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Finger millet (Eleusine coracana) polyphenols and their nutraceutical potential

Chethan, S. (2008) Finger millet (Eleusine coracana) polyphenols and their nutraceutical potential. PhD thesis, University of Mysore.

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Abstract

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 ii 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 iii 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. iv 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.

Item Type: Thesis (PhD)
Uncontrolled Keywords: Finger millet, millet polyphenols, antimicrobial activity, nutraceuticals
Subjects: 600 Technology > 08 Food technology > 21 Cereals > 05 Ragi (Finger Millet)
600 Technology > 08 Food technology > 18 Processed foods > 04 Functional foods
Divisions: Grain Science and Technology
Depositing User: Food Sci. & Technol. Information Services
Date Deposited: 24 Mar 2011 11:13
Last Modified: 28 Dec 2011 10:22
URI: http://ir.cftri.com/id/eprint/10003

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