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Drying of Foodstuff Using Supercritical Carbon Dioxide

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Drying of Foodstuff Using Supercritical Carbon Dioxide THE DRYING OF FOODS USING SUPERCRITICAL CARBON DIOXIDE ZOË KATHERINE BROWN A Thesis Submitted to The University of Birmingham for the Degree of Doctor of Engineering Department of Chemical Engineering The University of Birmingham October 200 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. SUMMARY Food drying techniques such as air and freeze drying are not ideal: high temperatures used during air drying result in degradation of nutrients and sensorial properties, while freeze drying is expensive and therefore only applicable to high value foods. As an alternative to such drying techniques, drying with supercritical carbon dioxide was investigated here. Initially, carrot was dried using this technique. Addition of a co-solvent (ethanol) to the supercritical fluid was used as a method to increase the water solubility in the supercritical fluid and therefore aid drying. Analysis of the dried and rehydrated product structure, rehydration properties and mechanical properties was carried out which gave an indication of product quality. Drying of agar, containing varying concentrations of sugar was carried out on a laboratory and pilot plant scale. Gel structure and gel properties were studied. Addition of sugar to agar gel pieces improved structural retention considerably during drying. Fourier transform infrared analysis was used to investigate interactions that may be responsible for structural differences seen during supercritical drying. Changes in experimental parameters such as flow rate and depressurisation rate did not appear to have a significant effect on the dried gel structure. The supercritical drying technique investigated allowed food products to be dried and unique structures to be created with different rehydration and textural properties to the equivalent food products dried by air or freeze drying. I ACKNOWLEDGEMENTS This thesis owes its existence to the help, support and inspiration of many people. In the first place, I wish to acknowledge the EPSRC, Unilever and the University of Birmingham for financial support during the four years of study and the use of laboratory facilities which enabled this work to be carried out. I would especially like to thank my primary supervisor Dr Rachel Bridson for her continued support, guidance and invaluable input. I am also grateful to my associate supervisors, Professor Peter Fryer and Professor Ian Norton, without whose knowledge and assistance this study would not have been successful. I would like to thank Dr Richard Greenwood for his assistance with all EngD matters. Thanks also to colleagues at Unilever, Vlaardingen for assisting me in the work that I carried out at their research facilities, in particular I would like to thank Stephan Schumm. Many thanks are extended to my friends and colleagues at the University of Birmingham for their encouragement and advice on technical matters. In particular, I would like to thank Dr Teijun Lu, Dr Serafim Bakalis, Dr Phil Robbins and Dr Gary Leeke. I would also like to acknowledge all those in the workshop within the school of engineering for their endless assistance and support. Finally, I extend my gratitude to my friends and family for their continued support, encouragement, motivation and understanding throughout the duration of my studies. II CONTENTS Page Summary…………………………………………………………………………………….……I Acknowledgements………………………………………………………………………………II Contents…………………………………………………………………………………………III List of figures…………………………………………………………………………….........VIII List of tables…………………………………………………………...………………………XIV Abbreviations…………………………………………………………………………………..XV 1.0 Literature Review .................................................................................................................. 1 1.1 Introduction to supercritical fluids....................................................................................... 2 1.1.1 Properties and characterisation of supercritical fluids.................................................. 3 1.1.2 Carbon dioxide as a solvent .......................................................................................... 4 1.1.3 Addition of co-solvents to supercritical systems .......................................................... 5 1.2 Uses and applications of supercritical fluids ....................................................................... 6 1.2.1 Supercritical extraction ................................................................................................. 6 1.2.2 Supercritical drying....................................................................................................... 8 1.2.3 Polymer processing using supercritical fluids ............................................................ 11 1.2.3.1 Fourier transform infrared analysis to study the interactions of supercritical carbon dioxide with polymers.......................................................................................... 14 1.3 Introduction to drying of foods.......................................................................................... 16 1.3.1 Advantages and disadvantages of conventional methods for drying of foods............ 18 1.3.2 Drying kinetics............................................................................................................ 21 1.3.3 Water content measurement........................................................................................ 23 1.3.4 Drying of fruit and vegetables .................................................................................... 25 1.3.4.1 Drying of carrots.................................................................................................. 26 1.3.5 Gels in the food industry............................................................................................. 27 1.3.5.1 Introduction to agar.............................................................................................. 28 1.3.5.2 Drying of gels ...................................................................................................... 31 1.3.5.3 Addition of sugar to biopolymer gels and their effect on microstructure and physical properties ........................................................................................................... 32 III 1.4 Importance of food structure resulting from drying .......................................................... 34 1.5 Analysis of internal microstructure.................................................................................... 37 1.5.1 Microscopy ................................................................................................................. 38 1.5.2 X-ray micro-computed tomography ........................................................................... 38 1.6 Macrostructural measures of quality.................................................................................. 40 1.6.1 Texture analysis .......................................................................................................... 40 1.6.2 Colour analysis techniques ......................................................................................... 42 1.6.3 Rehydration................................................................................................................. 43 1.7 Aims of this work............................................................................................................... 45 2.0 Drying of carrots using supercritical carbon dioxide............................................................. 46 2.1 Introduction........................................................................................................................ 47 2.1.1 Solvent systems and the use of ethanol as a co-solvent.............................................. 48 2.2 Materials ............................................................................................................................ 49 2.2.1 Rig components .......................................................................................................... 49 2.2.2 Carrot preparation and storage.................................................................................... 49 2.2.3 Carbon dioxide and ethanol ........................................................................................ 49 2.3 Apparatus and methodology .............................................................................................. 49 2.3.1 High pressure equipment and supercritical drying method ........................................ 49 2.3.2 Initial sample characterisation and preparation of carrots .......................................... 53 2.3.3 Method for supercritical drying of carrot pieces using supercritical carbon dioxide . 55 2.3.3.1 Addition of ethanol as a co-solvent ..................................................................... 57 2.3.4 The use of liquid carbon dioxide for drying experiments........................................... 60 2.3.5 Method for air drying of carrot pieces .......................................................................
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