UNIVERSITI PUTRA MALAYSIA

OSMOTIC DEHYDRATION COMBINED WITH AIR DRYING OF RED PITAYA FRUIT CUBES

ASSAL HAJ NAJAFI

FK 2010 99

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OSMOTIC DEHYDRATION COMBINED WITH

AIR DRYING OF RED PITAYA FRUIT CUBES

ASSAL HAJ NAJAFI

MASTER OF SCIENCE

UNIVERSITI PUTRA MALAYSIA

2010

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OSMOTIC DEHYDRATION COMBINED WITH AIR DRYING OF RED

PITAYA FRUIT CUBES

By

ASSAL HAJ NAJAFI

Thesis Submitted to the School of Graduate Studies, Universiti Putra Malaysia,

in Fulfilment of the Requirements for the Degree of Master of Science

March 2010

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Abstract of thesis presented to senate of Universiti Putra Malaysia in fulfilment of

the requirement for the degree of Master in Science

OSMOTIC DEHYDRATION COMBINED WITH AIR DRYING OF RED

PITAYA FRUIT CUBES

By

ASSAL HAJ NAJAFI

March 2010

Chairman :Yus Aniza Yusof, PhD

Faculty : Engineering

The main objectives of this study are to investigate the effects of different

temperatures on osmotic dehydration of red cubical pitaya fruit and subsequently

physical quality evaluations, in developing dried pitaya cubes as a new healthy

snack product. Two major steps were involved (i) an osmotic dehydration process

was used as a pre-treatment and (ii) an air drying process in a cabinet dryer was used

for further drying. The effect of sugar solution concentration (40, 50 and 60%),

temperature (25, 30 and 35 ˚C) and air velocity (1 and 3 ms¹־) and also air

temperature for the air drying process (40, 50 and 60 ˚C) were studied. Sampling

was performed every 15 minutes for 2 hours, then at 4, 6, 24, 48 and 72 hours of

immersion. Then pitaya slices were removed from the solution in order to

investigate dehydration efficiency and equilibrium stage of dehydration. Osmotic

dehydration kinetics was modelled according to Peleg, and Page equations. Both

models were evaluated using two statistical measures, correlation coefficient, and

root means square error. The statistical parameters (R² and RMSE) indicated that

both models can predict good fitting with moisture content, weight reduction, and

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sugar gain and water loss. But the best fitting for weight reduction and sugar gain

were obtained using Peleg equation. The Page empirical model presented a good fit

of the water loss experimental data.

Addition of sucrose to osmotic solutions decreased the driving force of the process

and resulted in higher water loss and sugar gain. Colour saturation values increased,

denoting colour intensification during the process of osmotic dehydration. Lightness

of the pitaya cubes decreased as the sugar concentration increased. The greatest

changes in Total Colour Difference of osmotic dehydrated samples occurred in 50

and 60% sugar solutions. An increase of concentration and passing time cause softer

texture in product compared to the fresh pitaya. However, based on the air drying

process, the best osmotic dehydration condition was a sugar concentration of 60% at

35 ˚C with a contact time of 2 hours. This treatment could remove more water of the

samples, therefore air drying time reduced.

Pitaya samples were air dried in a cabinet dryer at 40, 50 and 60 ˚C with two

different air velocities of 1 and 3 ms¹־ for 8 hours. Among the pre-treatment

conditions, the sucrose concentration, temperature and immersion time significantly

(p < 0.05) influenced the air drying time. Osmotic dehydrated pitayas that were air

dried at 60 ˚C showed a large moisture decline in the early drying periods similar

with the drying rates of untreated samples. At the beginning of the drying process of

fresh pitaya, drying rate was influenced by air temperature. Air dried samples at 40

˚C showed lower drying rates attributed to sugars concentration on the outer layers

of pitaya tissue and their crystallization during drying, but had better colour

retention during drying. In the air dried osmotic dehydrated samples at 60 ˚C a

greater texture hardening was observed.

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The best product was obtained in the following operational condition: air

temperature of 60 ˚C, air velocity of 1 and 3 ms¹־ and contact time of around 5 hours

(with 22.78% to 22.3% moisture content) for air drying. Because of the short time of

drying these conditions help to improve colour and texture of the osmotic

dehydrated pitaya cubes. This study provides an extensive understanding of osmotic

dehydration of red cubical pitaya at lower temperatures. The results indicate that the

process is feasible and may represent a new product for pitaya fruit.

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Absrak tesis yang dikemukakan kepada senat Universiti Putra Malaysia sebagai

memenuhi keperluan untuk ijazah Master Sains

PENYAHHIDRATAN OSMOSIS BERGABUNG DENGAN PENGERINGAN

UDARA KE ATAS KIUB BUAH NAGA MERAH

Oleh

ASSAL HAJ NAJAFI

March 2010

Pengerusi : Yus Aniza Yusof, PhD

Fakulti : Kejuruteraan

Tujuan utama kajian ini dijalankan adalah untuk mengkaji kesan perbezaan suhu

pada penyahhidratan osmosis ke atas kiub buah naga merah dan seterusnya penilaian

kualiti fizikal, dalam mempertingkatkan kiub buah naga kering sebagai salah satu

produk makanan ringan yang sihat. Dua langkah utama yang terlibat ialah (i) proses

penyahhidratan osmosis yang digunakan sebagai pra-eksperimen dan (ii) proses

pengeringan udara di dalam kabinet kering sebagai proses pengeringan seterusnya.

Kesan kepekatan larutan gula (40, 50 dan 60%), suhu (25, 30 dan 35%), dan halaju

udara (1 dan 3 ms-1

) dan juga suhu udara dalam proses pengeringan (40, 50 dan 60 o

C) telah dikaji. Sampel diuji setiap 15 minit dalam tempoh 2 jam. Kemudian pada 4,

6, 24, 48 dan 72 jam rendaman, hirisan buah naga dikeluarkan dari larutan untuk

dikaji kecekapan dan tahap keseimbangan penghidratan. Kinetik penghidratan

osmosis telah dimodelkan mengikut persamaan “Peleg” dan “Page”. Kedua-dua

model dinilai menggunakan penilaian statistik, hubungan pemalar dan perbezaan

punca kuasa dua. Parameter statistik (R2 dan RMSE) menunjukkan kedua-dua model

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boleh memberi hubungan yang baik dengan kelembapan, pengurangan berat,

penambahan gula dan kehilangan air. Tetapi hubungan yang terbaik untuk

pengurangan berat dan penambahan gula diperolehi menggunakan persamaan

“Peleg”. Secara empirikal, model “Page” menunjukkan hubungan yang baik dengan

data eksperimen kehilangan air. Penambahan sukrosa kepada larutan osmosis telah

menurunkan kuasa memandu proses dan menyebabkan peningkatan dalam

kehilangan air dan penambahan gula. Nilai ketepuan warna meningkat, menandakan

peningkatan warna berlaku dalam proses penghidratan osmosis. Kecerahan kiub

buah naga menurun dengan peningkatan kepekatan gula. Perubahan terbesar dalam

Jumlah Perbezaan Warna daripada sampel penghidratan osmosis berlaku pada 50

dan 60% larutan gula. Peningkatan kepekatan dan pemanjangan tempoh

menyebabkan tekstur produk lembut berbanding buah naga segar. Namun

demiluian, dengan berasasken proses pengeringan, keadaan penghidratan osmosis

terbaik adalah kepekatan gula 60% pada suhu 35oC dalam masa 2 jam. Keadaan ini

boleh mengeluarkan lebih air dari sampel dalam seterusnya mengurangkan masa

pengeringan udara.

Sampel kiub buah naga telah dikeringkan dalam kabinet pengering pada suhu 40,50

dan 60oC pada dua jenis halaju udara 1 dan 3 ms

-1 untuk tempoh 8 jam. Antara

keadaan pra-eksperimen, kepekatan sukrosa, suhu dan tempoh rendaman

mempengaruhi masa pengeringan udara dengan kepentingan (p < 0.05). Buah naga

ternyahhidrat osmosis yang telah dikeringkan pada 60oC menunjukkan penurunan

kelembapan terbesar pada peringkat permulaan pengeringan dan kadar yang sama

juga dilakukan pada sampel yang tidak ternyahhidrat. Pada permulaan proses

pengeringan buah naga segar, kadar pengeringan mempengaruhi suhu udara. Sampel

yang dikeringkan pada 40oC menunjukkan kadar pengeringan terendah disebabkan

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kepekatan gula pada permukaan luaran buah naga dan penghabluran semasa

pengeringan, tetapi mempunyai pengekalan warna terbaik semasa pengeringan.

Sampel ternyahhidrat osmosis yang telah dikeringkan pada 60oC didapati teksturnya

telah mengeras.

Produk terbaik telah diperolehi melalui keadaan operasi seperti berikut: suhu udara

pada 60oC, halaju udara di antara 1 dan 3 ms

-1 dalam tempoh masa selama 5 jam

(22.78 sehingga 22.3 kelembapan) untuk proses pengeringan udara. Ini disebabkan,

pengeringan dalam tempoh masa yang singkat dapat membantu memperbaiki warna

dan tekstur kiub buah naga yang ternyahhidrat secara osmosis. Kajian ini

menyediakan pemahaman yang mendalam tentang penyahhidratan osmosis ke atas

kiub buah naga merah pada suhu rendah. Keputusan kajian telah menunjukkan

proses ini boleh dilaksanakan dan boleh dijadikan sebagai alternatif baru dalam

pengeluaran produk buah naga.

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ACKNOWLEDGEMENTS

I pray to Almighty ALLAH who give me the thoughts, the will, and guided me to

complete this work. I pray that ALLAH will bless this work and make it useful for

mankind, and that He will forgive us.

My sincere and deepest gratitude to Dr.Yus Aniza Yusof, the chairman of my

supervisory committee for her guidance, encouragement, patience and continuous

follow up during the course of this study. My appreciation and gratitude is also

extended to members of my supervisory committee, Professor Dr Russly Abdul

Rahman and Dr. Chin Nyuk Ling for their advice, punctuate comments and support.

My gratitude is also due to all the staff of the Faculty of Food Science and

Technology, UPM for their cooperation. My special appreciation is extended to my

friends for their kind help and friendly attitude.

I would also like to give my thanks to my dearest husband, Hadi Mohammad Zadeh

who brightens my life with his support, encouragement, sacrifice and patience.

Last but not least, I must express my deepest gratitude to my wonderful parents who

continuously encouraged me and presented me the most beautiful World. At the

same time, I would also like to thank my family for all supports throughout this

period.

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I certify that a Thesis Examination committee has meet on 31.March.2010 to

conduct the final examination of Assal Haj Najafi on her thesis entitled “Osmotic

Dehydration Combined with Air Drying of Red Pitaya Fruit Cubes” in accordance

with the Universities and University Colleges Act 1971 and the Constitution of the

Universiti Putra Malaysia [P.U.(A) 106] 15 March 1988. The committee

recommends that the student be awarded the Master of Science.

Members of the Thesis Examination Committee were as follows:

Mohd Nordin B. Ibrahim, PhD

Professor

Faculty of Engineering (Food Engineering)

Universiti Putra Malaysia

(Chairman)

Johari Endan, Phd

Associate Professor

Faculty of Engineering (Food Engineering)

Universiti Putra Malaysia

(Internal Examiner)

Farah Saleena Taip, PhD

Faculty of Engineering (Food Engineering)

Universiti Putra Malaysia

(Internal Examiner)

Azila Abdul Aziz, PhD

Associate Professor

Faculty of Engineering (Food Engineering)

Universiti Technology Malaysia

Malaysia

(External Examiner)

_______________________________

BUJANG BIN KIM HUAT,

Professor and Deputy Dean

School of Graduate Studies

Universiti Putra Malaysia

Date:

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This thesis was submitted to the Senate of Universiti Putra Malaysia and has been

accepted as fulfillment of the requirement for the degree of Master with dissertation.

The members of the supervisory were as follows:

Yus Aniza Yosuf, PhD

Faculty of Engineering

Universiti Putra Malaysia

(Chairman)

Russly Abdul Rahman, PhD

Professor

Faculty of Engineering

Universiti Putra Malaysia

(Member)

Chin Nyuk Ling, PhD

Faculty of Engineering

Universiti Putra Malaysia

(Member)

________________________________

HASANAH MOHD GHAZALI, PhD

Professor and Dean

School of Graduate Studies

Universiti Putra Malaysia

Date: 15 July 2010

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DECLARATION

I declare that the thesis is my original work except for quotations and citations

which have been duly acknowledged. I also declare that it has not been previously,

and is not concurrently, submitted for any other degree at Universiti Putra Malaysia

or at any other institution.

_________________________

Assal Haj Najafi

Date: 3 June 2010

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TABLE OF CONTENTS

Page

ABSTRACT i

ABSTRAK iv

ACKNOWLEDGEMENTS vii

APPROVAL viii

DECLARATION x

LIST OF TABLES xiii

LIST OF FIGURES xiv

LIST OF ABBREVIATIONS xvii

CHAPTER

1 INTRODUCTION

1.1 Introduction 1

1.2 Osmotic Dehydration 1

1.3 Air Dehydration 3

1.4 Pitaya Fruit 5

1.5 Problem Statement 5

1.6 Hypothesis 6

1.7 Objectives 6

2 LITERATURE REVIEW

2.1 Introduction 8

2.2 Pitaya Fruit 8

2.2.1 Osmotic Dehydration of Pitaya 9

2.3 History of Drying 12

2.3.1 General Drying Methods 12

2.4 Food Additives in Osmotic Dehydration 16

2.4.1 Use of Sugar as a Food Additive 17

2.5 Theory of Osmotic Dehydration 18

2.6 Drying Systems for Osmotic Dehydration 21

2.7 Effect of Osmotic Dehydration on Air Drying 24

2.7.1 Drying Curves and Rates 24

2.8 Empirical Modelling 25

2.9 Quality Assessment 30

2.9.1 Theory of Texture Analysis 31

2.9.2 Theory of Colour Measurement 32

2.9.3 Theory of Moisture Content and Water Activity 33

2.10 Product Quality During Osmotic Dehydration 34

2.11 Applications During Osmotic Dehydration 34

2.12 Energy Saving during Osmotic Dehydration 35

3 EXPERIMENTAL DESIGN AND METHODOLOGY

3.1 Introduction 36

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3.2 Materials 36

3.2.1 Sample Preparation 36

3.2.2 Osmotic Solution 37

3.3 Methodology 38

3.3.1 Osmotic Pre-treatment 38

3.3.2 Air Drying 39

3.4 Analytical Methods 40

3.4.1 Moisture Content, Water Loss, Solid Gain and Weight Reduction

Measurements

40

3.4.2 Water Activity Measurements 41

3.4.3 Texture Measurements 42

3.4.4 Colour Measurements 43

3.5 Mathematical Models 45

3.5.1 Peleg’s model 45

3.5.2 Page’s model 47

3.6 Experimental Design 48

3.6.1 Studying on Equilibrium Condition of Osmotic Dehydration of Red

Pitaya

50

3.6.2 Determining Colour and Texture of Red Pitaya 51

3.6.3 Investigating Drying of Fresh and Osmotic Dehydrated Red Cubical

Pitaya and Physical Quality Evaluations

51

3.6.4 Statistical Analysis 52

4 RESULTS AND DISCUSSION

4.1 Introduction 53

4.2 Preliminary Studies 53

4.3 Osmotic Dehydration of Red Cubical Pitaya 54

4.3.1 The Effects of Time, Temperature and Concentration 55

4.3.2 Colour Analysis of Osmotic Dehydrated Pitaya Fruit 90

4.3.3 Texture Analysis of Osmotic Dehydrated Pitaya Fruit 94

4.4 Air Drying of Osmotic Dehydrated Red Cubical Pitaya Fruit 97

4.4.1 Osmotic Dehydration 97

4.4.2 Air Drying 99

4.5 Summary of Osmotic Dehydrated and Air Dried Pitaya Samples 120

5 CONCLUSION AND RECOMMENDATION

5.1 Conclusion 123

5.2 Recommendation for Future Works 126

REFERENCES 128

APPENDICES 143

BIODATA OF STUDENT 169