EVALUATION OF NUTRIENT COMPOSITION AND ANTIOXIDANT PROPERTIES OF SELECTED COMMONLY CONSUMED AND UNDERUTILISED SEASONAL FRUITS IN NSUKKA METROPOLIS

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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.



This material content is developed to serve as a GUIDE for students to conduct academic research


EVALUATION OF NUTRIENT COMPOSITION AND ANTIOXIDANT PROPERTIES OF SELECTED COMMONLY CONSUMED AND UNDERUTILISED SEASONAL FRUITS IN NSUKKA METROPOLIS

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