and was not suitable for preparation of tortillas.[103] The differential scanning calorimeter (DSC) has been used as an effective tool in determining the thermal properties of a wide range of food products. A quantitative measure of phenomena like gelatinization and glass transition is provided based on the heat flow associated with order-disorder transitions.[104] Abundant research has been done on the use of the DSC for measuring thermal characteristics of dough and flour samples. Akingbala et. al.[93] elucidated the gelatinization temperature of starches isolated from 24 non waxy varieties of sorghum using a Differential Scanning Calorimeter and have reported the onset, peak, and end gelatinization temperatures to be 71.0 + 1.0, 75.6 + 0.9, and 81.1 + 1.1°C, respectively, and the gelatinization energies to be in the range of 2.51 to 3.96 cal/g. They found no significant relationship between the thermal properties and grain characteristics like endosperm texture and type, pericarp thickness and color, kernel mass, and grain density. No significant relationship between the thermal properties of sorghum and physicochemical properties of their starches was reported.[90,93] Future Perspectives The importance of sorghum in terms of its nutritional significance and agronomic advantages especially with reference to developing countries in the semi-arid tropics is immense. Sorghum is less favored in developed countries owing to its lower nutritive NUTRITIONAL AND RHEOLOGICAL PROPERTIES OF SORGHUM 65 value compared to other cereals. Although the absence of gluten makes it highly favorable in the diet of gluten-intolerant populations, it also leads to poor rheological properties. 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