TABLE OF CONTENTS
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
LIST OF PLATES AND FIGURES
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
LIST OF TABLES
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
ABSTRACT
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.
CHAPTER ONE
INTRODUCTION
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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