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UNIVERSITI PUTRA MALAYSIA
PREPARATION OF FROZEN SWEET POTATO (Ipomoea batatas.) DOUGH BASE for
TRADITIONAL MALAYSIAN FRIED CAKES
HUSNI HAYATI MOHD. RAFDI
FSTM 2008 4
PREPARATION OF FROZEN SWEET POTATO
(Ipomoea batatas.) DOUGH BASE for
TRADITIONAL MALAYSIAN FRIED CAKES
HUSNI HAYATI MOHD. RAFDI
MASTER OF SCIENCE
UNIVERSITI PUTRA MALAYSIA
2008
i
Abstract of thesis presented to the Senate of Universiti Putra Malaysia in
fulfillment of the requirement for the degree of Master of Science
PREPARATION OF FROZEN SWEET POTATO (Ipomoea batatas)
DOUGH BASE for TRADITIONAL MALAYSIAN FRIED CAKES
By
HUSNI HAYATI BT. MOHD. RAFDI
July 2008
Chairman : Roselina Karim, PhD
Faculty : Food Science and Technology
Sweet potato dough base is popular for making traditional fried cakes in
Malaysia but there has been no specific quality standard as reference for the
dough preparation. The independent variables to produce the dough, i.e.
mashing time (11.36 – 138.64s) and wheat flour concentration (3.79 –
46.21%) were designed using the central composite design based on
Response Surface Methodology (RSM). The dough and fried cakes
produced were examined for quality on the basis of physical parameters and
sensory attributes. Optimal conditions for making sweet potato dough for
fried cakes production were determined, by Response Surface Methodology
(RSM), to be 45s of mashing time at 150rpm and 25% of wheat flour
concentration. The dough base was then produced using the optimum
condition and stored for 6 months at -20±2oC for quality changes
observation at 1 month interval. Sweet potato paste was also included in the
storage study for comparison. Unfrozen samples (dough and paste) were
used as control in this study. Moisture content of frozen samples decreased
ii
with increasing of storage time and caused dehydration in samples. The
water holding capacity of the samples reduced reflected by drip loss
produced in paste, also by increased in paste and dough adhesiveness after
frozen storage. These suggested that structure damage occurred due to
frozen and frozen storage. Thus, caused the firmness and elasticity (G’) of
dough and paste to increase. As the consequence, the Texture Profile
Analysis (TPA) of fried cakes (firmness, cohesiveness, springiness and
chewiness) were decreased. Micrographs obtained showed that all the
structure of dough, paste and fried cakes collapsed, ruptured and were drier
after 6 months of storage. Based on the Quantitative Descriptive Analysis
(QDA), frozen dough and paste became harder to handle due to increase of
adhesiveness. However, those negative changes in instrumental analysis did
not influence the overall acceptability on the fried cakes. Fried cakes were
still acceptable with overall acceptability score was 5.52, denotes as slightly
acceptable. Freeze-thaw cycle study was conducted on sweet potato dough
to investigate the stability of the dough against temperature fluctuation.
Sweet potato dough and paste were exposed to 5 freeze-thaw cycles
treatment for 5 weeks and the quality changes on the physical and sensory
characteristics of samples were observed. Samples that stored for 5 weeks
without undergoing freeze-thaw cycles were used as control. The freeze-
thaw cycles resulted in severe damages to sweet potato dough and paste
structure as the changes in moisture, drip loss, firmness, adhesiveness,
storage modulus (G’) and complex viscosity (µ*) were significant (P<0.05).
Due to dough changes, fried cakes became firmer with increasing in number
of freeze-thaw cycles. The dough, paste and fried cakes structure became
iii
drier and many holes were developed after been exposed to 5 freeze-thaw
cycles as shown in micrographs. Adhesiveness and handling ability from
QDA assessment was the most affected physical characteristics. Even
though freeze-thaw cycles caused severe damaged to dough structure, the
fried cakes made from the dough were slightly acceptable.
Keywords: Sweet potato dough, traditional fried cakes, frozen storage,
freeze-thaw cycle, physical changes, sensory evaluation
iv
Abstrak tesis yang dikemukakan kepada Senat Universiti Putra Malaysia
sebagai memenuhi keperluan untuk ijazah Master Sains
PENYEDIAAN DOH SEJUKBEKU BERASASKAN KELEDEK
(Ipomoea batatas) untuk KUIH GORENG TRADISIONAL
MALAYSIA
Oleh
HUSNI HAYATI BT. MOHD. RAFDI
Julai 2008
Pengerusi : Roselina Karim, PhD
Fakulti : Sains dan Teknologi Makanan
Doh ubi keledek sangat popular sebagai asas doh bagi pembuatan kuih
goreng tradisional di Malaysia tetapi tiada sebarang piawai kualiti untuk
dijadikan rujukan bagi pembuatan doh tersebut. Nilai jangkaan spesifik bagi
pembolehubah tidak bersandar untuk menghasilkan doh tersebut; i.e masa
lecekan (11.36 – 138.64s) dan amaun tepung gandum (3.79 – 46.21%) telah
diperolehi menggunakan rekaan komposit titik tengah daripada perisian
Response Surface Methodology (RSM). Doh dan kuih goreng yang
dihasilkan dianalisa kualitinya berdasarkan sifat-sifat fizikal dan penilaian
deria. Keadaan optimum untuk membuat doh keledek bagi penghasilan kuih
goreng akhirnya diperolehi, menggunakan RSM, ialah 45s bagi masa
lecekan pada 150ppm dan 25% bagi amaun tepung gandum. Doh keledek
kemudiannya dihasilkan menggunakan masa lecekan dan formulasi yang
optimum yang kemudiannya disimpan selama 6 bulan pada 20±2oC dan
pemerhatian ke atas perubahan kualiti doh dijalankan pada setiap 1 bulan.
v
Pes keledek turut dikaji sebagai perbandingan. Sampel (doh dan pes) yang
tidak disejukbekukan telah dijadikan sampel kawalan dalam kajian ini.
Penyimpanan sejukbeku selama 6 bulan telah menyebabkan pengeringan
pada sampel disebabkan oleh penurunan kandungan lembapan. Kebolehan
memegang air dalam sampel menurun berdasarkan penghasilan cecair
dalam pes dan meningkatnya kadar kelekitan dalam doh dan pes selepas
penyimpanan sejukbeku. Ini menunjukkan berlakunya kerosakan struktur
dalam sampel yang disejukbekukan. Hasilnya, kekerasan dan kekenyalan
(G’) doh dan pes meningkat. Kesinambungan daripada itu, Analisis Profil
Tekstur (TPA) kuih goreng (sifat kekerasan, keanjalan, kekenyalan dan
kekunyahan) yang dihasilkan daripada doh sejukbeku menunjukkan
penurunan. Gambar mikro mununjukkan struktur kesemua doh, pes dan
kuih goreng menjadi rosak, musnah dan kering selepas 6 bulan
penyimpanan. Berdasarkan Analisis Diskriptif Kuantitatif (QDA), doh dan
pes sejukbeku semakin sukar dibentuk disebabkan oleh peningkatan sifat
kelekitan. Bagaimanapun, kesemua perubahan negatif yang ditunjukkan
oleh analisis menggunakan instrumen tidak mempengaruhi penerimaan
keseluruhan kuih goreng. Kuih goreng masih diterima dengan nilai 5.52,
iaitu penerimaan minimum. Kajian sejukbeku-cair telah dijalankan ke atas
doh keledek untuk mengkaji kestabilan doh terhadap ketidakstabilan suhu.
Doh dan pes telah didedahkan kepada 5 kitaran sejukbeku-cair selama 5
minggu dan perubahan kualiti terhadap sifat-sifat fizikal dan penilaian deria
sampel telah dikaji. Sampel yang disimpan selama 5 minggu tanpa
didedahkan kepada kitaran sejukbeku-cair dijadikan sebagai sampel
kawalan. Kitaran sejukbeku telah menyebabkan kerosakan serius kepada
vi
struktur doh dan pes memandangkan perubahan nilai kandungan lembapan,
kehilangan cecair, sifat kekerasan, sifat kelekitan, sifat kekenyalan (G’) dan
sifat kelikatan kompleks (µ*) adalah jelas (P<0.05). Perubahan dalam doh
mempengaruhi tekstur kuih goreng; iaitu kekerasan kuih goreng meningkat
dengan peningkatan masa dan bilangan kitaran sejukbeku-cair. Kerosakan
struktur dilihat jelas dibawah mikroskop, di mana struktur doh, pes dan kuih
goreng menjadi lebih kering dan banyak ruang kosong terbentuk di antara
struktur sampel selepas didedahkan kepada 5 kitaran sejukbeku-cair.
Penilaian deria QDA menunjukkan bahawa sifat – sifat fizikal yang paling
terjejas ialah sifat kelekitan dan sifat kebolehan membentuk doh. Walaupun
kitaran sejukbeku-cair menyebabkan kerosakan serius kepada struktur doh,
kuih goreng yang dihasilkan daripada doh tersebut masih lagi diterima
dengan penerimaan minimum.
Kata kunci: Doh keledek, kuih goreng tradisional, penyimpanan sejukbeku,
kitaran sejukbeku-cair, perubahan fizikal, penilaian deria
vii
ACKNOWLEDGEMENTS
I would like to express my appreciation and gratitude to my supervisor, Dr. Roselina
Karim for her invaluable guidance, suggestions, constructive criticisms and constant
encouragement throughout the course of my study and in the dissertation preparation,
and for the financial support. My deep appreciation and gratitude also to Mr.
Dzulkifly Mat Hashim and Associate Professor Dr. Sharifah Kharidah Syed
Muhammad for their support, invaluable guidance and suggestions.
I would like to thank the Dean and all members of the Faculty of Food Science and
Technology, especially staffs in Processing and Engineering Laboratory for
providing the research facilities and technical assistance during the course of my
study.
Special thanks to my friends especially the Packaging Research Group; Zauyah,
Stella Marie, Nora Zeti, Wan Azlina and Shafie for their kind assisstance,
suggestions and encouragement.
I am greatly indebted to my beloved parents, sisters and brothers for their loving
support. Finally, I would like to thank all my friends who gave me the needed
encouragement during my study.
viii
I certify that an Examination Committee has met on 22 July 2008 to conduct the final
examination of Husni Hayati binti Mohd. Rafdi on her Master of Science thesis
entitled “Preparation of Frozen Sweet Potato (Ipomoea batatas) Dough Base for
Traditional Malaysian Fried Cakes” in accordance with Universiti Putra Malaysia
(Higher Degree) Act 1980 and Universiti Pertanian Malaysia (Higher Degree)
Regulation 1981. The Committee recommends that the candidates be awarded the
relevant degree. Members of the Examination Committee are as follows:
Badlishah Sham Baharin, PhD
Associate Professor
Faculty of Graduate Studies
Universiti Putra Malaysia
(Chairman)
Suhaila Mohamed, PhD
Professor
Faculty of Food Science and Technology
Universiti Putra Malaysia
(Internal Examiner)
Noranizan Mohd. Adzahan, PhD
Lecturer
Faculty of Food Science and Technology
Universiti Putra Malaysia
(Internal Examiner)
Amiza Mat Amin, PhD
Associate Professor
Faculty of Agriculture and Food Science
Universiti Malaysia Terengganu
(External Examiner)
HASANAH MOHD GHAZALI, PhD
Professor and Deputy Dean
School of Graduate Studies
Universiti Putra Malaysia
Date: 25 September 2008
ix
This thesis submitted to the Senate of Universiti Putra Malaysia and has been
accepted as fulfilment of the requirement for the degree of Master of Science. The
members of the Supervisory Committee are as follows:
ROSELINA KARIM, PhD
Lecturer
Faculty of Food Science and Technology
Universiti Putra Malaysia
(Chairman)
SH. KHARIDAH SY. MUHAMMAD, PhD
Associate Professor
Faculty of Food Science and Technology
Universiti Putra Malaysia
(Member)
DZULKIFLY MAT HASHIM
Lecturer
Faculty of Food Science and Technology
Universiti Putra Malaysia
(Member)
AINI IDERIS, PhD
Professor and Dean
School of Graduate Studies
Universiti Putra Malaysia
Date: 16 October 2008
x
I hereby declare that the thesis is based on my original work except for quotations
and citations which have been duly acknowledge. I also declare that it has not been
previously or concurrently submitted for any other degree at UPM or other
institutions.
HUSNI HAYATI MOHD RAFDI
Date: 26 August 2008
xi
TABLE OF CONTENTS
Page
ABSTRACT i
ABSTRAK iv
ACKNOWLEDGEMENTS vii
APPROVAL viii
DECLARATION x
LIST OF ABBREVIATIONS xiv
LIST OF TABLES xv
LIST OF FIGURES xvi
LIST OF PLATES xviii
LIST OF APPENDICES xix
CHAPTER
1 INTRODUCTION 1
II LITERATURE REVIEW 5
2.1 Sweet Potato 5
2.1.1 Sweet potato in Malaysia 7
2.2 Wheat Flour 7
2.3 Freezing 9
2.3.1 Factors affecting frozen foods quality 9
2.3.2 Effect of frozen storage and freeze-thaw cycles on
frozen foods 12
2.4 Thawing 21
2.5 Scanning Electron Microscopy (SEM) 22
2.6 Response Surface Methodology (RSM) 23
2.7 Sensory evaluation 24
III OPTIMIZATION OF MASHING TIME AND WHEAT FLOUR
CONCENTRATION FOR THE PRODUCTION OF SWEET
POTATO DOUGH USING RESPONSE SURFACE
METHODOLOGY (RSM)
3.1 Introduction 27
3.2 Materials and Methods 28
3.2.1 Raw materials 28
3.2.2 Sample preparation 28
3.2.3 Instrumental Analysis 30
3.2.4 Sensory Analysis 30
3.2.5 Experimental design and statistical analysis 33
3.3 Results and Discussion 34
3.3.1 Proximate composition of raw sweet potato 34
3.3.2 Model fitting from RSM 34
xii
3.3.3 Instrumental analysis 35
3.3.4 Sensory evaluation 37
3.3.5 Optimization of basic formulation and verification
of model 39
3.4 Conclusion 40
IV EFEECT OF FROZEN STORAGE ON THE PHYSICAL AND
SENSORY CHARACTERISTICS OF FROZEN SWEET POTATO
PASTE AND DOUGH
4.1 Introduction 51
4.2 Materials and Methods 52
4.2.1 Raw materials 52
4.2.2 Samples preparation 52
4.2.3 Freezing of sweet potato paste and dough 53
4.2.4 Thawing of frozen samples 52
4.2.5 Analysis 53
4.2.6 Physical analysis 53
4.2.7 Instrumental analysis 54
4.2.8 Sensory analysis 55
4.2.9 Microstructural observation 56
4.2.10 Statistical analysis 57
4.3 Results and Discussion 57
4.3.1 Physical changes of frozen sweet potato paste and dough
during frozen storage 57
4.3.2 Textural changes of frozen sweet potato paste and dough
during storage 59
4.3.3 Rheological properties of sweet potato paste and
sweet potato dough 61
4.3.4 Texture Profile Analysis (TPA) of sweet potato fried
cakes using Texture Analyzer 63
4.3.5 Sensory characteristics of sweet potato paste, dough
and fried cakes 64
4.3.6 Microstructural changes using SEM 67
4.4 Conclusion 71
V EFEECT OF FREEZE-THAW CYCLES ON THE QUALITY OF
FROZEN SWEET POTATO PASTE AND DOUGH
5.0 Introduction 85
5.2 Materials and Methods 86
5.2.1 Raw materials 86
5.2.2 Samples preparation 86
5.2.3 Freezing of sweet potato paste and dough 86
5.2.4 Thawing of frozen samples 87
5.2.5 Freeze-thaw cycle study 87
5.2.6 Physical analysis 87
5.2.7 Instrumental analysis 87
5.2.8 Sensory analysis 88
5.2.9 Microstructural observation 88
xiii
5.2.10 Statistical analysis 88
5.3 Results and Discussion 88
5.3.1 Effect of freeze-thaw cycles on the physical
characteristics of sweet potato paste and dough 88
5.3.2 Textural changes of frozen sweet potato paste and
dough during freeze-thaw study 90
5.3.3 Changes on rheological properties of frozen sweet
potato paste and dough during freeze-thaw cycles 91
5.3.4 Effect of freeze-thaw cycles on the texture of fried
sweet potato cakes 93
5.3.5 Effect of freeze-thaw cycles on the sensory characteristics
of sweet potato paste, dough and fried cakes 93
5.3.6 Microstructure changes during freeze-thaw study 95
5.4 Conclusion 97
VI GENERAL CONCLUSION AND RECOMMENDATIONS
General conclusion 114
Recommendations 116
REFERENCES 118
APPENDICES 131
BIODATA OF STUDENT 144
xiv
ABBREVIATIONS AND SYMBOLS
g gram
mm millimeter
cm centimeter
µm micrometer
ha hectare
s second
min minute
hr hour
rpm round per minute
Pa Pascal
N Newton
kJ kilo joule
Kg Kilogram
G’ Storage modulus
G” Loss modulus
η* Complex viscosity
δ Phase angle
FT freeze-thaw
SEM Scanning Electron Microscopy
TPA Texture Profile Analysis
QDA Quantitative Descriptive Analysis
RSM Response Surface Methodology
PP Polypropylene
FAO Food Act Organisation
AOAC American Organisation Association Chemist
IIR International Institution of Refrigeration
xv
LIST OF TABLES
Table Page
1 Effects of frozen storage on dough properties and bread quality 26
2 The RSM experimental design (in coded and uncoded level of
two variables) employed for processing sweet potato dough 41
3 Proximate composition of sweet potato tuber and wheat flour 42
4 Effects of wheat flour concentration and mashing time on six
dependent variables 43
5 Regression coefficients and analysis of variance for 6 response
variables 44
6 Predicted and experimental values of the response variables at
optimum formulation 45
7 Effect of frozen storage on the rheological properties of frozen sweet
potato paste and dough 73
8 Effect of frozen storage on the sensory attributes of sweet
potato paste and dough 74
9 Mean score of QDA of fried sweet potato cakes made from frozen
sweet potato dough at different storage period 75
10 Mean score for sensory acceptability of fried cakes attributes using
Hedonic test 76
11 Physical changes of sweet potato paste and dough during freeze-thaw
study 99
12 Effect of freeze-thaw cycle on the rheological properties of frozen
sweet potato paste and dough 100
13 Effect of freeze-thaw cycles on the acceptability of fried sweet
potato cakes 101
xvi
LIST OF FIGURES
Figure Page
1 Flow chart of sweet potato paste, dough and fried cakes
processing method 46
2 Response surface plot of the effects of wheat flour concentration
and mashing time on the firmness of sweet potato dough 47
3 Response surface plot of the effects of wheat flour concentration
and mashing time on the adhesiveness of sweet potato dough 47
4 Response surface plot of the effects of wheat flour concentration
and mashing time on the chewiness of sweet potato fried cakes. 48
5 Response surface plot of the effects of wheat flour concentration
and mashing time on the handling ability of sweet potato dough 48
6 Response surface plot of the effects of wheat flour concentration
and mashing time on the sweet potato flavour of sweet potato
fried cakes 49
7 Response surface plot of the effects of wheat flour concentration
and mashing time on the overall acceptability of sweet potato
dough and fried cakes 49
8 Superimposed plots of the six response variables of sweet potato
dough and fried cakes 50
9 Effect of frozen storage on moisture content of sweet potato paste
and dough 77
10 Effect of frozen storage on the drip loss of sweet potato paste and
dough 77
11 Effect of frozen storage on the firmness of sweet potato paste and
dough using Texture Analyzer 78
12 Effect of frozen storage on the adhesiveness of sweet potato paste
and dough using Texture Analyzer 78
13 (a) Rheological spectrum of unfrozen sweet potato paste at frequency
range from 0.1 to 10 Hz 79
13 (b) Rheological spectrum of frozen sweet potato paste after 6 months
of frozen storage at frequency range from 0.1 to 10 Hz. 79
13 (c) Rheological spectrum of unfrozen sweet potato dough at frequency
range from 0.1 to 10 Hz 80
xvii
13 (d) Rheological spectrum of frozen sweet potato dough after 6 months of
frozen storage at frequency range from 0.1 to 10 Hz 80
14 (a) Effect of frozen storage on the firmness of the fried cakes 81
14 (b) Effect of frozen storage on the springiness of the fried cakes 81
14 (c) Effect of frozen storage on the cohesiveness of the fried cakes 81
14 (d) Effect of frozen storage on the chewiness of the fried cakes 81
15 Effect of freeze-thaw cycle on the firmness of frozen sweet potato
paste and dough 102
16 Effect of freeze-thaw cycle on the adhesiveness of frozen sweet potato
paste and dough 102
17(a) Dynamic mechanical spectrum of frozen sweet potato paste without
FT cycle treatment (control) 103
17(b) Dynamic mechanical spectrum of frozen sweet potato paste after 5
freeze-thaw cycles 103
17(c) Dynamic mechanical spectrum of frozen sweet potato dough without
freeze-thaw cycle treatment 104
17(d) Dynamic mechanical spectrum of frozen sweet potato dough after
5 freeze-thaw cycles 104
18(a) Effect of freeze-thaw cycle on the firmness of the fried sweet
potato cakes 105
18(b) Effect of freeze-thaw cycle on the springiness of the fried sweet
potato cakes 105
18(c) Effect of freeze-thaw cycle on the cohesiveness of the fried sweet
potato cakes 105
18(d) Effect of freeze-thaw cycle on the chewiness of the fried sweet
potato cakes 105
19 Effect of freeze-thaw cycle on the sensory characteristics of frozen
sweet potato paste using QDA 106
20 Effect of freeze-thaw cycle on the sensory characteristics of frozen
sweet potato dough using QDA 107
21 Effect of freeze-thaw cycle on the sensory characteristics of fried
sweet potato cakes made from frozen dough undergoing different
freeze-thaw cycle using QDA 108
xviii
LIST OF PLATES
Plate Page
1 Micrograph of unfrozen sweet potato paste at 300x magnification 82
2 Micrograph of frozen sweet potato paste after 6 months of
storage at 300x magnification 82
3 Micrograph of unfrozen sweet potato dough at 300x magnification 83
4 Micrograph of frozen sweet dough after 6 months of frozen storage at
300x magnification 83
5 Cross-section of sweet potato fried cakes made from unfrozen
sweet potato dough at 150x magnification 84
6 Cross-section of sweet potato fried cakes made from frozen
sweet potato dough after 6 months of frozen storage at 150x
magnification 84
7 Micrograph of frozen sweet potato paste after 5 weeks of
frozen storage without freeze-thaw cycle at 300x magnification 109
8 Micrograph of frozen sweet paste after 3 freeze-thaw cycles at 300x
magnification 109
9 Micrograph of frozen sweet paste after 5 freeze-thaw cycles at 300x
magnification 110
10 Micrograph of frozen sweet potato dough after 5 weeks of
frozen storage without freeze-thaw cycle at 300x magnification 110
11 Micrograph of frozen sweet dough after 3 freeze-thaw cycles
at 300x magnification 111
12 Micrograph of frozen sweet dough after 5 freeze-thaw cycles at 300x
magnification 111
13 Micrograph of fried cakes made of frozen dough after 5 weeks of
frozen storage without freeze-thaw cycle at 150x magnification 112
14 Micrograph of fried cakes made of frozen dough after 5
freeze-thaw cycles at 150x magnification 112
15 Micrograph of fried cakes made of frozen dough after 5
freeze-thaw cycles at 150x magnification 113
xix
LIST OF APPENDICES
Appendix Page
1A Definition of terms for QDA test on sweet potato dough and fried
cakes 131
1B Questionnaire for QDA of sweet potato dough 132
1C Questionnaire for QDA of fried sweet potato cakes 133
1D Questionnaire for Hedonic test of sweet potato dough and fried cakes 134
2A Definition of terms for QDA test on frozen sweet potato dough and
fried cakes 135
2B Questionnaire for QDA of frozen sweet potato paste and dough 136
2C Questionnaire for Hedonic test of sweet potato dough and fried cakes 137
3 Table 14: Effect of frozen storage on the physical, textural and
rheological properties of frozen sweet potato paste and dough 138
4 Table 15: Effect of frozen storage on fried sweet potato cakes on
texture profile using Texture Analyzer 139
5 Table 16: Physical and textural changes of sweet potato paste and
dough during freeze-thaw study 140
6 Table 17: Texture profile analysis of fried cakes during freeze-thaw
study using a Texture Analyzer 141
7 Table 18: Effect of freeze-thaw cycles on the sensory characteristics
of sweet potato paste and douGh using QDA 142
8 Table 19: Effect of freeze-thaw cycles on the sensory characteristics
of sweet potato fried cakes using QDA 143
CHAPTER 1
INTRODUCTION
Sweet potato (Ipomoea batatas) is a dicotyledonous plant that is rich in fiber and
carbohydrate. In Malaysia, sweet potato can be processed into tasty traditional fried
cakes such as kuih keria, cucur badak, kuih bom and cek mek molek. The boiled and
mashed sweet potatoes are used as the base for making these Malaysian fried cakes. The
base is prepared by mixing the boiled and mashed sweet potato with wheat flour to form
a dough. The dough is molded and either filled with or without fillings in the centre
before they are fried (Hasidah and Khatijah, 1994).
The preparation of the sweet potato dough is very tedious and time consuming. This is
because it involved several processing stages such as peeling, boiling, mashing the sweet
potato tuber, mixing it with flour and molding into desired shape prior to deep-frying.
Therefore, the introduction of a new semi finished product such as frozen sweet potato
dough will provide an alternative for consumers who do not have much time but wish to
make fresh sweet potato cakes at anytime they want to. It is quick and easy to use dough
as consumers need only to thaw the product before it can be moulded and then fried into
the desired type of cake.
Although the sweet potato traditional cakes recipe is easily accessible, to date there is no
published scientific information on the preparation of a desirable and an optimal quality
of traditional fried sweet potato cakes. Besides, very limited reports are published on
sweet potato, especially on frozen sweet potato products compared with other tubers like
cassava. Therefore the optimization of sweet potato dough processing method is
important for processing improvements in the industry.
2
Freezing keeps food safe by slowing the movement of molecules, causing microbes to
enter a dormant stage. Freezing preserves food for extended periods because it prevents
the growth of microorganisms that cause both food spoilage and foodborne illness
(USDA, 2005), therefore lengthen the shelf life of a product. Developing the frozen semi
finished sweet potato dough will provide diverse choice to consumers as Malaysian
consumers are increasingly recognizing convenient food such as frozen processed food
due to the increasingly busy and hectic lifestyles, and also widespread ownership of
freezers and microwave ovens. Few internationally established brands that have entered
the frozen bakery products subsector in the country offer Asian-style frozen bakery
products, like Chinese pau (buns), roti prata/canai (a local crispy savoury pancake
usually consumed with curry) and curry puffs (Euromonitor, 2005).
Although freezing extended the shelf life of most foods, it also affects the food quality.
An understanding of the rheological behavior of food materials is important for new
product development, equipment design, process improvement and food quality control
(Rodriguez-Sandoval et al., 2008). Not much work have been done on the quality of
frozen sweet potato paste or dough but, abundant of information on the quality of frozen
bread dough and potato are available. The quality of bread made from frozen dough is
influenced by the dough formulation, process parameters (Rouille et al., 2000), freezing
rate, storage duration and thawing rate (Inoue and Bushuk, 1991; Neyreneuf and
Delpuech, 1993; Lu and Grant, 1999). Dough was reported to suffer from loss of
strength in frozen storage attributed to various factors such as ice recrystallization and
the water redistribution (Berglund et al., 1991; Bot and de Bruijne, 2003; Selomulyo and
Zhou, 2007). These factors may either act independently or synergistically to damage,
disrupt and resulted in a less continuous gluten network (Berglund et al., 1991; Bot and
de Bruijne, 2003; Lucas et al., 2005) in a bread dough system.
3
In the case of mash potato products, Hopkins and Gormley (2000) reported that frozen
stored mashed potato had higher compressive strength but lower shear and adhesiveness
values compared to granules that were not frozen. Canet et al. (2005) indicating that,
TPA and firmness parameters were lower in the frozen/thawed mashed samples than in
the fresh samples. Therefore, the effect of frozen storage on sweet potato dough will be
investigate in term of rheological, texture, sensory evaluation and microstructure
changes for quality determination.
During frozen storage, the probability of temperature fluctuation to occur is high.
Temperature fluctuation occurred normally due to temperature gradients from within the
products during freezing and thawing, also during storage or transportation (Breton et
al., 2000). Such temperature fluctuations cause the available water to form ice crystals
and gone through recrystallization. This creates a number of undesirable effects,
including moisture migration, dehydration, syneresis, structural breakdown, and the
formation of large ice crystals which impart a gritty mouthfeel to frozen foods (Kuntz,
1995). According to Lee et al. (2000), the drip loss obtained from three repeated freeze-
thawed sweet potato starch gel was more than doubled. Bhattacharya et al. (2003)
reported that repeated freeze-thaw cycles produced significant decrease in dough
resistance, but increases the dough extensibility. Determining the appropriate
temperatures and cycling procedure to ensure that a product truly is "freeze/thaw-stable"
is a matter of making assumptions of the number and degree of thaw and refreezing
cycles a product will undergo before it is consumed. Typically this is based on the
conditions the product is likely to encounter after it leaves the manufacturer. Freeze-
thaw cycle test is crucial to the food formulation to see if it met the shelf life and storage
requirements (Kuntz, 1995).
4
Therefore, the objectives of this study are:
1. Optimize the mashing time and wheat flour concentration of sweet potato dough
base using RSM.
2. Study the effect of frozen storage on the quality of sweet potato paste and dough
base for traditional fried cakes preparation.
3. Study the effects of freeze-thaw cycles on the quality of sweet potato paste and
dough base for traditional fried cakes preparation.