In the past few years, there has been increasing interest in the study of mango phenolics from mango fruits, peels, seeds, leaves, flowers, and stem bark due to their antioxidative and health-promoting properties that make consumption of mangoes and derived products a healthy habit. Bioactive compounds found in the mangos, among other plants and herbs have been shown to have possible health benefits with antioxidative, anticarcinogenic, antiatherosclerotic, antimutagenic, and angiogenesis inhibitory activities (Cao and Cao 1999). Interestingly, many herbs, fruits, and vegetables are known to contain large amounts of phenolic antioxidants other than the well-known vitamins C, E, and carotenoids.
Polyphenols are secondary metabolites of plants and are widely distributed in beverages and plant-derived foods. Human consumption studies indicate 1 g of total polyphenols is frequently consumed per day and it is not anticipated that any acute or lethal toxicity would be observed through the oral intake route (Scalbert and Williamson 2000). Phenolic compounds have the capacities to quench lipid peroxidation, prevent DNA oxidative damage, scavenge free radicals (Cao and Cao 1999), and prevent inhibition of cell communication (Sigler and Ruch 1993), all of which are precursors to degenerative diseases. Free radicals cause depletion of the immune system antioxidants, change in gene expression, and induce abnormal proteins resulting in degenerative diseases and aging.
Antioxidant nutrients and phytonutrients inhibit the oxidation of living cells by free radicals by protecting the lipids of the cell membranes through free radical scavenging, blocking the initiators of free radical attack, neutralizing or converting free radicals into less active, stable products thus breaking the chain reaction and assisting in salvaging oxidized antioxidants enabling them to continue to be of benefit (Halliwell and others 1992). There are 2 main antioxidant defense mechanisms developed by living organisms: enzymatic and nonenzymatic components defense systems. An array of small molecules including polyphenols fall under the later system (Shahidi and others 1992; Rice-Evans and others 1997). Polyphenols have the ability to scavenge free radicals via hydrogen donation or electron donation (Shahidi and others 1992). A phenolic molecule is often characteristic of a plant species or even of a particular organ or tissue of that plant. The antioxidant activity of polyphenols is governed by the number, reactivity, and location of their aromatic hydroxyl groups (Chen and others 1996).
The main classes of polyphenols are defined according to the nature of their carbon skeleton and they are: phenolic acids, flavonoids, stilbenes, and lignans (Lee and others 2003). Other dietary polyphenols are not well-defined chemical entities and result from the oxidative polymerization of flavonoids and phenolic acids (Santos-Buelga and Scalbert 2000). The means of extracting polyphenols from plants is crucial as some polyphenols can be denatured by heat and lost by some solvents. Besides, some solvents are toxic and render the extracts unsafe for human consumption. Decoction is an extraction method of choice due to the absence of any organic solvent, as is the case with the industrial production of (Vimang), a mango stem bark extract in Cuba. Specific polyphenolic compounds can be determined and quantified by chromatographic techniques, while total phenols can be estimated by reduction of the Folin–Ciocalteu reagent (Singleton and Rossi 1965). Besides these, antioxidative capacity assays of plant extracts can also be used to predict their polyphenolic quantity and/or activity.
Polyphenolic composition of mango pulp Mangiferin, gallic acids (m-digallic and m-trigallic acids), gallotannins, quercetin, isoquercetin, ellagic acid, and β-glucogallin are among the polyphenolic compounds already identified in the mango pulp (Schieber and others 2000). Gallic acid has been identified as the major polyphenol present in mango fruits, followed by 6 hydrolysable tannins and 4 minor compounds, p-OH-benzoic acid, m-coumaric acid, p- coumaric acid, and ferulic acid (Kim and others 2007). Schieber and others (2000) found 6.9 mg/kg of gallic acid and 4.4 mg/kg of mangiferin in mango pulp. In a polyphenol screening of 20 mango varieties, Saleh and El-Ansari (1975) reported the co-occurrence of mangiferin, isomangiferin, and homomangiferin in mango fruit pulp. Mangiferin has been shown elsewhere to be the main compound of leaves and stem bark with great medicinal values. It has been reported that phenolic compounds and their associated antioxidant capacity decrease as fruit ripens (Kim and others 2007). Gallotannins represent the major components of unripe fruits and seeds. According to Prabha and Patwardhan (1986) gallic acid is the substrate of polyphenol oxidase in the fruit pulp, whereas ellagic acid is the predominant substrate in mango peel.
Polyphenolic composition of mango peel During mango fruit development, the total phenols have been found to be higher in the peel than in the flesh at all stages (Lakshminarayana and others 1970), with an estimated total polyphenol content in mango peel of 4066 mg (GAE)/kg (dry matter) (Berardini and others 2005b). Generally, ripe peels contain higher total polyphenols than raw peels (Ajila and others 2007). The polyphenolic constituents of mango peel include mangiferin, quercetin, rhamnetin, ellagic acid, kaempferol, and their related conjugates as shown in Table 1 where it can be seen that the 2 main polyphenols in mango peel are mangiferin and quercetin 3-0-galactoside. Berardini and others (2005b) found that, while mangiferin contents slightly decreased at elevated temperatures, the contents of the other xanthone derivatives significantly increased. The observed changes may be attributed to the formation of xanthones from benzophenone derivatives, which were recently identified in mango peels (Berardini and others 2004) and which are considered precursors of xanthone C-glycosides (Larrauri 1999). Anthocyanins have also been identified in the mango peel and estimated to range from 203 to 565 mg/100 g (dry matter) depending on variety and stage of maturity (Berardini and others 2005b). In their study on the antioxidative activity of mango peel extract, Berardini and others (2005b) established that the antioxidative capacity of the extract was higher than that of standard mangiferin and quercetin 3-O-glucoside, thus indicating that the antioxidative capacity of the peel extract cannot be attributed to a single component but to the synergistic effect of all the compounds present.
|Rhamnetin 3-O galactoside/glucoside||94.4|
Polyphenolic composition of mango seed kernels Besides the pulp and the peel, mango seed kernels are equally rich in polyphenols with potent antioxidative activity, but ironically the seeds are always discarded as waste during processing and consumption of the mango fruit. As an example, in India about 300000 metric tons of mango seed kernels are discarded every year (Char and Azeemoddin 1989). Ahmed and others (2007) identified and quantified various polyphenolic compounds in the mango seed kernel: tannin 20.7 mg/100 g, gallic acid 6.0 mg/100 g,coumarin 12.6 mg/100 g, caffeic acid 7.7 mg/100 g, vanillin 20.2 mg/100 g, mangiferin 4.2 mg/100 g, ferulic acid 10.4 mg/100 g, cinnamic acid 11.2 mg/100 g, and unknown compounds 7.1 mg/100 g. The total polyphenolic content of the mango seed kernel extract was estimated to be 112 mg (GAE)/100 g (Ahmed and others 2007). Soong and Barlow (2004) assayed the antioxidant activity of a variety of fruit seeds, namely, mango, jackfruit, longan, avocado, and tamarind and found that the antioxidant activity of the mango seed kernel was the highest, a fact attributed to its high polyphenolic content. These point to a reason to industrially utilize the mango seed kernel as a functional food ingredient.
Polyphenolic composition of mango leaves and stem bark Galloyl, hydroxy benzoyl esters, and epicatechin have been identified in mango leaves. Chemical studies performed with a standard aqueous extract of the stem bark from M. indica, which has been used in nutraceutical formulations in Cuba under the brand name vimang, have enabled the isolation and identification of phenolic acids (gallic acid, 3,4 dihydroxy benzoic acid, benzoic acid), phenolic esters (gallic acid methyl ester, gallic acid propyl ester, benzoic acid propyl ester), flavan-3-ols (catechin and epicatechin), and the xanthone mangiferin, which is the predominant component of this extract (10%) (Sanchez and others 2000). The total polyphenolic content of mango stem bark extract was found to be 10.61 g (GAE)/100 g of dry weight by the HPLC method and 9.4 g (GAE)/100 g dry weight by the Folin–Ciocalteu method. Thus, no significant difference was found between the 2 methods.
At this juncture, it is worth pointing out that environmental and developmental factors have been reported to affect the accumulation and eventual concentration of polyphenols in plant parts. Saleh and El-Ansari (1975) reported on the polyphenolic composition of mango leaves, twigs, bark, fruits, and seeds. Mangiferin was the major component of the leaves, twigs, and bark, with the bark having the highest content, while the gallotannins were the major components of the unripe fruits and their seeds besides being detected in all parts of the mango, isomangiferin, and homomangiferin were mainly present in the leaves and twigs, the former was also detected in fruits of some varieties. Fisetin was high in twigs, as was quercetin in the fruits. Quercetin-3-glucoside and kaempferol-3- glucoside were mainly found in the leaves. Gallic acid was found throughout all parts of the mango and so was ellagic acid; however, the latter was found in higher concentration in twigs, fruits, and seeds as compared to the leaves. m-Digallic acid was high in unripe fruits, as was β-glucogallin in the seeds.
Mango extracts from leaves, fruit, seed kernel, fruit pulp, roots, and stem bark for medicinal purposes in many countries have been widely documented in the Napralet database. The ethnomedical use of the mango stem bark extract in Cuba has been extensively researched for over 10 y on more than 7000 patients and has been found effective against cancer, diabetes, asthma, infertility, lupus, prostatitis, prostatic hyperplasia, gastric disorders, arthralgies, mouth sores, among others (Nunez-Selles 2005). The various research studies on mango antioxidative bioactivity underscore that mango polyphenols are the active compounds in the extracts.
The different parts of the mango (fruit pulp, peel, seed kernel, leaves, and stem bark) are a rich source of various polyphenols with quantities of the different polyphenols varying in the different parts or missing totally in some parts. Significant research work has been done on mangoes and many articles have been published, but most of them have discussed mango polyphenols from 1 part only and none has discussed polyphenols in the mango as a whole. In this article, we review for the 1st time the various mango polyphenolic compounds from the different parts and their related antioxidative and medical significance to human health paying special attention to mangiferin.