TABLE OF CONTENTS                                                                                        

Title Page    –         –         –         –         –         –         –         –         –

Certification           –         –         –         –         –         –         –         –

Dedication   –         –         –         –         –         –         –         –         –

Acknowledgements         –         –         –         –         –         –         –

Table of Contents  –         –         –         –         –         –         –         –

CHAPTER ONE: INTRODUCTION

1.1     Classification and Dietary Sources of

Phenolic Compounds      –         –         –         –         –         –

1.1.1  Classification of Phenolic Compounds          –         –         –

1.1.1.1 Flavonoids –         –         –         –         –         –         –         –

1.1.1.2 Non-Flavonoids   –         –         –         –         –         –         –

1.2     Dietary Sources of Phenolic Compounds      –         –         –

1.2.1  Cereals         –         –         –         –         –         –         –         –

1.2.2  Fruits –         –         –         –         –         –         –         –         –

1.2.3  Vegetables   –         –         –         –         –         –         –         –

CHAPTER TWO: FACTORS AFFECTING BIOAVAILABILITY OF DIETARY PHENOLIC COMPOUNDS

2.1     Factors Related to Phenolic Structure            –         –         –

2.1.1  Solubility     –         –         –         –         –         –         –         –

2.1.2  Glycosylation        –         –         –         –         –         –         –

2.1.3  Acetylation and Methylation      –         –         –         –         –

2.1.4  Polymerization       –         –         –         –         –         –         –

2.2     Factors Related to Food   –         –         –         –         –         –

2.2.1  Food Matrix –         –         –         –         –         –         –         –

2.2.2  Food Processing    –         –         –         –         –         –         –

2.3     Factors Related to Host   –         –         –         –         –         –

2.3.1  Dietary Intake        –         –         –         –         –         –         –

2.3.2  Absorption and Metabolism       –         –         –         –         –

2.4     Other Factors         –         –         –         –         –         –         –

2.4.1  Distribution and Food Content   –         –         –         –         –

2.4.2  External Factors     –         –         –         –         –         –         –

CHAPTER THREE: HEALTH BENEFITS OF PHENOLIC COMPOUNDS

3.1     Cardio-Protective Effect  –         –         –         –         –         –

3.2     Anti-Cancer Effect           –         –         –         –         –         –

3.3     Anti-Diabetic Effect        –         –         –         –         –         –

3.4     Anti-Aging Effect            –         –         –         –         –         –

3.5     Neuro-Protective Effect   –         –         –         –         –         –

CHAPTER FOUR: SUMMARY AND RECOMMENDATIONS

4.1     Summary     –         –         –         –         –         –         –         –

4.2     Recommendations –         –         –         –         –         –         –

             References

CHAPTER ONE: INTRODUCTION

Phenolic compounds are secondary metabolites that are derivatives of the pentose phosphate, shikimate and phenylpropanoid pathways in plants. They are essential to the physiology of plants, because of their involvement in various important functions (growth, structure, defense, pigmentation, lignifications etc). The majority of polyphenols are synthesized by the highly branched phenyl propanoid pathway, which is responsible for the biosynthesis of a large number of chemical compounds with considerable structural diversity. They are largely found in fruits, vegetables, cereals and beverages (Huang et al., 2005).

They are a diverse group of chemicals having one feature in common that is the presence of at least one aryl ring to which a minimum one hydroxyl group is attached. Fontana et al. (2002) reported that plant phenolics and terpenoids are being widely used because of their strong antimicrobial property against food borne pathogens and therefore, could be applied as novel preservatives in the food industry. In food, polyphenols may contribute to the bitterness, astringency, colour, flavor, odour and oxidative stability. Phenolic compounds exhibits a wide range of phsyiolgoical properties, such as anti-allergenic, anti-artherogenic, anti-inflammatory, anti-microbial, antioxidant, anti-thrombotic, cardio-protective and vasodilatory effects (Arts et al., 2000; Graf et al., 2005). Polyphenols can act as metal chelators which adds to the antioxidants effects of these compounds through inhibition of transition metal catalyzed free radical formation. They also act as inhibitor of pre harvest seed germination (Bravo, 1998; Haslam, 1998). In view of the fact that these compounds are present in all plant foods, this paper was designed to highlight the phenolic compounds and its  possible health benefits on humans.

1.1 Classification and Dietary Sources of Phenolic Compounds

1.1.1 Classification of Phenolic Compounds

Phenolic compounds are derived from the secondary metabolites of plants and they are chemical compounds that have at least one aromatic ring to which one or more hydroxyl groups are bounded to aromatic or aliphatic structures (Bravo, 1998). There is a wide variety of phenolic compound and are classified on the basis of the number of phenol rings that they contain into two main groups: flavonoids and non flavonoids .

1.1.1.1 Flavonoids

Flavonoids constitute the largest class of phenolic compounds with more than 3,000 structures. These consist of two aromatic rings bound together by three carbon atoms that form an oxygenated heterocycle (Tsao, 2012). Due to the hydroxylation pattern and variation in the chromane ring (Ring C), flavonoids can be further divided into different sub-groups such as anthocyannins, flavan-3-ols, flavones, flavanones and flavonol (Merken and Beecher, 2000) of which flavones and flavonol are most widely occurring. Individual differences within each group arise from the variation in number and arrangement of the hydroxyl groups and their extent of alkylation and/or glycosylation (Spencer et al., 2008). More than 4,000 varieties of flavonoids have been identified which are responsible for the attractive colours of the flowers, fruits, and leaves. Quercetin, epicatechin, myricetin, catechins, etc are some of the common flavonoids.

Flavonoids accumulate in the outer and aerial tissues (skin and leaves) in fruits since their biosynthesis is stimulated by light. In leafy vegetables such as lettuce and cabbage, the glycoside concentration is ≥ 10 times as high in the green outer leaves as in the inner light coloured leaves.

Flavones on the other hand are much less common than flavonols in fruits and vegetables. Flavnones consist chiefly of glycosides of luteolin and apigemin and are found in parsley and celery. Flavanones are found in human foods such as tomatoes and certain aromatic plants like mint. However, they are present in high concentrations only in citrus fruits. In foods, flavanones are generally glycosylated by a disaccharide at position 7 and this imparts a bitter tastes (Tomas-Barberan and Clifford, 2000).

Flavanols exist in monomer (catechines) as well as polymer form (proanthocyanidins). Catechines are found in many types of fruits (e.g apricots) and red wine, but green tea and chocolate are by far the good sources. In contrast to other classes of flavonoids, flavanols are not glycosylated in foods. For example, the tea epicatechins are remarkably stable when exposed to heat as long as the pH is acidic (Zhu et al., 1997).

1.1.1.2 Non-Flavonoids

Non-flavonoids like benzoic acid and cinnamic compound which are commonly called phenolic acids contain an aromatic ring that can be attached to different functional groups or esterified to organic acids.

Phenolic acids are widely dispersed in plant kingdom. They contain two distinguishing constitutive carbon frame-works, hydroxycinnamic (C₆ C₃) and hydroxybenzoic (C₆C₁) structure. The hydroxybenzoic acid content of edible plant is generally low, with the exception of certain red fruits, black radish and onions which have high concentration (Shahidi and Naczk, 1995). The hydroxycinnamic acids are more common than hydroxybenzoic acids and consist chiefly of p-coumaric, caffeic, ferulic and sinapic acids (Han et al., 2007). Phenolic acid have antioxidant properties due to their high redox potential, which allows them to act as reducing agents and singlet oxygen quencher (Ignat, 2011). Some other phenolic compounds are stilbenes, tannins, lignins and lignans which cause astringency,colour  and flavor in plants. Stilbenes contain two phenyl moieties connected by a two-carbon methylene bridge. They exist as stereo-isomers and naturally occurring stilbenes are present in the fransforms. They also occur in free and glycoylated forms and as dimeric, trimeric and polymeric stilbenes occurrence of stilbenes in the human diets is quite low since they are produced in plants in response to infection by pathogens or to a verity of stress conditions.

Lignans are diphenolic compound that contain a 2,3-dibenzylbutane structure which is formed by the dimerization of two cinnamic acid residues Several lignans are considered to be phytoesterogens and the riches dietary source is linseed.

1.2     Dietary Sources of Phenolic Compounds

The rich sources of phenolic compounds are cereals, fruits and vegetables.

1.2.1  Cereals

Phenolic compounds are among the health-promoting phytochemicals present in cereals. Records have shown that phenolic acids and flavonoids are the most common types of phenolic compounds found in whole grains. In cereals, phenolic compounds can be present in the free or bounded form. Bound phenolics are mostly attached to arabinosyl chains of cell wall arabinoxylans and most of these bound phenolic compounds are located in the aleurone layer, but can also be found in seed and embryos (Bondia-Pons et al., 2009; Vitaglione et al., 2008). Phenolic acids such as coumaric, gallic, hydoxybenzoic, vinillic and sinapic acid are reported to be found in both free and bounded form in durum wheat, bread wheat, barley, oat, rye, rice, corn etc. (Irakli et al., 2012). In cereals, the free phenolic acids, constitute a small prortion of the total phenolic content while bound phenolic acids are the most predominant. For example, the total bound phenolic acids constitutes from 88% (rye) to 99.5% (corn) of the total phenolic acids.

1.2.2  Fruits

Apples are one of the most popular fruits whose health benefits are attributed to phenolic compounds. The four polyphenol groups predominant in apples are flavan-3-.1s, phenolic acids, dihydrochalcones and flavonols (Ceymann et al., 2012). Some phenolics such as chlorogenic acid, phloretin, epicatechin, quercetin and procy anidin B2 have been identified as major antioxidants in apple. (Lee et al., 2003) Records have also shown that mango fruits contain several bioactive compounds such as vitamins, carotenoids, terpenoids and phenolic compounds. Phenolic acids like gallic, protocatechin, chlorogenic and vanillic acids are found predominant in mango pulp (Palafox-Carlos et al., 2012). Furthermore, hydrolysable tannins and mangiferin are reported to be present in mango pulp of different varieties (Naratto et al., 2014).

Citrus fruits which are rich in various nutrients like vitamins A and C, folic acid and dietary fiber are known to have considerable amounts of flavonoids such as flavones, flavonols and anthocyanins. Other phenolics often found in citrus are p-caumaric, ferulic, caffeic and sinapic acids (Benevente-Garcia et al., 1997; Sun et al., 2013).

1.2.3  Vegetables

Phenolic acids and isocoumarins were the predominant phenolics in carrots (Kammerer et al., 2014). Among the most common phenolic compounds found in vegetables are flavonoids, phenolic acids and isocoumarins. For example. most compounds detected in black carrot roots and black carrot juice are composed of p-coumaric, caffeic and ferulic acids, although 5-caffeoylquinic acid was the predominant phenolic acid.

Apart from carrot, tomatoes was also found to contain a total of 38 phenolic compounds, among which gallic acid, protocatechuic acid caffeic acid derivatives, ferulic acid derivatives and quercetin were present (Vallrerdu-Queralt et al., 2014). Likewise lettuce is a dietary source of phenolic compounds with flavonoils present in red varieties in higher quantities while the main phenolic compounds in green varieties include caffeoylmalic acid, dicaffeoyltartaric acid, flavonoid glycosides etc. (Llorach et al., 2008). Other vegetables like onions, spinach and pepper fruits are also a rich source of flavonoids as reported by Naczk and Shahidi, 2006).