Flavonoids, a group of natural substances with variable phenolic structures, are found in fruits, vegetables, grains, bark, roots, stems, flowers, tea and wine. These natural products are well known for their beneficial effects on health and efforts are being made to isolate the ingredients so called flavonoids. Flavonoids are now considered as an indispensable component in a variety of nutraceutical, pharmaceutical, medicinal and cosmetic applications. This is attributed to their anti-oxidative, anti-inflammatory, anti-mutagenic and anti-carcinogenic properties coupled with their capacity to modulate key cellular enzyme function. Research on flavonoids received an added impulse with the discovery of the low cardiovascular mortality rate and also prevention of CHD. Information on the working mechanisms of flavonoids is still not understood properly. However, it has widely been known for centuries that derivatives of plant origin possess a broad spectrum of biological activity. Current trends of research and development activities on flavonoids relate to isolation, identification, characterisation and functions of flavonoids and finally their applications on health benefits. Molecular docking and knowledge of bioinformatics are also being used to predict potential applications and manufacturing by industry. In the present review, attempts have been made to discuss the current trends of research and development on flavonoids, working mechanisms of flavonoids, flavonoid functions and applications, prediction of flavonoids as potential drugs in preventing chronic diseases and future research directions.

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Flavonoids: an overview

Phenolic acids are aromatic secondary plant metabolites, widely spread throughout the plant kingdom. Existing analytical methods for phenolic acids originated from interest in their biological roles as secondary metabolites and from their roles in food quality and their organoleptic properties. Recent interest in phenolic acids stems from their potential protective role, through ingestion of fruits and vegetables, against oxidative damage diseases (coronary heart disease, stroke, and cancers). High performance liquid chromatography (HPLC) as well as gas chromatography (GC) are the two separation techniques reviewed. Extraction from plant matrixes and cleavage reactions through hydrolysis (acidic, basic, and enzymatic) are discussed as are the derivatization reagents used in sample preparation for GC. Detection systems discussed include UV-Vis spectroscopy, mass spectrometry, electrochemical, and fluorometric detection. The most common tandem techniques are HPLC/UV and GC/MS, yet LC/MS is becoming more common. The masses and MS fragmentation patterns of phenolic acids are discussed and tabulated as are the UV absorption maxima.

Phenolic Acids in Foods: An Overview of Analytical Methodology

The impetus for developing analytical methods for phenolic compounds in natural products has proved to be multifaceted. Hundreds of publications on the analysis of this category of compounds have appeared over the past two decades. Traditional and more advanced techniques have come to prominence for sample preparation, separation, detection, and identification. This review provides an updated and extensive overview of methods and their applications in natural product matrices and samples of biological origin. In addition, it critically appraises recent developments and trends, and provides selected representative bibliographic examples.

Extraction, separation, and detection methods for phenolic acids and flavonoids

Wine has been part of human culture for 6,000 years, serving dietary and socio-religious functions. Its production takes place on every continent, and its chemical composition is profoundly influenced by enological techniques, the grape cultivar from which it originates, and climatic factors. In addition to ethanol, which in moderate consumption can reduce mortality from coronary heart disease by increasing high-density lipoprotein cholesterol and inhibiting platelet aggregation, wine (especially red wine) contains a range of polyphenols that have desirable biological properties. These include the phenolic acids (p-coumaric, cinnamic, caffeic, gentisic, ferulic, and vanillic acids), trihydroxy stilbenes (resveratrol and polydatin), and flavonoids (catechin, epicatechin, and quercetin). They are synthesized by a common pathway from phenylalanine involving polyketide condensation reactions. Metabolic regulation is provided by competition between resveratrol synthase and chalcone synthase for a common precursor pool of acyl-CoA derivatives. Polymeric aggregation gives rise, in turn to the viniferins (potent antifungal agents) and procyanidins (strong antioxidants that also inhibit platelet aggregation). The antioxidant effects of red wine and of its major polyphenols have been demonstrated in many experimental systems spanning the range from in vitro studies (human low-density lipoprotein, liposomes, macrophages, cultured cells) to investigations in healthy human subjects. Several of these compounds (notably catechin, quercetin, and resveratrol) promote nitric oxide production by vascular endothelium; inhibit the synthesis of thromboxane in platelets and leukotriene in neutrophils, modulate the synthesis and secretion of lipoproteins in whole animals and human cell lines, and arrest tumour growth as well as inhibit carcinogenesis in different experimental models. Target mechanisms to account for these effects include inhibition of phospholipase A2 and cyclo-oxygenase, inhibition of phosphodiesterase with increase in cyclic nucleotide concentrations, and inhibition of several protein kinases involved in cell signalling. Although their bioavailability remains to be fully established, red wine provides a more favourable milieu than fruits and vegetables, their other dietary source in humans.

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Wine as a biological fluid: History, production, and role in disease prevention

USDA Database for the Flavonoid Content of Selected Foods Release 3.1

Analytical applications of some flotation techniques-a review.

A method for the determination of low-molecular-mass polyphenolic compounds in vinegars has been optimized and applied to different samples. Ethyl acetate has been used for the extraction of the polyphenolic compounds in a continuous rotatory system, furnished with a special device that allows for the simultaneous extraction of twelve samples. The extracts obtained were submitted to HPLC separation using a C18 reversed-phase column and gradient elution. The identification of polyphenolic compounds was carried out using a photodiode detector. Important differences were found in the phenolic content of the different samples. Best extraction yields were obtained when using 80 ml ethyl acetate and 300 steps (2 h 5 min) at a velocity of 0.8 rpm. The reproducibility of the method may be considered quite good. The method has been successfully applied to the analysis of real samples of very different kinds of vinegar. There are very interesting differences between the low-molecular-mass phenolic fraction of the vinegar analysed. Some interesting low-molecular-mass phenolic compounds have been identified. Their concentration, as well as the concentrations of other so far non-identified substances, depend upon the type of sample. The individual investigation of these phenolic compounds may furnish hints enabling the origin and type of vinegar to be traced. The elaboration type used in connection with the starting wine can be ascertained and differentiated, together with the ageing system used, through inference from the analytical data obtained from different vinegars.

Analysis of polyphenolic compounds of different vinegar samples

Objective functions in experimental and simulated chromatographic optimization

Automated on-line-solid-phase extraction—high-performance liquid chromatography-diode array detection of phenolic compounds in sherry wine

The synthesis of the electronic and infrared spectra of di-2-pyridyl ketone salicyloylhydrazone (DPKSH) and di-2-pyridyl ketone benzoylhydrazone (DPKBH) is reported. Ultraviolet absorption spectra have been applied for determining the dissociation constants: DPKSH, pK1 = 3.5 and pK2 = 6.85; DPKBH, pK1 = 2.75 and pK2 = 10.6. These pKa values are interpreted as a function of the ortho-position of the OH group. Chelating properties of both tridentate ligands have been investigated. DPKSH forms a larger number of metal chelates in acidic media than does DPKBH. Also, it is concluded that DPKSH is a more suitable compound than DPKBH to preconcentrate and to determine heavy metal ion traces as it is inferred from the data obtained by flame AAS.

J.A. Pérez-Bustamante

Papers

Analytical applications of some flotation techniques-a review.

Analysis of polyphenolic compounds of different vinegar samples

Objective functions in experimental and simulated chromatographic optimization

Automated on-line-solid-phase extraction—high-performance liquid chromatography-diode array detection of phenolic compounds in sherry wine

Ligands with the “ONNN” Group as Chelating and Preconcentrating Substances of Heavy Metal Ions: Aroylhydrazone Derivatives from Di-2-Pyridyl Ketone