ABSTRACT
The increased interest in antioxidant activity of plant phytochemicals has necessitated their determination in rarely consumed fruits. The aim of this study was to determine the antioxidant capacity as well as the vitamin and mineral content of the selected commonly consumed and underutilized fruits. Samples of six selected commonly consumed fruits; pineapple (Ananas comosus) and banana (Musa acuminata) and underutilized fruits; soursop (Annona muricata), African canarium (Canarium schweinfurthii), african star apple (Chrysophyllum albidum) and tangerine (Citrus tangerina) were collected from the local market and analysed for antioxidant capacity using the free radical scavenging activity. The stable radical 1,1-diphenyl-2- picrylhydrazil (DPPH) and ferrous reducing antioxidant power (FRAP) assay were used.Antioxidant vitamins A,C,E and trace minerals were determined. The proximate analysis of the fruit pulps showed that C. albidum and C. schweinfurthii had the highest percentage of carbohydrate (22.41± 0.00 % and 21.37 ± 0.00 % respectively) while C. tangerina had the least (2.34 ± 0.00 %). C. schweinfurthii had the highest percentage of fat (19.41 ± 0.00 %) relative to the other fruit pulps. Tannins level was significantly (p<0.05) higher in M. acuminata(7.99 ± 0.00 mg/100g), terpenoids and saponin levels were also significantly (p<0.05) higher in C. schweinfurthii (56.92 ± 0.15 mg/100g and 1.03 ± 0.02 mg/100g respectively) when compared to control and other underutilized fruit pulps. For flavonoid content, C. schweinfurthii and A. muricata had the highest values (32.27 ± 0.16 mg/100g and 30.13 ± 0.04 mg/100g respectively) while A. comosus had the least (7.20 ± 0.03 mg/100g). Vitamin C level was significantly (p<0.05) higher in C. schweinfurthii and C. albidum (484.80 ± 2.1 mg/100g and 479.41 ± 0.7 mg/100g) respectively compared to the control. C. tangerina had the highest vitamin A levels (206.89 ± 4.9 mg/100g) while M. acuminata and A. comosus showed the highest level of vitamin E (74.48 ± 0.0 mg/100g and 59.42 ± 0.0 mg/100g) respectively. Selenium, zinc, potassium, calcium and iron levels were significantly (p<0.05) higher in C. albidum and C. schweinfurthii relative to the other fruit pulps studied. C. albidum had the highest level of % inhibition (73.07%) relative to other fruit pulps while M. acuminata had the least % inhibition (31.12%). Ferric reducing power activity of the fruit pulps revealed significant increase in A. comosus and M. acuminata with increasing concentrations. M. acuminata had the highest reducing power activity (0.655 mg/ml) at the highest concentration (1mg/ml) while A. muricata had the least at (0.01mg/ml). In conclusion, among selected fruits, underutilized fruits have shown relatively higher level of antioxidant capacityand contain appreciable amount of essential nutrients, vitamins and minerals than the commonly consumed fruits. Especially African star apple and African canarium are good sources of antioxidants. The study further showed that no single plant food could provide all the required nutrients.
CHAPTER ONE
INTRODUCTION
It is widely accepted that a plant-based diet with highintake of fruits, vegetables, and other nutrient-rich plant foods may reduce the risk of oxidative stress-related diseases (Riboli and Norat, 2003; Johnson 2004; Stanner et al 2004). Nutritionists are worried about the nutritive value of cooked food because the quality of most nutrients like protein, carbohydrates, vitamins and minerals are very poor (Reis et al., 1987). Fruits have been included in the human diet since prehistoric time and now in the developed and developing countries, there is the habit of taking fresh fruits after meal. In Nigeria, different kinds of seasonal fruits are available which are important sources of fiber, vitamins and minerals which provide essential nutrients for the good health of humans. Fruits are a major source of above mentioned food supplements.Fruits are referred to as juicy seed bearing structure of flowering plant that may be eaten as food (Hyson, 2002). Increased consumption of fruit and vegetables significantly reduces the incidence of chronic diseases, such as cancer, cardiovascular diseases and other aging-related pathologies. Fruits are not accorded theimportance they deserve in the diet of Nigerians due to ignorance of their nutritive value, cost and difficulty in storage and distribution (Sai, 1997). In developing nations, numerous types of edible wild plants are exploited as sources of food to provide supplementary nutrition to the inhabitants (Aberoumand and Deokule, 2009). Fruits offer protection against free radicals that damage lipids, proteins, and nucleic acids. Polyphenols, carotenoids (pro-vitamin A), vitamins C and E present in fruits have antioxidant and free radical scavenging activities and play a significant role in the prevention of many diseases (Veliogluet al., 1998; Spiller, 2001; Prakash and Kumar, 2011).Food and Agricultural Organization (FAO) reported that at least one billion people are thought to usewild food in their diet (Burhingame, 2000).
1.1 Antioxidants
An antioxidant is a substance, generally an organic compound, that is more readily oxidized than a second substance and hence can retard or inhibit the autoxidation of the second substance when added to it (Stenesh, 1989).Antioxidants are the body’s first line of defense against oxidative damage, and are critical for maintaining optimum health and wellbeing. Antioxidants are chemicals that interact with and neutralize free radicals, thus preventing them from causing damage. Oxidation is a chemical reaction that transfers electron from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. Free radicals
are capable of attacking the healthy cells of the body, causing them to lose their structure and function (Mittler, 2002). In turn, these free radicals can start a chain reaction that damage cells. Fortunately, free radical formation is controlled naturally by various beneficial compounds known as antioxidants. It is when the availability of antioxidants is limited that this damage can become cumulative and debilitating (Cheeseman and Slater, 1993). Antioxidants terminate these reactions by terminating free radical intermediates, and inhibit other oxidation reactions. They do this by being oxidized themselves. Thus, antioxidants are often reducing agents such as thiols, ascorbic acid or polyphenols (Benzie, 2003).
Antioxidants are capable of stabilizing, or deactivating, free radicals before they attack cells. Antioxidants are absolutely, critical for maintaining optimal cellular and systemic health and well-being (Traber and Atkinson, 2007). To protect the cells, organ and systems of the body against reactive oxygen species, humans have evolved a highly sophisticated and complex antioxidant protection system (Vertuaniet al., 2004). Although oxidation reactions are crucial for life, they can also be damaging; hence, plants and animals maintain complex systems of multiple types of antioxidants, such as glutathione, vitamins C and E as well as enzymes such as catalase, superoxide dismutase and various peroxidases. Low levels of antioxidants, or inhibition of the antioxidant enzymes, causes oxidative stress and may damage or kill cells. Antioxidants are also widely used as ingredients in dietary supplements in the hope of maintaining health and preventing diseases such as cancer and coronary heart disease. Although, initial studies suggested that antioxidant supplements might promote health, later large clinical trials did not detect any benefit and suggested instead that excess supplementation may be harmful. In addition to these uses of natural antioxidants in medicine, these compounds have many industrial uses, such as preservatives in food and cosmetics and preventing the degradation of rubber and gasoline. For many years chemists have known that free radicals cause oxidation which can be controlled or prevented by a range of antioxidants substances (Bjelakovic et al., 2007).
1.1.1 Classification of Antioxidants Antioxidants are grouped into two namely; (1) Primary or natural antioxidants.
(2) Secondary or synthetic antioxidants.
1.1.1.1Primary or Natural Antioxidants
They are the chain breaking antioxidants which react with lipid radicals and convert them into more stable products. Antioxidants of this group are include the following (Hurrell, 2003):
(1) Antioxidant minerals – These are co-factors of antioxidants enzymes. Their absence will definitely affect metabolism of many macromolecules such as carbohydrates. Examples include selenium, copper, iron, zinc and manganese.
(2) Antioxidant vitamins – It is needed for most body metabolic functions. They include- vitamin C, vitamin E and vitamin B.
(3) Phytochemicals – These are phenolic compounds that are neither vitamins nor minerals.
1.1.1.2Secondary or Synthetic Antioxidants
These are phenolic compounds that perform the function of capturing free radicals and stopping the chain reactions(Hurrell, 2003), the compounds include:
i. Butylated hydroxyanisole (BHA). ii. Butylated hydroxytoluene (BHT).
iii. Propyl gallate (PG) and metal chelating agent (EDTA). iv. Tertiary butyl hydroquinone (TBHQ)
v. Nordihydro guaretic acid (NDGA).
1.1.2 Antioxidant Vitamins
1.1.2.1Vitamin A
Vitamin A is a group of unsaturated nutritional organic compounds, that includes retinol, retinal, retinoic acid and several provitamin A carotenoids and beta-carotene (Fennema,
2008). Vitamin A is important for growth and development, for the maintenance of the immune system and good vision (Tanumihardjo, 2011). Vitamin A is needed by the retina in the form of retinal, which combines with protein opsin to form rhodopsin, the light-absorbing molecule (Wolf, 2001) necessary for both low-light (scotopic vision) and color vision (Tanumihardjo, 2011). When there is insufficient amount of retinol available, rhodopsinsynthesis is affected and night blindness may result. The condition can, however, also be due to a lack of other nutrients which are critical to the regeneration of rhodopsin such as protein and zinc. Vitamin A also functions in a very different role as retinoic acid (an irreversibly oxidized form of retinol), which is an important hormone-like growth factor for
epithelial and other cells (Tanumihardjo, 2011). Thus, in vitamin A deficiency, the number of goblet cells are reduced in epithelial tissues, resulting in a reduction in mucous secretions with their antimicrobial components. Cells lining protective tissue surfaces flatten and accumulate keratin because they fail to regenerate and differentiate. All these changes result in diminished resistance to invasion by potentially pathogenic organisms. The immune system is also adversely affected by direct interference with production of some types of protective secretion and cells. As these changes in internal epithelial tissues occur, the external reflections of such changes are seen in the classical eye changes in xerophthalmia and xerosis.Ocular manifestations of vitamin A deficiency (VAD), termed “xerophthalmia” or “dry eye” has been recognized for a long time. The most frequently encountered of these signs is night blindness, which is the earliest manifestation of xerophthalmia. In VAD, the time required to regenerate rhodopsin is prolonged, thereby delaying adaptation time to dark environments. Night-blind young children tend to stumble when going from bright to dimly- lit areas and they tend to remain inactive at dusk and at night. No field applicable objective tool is currently available for measuring night blindness. There is also no clearly defined blood retinol level that is associated with the occurrence of the symptom.
Vitamin A is found naturally in cod liver oil, liver, sweet potato, carrot, broccoli, butter, kale, spinach, pumpkin, egg, papaya, mango, pea, milk, tomatoes, seaweed etc. Conversion of carotene to retinol varies from person to person (Borel et al., 2005; Tang et al., 2005).
1.1.2.2Vitamin C
Vitamin C (ascorbic acid) is a six-carbon lactone that issynthesized from glucose in the liver of most mammalian species, but not by humans, non-human primates and guinea pigs. These species do not have the enzyme gulonolactone oxidase, which is essential for synthesis of the ascorbic acid immediate precursor 2-keto-l-gulonolactone. The gene encoding for gulonolactone oxidase has undergone substantial mutation, resulting in the absence of a functional enzyme (Nishikimi et al., 1994; Nishikimi and Yagi 1996). Consequently, when humans do not ingest vitamin C in their diets, a deficiency state occurs with a wide spectrum of clinical manifestations. Clinical expression of vitamin C deficiency, scurvy, is a lethal condition unless appropriately treated. Thus, humans must ingest vitamin C to survive. Vitamin C is the most important vitamin in fruits and vegetables. Except human and other primates, most of the phylogenetically higher animals can synthesize vitamin C (L– ascorbate). More than 90% of the vitamin C in human diets is supplied by fruits and vegetables. Vitamin C is the generic termfor all compounds exhibiting the biological activity
of L-ascorbic acid. Ascorbic acid is the principal biologically active form but L– dehydroascorbic acid, an oxidation product, also exhibits biological activity. Vitamin C is required for the prevention of scurvy and maintenance of healthy skin, gums and blood vessels. It functions in collagen formation, absorption of inorganic iron, reduction of plasma cholesterol level, inhibition of nitrosoamine formation, enhancement of the immune system, and reaction with singlet oxygen and other free radicals. As an antioxidant, it reportedly reduces the risk of arteriosclerosis, cardiovascular diseases and some forms of cancer (Tan,
2012, Choi et al., 2007 and Kaviarasanet al.,2007). Vitamin C is an electron donor and therefore a reducingagent. All known physiological and biochemical actions of vitamin C are due to its action as an electron donor. Ascorbic acid donates two electrons from a double bond between the second and third carbons of the 6-carbon molecule. Vitamin C is called an antioxidant because, by donating its electrons, it prevents other compounds from being oxidized. However, by the very nature of this reaction, vitamin C itself is oxidized in the process.
1.1.2.3Vitamin E
Figure 1:Struture of Vitamin E Source: Atkinson et al., 2007
Vitamin E is a fat soluble vitamin with antioxidant properties. It is mostly found in green vegetables, grains, nuts and various vegetable oils, as well as in eggs and milk. It exists in eight different forms (alpha, beta, gamma and delta tocopherol and tocotrienol). All featured a chromanol ring, with a hydroxyl group that can donate a hydrogen atom to reduce free radicals and a hydrophobic side chain which allow for penetration into biological membrane. Of these, α-tocopherol has been most studied as it has the highest bioavailability, with the body preferentially absorbing and metabolizing it. It has been claimed that the alpha
tocopherol form is the most important lipid soluble antioxidants, and that it protects membranes from oxidation by reacting with lipid radicals produced in lipid peroxidation chain reaction (Traber and Atkinson, 2007). This removes the free radical intermediates and prevents propagation reaction from continuing. This reaction produces alpha tocopheroxyl radical that can be recycled back to the active reduced form through reduction by antioxidants, such as ascorbate, retinol or ubiquinol (Atkinson et al., 2007).
Figure2: Reaction of Vitamin E Source: Atkinson et al., 2007
The presence of the phenolic –OH group on the 6th carbon of the chromane ring is the most important group for its antioxidant activity. Dietary sources of vitamin E which include cotton seed oil, corn oil, sun flower oil, wheat germ oil and margarine are rich sources of the vitamin. The normal value of blood vitamin E is around 1mg/dl and it is transported chiefly in the lipoprotein fraction. Thus, the serum alpha tocopherol level of breastfeeding infants increases more rapidly than that of bottle fed infants (Chaterjea and Shinde, 2007). However, the role and importance of the various forms of vitamin E are presently unclear (Atkinson et al., 2007), and it has been suggested that the most important function of alpha tocopherol is as a signaling molecule, with this molecule having no significant role in antioxidant metabolism (Wager, et al., 2004).
1.2 Trace Minerals
1.2.1 Selenium
Selenium functions in the antioxidant system as a component of a family ofglutathione peroxidase enzymes. These enzymes prevent cellular damage by destroying hydrogen
peroxide and lipid hydroperoxides. Selenium also is involved in the deiodination of thyroxine (T4) to the more metabolically active triiodothyronine (T3) in tissues. The immune system is adversely affected by selenium deficiency, and it is well documented that selenium deficiency increases the incidence of mastitis and retained placenta in dairy cows. Sulfur and selenium have similar chemical properties and increasing dietary sulfur reduces the absorption of selenium (Ivancic and Weiss, 2001).It is best to get selenium through foods, as large doses of the supplement form can be toxic. Good food sources include fish, shellfish, red meat, grains, eggs, chicken, and garlic. Vegetables can also be a good source if grown in selenium-rich soils.There is also evidence that selenium is less bioavailable in legume hay than in grass hay or concentrates (Spears, 2003). Selenomethionine, which is the major form of selenium found naturally in feedstuffs and in selenized yeast, is more bioavailable in cattle than sodium selenite (Pehrson et al., 1989).
1.2.2 Zinc
Zinc is an essential component of over 70 enzymes found in mammalian tissues. Enzymes that require zinc are involved in protein, nucleic acid, carbohydrate, and lipid metabolism. Zinc is also important for normal development and functioning of the immune system, in cell membrane stability, and gene expression. Responses of cattle to zinc supplementation of principal diets have been highly variable, suggesting that dietary factors affect zinc bioavailability. However, dietary factors that may affect zinc bioavailability in ruminants are not well defined. Some studies suggest that high dietary calcium reduces zinc status in cattle (Spears, 2003).
1.2.3 Iron
Iron plays a vital role in oxygen transport in the blood as a component of hemoglobin and in oxygen storage and transport in muscle as a component of myoglobin. A number of cytochromes and iron-sulfur proteins involved in the electron transport chain also contain iron as an integral component. In addition several enzymes either contain iron or are activated by iron. Most principal diets are more than adequate in iron, and iron deficiency is unlikely in cattle unless parasite infestations or diseases exist that cause chronic blood loss.Factors affecting bioavailability of iron in ruminants have received little attention because of the abundance of iron in ruminant diets. High dietary iron, when provided in a form such as ferrous sulfate, that is highly bioavailable, has been associated with reduced performance, elevated liver and spleen iron concentrations, and decreased copper status ( Mullis et al.,
2003). Iron absorption is well regulated, but exposure to high dietary iron may overwhelm
homeostatic control mechanisms resulting in iron accumulation in tissues, especially the liver and spleen.
1.3Phytochemicals
Phytochemicals are secondary metabolites produced by plants. They give plants their color, flavor and smell and are part of a plant’s natural defense system (Ejele and Akujobi, 2011). These compounds have been linked to human health by offering protection against degenerative diseases (Liu, 2004; Dandjessoet al., 2012). Phytochemicals are present in varieties of plants utilized as important components of both human and animal diets. These include fruits, seeds, herbs and vegetables (Okwu, 2005). Different mechanisms have been suggested for the action of phytochemicals. They may act as antioxidants, or modulate gene expression and signal transduction pathways (Dandjessoet al, 2012). They may be used as chemotherapeutic or chemo preventive agents (Paolo et al., 1991).
1.3.1 Phytochemical Constituents of Plants
Phytochemicals are chemical compounds formed during the plant normal metabolic processes. These chemicals are often referred to as “secondary metabolites” of which there are several classes including alkaloids, flavonoids, coumarins, glycosides, gums, polysaccharides, phenols, tannins, terpenes and terpenoids (Harborne, 1973; Okwu, 2005). Phytochemicals are naturally occurring and are believed to be effective in combating or preventing diseases due to their antioxidant properties (Halliwell and Gutteridge, 1992; Ejeleet al., 2012). The medicinal values of these plants lie in their component phytochemicals, which produce the definite physiological actions on human body. The most important of these phytochemicals are alkaloids, tannins, flavonoids and phenolic compounds (Iwu, 2000).Some of these naturally occurring phytochemicals are anticarcinogenic and some others possess other beneficial properties, and are referred to as chemopreventers. Among the most investigated chemopreventers are some vitamins, plant polyphenols, and pigments such as carotenoids, chlorophylls, flavonoids, and betalains (Liu, 2003; Ejeleet al., 2012).
1.3.1.1Terpenoids
Terpenoids, also known as Isoprenoids are the major family of natural compounds, comprising of more than 40,000 different molecules (Okafor, 1983). The isoprenoid biosynthetic pathway produces both primary and secondary metabolites that are of great significance to plant growth and persistence (Trease and Evans, 2002). Terpenoids are
secondary metabolites that have molecular structures whose carbon backbones are made up of isoprene (2-methylbuta- 1, 3-diene) units. The terpenoids comprise of two isoprene units, containing ten carbon atoms. Among the primary metabolites produced by this pathway are: the phytohormones- abscisic acid (ABA); gibberellic acid (GAs) and cytokinins; the carotenoids; plastoquinones and chlorophylls involved in photosynthesis; the ubiquinones required for respiration; and the sterols that impact membrane structure (Harborne, 1973). Many of the terpenoids are important for the quality of agricultural products, such as the flavor of fruits and the fragrance of flowers like linalool (Singh, 2009). In addition, terpenoids can have medicinal properties such as anti-carcinogenic (e.g. Taxol and perilla alcohol), antimalarial (e.g. artemisinin), anti-ulcer, antimicrobial or diuretic (e.g. glycyrrhizin) activity (Harrawijnet al., 2001). The steroids in animals are biologically produced from precursors of terpenoid and sometimes terpenoids are added to proteins to increase their attachment to the cell membrane, a process known as isoprenylation (Singh, 2009).
1.3.1.2Flavonoids
Figure 3: Structure of Flavonoids
Source : Bergman et al., 2003
Flavonoids are polyphenolic compounds that are ubiquitous in nature and are categorized according to their chemical structure into flavones, anthocyanidins, isoflavones, catechins, flavonols, chalcones and flavanones (Robak and Gryglewski, 1988). They occur mostly in vegetables, fruits and beverages like tea, coffee and fruit drinks. They accumulate in plants as
phytoalexins defending them against microbial attack (Harborne, 1973) and fungal attack (Oloyede et al., 2010). Flavonoids have been found to possess many useful effects on human health. They have been shown to have several biological properties including anti- inflammatory activity, enzyme inhibition, antimicrobial activity, oestrogenic activity (Oliver- Bever, 1986; Malairajan et al., 2006), antioxidant and free-radical-scavenging ability (Robak and Gryglewski, 1988). Flavonoids have also been shown to exhibit anti-leukemic properties and mild vasodilatory properties useful for the treatment of heart disease (Odugbemi et al.,
2007).
1.3.1.3Saponins
Saponins are groups of secondary metabolites found widely distributed in the plant kingdom as plant glycosides, characterized by a skeleton of 30-carbon precursor oxidosqualene to which glycosyl residues are attached. They have sturdy foaming property (Harborne, 1973). They are subdivided into triterpenoid and steroid glycosides and are stored in plant cells as inactive precursors but are readily converted into biologically active antibiotics by plant enzymes in response to pathogenic attack (Okwu, 2005). Saponins protect plants against attack by pathogens and pets (Jerutoet al., 2011). These molecules also have substantial marketable value and are processed as drugs and medicines, foaming agents, sweeteners, taste converters and cosmetics (Kensil, 1996). Saponin containing plants are used as traditional medicines, especially in Asia, and are intensively used in food, veterinary and medical industries (Kensil,1996). The pesticidal activity of saponins has long been reported (Irvine,
1961). Saponin-glycosides are very lethal to cold-blooded organisms, but not to mammals (Kensil,1996). Plant extracts containing a high percentage of saponins are commonly used in Africa to treat water supplies and wells contaminated with disease vectors; after treatment, the water is safe for human drinking (Kensil,1996). Saponins induce a strong adjuvant effect to T-
dependent as well as T-independent antigens and also induce strong cytotoxic CD8+
lymphocyte responses and potentiate the response to mucosal antigens (Kensil, 1996). They have both stimulatory effects on the components of specific immunity and non-specific immune reactions such as inflammation (Chukwujekuet al., 2005) and monocyte proliferation (Aggarwal and Shishodia, 2006).
Saponins have long been known to possess lytic action on erythrocyte cell membranes and this property has been used in their detection. The haemolytic actions of saponins are alleged to be due to their affinity for the aglycone moiety of membrane sterols, mainly cholesterol with which they form undissolvable complexes (Davies, 1995).
1.3.1.4.Tannins
Tannins are an exceptional group of water soluble phenolic metabolites of relatively high molecular weight and having the ability to complex strongly with carbohydrates and proteins (Heldt and Heldt, 2005). Tannins are astringent, bitter plant polyphenols and the astringency from tannins is what causes the dry and pucker feeling in the mouth following the consumption of unripened fruit or red wine (Serafiniet al., 1994). They are grouped into two forms, hydrolysable and condensed tannins (Nityanand, 1997). Hydrolysable tannins are potentially toxic and cause poisoning if large amounts of tannin-containing plant material such as leaves of oak (Quercusspp.) and yellow wood (Terminalia oblongata) are consumed (Heldt and Heldt, 2005). Tannins are considered as one of the anti-nutrients of plant origin because of their ability to precipitate proteins, inhibit the digestive enzymes and impair the absorption of vitamins and minerals (Khattabet al., 2010).
Several health benefits have been attributed to tannins and some epidemiological associations of tannins with decreased frequency of chronic diseases have been established (Serrano et al.,
2009). Several studies have shown significant biological effects of tannins such as antioxidant or free radical scavenging activity as well as inhibition of lipid peroxidation and lipoxygenases in vitro (Amarowiczet al., 2000). They have also been shown to possess antimicrobial, antiviral antimutagenic and antidiabetic properties (Gafneret al., 1997). The antioxidant activity of tannins results from their free radical and reactive oxygen species- scavenging properties, as well as the chelation of transition metal ions that modify the oxidation process (Serrano et al., 2009).
1.3.1.5Steroids
Sterols are triterpenes which are based on the cyclopentanoperhydrophenanthrene ring system (Harborne, 1973). Sterols in plants are generally described as phytosterols with three known types occurring in higher plants: sitosterol (formerly known as β-sitosterol), stigmasterol and camposterol (Harborne, 1973). These common sterols occur both as free and as simple glucosides. Sterols have essential functions in all eukaryotes. Free sterols are integral components of the membrane lipid bilayer where they play important role in the regulation of membrane fluidity and permeability (Irvine, 1961). While cholesterol is the major sterol in
animals, a mixture of various sterols is present in higher plants, with sitosterol usually predominating. However, certain sterols are confined to lower plants such as ergosterol found in yeast and many fungi while others like fucoterol, the main steroid of many brown algae is also detected in coconut (Harborne, 1973).
1.3.1.6Alkaloids
Alkaloids play very important roles in an organism’s metabolism and functional activity. They are metabolic products in plants, animals and micro-organisms. They occur in both vertebrates and invertebrates as endogenous and exogenous compounds. Many of them have a disturbing effect on the nervous systems of animals. Alkaloids are the oldest successfully used drugs throughout the historical treatment of many diseases (Aladesanmiet al., 1998) and are one of the most diverse groups of secondary metabolite found in living organism. They have an array of structural types, biosynthetic pathways, and pharmacological activities (Tankoet al., 2008). In plants and insects, toxic alkaloids are sequestered for use as a passive defense mechanism by acting as deterrents for predating insects (Eyonget al., 2006).
Alkaloids have been used throughout history in folk medicine in different regions around the world. They have been constituent parts of plants used in phytotherapy. Many of the plants that contain alkaloids are just medicinal plants and have been used as herbs. Some alkaloids that have played an important role in this sense include aconitine, atropine, colchicine, coniine, ephedrine, ergotamine, mescaline, morphine, strychnine, psilocin and psilocybin (Aladesanmiet al., 1998).
Many alkaloids are known to have effect on the central nervous system. Some alkaloids act as antiparasitic (such as morphine, a pain killer). For example, quinine was widely used against Plasmodium falciparum. In this respect, it has been found from phytochemical screening that most plants traditionally used to treat malaria contain alkaloids among other things (Tor- anyiinet al., 2003; Mahesh and Statish, 2008; Jerutoet al., 2011).
1.4Fruits
Fruits are referred to as the juicy seed bearing structure of a flowering plant that may be eaten as food (Hyson, 2002). In botany, a fruit is a part of a flowering plant that derives from specific tissues of the flower, one or more ovaries, and in some cases accessory tissues (Lewis, 2002). Fruits are the means by which these plants disseminate seeds. Many of them that bear edible fruits, in particular, have propagated with the movements of humans and
animals in a symbiotic relationship as a means for seed dispersal and nutrition, respectively; in fact, humans and many animals have become dependent on fruits as a source of food (Lewis, 2002). Fruits account for a substantial fraction of the world’s agricultural output.
Fruits are generally high in fiber, water, vitamin C and sugars, and also contain various phytochemicals that do not yet have an RDA/RDI listing under most nutritional factsheets, and which research indicates are required for proper long-term cellular health and disease prevention. Regular consumption of fruit is associated with reduced risks of cancer, cardiovascular disease (especially coronary heart disease), stroke, Alzheimer disease, cataracts, and some of the functional declines associated with aging (Rui, 2003).
Diets that include a sufficient amount of potassium from fruits and vegetables also help reduce the chance of developing kidney stones and may help reduce the effects of bone-loss. Fruits are also low in calories which would help lower one’s calorie intake as part of a weight-loss diet.
1.4.1 Fruit Structure and Development
The outer, often edible layer, is the pericarp, formed from the ovary and surrounding the seeds, although in some species other tissues contribute to or form the edible portion. The pericarp may be described in three layers from outer to inner, the epicarp, mesocarp and endocarp.
Inside the ovary/ovaries are one or more ovules where the mega gametophyte contains the egg cell. After double fertilization, these ovules will become seeds. The ovules are fertilized in a process that starts with pollination, which involves the movement of pollen grain from the stamens to the stigma of flowers. After pollination, a tube grows from the pollen grain through the stigma into the ovary to the ovule and two sperm are transferred from the pollen to the mega gametophyte. Within the mega gametophyte one of the two sperm unites with the egg, forming a zygote, and the second sperm enters the central cell forming the endosperm mother cell, which completes the double fertilization process (Mauseth, 2003a). Later the zygote will give rise to the embryo of the seed, and the endosperm mother cell will give rise to endosperm, a nutritive tissue used by the embryo. As the ovules develop into seeds, the ovary begins to ripen and the ovary wall, the pericarp, may become fleshy (as in berries or drupes), or form a hard outer covering (as in nuts). In some multiseeded fruits, the extent to which the flesh develops is proportional to the number of fertilized ovules (Mauseth, 2003b). The pericarp is often differentiated into two or three distinct layers called the exocarp (outer
layer, also called epicarp), mesocarp (middle layer), and endocarp (inner layer). In some fruits, especially simple fruits derived from an inferior ovary, other parts of the flower (such as the floral tube, including the petals, sepals, and stamens), fuse with the ovary and ripen with it. In other cases, the sepals, petals and/or stamens and style of the flower fall off. When such other floral parts are a significant part of the fruit, it is called an accessory fruit. Since other parts of the flower may contribute to the structure of the fruit, it is important to study flower structure to understand how a particular fruit forms (Mauseth, 2003a).
There are three general modes of fruit development:
Apocarpous fruits develop from a single flower having one or more separate carpels, and they are the simplest fruits.
Syncarpous fruits develop from a single gynoecium having two or more carpels fused together.
Multiple fruits form from many different flowers.
Plant scientists have grouped fruits into three main groups, simple fruits, aggregate fruits, and composite or multiple fruits (Singh, 2004). The groupings are not evolutionarily relevant, since many diverse plant taxa may be in the same group, but reflect how the flower organs are arranged and how the fruits develop.
1.4.2 Uses
Many hundreds of fruits, including fleshy fruits like apple, peach, pear, kiwifruit, watermelon and mango are commercially valuable as human food, eaten both fresh and as jams, marmalade and other preserves. Fruits are also used in manufactured foods like cookies, muffins, yogurt, ice cream, cakes, and many more. Many fruits are used to make beverages, such as fruit juices (orange juice, apple juice, grape juice, etc.) or alcoholic beverages, such as wine or brandy.Apples are often used to make vinegar. Fruits are also used as gifts.Fruit Basket and Fruit Bouquet are some common forms of fruit gifts.Many vegetables are botanical fruits, including tomato, bell pepper, eggplant, okra, squash, pumpkin, green bean, cucumber and zucchini.Olive fruit is pressed for olive oil. Spices like vanilla, paprika, allspice and black pepper are derived from berries.
1.4.3 Storage
The plant hormone ethylene causes ripening of many types of fruit. Maintaining most fruits in an efficient cold chain is optimal for post-harvest storage, with the aim of extending and ensuring shelf life. All fruits benefit from proper post-harvestcare.
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EVALUATION OF NUTRIENT COMPOSITION AND ANTIOXIDANT PROPERTIES OF SELECTED COMMONLY CONSUMED AND UNDERUTILISED SEASONAL FRUITS IN NSUKKA METROPOLIS
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