Banana Bioactives: Absorption, Utilisation and Health Benefits

Banana is an important fruit consumed globally and cultivated in humid and subtropical climes. The fruit comprises nutrients in its pulp and peel with beneficial properties. Banana in its unripe form, consists of indigestible compounds, resistant starch and dietary fibres. The starchy fruit degrades to fructose and sucrose when ripe, thus reducing its starch content. Aside its carbohydrate profile, essential macro and micro minerals, vitamins and phenolic compounds are other nutrients present in pulp and peel of the fruit. Resistant starch, an indigestible compound available in banana fruit, escapes absorption in the small intestine and is transported to the large intestine where digestion takes place. This occurrence makes banana a pre-ferred fruit for consumers suffering from diabetes. Polyphenols, present in minute concentration in the fruit, functions as antioxidants, thus contributing immensely to the prevention of metabolic degenerative diseases. This chapter further examines available nutrients present in banana fruit, their absorption and utilisation in the body. The chapter also brings to the fore, the health benefits of consumption of ripe, unripe and processed banana products.


Overview on banana
Banana is one of the world's most important fruit consumed globally. The fruit (Figure 1) grows well in a scale of about 1000 m altitude and sometimes even in the subtropics with higher altitudes. Its name Musa is derived from the word "Mouz" which is an Arabic name. The name was given to banana plant to honour a Roman physician named Antonia Musa who lived in the first century [1]. Sadia and Azizuddin [1] reported that Musa belongs to the family Musaceae and is divided into four edible cultivars namely: Rhodochlamys, Eumusa, Callimusa and Australimusa. There are presently diploid, triploid or tetraploid genome groups with the main genome groups represented as AA, AB, AAA, AAB and ABB [1]. Plantain, a cultivated banana variety and often referred to as the cooking-type banana is obtained from Musa acuminata (genome "A") and Musa balbisiana (genome "B") species [2]. Plantain belongs to the AAB genome, while the commercial export market Cavendish subgroup consists of the AAA genome [1]. Cooking and dessert banana contain substantial amount of nutrients thus qualifying the fruit to be the world's Banana Nutrition -Function and Processing Kinetics 4 recorded a concentration of 12.7 mg/100 g FW for vitamin C that was almost threefold than 'Williams' (4.5 mg/100 g); a β-carotene concentration of 96.9 μg α-carotene/100 g and 104.9 μg α-carotene/100 g, while 'Williams' averaged 55.7 μg β-carotene/100 g and 84.0 μg α-carotene/100 g. 'Apple' bananas were also reported to have more phosphorus (P), calcium (Ca), magnesium (Mg), manganese (Mn) and zinc (Zn) than 'Williams' [10]. Similar data for fully ripe banana fruit ( Table 2) were reported by the United States Department of Agriculture [23]. Other nutrients implicated to be present in cultivars of Musa spp. includes biogenic amines and polyphenols.

Carbohydrates
Banana and plantain are known to contain sugars, starch, fibre and cellulose compounds especially in its pulp. The peel portion of banana and plantain are rich in fibre. In banana fruit, inherent starch accumulates during development, with minimal changes in the principal carbohydrate metabolites observed during the preclimacteric phase [24]. Subsequently, the fruit starch is converted to sucrose, glucose and fructose as senescence sets in and progresses. Ripening in banana generally involves a decrease in starch concentration from 15 to 25% to less than 5% in the ripened pulp, together with an additional increase in sugar content [25,26]. At the onset of senescence, sucrose is the predominant sugar, with glucose and fructose predominating as ageing sets in [27]. The concentration of sugars in banana and plantain is associated with respiratory climacteric stage [1]. Starch conversion to sucrose is catalysed by activity of sucrose phosphate synthase, while acid hydrolysis causes starch conversion to non-reducing sugars from sucrose. It was observed that harvest maturity largely affects the conversion of starch to sugar. Changes like these have been reported in both diploid (Musa AA) [28] and triploid (Musa AAA) banana fruits [29]. Similarly, during the ripening stage of dessert banana, starch is completely broken down, unlike for plantains where starch is not totally broken down [30]. Due to the presence of pectin in banana, it was reported that consumption of the fruit can mitigate intestinal diseases [1]. Furthermore, green banana produces antidiarrheal activity in children which will help to fight against the incidence of diarrhoea, one of the main causes of high mortality and morbidity in children of third world countries. Research further revealed that low fasting blood glucose and glycogenesis in the liver can be increased due to presence of fibres in banana fruit [1]. Dessert and cooking banana in their ripe state are reported to be rich in resistant starch (RS), while at their unripe state, the fruit contains mostly digestible starch [31].

Protein and Amino Acids
In the works of John and Marchal [32], whole nitrogen in 'Cavendish' pulp has been reported to be 210 mg/100 g (FW) and 750 mg/100 g (DW). The protein signified a total nitrogen content of about 60-65%. This is in agreement with what was reported by the USDA [23], wherein the protein content for M. acuminata was 1.09 g/100 g (FW). John and Marchal [32] found an increased protein value of 4-7 g/100 g (DW) and 1.3-1.8 g/100 g (FW) upon development of 'Cavendish' banana, and also for 'Dwarf Cavendish' from 4 to 8 g/100 g (FW), with the concentration increasing as ripening progresses. Unripe bananas are rich in proteins, with chitinase enzymes being the most abundant protein [32]. During ripening of bananas, starch phosphorylase, malate dehydrogenase and pectate lyase are accumulated. Dopamine, a water-soluble antioxidant reported in both pulp and peel of 'Cavendish' banana is one of the catecholamines that suppress the oxygen uptake of linoleic acid [33]. Similarly, various bioactive amines including putrescine, spermidine and serotonin are reported to be present in high concentrations in banana. In addition, bananas have been found to contain physiologically relevant amounts of biogenic amines, nitrogenous compounds that include serotonin, dopamine and norepinephrine that vary relative to the ripening cycle, as well as the cultivar-dependent phytosterols cycloeucalenone, cycloeucalenone, cycloeucalenol, cycloartenol, stigmasterol, campesterol and β-sitosterol [33]. Bananas have some potential health benefits for cancer, cholesterol metabolism and related markers of cardiovascular disease risk.

Vitamins
Dessert and cooking banana contain different forms of vitamins in varying concentrations among the known cultivars worldwide. According to the reports of USDA [23] and Wills et al. [34], the vitamin C levels for 'Cavendish' banana estimated using high-performance liquid chromatography (HPLC), ranged from 2.1 to 18.7 mg/100 g (DW/FW), which however, varies considerably among different cultivars. The average vitamin C ( Table 2) content for 'Dwarf Brazilian' (Musa AAB 'Santa Catarina Prata') was found to be 12.7 mg/100 g and 4.5 mg/100 g for 'Williams' [13]. These results agree with the report of Wenkam [35], who reported vitamin C values of 5.1 mg/100 g (DW/FW?) for 'Williams' and 14.6 mg/100 g (DW/FW?) for 'Dwarf Brazilian' banana cultivar.
Dessert banana contains substantial levels of provitamin A carotenoids (PVAC), but few cultivars with orange-or yellow-coloured pulp are known to have higher concentration of carotenoids [15]. Vitamin A deficiency has been reported to be a public health concern identified in almost 118 countries in the world, with highest prevalence in Asia and Africa, as their diets are mainly cereals and tubers [1]. Researchers from Australia (Queensland University of Technology) have concocted Cavendish banana with high amounts of β-carotene, a precursor of vitamin A. The carotenoids mostly found in fully developed bananas are lutein, α-and β-carotene. The fruit pulp is also rich in carotenoids, while the peel contains low amounts [36]. It is reported that during ripening the amount of carotenoids increase [20,34]. Similar observation was reported by Kanazawa and Sakakibara [37], which contradicts the report of Gross and Flugel [38] who found carotenoid content decreasing during initial stage of ripening. As can thus be seen, different cultivars of Musa spp. contain different amounts of carotenoids. Banana and plantain fruits with orange flesh are rich in VAC [38,39]. One variety of banana, 'Karat' of Micronesia, was reported to accumulate β-carotene of about 2230 mg/100 g [39].

Minerals
Banana and plantain are very rich in K, an essential element for maintaining human blood pressure and for proper functioning of the heart [40]. The fruit is also rich in Mg, Fe, Cu and Mn [20]. In the works of Pareek [10], the average K content for Hawaii's bananas ('Dwarf Brazilian' and 'Williams') were reported to be 330.6 mg/100 g (FW). Similarly, it was reported that fruits cultivated in Tenerife, were observed to contain a K content of 5.09 mg/g (FW), P content of 0.59 mg/g (FW), Ca content of 0.38 mg/g (FW) and Mg content of 0.38 mg/g (FW) [10,41], though it was reported by the authors that the area of origin had a major effect on the occurrence of the minerals. Varietal differences had no effect on concentration of minerals present except for Fe. In the works of Forster et al. [42], differences were observed in the mineral content of bananas grown in both Ecuador and Tenerife. High Na, K, Mg and Fe contents were found for bananas grown in Tenerife, while high Ca, Cu, Zn and Mn contents were reported for bananas grown in Ecuador. This difference was attributed to agricultural practices, geographical location and soil composition [42]. Mineral composition of banana samples according to degree of ripeness was reported for the fruits obtained from Nigeria: 73.47% ash, 0.68% Zn, 0.146% Mn for unripe samples; 77.19% ash, 0.80% Zn, 0.271% Mn for ripe samples and 79.22% ash, 0.78% Zn for overripe samples [43]. Similarly, the peel of banana cultivars obtained from Cameroon contained relatively high minerals: K (50.0 mg/g DW), P (22.2 mg/g DW), Mg (11 mg/g DW) and Ca (18 mg/g DW) [44]. Generally, bananas contain low amounts of Ca, however, Micronesian cv. 'Krat' is relatively high in Ca [39].

Banana phytochemicals and bioactives
Phytochemicals have been reported to be an immense source of anticancer medications and chemopreventive agents [45]. These plant chemicals exert some of their actions through interactions with essential enzymes that regulate the activities in genes. Banana contains various bioactive compounds such as phenolics, carotenoids, alkaloids, glycosides, phlobatannins, tannins, terpenoids, saponins, steroids, biogenic amines and phytosterols, which are highly desirable in diet as they exert health beneficial effects [11,32,46]. These composites are helpful in protecting the body against oxidative stress due to their antioxidative activities [47], controlling gene expression in cell proliferation and apoptosis and important in controlling blood pressure [48]. The incorporation of banana pulp and peel in various food products could add value since they have health benefits [47]. Thus, banana pulp and peel can be used as natural sources of antioxidants and provitamin A. Banana peel is reported to have higher antioxidant capacity than banana pulp [49][50][51][52][53]. Furthermore, phenolic content of banana peel was higher compared to other fruits such as avocado, pineapple, papaya, passion fruit, water melon and melon [50]. However, it is recommended that to effectively recover and utilise phenolic compounds from banana peels, it is important to evaluate its chemical profile, factors affecting the levels of phenolic compounds in the peels such as antinutrients, and potential use of these compounds as food ingredients or nutraceuticals. It is important to understand how these bioactive compounds found in fruits and vegetables limit or prevent oxidative stresses as a free-radical scavengers or metalchelating agents. In the report of Liu [53], it was emphasised that there has to be a balance between oxidants and antioxidants for normal functioning of the body cell and/or sustaining optimal physical condition in the human body. Too much oxidants in the human body result in damage to the biomolecules such as proteins, lipids carbohydrates and DNA. Hence, to understand the mechanism of action of antioxidant, it is imperative to understand the formation of free radicals and their ability to damage macromolecules and nucleic chains.
Phenolic compounds have been implicated to be present in banana fruits. Although, banana peels contain more tannin compared with its pulp, tannins in the fruit confer an unpleasant astringent taste on the fruit. The astringency in ripe fruit is reduced, which is associated with a change in the structure of the tannins, rather than a reduction in their levels, as they form polymers [28]. When banana fruit is cut, oxidative browning occurs due to the presence of polyphenols. Report of different studies has shown that banana peels of different cultivars contain varying concentrations of total phenolic compounds (TPC). In the works of Nguyen et al. [54], it was reported that total phenolics, flavonoids and antioxidant activity of banana pulp and peel flours, Cv. 'Kulai Hom Thong' was shown to contain 3.0 mg of gallic acid equivalent (GAE)/gFW, while 'Kulai Khai' was reported to contain 0.9 mg of (GAE)/ gFW. Similarly, banana cv. 'Pisangmas' from Malaysia was reported to contain TPC ranging from 0.24 to 0.72 mg GAE/g FW, depending on the extraction method [32]. Sulaiman et al. [52], also reported significant differences in the antioxidant activity, total phenolic and mineral contents of eight Malaysian banana cultivars. Flavonoids epicatechin and myricetin 3-O-rhamnosyl-glucoside were identified in flour of organic acid pretreated "Mabonde", "Luvhele" and "M-red" cultivars at different concentrations [55]. Authors Bennett et al. [56] and Borges et al. [57] also reported the presence of catechin and gallic acid in pulp and peel of ripe and unripe banana cultivars. Similarly, plantain a banana cultivar belonging to the AAB, ABB or BBB group [58] have also been reported to contain high concentration of hydroxycinnamic acids (ferulic acid-hexoside with 4.4-85.1 μg/g DW) in its pulp [32]. Plantain peels are rich in flavonol glycosides and rutin ranging from 242.2 to 618.7 μg/g DW. Polyphenols are extremely diverse group of secondary metabolites, having sweetsmelling ring with one or more hydroxyl groups. Polyphenols have large range of structures and functions and can be classified as subgroups of flavonoids, phenolic acids, tannins, stilbenes and coumarins. These secondary metabolites are very essential in the metabolism, reproduction and growth of plants. Polyphenols also protect plants against pathological parasites, predators, fungal infections and viruses [53].

Banana volatiles
Tropical flavours including coconut, mango, mandarin as well as the combination of fruit and vegetable flavours are gaining popularity, with the African marula fruit and mangosteen being strong super fruit contenders [59]. Banana fruit flavour has been attributed to the presence of esters with inherent alcohols which contribute to flavour enhancement [60]. Alcohols and insignificant carbonyl composites present in banana are called green and woody notes [61], while the ester fraction contributes to fruity notes [62][63][64]. The presence of esters cause the sweet-smelling profile in ripe bananas, while that of unripe banana is determined by the presence of pentyl and hexyl alcohols, aldehydes and ketones [65]. Components of the fruity notes present in banana fruit includes 3-methylbutyl acetate, isoamyl butanoate and isoamyl isovalerate [66][67][68]. Esters constitute a major fraction of emitted volatiles from fresh banana fruits [64]. Esters can also be used to differentiate cooking bananas from desert bananas; in that cooking bananas lack esters, whereas the same form a major component of flavour present in desert bananas [69].
Aroma in banana fruit is characterised by the presence of various volatile compounds varying in concentration among cultivars [70,71]. Pino [72] reported the presence of 250 volatiles in fresh and processed banana products, though few of these volatiles have been isolated as flavour contributors. Essential components affecting taste of banana fruit includes d-glucose, d-fructose and sucrose for sweetness, while citric, l-malic, oxalic and succinic acids have been implicated for sourness [65,73]. Determination of volatiles that has unique character of the fruit is important as it produces the principal characteristic flavour of the fruit [74,75]. The most appealing property of banana required by most consumers apart from their nutritional and health benefits is the flavour [76,77].

Nutrigenomics of banana nutrients
With the ever changing nutrition-related health problems in developing economies, there is a gradual shift in nutrition research that focuses on how nutrition can be maximised in maintaining homeostasis at the cellular, tissue, organ and system level of the body [75]. This, however, requires the understanding of nutrient interactions at the molecular level. Nutrigenomics is the research into nutritional genomics, which also include nutrigenetics. In the works of Neeha and Kinth [78], nutrigenomics is defined as the study of the interaction between nutrients and genes, proteins and metabolic processes such as DNA and RNA synthesis and glycogenesis. Nutrigenomics focuses on the effects of nutrients on genome, proteome and metabolome as well as the interactions among these nutrients and nutrient-regimes in the body [78]. Through the application of molecular biology and genomic tools, researchers have identified genes responsible for the production of nutritionally significant proteins such as digestive enzymes, transport molecules and cofactors at their site of use [79]. Studies on genetic improvement of banana fruit is advancing at a rapid pace, using modern biotechnology which includes genetic engineering. Other programmes such as the use of banana as edible vaccine delivery system and biofortification of bananas to increase their β-carotene, α-tocopherol and iron contents are on the way [79]. These improvements will be most beneficial for regions of the world that consume bananas as their major staple. However, genetic improvement of banana fruit is a major challenge as cultivated banana fruits are basically sterile or possess low fertility with conventional breeding, though possible in delivering a few acceptable cultivars [79]. In the WHO [80] guideline on vitamin A supplementation in infants and children 6-59 months of age, about 19 million pregnant women and 190 million preschool age children from parts of Africa and South-East Asia were reported to be affected by vitamin A deficiency (VAD). Similarly, VAD is reported to be responsible for about 6% of child mortality below the age of five in Africa and 8% in South-East Asia [79]. Some banana cultivars have been implicated for the presence of dietary provitamin A carotenoid (PVAC) due to their characteristic orange-coloured fruit flesh. Carotenoid composition of the PVAC implicated banana cultivars has shown the presence of trans-α-, trans-β-, cis-β-carotene and lutein [21]. These notable differences in PVAC composition of the fruit could have significant consequences on the nutritional profile of banana varieties [79]. Banana bioactive compounds have potential for preventing various diseases when used as an ingredient in the food industry [81,82].

Utilisation of banana bioactives as nutraceuticals and food ingredients
According to the Science Forum, nutraceuticals is defined as a "diet supplement that delivers a concentrated form of a biologically active component of food in a non-food matrix to enhance health" [83]. Anyasi et al. [58] in their study further added that nutraceutical can also be extracted from a different product derived from the food and pharmaceutical industry, herbal and dietary supplement market, and the pharmaceutical/agribusiness and nutrition conglomerates. Raw banana shows higher amount of functional ingredients such as dietary fibre, resistant starch and total starch, which allows banana to impart health benefits to humans when incorporated in food products [84]. Table 3 highlighted the effects of banana bioactive compounds as a value-added ingredient in food processing. It is important to understand fruit maturity during preparation of banana flour to produce desirable food products. Ripe banana can be considered for industrial processing, which could result to products that are comparable to those obtained from apple, juice, fruit drinks, fermented drinks, stewed fruit, puree, marmalade, jam, flakes, confectionery, pastry, sorbets and ice-cream. Raw bananas can be considered as a source for new food innovation and development for partial or preprocessed food products like snacks and breakfast cereals [82]. Mixed pulp and peel flour from green banana has higher ash, total fibre and total phenolics than traditional wheat flour [85]. The addition of banana flour increases the indigestible fraction and the content of phenolic compounds in spaghetti [86]. Crackers containing greater amount of green banana flour showed increased antioxidant capacity [87]. The influence of green banana flour as a substitution for cassava starch on the nutrition, colour, texture and sensory qualities of snacks was reported for raw banana flour [87]. In the study, increased nutritional value including dietary fibre, polyphenol content and antioxidant capacity of the snacks was noted. Unripe banana peel can be incorporated in a sponge cake without imparting negative effects on the sensory quality [88]. The application of banana by-products, an underutilised renewable food biomass with potential in food and nutraceutical industry as a means of promoting green technology was reported [89][90][91]. Banana by-products such as peels are readily available for use as a source of raw materials, as they are regarded as waste during processing of foods such as jams, chips and noodles for green technology industry. Since the discovery of banana as a fruit for human consumption, there are no reports in the literature which show that it contains hazardous phytochemicals. Therefore, banana by-products preparation for usage in the food industry does not require excessive treatment as compared with other fruits by-products with potent hazardous constituents [89]. Banana peel extracts are rich in antioxidant capacity using multiple antioxidant assays. These peels are helpful in exhibiting antimicrobial activity against a wide range of bacteria and fungi [92].

Health benefits associated with banana bioactives
Banana is a very common fruit in most continents of the world and is consumed essentially as food. It is the fifth most significant food source with respect to world trade. Banana has more than a few bioactives which include biogenic amines, phenolics, phytosterols and carotenoids [46,93]. These compounds are of immense benefit to consumers due to countless positive effects they have on human health ( Table 4). The positive effects of these compounds on human health is expected because they have antioxidative properties, hence they are efficient in reducing oxidative stresses. Banana pulp has been reported to have high anti-tumour and antioxidant potentials [57]. Consumption of bananas is advantageous to body muscles due to its high content of K. Banana is usually suggested for patients suffering from anaemia due owing to its high content of Fe. Banana is low in Na, hence it helps in regulating blood pressure [46]. The presence of syringic acid in banana was reported [92]. This compound has an antidiabetic effect and could be used in managing glycoprotein abnormalities [92]. The consumption of catechin rich banana can help build up resistance to oxidation of low density lipoprotein (LDL), brachial artery dilation, increase plasma antioxidant activity and fat oxidation [94]. The gallic acid in banana was reported to exhibit hepatoprotective effects [95]. Serotonin in banana helps to avoid depression by altering mood and calming the body. The consumption of carotenoid rich banana is also effective in the treatment of vitamin A deficiency disorders and chronic diseases [39]. Dopamine and ascorbic acid present in banana is useful in reduction of plasma oxidative stress and enhancement of resistance to oxidative modification of LDL [56,96]. In many  animal trials, banana has been shown to be useful in the treatment of diabetes, due to its antihyperglycemic effect [97]. Banana is also utilised as a source of energy for sports athletes as it forms a valuable constituent in various energy drinks and dried banana bars [1]. Consumption of the fruit has also been used in the prevention of muscular contractions in athletes due to its vitamins, K and Mg contents [1].

Current trends in banana bioactive utilisation
A number of essential bioactive compounds in bananas have been reported by different researchers. Banana holds adequate quantity of valuable bioactive compounds for health promotion. Several studies have established and verified antioxidant activity of these compounds and efficaciously used bananas in treatment of diseases and promotion of wellbeing [46]. Bananas are being currently used to produce variety of food items which are of benefit to human health. Moreover, bananas are being composited with other food products to improve the micro and macro-nutrient values, especially for food low in micro and macro-nutrients [75,97].

Conclusion
Bananas are cultivated and utilised at different stages throughout the globe because the fruit contains both therapeutic and nutritional properties. Banana is composed of a substantial amount of beneficial bioactives which are essential for health and disease prevention. Bioactive compounds (phenolics, biogenic amines

Bioactive compounds Uses References
Phenolic compounds-gallic acid, catechin, epicatechin, tannins and anthocyanins, gallocatechin, epigallocatechin, quercetin, myricetin, kaempferol, ferulic, sinapic, salicylic, gallic, p-hydroxybenzoic, vanillic, syringic, gentisic and p-coumaric acids Act as protective scavengers against oxygenderived free radicals and reactive oxygen species responsible for ageing and various diseases [56,[98][99][100][101] Carotenoids-lutein, β-carotene, α-carotene, violaxanthin, auroxanthin, neoxanthin, isolutein, β-cryptoxanthin and α-cryptoxanthin Act as antioxidants, especially in scavenging singlet oxygen, decreases the risk of certain cancers, heart problems and eye diseases; improves immunity [102][103][104][105] Biogenic amines-serotonin, dopamine and norepinephrine Reduce the plasma oxidative stress and enhance the resistance to oxidative modification of low density lipoproteins; contributes towards the feelings of well-being and happiness; plays an important role in the human brain and body as a neurotransmitter with great impact on our mood, ability to concentrate and emotional stability [106,107] Phytosterols-cycloeucalenone, cycloeucalenol, cycloartenol, stigmasterol, campesterol and b-sitosterol Lowers cholesterol level in the blood and reduce its absorption in the intestine; act as immune system modulators and also have anticancer properties [108][109][110]  and phytosterols) in bananas could be enriched through genetic engineering, while developing bio-fortified cultivars for improvement of micronutrients. Banana peel contains substantial amounts of dietary fibre and bioactives at various stages of maturation. Industrial utilisation of the fruit in variety of food products will reduce occurrence of health related diseases even as its use in genomics will mitigate the effect of micro nutrient deficiency in the body.