Characterization of Physicochemical and Micromeritics Properties of Carboxymethylated Starches Derived from Vigna unguiculata Seeds and Pennisetum glaucum Grains with an Intermediate Degree of Substitution

Conventional commercial sources of starch for industrial applications are mainly based on staple foods, due to the worsening economic situation, there is a need to explore other underutilized unconventional sources. This study aims to extract, modify, and characterize starch from unconventional sources: cowpea (Vigna unguiculata) seeds and pearl millet (Pennisetum glaucum) grains. Starch was extracted from cowpea seeds and pearl millet grains by milling and precipitation in sodium hydroxide solution and water respectively. Carboxymethylation of the starch was carried out using monochloroacetic acid. The native starch, modified starches, and commercial brand-sodium starch glycolate were subjected to physicochemical and micromeritic characterization as well as spectroscopic and thermal analysis. The starch yields of 16.01 and 46.60% were obtained from cowpea seeds and pearl millet grains respectively. Starch samples with an intermediate degree of substitution (0.52-0.60) were obtained from both sources. The starch samples complied with British Pharmacopoeia specification and carboxymethylation generally resulted in improved properties (reduced gelatinization temperature, increased swelling capacity and flow properties). The pearl millet was made of oval-shaped granules with higher starch yield, reduced moisture content, and improved flow than the owpea starch, however, the latter contained cuboid-shaped granules, had higher densities, and reduced redispersion time. Carboxymethyl functional group was introduced, and the structure of starch was intact upon modification. Carboxymethyl starches obtained from cowpea seeds and pearl millet grains have desirable properties and can be potential sources of low-cost excipients for pharmaceutical formulations.