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Starch is the major storage form of carbohydrate in sorghum and millets. It consists of amylopectin, a branched-chain polymer of glucose, and amy-lose, a straight-chain polymer.
The digestibility of the starch, which depends on hydrolysis by pancreatic enzymes, determines the available energy content of cereal grain. 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 (McNeil! et al., 1975; Harbers, 1975).
The physico-chemical properties of the starch affect the textural characteristics of the food preparations made from grain. The behaviour of starch in water is temperature and concentration dependent (Whistler and Paschall, 1967). Grain starches in general show very little uptake of water at room temperature, and their swelling power is also small. At higher temperature 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. Genetic and environmental factors affect the gelatinization temperature of grain starch (Freeman, Kramer and Watson, 1968). 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. Some characteristics of sorghum and millet starches are presented in Table 19; soluble sugar composition and total sugar content are given in Table 20.
TABLE 19: Characteristics of isolated starches of sorghum and millets
|Grain||Amylose (%)||Gelatinization temperature (°C)||Water-
Binding capacity (%)
|Swelling at 90³C (%)||Solubility at 90°C (%)||
at 95 °C
|Cooled to 35° or 50°C||After
at 35°C or 50°C
|Proso millet||28.2||56.1||61.2||108.0||12.0||6.89||688||520||826||1 203|
|Foxtail millet(a)||-||53.5||59.5||128.5||11.2||4.65||840||620||1 100||1 220|
|Foxtail millet (b)||17.5||55.0||62.0||-||9.8||4.80||1 780||1 540||2000||-|
|Finger millet||16.0||64.3||68.3||-||11.4||6.50||1 633||1 286||1 796||-|
a Peak viscosity achieved at 83.5°C.
Sources: Rooney and Serna-Saldivar. 1991: Leach. 1965: Horan and Heider. 1946: Subramanian et al. . 1982; Beleia. Varriano-Marston and Hoseney, 1980: Yanez and Walker. 1986: Lorenz and Hinze. 1976: Wankhede. Shehnaj and Raghavendra Rao. 1979b: Paramahans and Taranathan. 1980.
TABLE 20: Soluble sugar composition of sorghum and millets (g per 100 g, dry-matter basis)
|Grain||Number of cultivars||Total sugar||Sucrose||Glucose + fructose||Raffinose||Stachyose|
|Sorghum, normal (a)||10||2.25||1.68||0.25||0.23||0.10|
|Sorghum, normal (b)||-||1.34||0.61||0.52||0.15||0.06|
|Sorghum, high Iysine||-||2.57||0.94||1.13||0.39||0.11|
Sources: Subramanian. Jambunathan and Suryaprakash. 1980: Murty et al . 1985: Subramanian. Jambunathan and Suryaprakash, 1981: Wankhede, Shehnaj and Raghavendra Rao. 1979a: Becker and Lorenz. 1978.
With values ranging from 56 to 73 percent, the average starch content of sorghum is 69.5 percent (Jambunathan and Subramanian, 1988). About 70 to 80 percent of the sorghum starch is amylopectin and the remaining 20 to 30 percent is amylose (Deatherage, McMasters and Rist, 1955). Both genetic and environmental factors affect the amylose content of sorghum (Ring, Akingbala and Rooney, 1982). Waxy or glutenous sorghum varieties are very low in amylose; their starch is practically 100 percent amylopectin (Ring, Akingbala and Rooney, 1982; Deatherage, McMasters and Rist, 1955). But in sugary sorghum the amylose content of the starch is about 5 to 15 percent higher than in normal sorghum (Singh and Axtell, 1973b). The total carbohydrate content of sugary sorghum is normal, however, since it contains exceptionally high levels of water-soluble polysaccharides (29.1 percent)
The digestibility of isolated starch of sorghum cultivars ranged from 33 to 48 percent as against 53 to 58 percent for corn starches (Sikabbubba, 1989). The texture of the grain endosperm, the particle size of the flour and starch digestibility were found to be strongly correlated with each other. Starch in floury sorghum was found to be more digestible than that in corneous sorghum. Particles of ground floury sorghum were smaller than those of similarly ground corneous sorghum. The smaller particle size and correspondingly greater surface area facilitate the enzyme action and thus improve starch digestibility.
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, corn and pearl millet grains (Hibberd et al., 1982). Feeding trials in rats (Elmalik et al., 1986) and other animal species (Sherrod, Albin and Furr, 1969; Nishimuta, Sherrod and Furr, 1969) have confirmed the superiority of waxy sorghum over normal grain types in terms of dry matter and gross energy digestibility.
The presence of tannins in the grain contributes to the poor digestibility of starch in some varieties of sorghum (Dreher, Dreher and Berry, 1984). Tannins isolated from sorghum grain were shown to inhibit the enzyme X-amylose, and they also bind to grain starches to varying degrees (Davis and Hoseney, 1979).
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.
The quality of cooked sorghum has been strongly associated with the total and soluble amylose content of the grain and also the soluble protein content (Cagampang and Kirleis, 1984). The swelling power of starch and its solubility significantly influenced the cooking quality of sorghum (Subramanian et al., 1982). 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 (Cagampang, Griffith and Kirleis, 1982).
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 (Desikachar and Chandrashekar, 1982). 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, the unleavened bread that is the most common form in which sorghum and pearl millet are consumed on the Indian subcontinent. On the other hand, for stiff porridges such as Indian mudde or sankhati and African tô, the desirable characteristics of the grain starch are high gelatinization temperature, low peak paste viscosity and low retrogradation tendency. In other words, the starch characteristics for goodquality roti were found to be exactly opposite to those desirable for goodquality porridge. Thus sorghum varieties that are not suitable for roti may be suitable for porridge. Almeida-Dominguez, Serna-Saldivar and Rooney (1991) found that low-amylose or waxy sorghum produced sticky dough (masa) and was not suitable for preparation of tortillas.
In different pearl millet genotypes the starch content of the grain varied from 62.8 to 70.5 percent, soluble sugar from 1.2 to 2.6 percent and amylose from 21.9 to 28.8 percent (Jambunathan and Subramanian, 1988). Lower values for starch (56.3 to 63.7 percent) and amylose (18.3 to 24.6 percent) have been found in some high-yielding Indian pearl millet varieties (Singh and Popli, 1973). Subramanian, Jambunathan and Suryaprakash (1981) found that the predominant component of total soluble sugar (2.16 to 2.78 percent ) was sucrose (66 percent), followed by raffinose (28 percent). Other sugars detected in measurable amounts were stachyose, glucose and fructose. The proportion of sucrose in total sugar was lower in pearl millet than in sorghum.
Pasting properties of pearl millet starch were generally similar to those of sorghum except when it was held for one hour at 95°C (Bad), Hoseney and Finney, 1976). Beleia, Varriano-Marston and Hoseney (1980) considered inherent molecular dissimilarities the primary factor in physico-chemical differences among five pearl millet starches examined. The amylose content of these starches varied within a narrow range (22 to 24 percent). Variation in the water-binding capacity (83.6 to 99.5 percent) was probably due to differences in the proportions of amorphous and crystalline starch in the granule; amorphous starch has greater water absorption capacity than crystalline starch. In the five starches, the initial gelatinization temperature ranged from 59° to 63°C, the mid-point from 65° to 67.5°C and the final gelatinization temperature from 68° to 70°C. The gelatinization of pearl millet starch occurred at a lower temperature than that of sorghum starch (Table 19). In general it was observed that starches having low solubility and swelling below 75°C showed greater solubility and swelling at and above 80°C. The peak pasting temperature of the five starches was the same, 76.5°C. Differences in paste viscosity were larger in magnitude after one hour's holding at 95°C and during the cooling cycle. This showed that some starches tended to retrograde more than others.
The peak paste viscosity of pearl millet flour starch was much lower than that of sorghum starch (Bad), Hoseney and Finney, 1976). Pearl millet was shown to have very high amylase activity, about ten times higher than that of wheat grain (Sheorain and Wagle, 1973), and this was probably responsible for the low peak viscosity observed. It is of interest that amylase of pearl millet was observed to be more active against wheat starch than against the starch from pearl millet grain itself (Beleia and Varriano-Marston, 1981 a,b). This observation is of great practical importance. Bread prepared from wheat flour blended with 10 percent pearl millet flour had better loaf volume than standard bread prepared from wheat flour containing malt and sugar (Bad), Hoseney and Finney, 1976). Thus pearl millet flour used in partial replacement of wheat flour can be successfully substituted for malt and sugar in the preparation of bakery products such as bread, biscuits and pasta. Subramanian, Jambunathan and Ramaiah (1986) observed that the quality of unleavened bread (roti) prepared from pearl millet flours was influenced by swelling capacity, water-soluble flour fraction, water-soluble protein and amylose content of the flour. The swelling capacity of the flour was highly and positively correlated with all the sensory dualities of roti, namely colour, texture, odour, taste and acceptability. On the other hand, the amylose content and water-soluble flour fraction were negatively correlated with all these characteristics.
In high-yielding varieties of finger millet analyzed by Wankhede, Shehnaj and Raghavendra Rao (1979a), mean starch content was 60.3 (59.5 to 61.25) percent; pentosan 6.6 (6.2 to 7.2) percent; cellulose 1.6 (1.4 to 1.8) percent, lignin 0.28 (0.04 to 0.6) percent; and free sugar 0.65 (0.59 to 0.69) percent. Sucrose (33 percent) glucose and fructose (each 12 percent) and maltose and raffinose (10 percent each) were the major components of the free sugar of finger millet. The amylose content of the starch in finger millet was 16 percent (Wankhede, Shehnaj and Raghavendra Rao, 1979b), which is lower than the values in normal sorghum and other millets. The swelling capacity and solubility in water at 90°C of the isolated starch of finger millet were lower than for sorghum and similar to those of other millet starches. The high peak viscosity and the increase in viscosity on cooling suggested a strong tendency of the starch to undergo retrogradation. The paste viscosity is reduced and the nutrient density, particularly energy density, is enhanced after malting of the grain, and on this basis weaning food containing 70 parts of malted finger millet and 30 parts of dehulled green gram has been developed (Malleshi and Desikachar, 1982).
Foxtail and prove millet have been reported to have both glutenous and nonglutenous endosperm types, while only the non-glutenous type of endosperm is reported to be present in finger and barnyard millets (Tomita et al., 1981). The starch in two foxtail millet varieties was 100 percent amylopectin. Starches of foxtail, prove and barnyard millets were more digestible than maize starch in terms of in vitro amylolysis by pancreatic amylase. The glutenous starches were more digestible than non-glutenous types as in other cereal grains.
The increase in paste viscosity on cooling to 35°C and the further rise after one hour's holding at that temperature indicated the strong tendency of these millet starches to undergo retrogradation. One of the prove varieties, namely Big red prove, was exceptional in that its starch had higher water-binding capacity and gelatinization temperature than that of five other varieties.
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