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Grapes are largely produced popular berry fruits potentially rich in some of the vital bioactive components which have several functional properties in general and specifically health-promoting medicinal properties. Grape contains a wide range of polyphenols along with wine as a popular grape product and is potentially rich in many phenolic compounds. Phenolic compounds are structurally different; linear and branched forms of the phenolic molecules perform many active biological functions. The grape polyphenols are traditionally classified into flavonoids and nonflavonoids, but it is subdivided into many subgroups according to the nature of chemical arrangement of the polyphenol molecule within its chemical structure. These subdivided groups have their different target-specific biological actions which increase the demand of characterization of these molecules since they have several important medicinal functions in human health. Many extraction and analytical techniques are used for the estimation, characterization, and isolation of these active polyphenolic components. Moreover, various analytical techniques are developed for the determination of total phenolic contents as well as for subcategorization of these extracted polyphenols. Some traditional processes and spectrophotometric analysis are ceaselessly improved so as to attain adequate separation of phenolic molecules, their further identification, and quantification.
KeywordsGrapes Polyphenols Flavonoids Antioxidants Anthocyanin Chemical structure Extraction Wine
Fruits and vegetables are rich with innumerable chemical compounds including vitamins, minerals, and other potentially helpful phytochemicals. The consumption of fruits and vegetables has since long been associated with lesser incidences of certain diseases like cataract, scurvy, cancer, and cardiovascular diseases (CVD) in populations consuming them. The disease prevention ability of fruits and vegetables is mainly attributed to the phytochemicals present in minor quantities. Plants synthesize several classes of antioxidants, including vitamin C, as well as phenolic compounds, e.g., flavonoid pigments, carotenoids, and tocopherols (principally vitamin E). There are innumerous compounds in fruits and vegetables that could individually or synergistically contribute to improvements in human health. The number of identified physiologically active phytochemical has increased considerably in the last decades.
The grape (Vitis vinifera) is one of the important fruits consumed across the globe. It is having potential health benefits and economic importance, and therefore it is widely cultivated around the globe. Over 72 million tons of grapes are grown each year worldwide, mostly to produce wine. Grapes are also a popular finger food since the nutrients in grapes offer a number of possible health benefits. They have been associated with prevention of cancer, heart disease, high blood pressure, and constipation. Grapefruit that is also called as pomelo belongs to citrus tree of the Rutaceae family [1, 2]. Grapefruit has become popular as breakfast fruit in various parts of the world, and production has expanded to most citrus growing countries, notably the India, the United States, Israel, Cyprus, South Africa, and Brazil. As a source of vitamin C, the grapefruit is exceeded among common fruits only by the orange and lemon .
Production Scenario of Grapes
As per the latest estimations of Food and Agriculture Organization, grape cultivation is carried out over 75,866 km2 area across the globe. Out of total grape production, 71% of the grapes are utilized for manufacturing of wines, 2% of the grapes are utilized in the dried form, and around 27% are consumed in fresh form. However, comparatively lesser portion of its production is utilized for preparation of the grape juice . There are no confirmed statistics for indicating variety wise production of the grapes. However, a very popular variety of grapes which is used for cultivation around the world is Sultana that is also known as Thompson Seedless. Other common grape varieties include Airen, Cabernet Sauvignon, Sauvignon blanc, Cabernet Franc, Merlot, Grenache, Tempranillo, Riesling, and Chardonnay [5, 6].
Grapes Production in India
Grape varieties cultivated in India
Bangalore Blue, Gulabi (Muscat)
Beauty Seedless and Sharad Seedless
Anab-e-Shahi, Dilkhush (clone of Anab-e-Shahi)
Perlette, Pusa Seedless, Thompson Seedless and its clones, Tas-A-Ganesh, Sonaka, and Manik Chaman
Grape is one of the important fruit crops covering an area of 123 thousand hectares occupying 2.01% of the total area. Currently, Thompson Seedless is the ruling grape Indian variety occupying major area with its clones followed by Bangalore Blue varieties. Major grape-growing states are Maharashtra, Karnataka, Telangana, Andhra Pradesh, Tamil Nadu, and the north-western region covering Punjab, Haryana, western Uttar Pradesh, Rajasthan, and Madhya Pradesh . Maharashtra ranks first in terms of production accounting for more than 81.22% of total production and highest productivity in the country . India is also a major exporter of fresh grapes to the world. The country has exported 1, 93,690.55 MT of grapes to the world for the worth of Rs. 2176.88 crores (298.05 USD millions) during the year 2019–2020. Major export destinations (2019–2020) are the Netherland, Russia, the UK, Bangladesh, and Germany [10, 11].
2 Chemical Composition and Medicinal Benefits of Grapes
Grapes (Vitis vinifera) are berries that grow in clusters are very delicious, and enticing with variable tastes, and colors. Grapes are consumed largely as whole fruits; but they are additionally utilized in preparation of diverse food merchandise and therefore the seeds of seeded varieties are utilized in medication, preponderantly antimicrobial medication and nematicides [12, 13]. Grapes cultivation dates back to 5000 BC; but, the wide unfold propagation of grapes was started in Europe, so unfold to Australia, America, and alternative countries through invaders and sailors. Egyptians used the grapes for wine, and later the technology was developed in varied western countries, notably Spain, France, and Federal Republic of Germany .
The history of grapes is protracted and ample. Wine making from grapes are recorded in the historic Greek and Roman civilization. European grapes (Vitis vinifera), North American Grapes (Vitis labrusca and Vitis rotundifolia), and French hybrid are mainly predominant species these days. Grapes are classified as table grapes, wine grapes (utilized in viniculture), raisin grapes, and whether to be eaten with seeds or seedless. The diverse varieties of the grapes are experienced by the peoples across the globe.
Grapes consist principally of two varieties; one contains seeds whereas the others are seedless. The seedlessness may be an extremely fascinating quality of the grape. Among these seedless grape varieties are natural and a few are genetically obtained from basic seedless varieties that all belong to basic styles of genus Vitis [14, 15]. The grapes which are rich in polyphenols help in pressure regulation and protection of epithelial cells. They conjointly facilitate to fight cancer and minimize the chance of cardiopathy. The essential oil extracted from grape seeds are used in several health care products like cosmetics. These oils conjointly contain vitamin E, an unsaturated carboxylic acid of n-3, n-6, and n-9 series that protects the body from atom attacks. The grape juice helps to fight cancer, reduces the risk of coronary failure, improves brain health, and permits the body to withstand aging issues with the passage of time [15, 16, 17]. Grape juice is also supposed to be normalizing the cardiovascular disorders. It is crucial to understand the composition of grape with its biological process and health properties very well [18, 19].
3 Grape Polyphenols
Grape includes diverse nutrient elements, inclusive of vitamins, minerals, carbohydrates, fit to be eaten fibers, and phytochemicals. The phytochemicals are effective antioxidants that prevent free radical-mediated degenerative diseases like cancer, coronary heart diseases (CHD), etc. Polyphenols are the maximum vital phytochemicals in grape due to the fact that they own many organic compounds and providing benifits for better health .
Phenolic compounds are the important and most plentiful secondary metabolites present in the plant kingdom. Chemical structural similarity is observed of these compounds containing aromatic benzene ring with one and more hydroxyl substituents. In plants, they not only play an essential role in growth, fertility, reproduction, and defense mechanism but also act as a shield against abiotic and biotic stresses [21, 22]. The basic components of pigments, essences, and flavors are contributed by the phenolics. The properties like anticarcinogenic, cardioprotective, anti-inflammatory, antibacterial, and antiviral along with major antioxidant and antiradical activity are active biological functions reported in polyphenols such as catechin, resveratrol, quercetin, and rutin [23, 24, 25, 26].
Polyphenolic compounds are very fundamental for the biological excellence of naturally derived food products through their contribution to oxidative stability and organoleptic characteristics. Indeed, the organoleptic properties of grapes and wines are largely related to phenolic compounds extracted from the grapes. The major compounds responsible for wine quality are flavonoids, including anthocyanins and flavan-3-ols. Moreover, the red color of wine is due to anthocyanins pigment which is present in grape skin. Flavan-3-ols exist not only as monomers but also as oligomers and polymers, called condensed tannins or proanthocyanidins. Condensed tannins are important quality parameter of wine due to their astringent, bitter taste [27, 28], and their role in the long-term color stability [29, 30, 31, 32, 33].
3.1 Forms and Structure of Grape Polyphenols
Flavonoids are a various cluster of phytochemicals found in the grapes in conjunction with carotenoids, and they impart vivid colors and also have potential antioxidant activities. Flavonoids are the most important cluster of phytonutrients, with quite more than half a dozen of types such as catechin, anthocyanin, proanthocyanin, flavonol, quercetin, etc.
The phenolic compounds obtained from grapes primarily encompass anthocyanins, flavanols, flavonols, stilbenes (resveratrol), and phenolic acids [36, 37]. The anthocyanins pigment are found in grape skin. Flavonoids have broad categorization in grapes such as seeds and stems which majorly contain catechins, epicatechin, and procyanidin polymers. Anthocyanins are the primary polyphenolics in Thompson Seedless grapes, whereas flavan-3-ols are also present in ample quantity among various grape varieties [37, 38, 39]. The researcher’s attention leans toward chemical compositions and health benefits of polyphenols from grapes and crimson wines .
The suggested scientific evidences have proved that polyphenols have potential of inhibiting few degenerative disorders, inclusive of cardiovascular diseases [41, 42, 43, 44], and some other varieties of grapes are effective in decreasing plasma oxidation strain and decreasing the momentum of aging of the fruits [31, 45]. Phenolic compounds also are appeared as preservatives in opposition to microbes and oxidation for food [46, 47, 48, 49] and in vivo assays confirmed that phenolic compounds are bioavailable [50, 51]. Moreover, a few scientific evidences have been additionally proven that at better concentrations, the effect of phenolic compounds on health becomes bad, and a few systems particularly showed the bad consequences [52, 53]. However, some high molecular weight phenolics showed poor absorption [54, 55].
Grapefruit, grape seed, and grape skin extracts along with fruit juice are all supposed to contain a various array of potent antioxidants within the sort of polyphenols, which include phenolic acids (e.g., gallic acid), anthocyanins, and flavonoids (e.g., proanthocyanidins). Counting on their localization within the grape tissues, the grape proanthocyanidins showed difference in the number, structure, and degree of polymerization . Moreover, grape contains higher concentrations of monomeric, oligomeric, and polymeric flavan-3-ols in seeds as compared to grape skins [56, 57]. Grape seeds contain approximately 2.3 to 8.2 mg/g of monomeric, oligomeric, and polymeric flavan-3-ols like catechin, epicatechin, and their gallates .
Grape skin contains approximately 20-fold less (on a milligram per gram basis) monomeric, oligomeric, and polymeric flavan-3-ols as compared to grape seeds . It’s also documented that the polyphenol composition and content of grapes varies between different cultivars, and is influenced by geographic location and the climate . Although the skin and seeds of grapes are reported to contain “cardioprotective” polyphenolic antioxidants, a recent animal study demonstrated that extracts from the flesh of grapes possessed cardioprotective actions . The entire polyphenolic index was lower within the grape flesh as compared with the grape skin; however, the anthocyanins were exclusively within the grape skin, whereas the reactive oxygen scavenging activities were similar within the two groups. The results indicated that the flesh of grapes could also be equally cardioprotecive despite the very fact that the grape flesh doesn’t contain anthocyanin activity.
Among the trans-isomer compounds, resveratrol, or 3,5,4-trihydroxy stilben, is known to be created in wines in response to a mycosis . Flavonoids, the foremost plethoric stilbene synthetic resin compounds in grapes and wines, own a typical C15-skeleton, composed of 3 rings (A, B, C). The family of this molecules is set up by totally different subcategories, flavones, flavonols, flavanones, flavanols and anthocyanins differs by the ring C in saturation degree and substituents. Flavylium ion, includes two benzene rings associated with unsaturated cationic ventilated heterocycle, derived from the 2-phenyl-benzopyrylium nucleus. These are glycosylated derivatives of 5 aglycones or anthocyanidins family such as cyanidin, peonidin, petunidin, delphinidin, and malvidin [61, 62]. The diversity in structure results from spontaneous chemical process of the aldohexose by carboxylic acid, p-coumaric, and caffeic acids. These compounds consist linear chain monomers however, additionally oligomers or polymers known as proanthocyanidins. Proanthocyanin structures shows variation within the nature of their organic subunits, mean degree of polymerization (mDP), and linkage position.
The grape skin contains relatively lower amounts of proanthocyanidins than seeds and their structural characteristics showed difference significantly. Grape seed proanthocyanidins are composed mainly of procyanidins, whereas grape skin proanthocyanidins contains both procyanidins and prodelphinidins. A better mDP and a lower proportion of galloylated subunits are observed in skin proanthocyanidins than in seeds. Condensed tannins derived from grapes have great importance in the quality of wines .
4 Antioxidant Activities of Some Polyphenolic Compounds
The antioxidant activity of the grapes has been studied in terms of inhibition of lipid oxidation, scavenging of free radicals, reduction of hydroperoxide formation [63, 64]. Various analytical methods are generally used to estimate the antioxidant capabilities of the various phenolic compounds extracted from various grape varieties or several parts of grapes. These generally includes crocin bleaching assay (CBA) , 1,1-diphenyl-2-picryhidrazyl (DPPH) method , oxygen radical absorbance capacity (ORAC) assay , the thiobarbituric acid reactant substances (TBARS), 2, 2′-azino-bis-(3- ethylbenzothiazoline-6-sulfonic acid) (ABTS) assay , ferric reducing antioxidant power (FRAP) assay, and Trolox equivalent antioxidant capacity (TEAC) assay [67, 68].
4.1 Analysis of Antioxidant Activity of Grapes
Many sophisticated techniques have been developed for estimation and evaluation of the antioxidant activities of the grapes. The available analytical techniques have helped in thorough analysis of these antioxidant activities of many grape polyphenols. The simplified techniques have the classification of these activities based on different analytical parameters. Certain general steps which are usually followed as standard processes for evaluating antioxidant activities.
Selection of the Grape Varieties
Almost all the varieties of the grapes yield nearly similar sort of polyphenols. Moreover, some popular grape varieties are generally selected for evaluation of the antioxidant activities of the polyphenols extracted from these varieties. Every region of the world has different popular varieties according to their regional climatic conditions . Although there are different grape varieties, but the chemical content of the grapefruit has much constant similarity compositional index as compared with chemical compositions of other fruits. Therefore, selection of grape varieties for the evaluation of antioxidant activities has significant regional impact [69, 70].
4.2 Measurement of Antioxidant Activity
The grapes of selected varieties are pretreated to make it ready for separation of the grape pomace. The obtained grape pomace is then freeze-dried followed by fine grinding, and then stored at 20 °C until its further use. The fine ground material is then utilized for extraction by using methanol as a solvent along with 1% HCL solution. The obtained extract is then concentrated by using hexanal followed by removal of methanol and hexanal residues. The percent yield is expressed in gram extract per 100 g of dry grape pomace . The obtained extracts are further subjected to HPLC analysis by using suitable combination of the eluents, and then ESI-MS used exclusively for the detection and characterization of the metabolites . Moreover, positive mode is applied for detection and characterization of anthocyanin whereas flavanols are characterized using negative mode followed by DAD analysis between 200 and 700 nm.
The antioxidant activity of the grape extracts is measured by using DPPH method in terms of radical scavenging activity. The extracted sample is prepared in appropriate dilution for analysis by using UV-spectrophotometer. Initially, standard curve is obtained at 515 nm by using standard solution of the DPPH at different concentrations. The freshly prepared DPPH solution and methanolic extract solutions are mixed at varied proportions, incubated at 25 °C for 5 hrs, and then kept in dark to bring the prepared solution concentration at steady state [73, 74].
Moreover, the Trolox Equivalent Antioxidant Capacity (TEAC) assay is used for evaluation of free radical scavenging activity. The ability of polyphenolic contents in grape extract to scavenge the free radicals of ABTS Azino-bis (3-ethylbenzothiazoline-6-sulfonate) is recorded in terms of the value of TEAC assay. The samples are initially diluted with methanol at different concentrations followed by addition of diluted ABTS solution, and then absorbance is measured at 734 nm wavelength .
5 Application of grape polyphenols
The polyphenols extracted from the grapes are having tremendous potential of preventing the lipid peroxidation and therefore it protects free radicals from undergoing damages caused by the oxygen scavenging, in this way these polyphenols helps to extend the shelf life of the foods which are vulnerable to oxidative damages during their storage .
Modern research findings suggest that application of the polyphenols extracted from the grapes is more suitable to food industry than pharmaceutical applications. Many polyphenols have proved their potency in extending the shelf life of foods which are prone to free radical oxygen scavenging damages that compromises their storage qualities. These polyphenols also shows antimicrobial activities which have proved effective particularly against Salmonella typhimurium which is responsible for staling of the some frozen food products [67, 68].
Many research studies have been undertaken by using grape polyphenols along with different combinations of nisin, EDTA, and other agents in which polyphenols are found influencing the antioxidant and antimicrobial potential. These functional properties of grape polyphenols help to improve the storage stability of the fried and frozen foods. These grape polyphenols found effective against Salmonella typhimurium, Listeria monocytogenes, and Escherichia coli. The research findings also proved the potential use of these polyphenolic compounds in extending the storage life of the different food products. Moreover, the increasing trend of application of grape polyphenols is observed as a modern food preservation technique in food processing industries .
6 Examples of Some Important Polyphenolic Compounds of Grapes
Catechin: (Fig. 3)
Class: Flavonoids .
Chemical name: 2-(3,4-dihydroxyphenyl)-3,4-dihydro-2Hchromene-3,5,7-triol .
Structural chemistry: The position and number of hydroxyl group in the moiety determines antioxidant activity. The presence of catechol moiety and unsaturation increases activity in catechins. (+) Catechin is more effective  .
Mode of action: Inhibits liquid peroxidation .
Application: Anti allergic, Anti-inflammatory, Antiviral, Antiproliferative, and Anticarcinogenic activities .
Class: Stilbenes 
Chemical name: 3,4’, 5 trihydroxystilbene  .
Mode of action: Increases plasma antioxidant activity and decreases lipid peroxidation .
Application: Anti-inflammatory, Neuroprotective, and Antiviral properties .
Name: Vanillic acid
Chemical name: 4-Hydroxy-3-methoxybenzoic acid .
Structural chemistry: The ester and hydroxyl groups present and their position affects the antioxidant capacity in the body .
Mode of action: Radical-scavenging activity .
Application: Showed chemopreventive against experimentally-induced carcinogenesis, Antibacterial, Antimicrobial properties, etc. .
Sources: Potato tuber, cereals like oats, wheat, barley, wine, etc. .
Grape is one of the major fruit crops cultivated across the globe with specific growing conditions, and its production is ever increasing. Grapes and its various value added processed products particularly wines are rich in many bioactive polyphenols, which are having potential health benefits to humans. The polyphenols from the grapes poses significant and versatile structural arrangement, some are flavonoids while others being nonflavonoids, and also includes many subcategories. Various novel techniques are developed for categorization and analysis of the versatile polyphenol composition within the various varieties of the grapes. This chapter reviews many modern techniques for the analysis and characterization of these active polyphenols from the grapes and its wines. These modern techniques are under continual improvement which includes spectrophotometric techniques, solvent extraction, supercritical extractions, and many more. Moreover, modern polyphenols manufacturing industries are in need of appropriate solutions to their industrial problems as far as the isolation of these polyphenolic compounds are concerned. Considering the tremendous versatility in the types of polyphenols in the different varieties of the grapes, many traditional spectrophotometric techniques are upgraded for identification, analysis, extraction, and characterization of the various polyphenolic compounds. Some advanced techniques are also developed by using HPLC, GC, and Mass Spectroscopy (MS) for target-specific phenolic components.
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