Finger millet (Eleusine coracana) polyphenols and their nutraceutical potential

Finger millet is one of the minor cereals and is known for its health benefits
and extremely good storage quality. These special features of the millet are generally
attributed to its polyphenols content. However, the scientific information to
substantiate theses attributes and also the information on the nature of the millet
polyphenols is scanty. Hence, R & D work towards isolation and characterization of
the millet polyphenols and its nutraceutical potential were undertaken, and the salient
features of the work presented in the thesis are as follows;
The scientific and technological information on cereals in general and finger
millet in particular with special reference to polyphenols, published in peer reviewed
scientific journals, proceedings of the scientific conferences, book chapters and
popular articles are presented in chapter I.
The investigations on the extraction, purification, fractionation and
characterization of the millet polyphenols are dealt in chapter II. Since, nearly 70 % of
the millet polyphenols are concentrated in the seed coat tissue, a special milling
protocol was developed to prepare the seed coat rich fraction of the millet and the
same was used for the studies. Various polar and non-polar solvents were tried for
extraction of the polyphenols and HCl–methanol (1:99 v/v) was identified as very
effective extractant. The polyphenols of the acidic methanol extract were sensitive to
the changes in pH and precipitated as well as denatured at alkaline conditions.
Fractionation of the extracted polyphenols by high performance liquid
chromatography (HPLC) showed a large number of constituent phenolics but only a
few of them namely, gallic, proto-catechuic, p-hydroxy benzoic, p-coumaric, syringic,
ferulic and trans-cinnamic acids and quercetin a flavonoid were identified. The
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identity of the component phenolics was confirmed by LC-MS and also by NMR
spectra. The information generated may facilitate isolation of specific phenolics of
pharmaconutritional importance.
The nature of inhibition of the millet polyphenols on some of the plant and
animal enzymes is reported in chapter III. Finger millet is extensively processed for
preparation of malt, and the malt amylases play a major role with respect to the
functional properties of malt based foods. Hence, the effect of millet polyphenols on
the nature of inhibition on its amylases was investigated by the kinetic studies using
Michaelis-Menten and Lineweaver-Burk equations. It was observed that the Km
remained constant (0.625 %), but the maximum velocity (Vmax) decreased in the
presence of a crude polyphenols extract, indicating mixed non-competitive type of
inhibition. In contrast to that, some of the constituent phenolics, namely, gallic acid,
vanillic acid, quercetin and trans-cinnamic acid isolated from the polyphenols extract
showed uncompetitive inhibition. Gallic acid and quercetin exhibited highest affinity
whereas trans-cinnamic acid showed lower affinity for the enzyme-substrate complex
with Ki¢ of 4.6×10-7 M and 7.3×10-7 M, respectively. The studies indicated the
presence of secondary binding sites in malted finger millet amylases similar to other
cereal amylases. These observations add to the basic science of the finger millet
enzymes and its phytochemicals besides may help for the usage of millet as a source
of amylase or as amylase rich food (ARF), in various food formulations.
Normally diabetes induced cataract is characterized by accumulation of polyol
which is mediated by the up regulation of a key enzyme aldose reductase (AR). Since,
the whole meal millet is consumed, it was of interest to know the positive role of the
millet polyphenols in minimizing the incidence as well as the intensity of
cataractogenesis. Hence, the crude polyphenols of the millet and also some of the
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individual phenolics isolated from it were evaluated for their AR inhibitory activity.
Out of the phenolics tested, quercetin was more potent inhibitor with IC50 of 14.8 hM.
The inhibition of AR was non-competitive. Results thus provide a stronger evidence
for the potential of the millet polyphenols in minimizing the cataractogenesis in
human eye lenses.
Exploratory investigations were conducted on the inhibitory activity of millet
polyphenols on snake venom PLA2, and it was observed that, the crude polyphenols
extract and also the gallic acid as well as quercetin isolated from that, exhibited
considerable inhibition with IC50 values of 83, 62 and 17 μg/ml, respectively.
The antioxidant and the antimicrobial activity of the millet polyphenols were
assayed and the same is described in chapter IV. The IC50 value for the antioxidant
potential of the millet phenolics on DPPH, hydroxyl and ABTS radical scavenging
activity were 62.7, 109 and 114 μg/ml, respectively. Gallic, caffeic, ferulic, protocatechuic
and p-hydroxy benzoic acids isolated from the crude extract of the millet
polyphenols exhibited inhibitory activity against K. pneumoniae, Y. enterocolitica, S.
pyogenes, P. mirabilis and S. marcescens however, the degree of inhibition varied
among the phenolics. Quercetin, inhibited the growth of all the pathogenic microbes
tested even at a very low concentration. The antioxidant and also the antimicrobial
activity of the polyphenols were dose dependent. These observations revealed that,
the millet polyphenols may serve as natural antioxidants as well as antimicrobial
agents, useful as a nutraceutical for reducing the intensity of some of the diseases
arising out of oxidative deterioration in humans if not preventing them, and also for
enhancing the safety of foods from microbial spoilage.
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From the studies it may be concluded that, finger millet seed coat is a rich
source of polyphenols, and acidic methanol is very effective solvent for extraction of
polyphenols. The millet polyphenols are complex mixture of several phenolics acids,
out of which only a few could be identified. The millet polyphenols inhibit the millet
malt amylases and aldose reductase from human eye lenses, the inhibition being
uncompetitive and noncompetitive, respectively. The polyphenols exhibit antioxidant
and antimicrobial activity also. Thus, the seed coat matter of the millet as such could
be utilized for biofortification of various foods as a source of antioxidant rich
ingredient or the polyphenols isolated from the millet may find application as a source
of nutraceuticals.