ABSTRACT
This study was carried out to determine the effect of variety and particle size on the quality characteristics of water yam flour and bread. Three mature improved varieties of water yam (TDa 11/00316, TDa 11/00201, and TDa 11/00432) were obtained from the Yam Programme of National Root Crops Research Institute (NRCRI), Umudike, Nigeria. Each of the yam varieties was processed into flour by peeling with knife, washing with clean water, slicing (0.1 inch thickness), sun drying (37 – 40⁰C, for 16 h at 55% RH), milling (coarse and fine milling) and sieving into 150 microns and 300 microns particle sizes. The flours were analyzed for functional, anti-nutrients, antioxidants, and pasting properties. They were then processed into gluten-free bread using xanthan gum and stevia to replace gluten and sugar respectively. The bread samples were analyzed for in vitro starch digestibility, physical properties, proximate composition, and sensory evaluation. Results showed that variety and particle size were significant factors affecting the quality characteristics of D. alata flour and bread. Functional properties ranged from 0.61 – 0.68 g/ml, 0.38 – 0.46 g/ml, 2.25 – 3.25 g/g, 1.39 – 1.64 g/g, 56.50 – 167.50 s, and 65.25 – 78.50⁰C for tapped bulk density, loose bulk density, water absorption capacity, oil absorption capacity, wettability and gelatinization temperature respectively. Anti-nutrients ranged from 1.37 – 1.74 %, 0.12 – 0.16 %, 0.12 – 0.15 mg/100g, and 16.93 – 23.41 mg GAE/100g for tannin, phytate, oxalate and total phenol respectively. Antioxidants ranged from 1.23 – 3.76 % and 12.11 – 45.36 % for DPPH and FRAP respectively. Pasting properties were reported as follows: pasting temperature: 84.40 – 86.90⁰C, peak time: 4.80 – 5.47 min, peak viscosity: 181.96 – 434.08 RVU, minimum viscosity: 142.21 – 351.79 RVU, breakdown viscosity: 32.29 – 171.84 RVU, final viscosity: 200.13 – 463.63 RVU, and setback viscosity: 56.46 – 167.50 RVU. In vitro starch digestibility was reported as follows: rapidly digestible starch (RDS): 0.59 – 2.59 g/100g, rapidly available glucose (RAG): 1.37 – 8.89 g/100g, slowly digestible starch (SDS): 0.23 – 6.49 g/100g, and resistant starch: 1.71 – 3.10 g/100g. Physical properties ranged from 175.50 – 190.50 cm3, 70.15 – 83.75 g, and 0.37 – 0.48 g/cm3 for loaf volume, weight and density respectively. Proximate composition were reported as follows: moisture content: 29.82 – 36.73 %, protein: 8.92 – 10.37 %, crude fibre: 0.53 – 0.95 %, fat: 1.33 – 5.46 %, ash: 2.32 – 2.92 %, and carbohydrate: 47.72 – 52.75 %. Sensory scores were reported as follows: appearance: 5.76 – 6.69, texture: 5.20 – 6.28, flavour: 5.20 – 6.32, taste: 4.92 – 6.52, aroma: 5.04 – 6.60, and general acceptability: 5.76 – 7.12. Bread made from coarser flour from TDa 11/00201 (WWP) was mostly accepted by the panelists. It was observed that interaction between variety and particle size had significant effect (p<0.05) on the anti-nutrient, antioxidant, pasting, physical and sensory properties of the flour and bread samples. It was concluded that smaller particle sizes enhance antioxidant properties of D. alata flour; however, coarser particle sizes are recommended for slower release of glucose especially that of the TDa 11/00201 variety.
TABLE
OF CONTENTS
Title page i
Declaration ii
Dedication iii
Certification iv
Acknowledgements v
Table of Contents vi
List of Tables xi
List of Figures xii
List of Plates xiii
Abstract xiv
CHAPTER
1: INTRODUCTION 1
1.1 BACKGROUND
OF THE STUDY 1
1.2 STATEMENT OF PROBLEM 2
1.3 JUSTIFICATION 3
1.4 OBJECTIVES
OF THE STUDY 3
CHAPTER 2: LITERATURE REVIEW 5
2.1 OVERVIEW
OF YAM 5
2.2 OVERVIEW
OF WATER YAM 6
2.2.1 Agronomy
of Water Yam 7
2.2.2 Varieties
of Water Yam 8
2.2.3 Processing
of Water Yam 9
2.2.3.1 Water
Yam Flour 10
2.2.3.2 Bread
Making 11
2.2.3.3 Gluten-Free
Bread 14
2.2.3.3.1 Stevia and its Food Applications 16
2.2.3.3.2 Xanthan Gum and its Food Applications 17
2.2.3.4 Water
Yam Bread 20
2.3 PARTICLE
SIZE DISTIBUTION OF FLOUR 21
2.4 STARCH
GRANULE MORPHOLOGY OF WATER YAM 23
2.5 IN
VITRO STARCH DIGESTIBILITY AND GLYCEMIC INDEX OF
WATER YAM 24
CHAPTER 3: MATERIALS
AND METHODS 27
3.1 PROCUREMENT
OF SAMPLE 27
3.2 PREPARATION OF SAMPLE 27
3.2.1 Production of Water Yam Flour 27
3.2.2 Recipe for Gluten-Free Water Yam Bread 29
3.2.2.1 Production of Gluten-Free Bread 30
3.2.3 Sample Coding 31
3.3 ANALYSES OF SAMPLES 31
3.3.1 Functional Properties 31
3.3.1.1 Bulk Density 31
3.3.1.2 Water/Oil
Absorption Capacity (WAC/OAC) 32
3.3.1.3 Wettability 32
3.3.1.4 Gelatinization
Temperature 32
3.3.2 Anti-nutritional Factors 33
3.3.2.1 Determination of Total Phenol 33
3.3.2.2 Determination of Phytate 33
3.3.2.3 Determination of Tannin 34
3.3.2.4 Determination of Oxalate 34
3.3.3 Antioxidant Properties 35
3.3.3.1 Measurement of Antioxidant Activity with DPPH
Method 35
3.3.3.2 Measurement of Antioxidant Activity with
FRAP Method 36
3.3.4 Pasting Properties 37
3.3.5 Determination of in vitro starch
digestibility 37
3.3.6 Determination of Physical Properties of
Bread 38
3.3.7 Proximate Composition 39
3.3.7.1 Moisture Content 39
3.3.7.2 Crude Protein 39
3.3.7.3 Fat Content 41
3.3.7.4 Crude Fibre 41
3.3.7.5 Ash Content 42
3.3.7.6 Carbohydrate Content 42
3.3.8 Sensory Evaluation 42
3.4 EXPERIMENTAL DESIGN 43
3.5 STATISTICAL ANALYSIS 43
CHAPTER 4:
RESULTS AND DISCUSSION 44
4.1 FUNCTIONAL PROPERTIES 44
4.2 ANTINUTRIENT COMPOSITION 47
4.3 ANTIOXIDANT PROPERTIES 52
4.4 PASTING PROPERTIES 55
4.5 WATER YAM BREAD 61
4.6 IN VITRO STARCH DIGESTIBILITY 64
4.7 PHYSICAL PROPERTIES OF BREAD 67
4.8 PROXIMATE COMPOSITION 70
4.9 SENSORY EVALUATION 75
CHAPTER 5: CONCLUSION AND RECOMMENDATIONS 78
5.1 CONCLUSION 78
5.2 RECOMMENDATIONS 79
CONTRIBUTION TO KNOWLEDGE 80
REFERENCES
APPENDICES
LIST
OF TABLES
Table
3.1: Recipe for Gluten-Free Water Yam Bread 29
Table
3.2: Sample Codes and Meaning 31
Table 4.1:
Functional Properties of D. alata
Flour Samples 45
Table 4.2:
Anti-nutrient Composition of D. alata
Flour Samples 48
Table 4.3: Pearson Correlation for Phenol and
FRAP 51
Table 4.4: Antioxidant Properties of D. alata Flour Samples 54
Table 4.5: Pasting
Properties of D. alata Flour 57
Table
4.6: Pearson Correlation for Breakdown and Setback Viscosities 60
Table 4.7: In
Vitro Starch Digestibility of D. alata
Bread 65
Table
4.8: Physical Properties of D. alata
Bread 69
Table
4.9: Proximate Composition of D. alata
Bread Samples 72
Table 4.10:
Sensory Scores of D. alata Bread
Samples 76
LIST
OF FIGURES
Figure
2.1: Chemical Structure of Xanthan Repeating Unit 19
Figure
3.1: Flowchart for the Production of Water Yam Flour 28
Figure
3.2: Flowchart for the Production of Water Yam (Gluten-Free) Bread 30
LIST
OF PLATES
Plate 4.1: Water
Yam Bread (WW 300 μm) 62
Plate 4.2: Water
Yam Bread (WY 150 μm) 63
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND OF THE STUDY
Water yam (Dioscorea alata) is one of the most widely cultivated species of
the genius Dioscorea, others being Dioscorea rotundata (white yam) and Dioscorea cayenensis
Lam (yellow yam). It is
found in various agro-ecological zones, from tropical to temperate and low to
high altitudes (Takada et al., 2017).
It has good agronomical characteristics such as ability to grow under low
fertile soil conditions, ability to produce fairly large tubers as well as better
tuber storability (Nwafor et al.,
2019). Dioscorea alata, D. rotundata
and D. cayenensis together account for at
least 95% of the total yam production of West Africa, the largest yam producing
area of the world.
Water yam could be a good source of nutrients,
with crude protein, ash, sugar, starch and total dietary fibre ranging from 4.3-8.7,
2.9-4.1, 3.6-11.0, 60.3-74.4 and 4.1-11.0%, on dry weight basis respectively
(Baah et al., 2009a). Also, studies
indicate that water yam has low glycemic index (< 55 on the 100-point
glucose scale) and may therefore be useful in the control of diabetes mellitus,
a degenerative disease which is prevalent around the world and emerging as a global
health burden that may reach pandemic levels by 2030 (Ampofo et al., 2021; Eyinla et al., 2021). Glycemic index (GI) is
affected by factors such as cultivar, flour particle size and processing method
amongst others (Wee and Henry, 2020; Ampofo et
al., 2021). The particle sizes of most commercially available flours vary
greatly and significantly affect the quality of baked products. For wheat
flour, the world’s number one baking flour, the particle sizes obtained from
the last stage of roller milling vary considerably due to differences in the
hardness of the endosperm in different wheat varieties and the milling
conditions of flour (Mirza Alizadeh et al., 2022).
The processing of tuber and plant components
into flour involves a milling operation which destroys the cell wall structure
in which starch granules are entrapped. Further milling reduces the flour
particle size and increases its specific surface area, thus increasing the
starch digestibility of the flour (Wee and Henry, 2020). Also, reports indicate
that wheat substitution with gluten-free flour increases the antioxidant
activity of bread (Duda et al.,
2021).
In the last few years, there has been growing
research interest in functional foods due to the prevalence of nutrient
toxicities and nutritional diseases such as type 2 diabetes, obesity, some
types of cancer and cardiovascular diseases (Awuchi et al., 2020). Some individuals are unable to metabolize foods that
contain gluten such as wheat bread; consumption of such food results in an
immune-mediated enteropathy called celiac disease (DeGeorge et al., 2017). A twin problem therefore
exists for people who are both diabetic and gluten-intolerant. Although
research has been done on the use of water yam flour for bread making, the
effect of particle size and variety on the quality of water yam flour and bread
has not been well investigated. Also, the use of 100% water yam flour to
produce gluten-free bread has not been successfully reported. This study
therefore seeks to investigate the effect of particle size and variety on the in
vitro digestibility, physicochemical and antioxidant properties of water yam
flour and gluten-free bread made from the flours.
1.2 STATEMENT OF PROBLEM
Water yam (Dioscorea
alata) has high productivity, high nutritional and good tuber storability, it
is not as highly regarded as Dioscorea
rotundata and Dioscorea cayanensis
in terms of food uses because of its unsuitability for making fufu and baked products. Not much has
been done to elucidate the physicochemical,
antioxidant and biochemical properties of water yam varieties; hence, the low
food uses. Also, the role
of uninformed food choices and bad nutrition on disease burden is well known
(Uloko et al., 2018). Studies have
shown that eating a diet of low in vitro starch digestibility (or low glycemic
index) can help to control blood sugar (Norris, 2021). Meanwhile, low-fibre, highly processed foods
which are already established risk factors are not frequently consumed by some
people suffering from elevated blood sugar levels; which raises the troubling
question as to what exactly are the metabolic properties of certain staples
(Eyinla et al., 2021). Although there
have been various levels of wheat substitution in bread making (even up to 40%)
(Duda et al., 2021), there is dearth
of information on the use of 100% water yam flours in the production of
gluten-free bread.
1.3 JUSTIFICATION
Extensive knowledge of the in vitro starch
digestibility (glycemic index), physicochemical, antioxidant and pasting
properties of flours of improved D. alata
varieties will encourage informed usage of these flours in the production of
value-added products. It is important to investigate the effect of particle
size and variety on the in vitro starch digestibility of D. alata flour to further elucidate its potentials for the control
of diabetes or otherwise, especially the improved varieties. The findings of
this study will provide a complete alternative food for diabetic and
gluten-intolerant individuals. Also, findings will provide more information
about the improved varieties of water yam.
1.4 OBJECTIVES OF THE STUDY
The
main objective of this study was to determine the effect of particle size and
variety on the quality characteristics (in vitro starch digestibility, physicochemical,
anti-nutrient, antioxidant and pasting properties) of water yam (D. alata) flour and bread.
The
specific objectives of the study were to:
I.
Produce flours of
different particle sizes (150 µm and 300 µm) from three improved varieties of
water yam;
II.
Characterize the flours
in terms of their functional, anti-nutrients, antioxidant and pasting
properties;
III.
Produce gluten-free bread
from the flours in (I) above;
IV.
Characterize the bread
samples in (III) above in terms of in
vitro starch digestibility (rapidly digestible starch, rapidly available
glucose, slowly digestible starch and resistant starch), physical properties,
and proximate composition;
V.
Evaluate the sensory
properties of the bread samples.
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