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Content                                                                                                                Page Number

Title page                                                                                                                                i

Certification                                                                                                                            ii

Dedication                                                                                                                              iii

Acknowledgment                                                                                                                   iv

Table of Contents                                                                                                                   v

List of plates                              ix

List of Figures                                                                                                                         ix

List of Tables                                                                                                                          x

Abstract                                                                                                                                  xi

CHAPTER ONE: Introduction                                                                                           1

  • Aim of Study                                                            2

1.2       Specific Objectives of Study                                                                                      2

CHAPTER TWO: Literature Reviews                                                                               3

2.1       Onions (Allium cepa)                                                                                                  4

2.1.1     Types of Onion                                                                                              4

2.1.2    Traditional Uses of Onions                                                                             7

2.1.3    Proximate and Chemical Composition of Onion                                            8

2.1.4    Antioxidant Potentials of Onion                                                                    9

2.1.5    Blood Sugar Regulation by Onions                                                                9

2.2       Processing of Onions                                                                                                  10

2.2.1    Effect of Processing on Antioxidant Activity                                                            11

2.3       Oxidative Stress                                                                                                          11

2.3.1    Mechanism of Free Radical and Oxidative Stress                                          12

2.3.2    Causes and Effect of Oxidative Stress                                                           13

2.3.3    Oxidative Stress and Diseases                                                                        14

2.3.4    Oxidative Stress, Free Radicals and Aging                                                    16

2.4       Antioxidant                                                                                                                 17

2.4.1    Sources of Antioxidants                                                                                 20

2.4.2    Classification of Antioxidants                                                                                    21

2.4.3    Examples of Antioxidants                                                                              23

2.4.4    Characteristics of Antioxidants                                                                      29

2.4.5    Chemistry of Antioxidants                                                                             30

2.4.6    Biological Activities of Antioxidants                                                             31

2.4.7    Antioxidant Assays                                                                                        32

CHAPTER THREE: Materials and Methods                                                                   37

3.1       Materials                                                                                                                     37

3.1.1    Apparatus                                                                                                        37

3.1.2    Reagents                                                                                                         37

3.1.3    Collection and Preparation of Sample                                                            37

3.2       Methods                                                                                                                      38

3.2.1    Method of Extraction                                                                                     38

3.2.2    Determination of Total Phenolic Content (TPC) of Allium cepa Extract       38

3.2.3    Determination of Total Flavonoid Content of Allium cepa L. Extract          39

3.2.4    Determination of Reducing Power of Allium cepa L. Extract                       39

3.2.5    Determination of DPPH Radical Scavenging Activity by Allium

cepaL. Extract                                                                                    40

3.2.6    Determination of ABTS+ Radical Scavenging Activity by Allium

cepa L. Extract                                                                                                40

3.2.7    Determination of Ascorbic acid (vitamins C) on Allium cepa L. Extract       41

3.4       Statistical Analysis                                                                                                      41

CHAPTER FOUR: Results                                                                                                 42

CHAPTER FIVE: Discussion and Conclusion                                                                  61

5.1       Discussion                                                                                                                   61

5.2       Conclusion                                                                                                                  64

References                                                                                                                             65












Plates and Figures                                                                                         Page Number

2.1:      Yellow Onions                                                                                                            4

2.2:      Red Onions                                                                                                                 5

2.3:      White Onions                                                                                                              5

2.4:      Pearl Onions                                                                                                                6

2.5:      Spring Onions                                                                                                             7

2.6:      Sprouted Onions                                                                                                         10

2.7:      Overview of Reactions leading to ROS Formation                                                    13

2.8:      Oxidative Stress and Antioxidant Pathway                                                               15

2.9:      Biochemical Mechanism of Aging                                                                              17

2.10:    Chemical Structure of Vitamin C                                                                               23

2.11:    Chemical Structure of Phenols                                                                                   26

2.12:    Chemical Structure of Flavonoid                                                                                27

2.13:    Chemical Structure of 2,2- diphenyl-1- picrylhydrazl                                                34

2.14:    Mechanism of Inhibition of Polymer chain, R, by DPPH                                          34

2.15:    Chemical Structure of ABTS                                                                                      35

4.1:      Ascorbic acid content of differently sprouted onions                                                            60

4.2:      Ascorbic acid content of differently boiled Onions                                                   60









Tables                                                                                                             Page Number

4.1:      Total Phenolic Content (mg/g PE) of Differently Sprouted Onions                          42

4.2:      Total Phenolic Content (mg/g PE) of Differently Boiled Onions                              42

4.3:      Total Flavonoid Content (mg/g RE) of Differently Sprouted Onions                       45

4.4:      Total Flavonoid Content (mg/gRE) of Differently Boiled Onions                            45

4.5:      Reducing Power (mg/AAE) of Differently Sprouted Onions                                                47

4.6:      Reducing Power (mg/AAE) Differently Boiled Onions                                            47

4.7:      Reducing Power (mg/AAE) of Different Concentrations of Methanol

Extract of Sprouted Onions                                                                                        49

4.8:      Reducing Power (mg/AAE) of Different Concentrations of Methanol Extract of

Boiled Onions                                                                                                             49

4.9:      DPPH Radical Scavenging Activity (%) of Differently Sprouted Onions                51

4.10:    DPPH Radical Scavenging Activity (%) of Differently Boiled Onions                    51

4.11:    DPPH Radical Scavenging Activity (%) of Different Concentrations of

Sprouted Onions Extract                                                                                            53

4.12:    DPPH Radical Scavenging Activity (%) of Different Concentrations of Boiled

Onions Extract                                                                                                            53

4.13:    ABTS+ Radical Scavenging Activity of Sprouted Onions                                         55

4.14:    ABTS+ Radical Scavenging Activity (%) of Differently Boiled Onions                   55

4.15:    ABTS+ Radical Scavenging Activity (%) of Different Concentrations of

Sprouted Onions Extract                                                                                            57

4.16:    ABTS+ Radical Scavenging Activity (%) of Differently Boiled Onions Extract      57





After sprouting of onions, the shoots are used as vegetables and bulbs discarded. These usually discarded onion bulbs may have improved antioxidant potentials resulting from sprouting. These improved properties could be harnessed to combat or manage some degenerative and non-communicable diseases. This study was therefore conducted to determine the effects of sprouting and boiling on the antioxidant potentials of onions (Allium cepa L.).  Samples were either sprouted for 0 – 10 days or boiled for 0 – 8 minutes. Phytochemical (total phenols, flavonoids, ascorbic acid) analyses and antioxidant activities such as reducing power, DPPH and ABTS radicals scavenging activities were used to assess antioxidant potentials using standard methods.The results show that regardless of the nature of solvent, boiling resulted in significant(P<0.05) reduction in total phenols content. The least reduction (36%) was observed in the chloroform extract of samples boiled for 8min. Aqueous and methanol extracts recorded 42% reductions. A significantly (P<0.05) higher total flavonoid content was expressed in methanol extract of onions sprouted for eight days (7.84mg/g RE). The reducing potential was found to be concentration dependent up to 0.4 mg/ml. From the EC50 values, the strongest reducing power was exhibited by the extract from onion bulbs sprouted for 6 days while the weakest reducing power was noticed with day 8 (EC50 = 0.73) and day 10 (EC50 = 0.74). This trend follows the same pattern with ascorbic acid content. In DPPH and ABTS, the strongest radical scavenging capacity was exhibited by the extract from onion bulbs sprouted for 4 days (EC50 = 0.01) and (EC50 = 0.002) respectively. Generally, sprouting for 2 – 8 days resulted in a significant (P<0.05) increase in all the antioxidant parameters tested. This was followed by a slight but significant decrease at the 10th day of sprouting. Boiling for up to 8 minutes resulted in significant losses in all antioxidant parameters tested.The present study shows that sprouted onions demonstrated higher antioxidant activity and can be considered as good sources ofnatural antioxidants.





Processing methods are known to have variable effects on total phenolic compound and antioxidant activity of plant samples. Effects include little or no change, significant losses, or enhancement in antioxidant activity (Nicoli et al., 1999). Food processing can improve the properties of naturally occurring antioxidants or induce the formation of new compounds with antioxidant activity, so that the overall antioxidant activity increases or remains unchanged (Tomaino et al., 2005).

Antioxidants present in vegetables are very useful and beneficial to health and have been associated with reduced risk of cardiovascular diseases and various forms of cancer (Kumud et al., 1990). These benefits have led to research studies in order to find antioxidants in plant material mainly used as foods (Yang et al., 2008). Among the compounds with antioxidant properties are the phenolics, which are believed to act as antioxidant, anti-carcinogenic, anti-microbial, anti-allergic, anti-mutagenic and anti-inflammatory, as well as in the reduction of cardiovascular diseases (Vali et al., 2007). Phenolics occur naturally in plants and are present in fruits, vegetables, leaves, nuts, seeds and flowers; therefore, they are present in the human diet, but are also used in some medicinal preparations (Madrau et al., 2008).

Onions (Allium cepa Linn) is used as foodstuff, condiments, flavouring agent, and in folk medicine (Ola-Mudathir and Maduagwu, 2014). It has been extensively studied for their therapeutic uses as antibiotic, antidiabetic, anti-atherogenic and anticancer (Augusti, 1996). It has been found that administration of onion products to diabetic rats significantly reduced hyperglycaemia (Kumud et al., 1990). Biological action of Allium products is ascribed to organosulfur compounds, which have also been shown to possess antioxidant and free radical scavenging activities. Onions have previously been shown to protect testis against cadmium induced oxidative stress in rats (Ola-Mudathir et al., 2008). Keeping this in mind, many studies have reported losses in total phenolic content (Ismail et al., 2004; Roy et al., 2007; Toor and Savage, 2006). These losses in antioxidant property of heat-treated samples were attributed to the leaching of phenolic compounds into water (Larrauri et al., 1997) as well as other methods of food processing. However, there still remains paucity of information on the effect of different processing methods on the antioxidant status of onions which is essentially used for the preparation of delicacies as well as in the preparation of decoctions used by trado-medical practitioners for treatment of some ailments. There is however a few reported studies on the effect of domestic processing on the antioxidant potentials of onions. Such information would be more relevant considering the fact that onions are rarely consumed raw without processing. Common processing methods include: remover of the outer layer, chopping into smaller pieces before boiling, grilling or flying in oil. Several cultures also subject onions to sprouting for the purpose of using the shoots as vegetables. After sprouting, the onions bulbs are usually discarded while the shoots are processed further. These usually discarded onion bulbs may have antioxidant potentials resulting from sprouting. These improved properties could be harnessed to combat or manage some degenerative and non-communicable diseases.

1.1       AIM OF STUDY

This study is geared towards determining the effects of sprouting and boiling on the antioxidant potentials of onions (Allium cepa L.).

            The Specific Objectives Are:

  • the total phenolic and flavonoid content of sprouted and boiled onions (aqueous, chloroform, and methanol).
  • the Vitamin C content of sprouted and boiled onions (extract of methanol, chloroform and water).
  • the antioxidant properties of sprouted and boiled onions (extract of methanol, chloroform and water).


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