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Functional and rheological properties of Bambara groundnut starch-catechin complex obtained by chemical grafting
The aim of this study was to produce Bambara groundnut (BGN) starch-catechin complex using chemical initiators (ascorbic acid and hydrogen peroxide) and cyclodextrin (alpha and beta) with the view to obtain a functional ingredient for the food industry. BGN starch was successfully extracted from BGN flour through dry milling method, yielding 32% of BGN starch. Native BGN starch was chemically modified using ascorbic acid (1% w/w) and hydrogen peroxide (165% w/w) as redox, biocompatible initiator for grafting catechin to the BGN starch. In addition, cyclodextrin (alpha and beta) were also used as initiators for modifying BGN starch through complexation methods. Complexation methods used included the microwave, co-evaporation and kneading. The characterization of native and modified BGN starches was carried out by performing scanning electron microscopy, powder X-ray diffraction, Fourier Transform Infrared (FTIR) and fluorescence spectroscopy analysis. Functional, thermal and rheological properties of native and modified BGN starches were evaluated. The pasting properties of BGN starches were determined using the Rapid Visco Analyser (RVA). According to the SEM profile, native BGN starch had round, oval and elliptical shapes typical for legume starches. Native BGN starch displayed a typical type-C crystallinity which is common among legumes with strong peaks at 2θ of 15o, 17o and 23o. BGN starches modified through complexation methods had sharper peaks indicating an increase in starch crystallinity; however, following chemical modification there was loss in starch crystallinity which was evidenced by the amorphous region in the chemically modified BGN starches. Structure of native and modified BGN starches was confirmed by FTIR. The FTIR spectra of native BGN starch showed variable peaks at 3285.34 cm-1, 2931.69 cm-1, 1634.36 cm-1, 1336.77 cm-1 which are attributed to OH stretching, C-H stretching, water bending vibrations and C-O stretching, respectively. Furthermore, the FTIR results confirmed that native BGN starch is made up of glucose molecules just like all other starches. All modified BGN starches displayed a new absorption peak at 1020 cm-1 wavelength, thus indicating that starch modification was successful. On the other hand, all BGN starch-catechin complexes displayed a new absorption peak in the range of 1520 -1560 cm-1, attributed to the C-C stretching within the aromatic ring of the catechin. The successful grafting of catechin to BGN starch was also confirmed by the fluorescence spectroscopy results, where all the BGN starch-catechin complexes had an emission peak at 320 nm while native BGN starch had an emission peak at 270 nm. Antioxidant capacity of BGN starch was determined through DPPH and ORAC antioxidant assays. Within the DPPH assay, the antioxidant activity ranged from 2.26 to 38.31 μmol TE/g. The antioxidant activity of modified BGN starch-catechin complexes was significantly (p ≤ 0.05) higher than the ones modified without catechin. On the other hand, within the ORAC assay, the antioxidant activity ranged from 0.07 to 126.71 μmol TE/g. As opposed to the results obtained in DPPH assay, the antioxidant activity of chemically modified BGN starch-catechin complexes was significantly (p ≤ 0.05) higher than that of complexed BGN starch-catechin complexes. Chemical modification significantly increased the swelling capacity of native BGN starch while complexation methods significantly reduced it.