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Product Code: 00007248

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This study investigated the effect of different cooking methods on glycemic index (GI) of three yam cultivars commonly eaten in Nigeria namely: Dioscorea rotundata, Dioscorea alata and Dioscorea dumentorum. Yam tubers were processed by, frying, pounding and boiling, and then immediately consumed by twenty-seven normal healthy adults comprising of males and females. Capillary blood samples were taken at the fasting state and at different intervals of 15, 30, 45, 60, 90 and 120 minutes from the consumption of each meal. The blood samples were analysed for plasma glucose concentration and incremental areas under plasma glucose curves were calculated. The GI varied between 22.31 –72.78 for the yam cultivars studied. Samples prepared by boiling recorded Dioscorea alata as low GI (22.31) and Dioscorea rotundata as high GI (72.78). While those processed by frying and pounding, recorded a low GI (20.95 – 44.35) and Dioscorea dumentorum having the least GI value (20.95). The study indicates that the glycemic index of the three yam cultivars (Dioscorea rotundata, Dioscorea alata and Dioscorea dumentorum) varies significantly by their methods of preparation and variety differences. Consumption of boiled and pounded Dioscorea alata could minimize postprandial blood glucose spikes and therefore, may prove to be more efficacious in the management of type 2 diabetes mellitus. Proximate composition (moisture, protein, fat, ash, fibre and carbohydrates) were determined for the raw and processed samples prior to glycemic analysis. The functional properties and amylose /amylopectin ratio of the flour sample were determined. The result of this study showed a significant difference (p<0.05) in the functional properties of the yam flour samples, the moisture content significantly differed (p<0.05) in different cooking methods and various varieties. There was no significant difference in the protein content of the different yam varieties. Low protein values observed in this study suggested that yam is not a good source of protein. At different levels of processing there was a significant difference in the fat content when compared to the raw samples with an increase on the fried samples and a decrease on boiled and pounded samples. There was a significance difference (p<0.05) in the crude fiber content of the different yam varieties at its raw state with Dioscorea dumentorum having the highest value (1.30). No significance difference was seen on the ash content of the boiled Dioscorea rotundata and Dioscorea dumentorum. The carbohydrate content differed significantly (p<0.05) amongst the yam varieties and their various processing methods. The variety main effect were significantly different (p<0.05) while the processing method main effect were also significantly different (p<0.05).


Title page                                                                                                                    i

Declaration                                                                                                                  ii

Certification                                                                                                               iii

Dedication                                                                                                                  iv

Acknowledgement                                                                                                     v

Table of Contents                                                                                                      vi

List of Tables                                                                                                              ix

List of figures                                                                                                             x

List of Plates                                                                                                               xi

Abstract                                                                                                                     xii



1.1       Background of the Study                                                                               1

1.2       Statement of Problem                                                                                      3

1.3       Justification of Study                                                                                      4

1.4       Objectives                                                                                                       4        

CHAPTER 2: LITERATURE REVIEW                                                             

2.1       Glycemic Index                                                                                               5

2.2       History of Glycemic Index                                                                              7

2.3       Coining of the Concept of Glycemic Index                                                    8

2.4       Classification of Glycemic Index                                                                    9

2.5       Measurement and Interpretation of Glycemic Index                                     12

2.6       Factors that Affect the Glycemic Index of a Food                                        13

2.6.1    Cooking method                                                                                             13

2.6.2    Processing method                                                                                          13

2.6.3    Type of starches                                                                                              14

2.6.4    Fibre                                                                                                                14

2.6.5    Sugar                                                                                                               15

2.6.6    Fat                                                                                                                   15

2.6.7    Acidity                                                                                                            15

2.6.8    Physical entrapment                                                                                        15

2.7       Carbohydrate Metabolism                                                                              16

2.8       Role of insulin in Diabetes Mellitus                                                                17

2.8.1    Type 1 diabetes mellitus                                                                                 18

2.8.2    Type 2 diabetes mellitus                                                                                 18

2.9       Application and Clinical use of Glycemic Index Values                                20

2.9.1    Glycemic index and obesity                                                                           20

2.9.2    Glycemic index and diabetes                                                                          21

2.9.3    Glycemic index and heart disease                                                                  24

2.9.4    Glycemic index and cancer                                                                             25

2.9.5    Enhancing satiety                                                                                            25

2.10     Yam                                                                                                                26

2.10.1 Background of yam                                                                                        26

2.11     Botanical and Agronomic Characteristics of Yams                                        27

2.12     Origin and Current Distribution of Major Food Yam Species                       28

2.13     Nutritional Value of Yam                                                                               30

2.14     Chemical Composition of Yam                                                                      31

2.15     Distribution of Yam in Nigeria                                                                       32

2.16     Constraints to Yam Production                                                                      33

2.17     Prospects of Yam Production                                                                                     34

2.18     Yam Food Preparation Method in West Africa                                             35

2.19     Production Process of Yam Flour                                                                   35

2.20     Yam Minisett Technology                                                                              37

2.21     Development of Non-Stake Yam Genotypes                                                 38



3.1       Sources of Raw Material                                                                                39

3.2       Sample Preparation                                                                                         39

3.3       Determination of Glycemic Index                                                                  42

3.4       Proximate Composition Analysis                                                                    42

3.4.1    Determination of moisture content                                                                 42

3.4.2    Determination of crude protein                                                                      43

3.4.3    Ash content determination                                                                             44

3.4.4    Crude fibre determination                                                                              44

3.4.5    Fat content determination                                                                               45

3.4.6    Determination of carbohydrate                                                                       45

3.5       Physicochemical Analysis                                                                               46

3.5.1    Determination of amylose/ amylopectin content                                            46

3.6       Functional Properties                                                                                      46

3.6.1    Bulk density                                                                                                    46

3.6.2    Water absorption capacity                                                                              47

3.6.3    Swelling index                                                                                                47

3.6.4    Gelatinization temperature                                                                              48

3.7       Statistical Analysis                                                                                          48


CHAPTER 4: RESULTS AND DISCUSSION                                                   

4.1       Bulk Density                                                                                                   50

4.2       Water Absorption Capacity                                                                            50

4.3       Swelling Index Capacity                                                                                51

4.4       Gelatinization Temperature                                                                             51

4.5       Amylose and Amylopectin                                                                             54

4.6       Moisture Content                                                                                            55

4.7       Protein Content                                                                                               58

4.8       Fat Content                                                                                                     60

4.9       Crude Fibre Content                                                                                       62

4.10     Ash Content                                                                                                    66

4.11     Carbohydrates                                                                                                 68

4.12     Glycemic Index                                                                                              69



REFERENCES                                                                                                        75





2.0:  Glycemic Index classification                                                                              11

4.1:  Functional properties of Dioscorea rotundata, Dioscorea alata

     and Dioscorea dumentorum                                                                       49

4.2:  Starch composition of Dioscorea rotundata, Dioscorea alata  

    and Dioscorea dumentorum Flour                                                               53

4.3:  Percentage moisture content of Dioscorea rotundata, Dioscorea

    alata and Dioscorea dumentorum                                                              57

4.4:  Percentage crude protein content of Dioscorea rotundata, Dioscorea

    alata and Dioscorea dumentorum                                                               59

4.5:  Percentage crude fat content of Dioscorea rotundata, Dioscorea

   alata and Dioscorea dumentorum                                                                61

4.6:  Percentage Crude Fiber content of Dioscorea rotundata, Dioscorea

   alata and Dioscorea dumentorum                                                                63

4.7:  Percentage ash content of Dioscorea rotundata, Dioscorea

    alata and Dioscorea dumentorum                                                               65

4.8:  Percentage Carbohydrate content of Dioscorea rotundata, Dioscorea

  alata and Dioscorea dumentorum                                                                 67

4.9:  Glycemic index of Dioscorea rotundata, Dioscorea

              alata and Dioscorea dumentorum                                                                 70









2.1:      Flow chart for yam flour production                                                              36

3.1:      Flow chart for production of yam samples                                                     40











1-3:  Pounded Samples of Dioscorea rotundata, Dioscorea                                      41

                   alata and Dioscorea dumentorum   


4-6:  Fried Samples of Dioscorea rotundata, Dioscorea                                            41

                     alata and Dioscorea dumentorum   


7-9:  Boiled Samples of Dioscorea rotundata, Dioscorea                                          41

                    alata and Dioscorea dumentorum 









The glycemic index (GI) is a dietary measuring system that ranks carbohydrate containing food and relates the rate at which the blood sugar is increased after two hours of consuming the food to a reference food usually glucose (Wolver, 1993).Consumption of low glycemic index foods is suggested to be effective for the prevention and control of diabetes (Little, 2003). Until recently, it was widely held that the blood glucose response to different diets is determined solely by the quantity of carbohydrate they contain and sugars being swiftly absorbed produce greater responses than starch during and after digestion (Wolver, 1993). This consequently led to the traditional diabetes diet plans in which the amount of food allowed were based upon their carbohydrate content (American Diabetes Association, 2002; WHO, 1985). This concept was however challenged by the findings of Jenkins et al. (1981) that sugars raise blood glucose to the same extent as several starchy foods and hence developed the glycemic index.

Glycemic index is expressed as percentages on an absolute scale. According to this system, carbohydrates containing foods are graded as either having low, intermediate and high glycemic index depending on the rate at which blood sugar level rises (Mendosa, 2000).  This in turn is related to the rate of digestion and absorption of sugars and starches available in that food (FAO, 1998). The low GI foods are ≤ 55 while Intermediate (medium) glycemic foods are those that are between 56 and 69 and high glycemic index ranges from 70 and above on the glycemic scale.

New studies have shown that the regular consumption of diets containing high glycemic index foods increases the risk of type 2 diabetes mellitus (Salmeron et al., 1997) and coronary heart disease (Ford and Liu, 2001). Type 2 Diabetes (previously known as non-insulin dependent Diabetes) is the most popular form of Diabetes characterized by hyperglycemia, insulin resistance and relative insulin deficiency (Maitra and Abbas, 2005). Type 2 Diabetes arises from interaction between genetic, environmental and behavioural risk factors (Chen et al., 2011). Individuals living with type 2 Diabetes are more prone to various forms of both short and long-term complications, which usually lead to their premature death. In contrast, the inclusion of low glycemic index foods in diet without a change in the total amount of carbohydrate consumed may improve blood glucose control, Brand et al. (1992); reduce serum triglycerols, Jenkins et al. (1981); prolong endurance during physical activity and improve insulin sensitivity, Frost et al. (1998).

Yams (Dioscorea  spp) are annual root tuber bearing plants with more than 600 species out of which six are socially and economically important in terms of food, cash and medicine (IITA,2009). Yam production has increased steadily in the last decade, from 18 million metric tonnes in 1990 to recent estimates of over 39 million (FAO 2006). Several edible species which are common are white yam (Dioscorea rotundata), yellow yam (Dioscorea esculentum), three leaved yam (Dioscorea dumentorum), water yam (Dioscorea alata) and aerial yam (Dioscorea bulbifera) are the most important of those commonly consumed in Nigeria and West African countries (Ihekoronye and Ngoddy, 1985; Okonkwo 1985). Moreover, the glycemic indices of these yam species are not quite known. Okonkwo (1985) opined that the recommendation of water yam and some cocoyam varieties as tolerable energy sources to diabetics is worthy of further investigation. Thus, it is important to have the knowledge of the glycemic indices of these commonly consumed yam species processed in different ways that yams are eaten (Dioscorea rotundata, Dioscorea  alata and Dioscorea dumentorum) as it will aid in evidenced meal planning and optimum food selection in West Africa and Nigeria in particular.



Obesity and metabolic diseases like diabetes mellitus and cardiovascular diseases (CVD) are on the increase, especially in certain young individuals. The World Health Organization reported 65% rise in world diabetic rate. In Nigeria, there are no existing data on the glycemic index of various commonly consumed foods, which leaves several individuals at risk as they consume both low and high glycemic index foods without putting their health status into consideration. According to experts, nutritional habits play an important role in increasing the burden of these chronic conditions. Therefore, modifying dietary habits could be beneficial in the prevention and management of these diseases. It has been suggested that the state of hyperglycaemia that is observed following food consumption under certain dietary regimes could be a risk factor for the development of various metabolic conditions especially in individuals with a poor glycemic control such as in diabetes mellitus and even healthy individuals.  Under such circumstances, it would be important to be able to reduce the amplitude and duration of hyperglycemia. Hyperglycemia or high blood sugar is a medical condition in which there is an excess amount of glucose circulating in the blood plasma. This is generally a blood sugar level higher than 11.1 mmol/l (200 mg/dl) while Hypoglycemia, also known as low blood sugar, is when blood sugar declines to below normal levels. Some carbohydrate rich foods cause less post-ingestive hyperglycemia than others. Therefore, having the right kind of carbohydrate foods could actually be a potential strategy in the prevention and management of chronic metabolic disorder. Hence, the need to know the glycemic indices of the commonly consumed foods in Nigeria. This will aid healthy people and patients with various nutritional disorders to choose the class of food that will be beneficial to their health status.



There is lack of information about the glycemic index of foods consmed in many African communities and this has led to the poor health conditions and associated dietary related disoders. The Glycemic index has been identified as a vital tool to measure biological and health effects of meal. The  management  of  dietary  induced  disorders  have  created  fear  of  consuming certain  foods by people suffering from metabolic diseases, yet little or no information exist on the glycemic index status of many commonly consumed foods. It is therefore extremely important for this information to be available to all in other to maintain a healthy status.


The primary objective of this work is to produce flour from the different yam cultivars and determine the blood glucose response.

Specific objectives of this research:

i. To process different yam cultivars (Dioscorea rotundata, Dioscorea alata and Dioscorea dumentorum) using different methods; boiling, frying and pounding.

ii. To determine the functional properties of the yam flour samples.

iii. To determine the amylose/amylopectin ratio of the yam flour samples

iv. To determine the proximate composition of the both the yam flour samples and test products.

v. To determine the glycemic index of boiled, fried and pounded yam samples.


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