UNIVERSITI PUTRA MALAYSIA PREPARATION OF FROZEN …psasir.upm.edu.my/id/eprint/5338/1/FSTM_2008_4.pdfDoh ubi keledek sangat popular sebagai asas doh bagi pembuatan kuih goreng tradisional

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


(Internal Examiner)

Noranizan Mohd. Adzahan, PhD

Lecturer



(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


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



(Chairman)

SH. KHARIDAH SY. MUHAMMAD, PhD

Associate Professor



(Member)

DZULKIFLY MAT HASHIM

Lecturer



(Member)

AINI IDERIS, PhD

Professor and Dean

School of Graduate Studies


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



4.2.2 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


4.2.8 Sensory analysis 55

4.2.9 Microstructural observation 56

4.2.10 Statistical analysis 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



5.2.2 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


5.2.8 Sensory analysis 88

5.2.9 Microstructural observation 88

xiii

5.2.10 Statistical analysis 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


and mashing time on the adhesiveness of sweet potato dough 47


and mashing time on the chewiness of sweet potato fried cakes. 48


and mashing time on the handling ability of sweet potato dough 48


and mashing time on the sweet potato flavour of sweet potato

fried cakes 49


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


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


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.

Documents

UNIVERSITI PUTRA MALAYSIA PREPARATION OF FROZEN …psasir.upm.edu.my/id/eprint/5338/1/FSTM_2008_4.pdfDoh ubi keledek sangat popular sebagai asas doh bagi pembuatan kuih goreng tradisional