Potential for sorghum and pearl millet for extruded products

Ragini Chaudhary, Scientific and Digital Systems, New Delhi, India
Bronwyn Elliott, Perten Instruments Australia P/L, Warriewood, Australia

There is interest in producing extruded food products based on pearl millet and sorghum, and with added grain legumes: pigeon pea, chickpea and groundnut. Dehulling sorghum and pearl millet improves the taste and eating quality.

Pearl millet is well adapted to growing areas characterized by drought, low soil fertility and high temperature. It performs well in soils with high salinity or low pH. Because of its tolerance to difficult growing conditions, it can be grown in areas where other cereal crops, such as maize or wheat, would not survive. The largest producer of pearl millet is India where it is known as bajra.

Sorghum is drought and heat tolerant and is especially important in arid regions. It has high levels of iron and zinc and is an important food crop in India. Grain legumes such as chickpea (chana), pigeon pea (toor dal) and groundnut (peanut) are important complements to cereals in dryland farming systems and in human diets, improving the nutritional value of the diet due to their high nutrient and protein contents.

Aim

To characterize the RVA pasting characteristics of pearl millet, sorghum and mixtures containing pigeon pea, chickpea and groundnut to evaluate their potential for extrusion.

Materials and Methods

Nine flour samples were tested on the RVA:

1. whole sorghum, 100%
2. dehulled sorghum, 100%
3. dehulled sorghum 70% + chickpea 30%
4. dehulled sorghum 70% + pigeon pea 30%
5. whole pearl millet, 100%
6. dehulled pearl millet, 100%
7. dehulled pearl millet 70% + pigeon pea 30%
8. dehulled pearl millet 70% + chickpea 30%
9. dehulled pearl millet + chickpea + groundnut

3.5g of sample and 25g of distilled water was weighed. The following RVA profile was used:

00:00:00 Temperature 50ºC
00:00:00 Speed 960 rpm
00:00:10 Speed 160 rpm
00:01:00 Temperature 50ºC
00:04:42 Temperature 95ºC
00:07:12 Temperature 95ºC
00:11:00 Temperature 50ºC
00:13:00 End

 

Results

Sorghum

Pearl Millet

Results and Discussion

Sorghum and pearl millet are coarse grains with similarities to corn. Their starches all gelatinize in a similar temperature range. Sorghum starch is similar to corn starch in size, shape and gelatinization properties. Pearl millet starch is also similar to that of corn, but with smaller granules.

Sorghum samples have higher RVA pasting temperatures and resisted shear thinning more than the pearl millet samples. This is most likely due to larger particle size as sorghum starch peak would generally be expected to be bigger than that for pearl millet. Other contributors may be elevated amylose content and starch-lipid-protein interactions.


The RVA graph indicates that the sorghum samples would be more suited to the harsh conditions of extrusion processing. Addition of chickpea and pigeon pea lowers the curve as they have lower starch content than sorghum and pearl millet. Addition of groundnut elevated both peak and final viscosity, with evidence of a second high viscosity peak in the cooling stage attributed to a three-way interaction between protein, starch and free fatty acid (FFA)1. Similar FFA effects have been reported for sorghum and corn.

References

1. Genyi Zhang and Hamaker, Bruce R (2003). ‘A three component interaction among starch, protein and free fatty acids revealed by pasting profiles’. J. Agric. Food Chem. 51, pp. 2797–2800.