DRYING AND REHYDRATION OF GELLAN GUM GELS Mattia Cassanelli A thesis submitted to The University of Birmingham For the degree of DOCTOR OF PHILOSOPHY School of Chemical Engineering College of Engineering and Physical Sciences University of Birmingham 2018 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties.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. Abstract Food drying is an essential process to extend product shelf life by water removal. In complex food formulations, single ingredients might dry and rehydrate differently, based on their microstructure and their interaction with water. In this context, hydrocolloids are often used as gelling agents, thickeners and stabiliser to modulate the product properties. This thesis aims to advance knowledge in food engineering, investigating the role of the drying process on gellan gum gel microstructure and the subsequent rehydration from a structural and molecular perspective. This research shows, for the first time, the freeze-dried low acyl (LA) gellan gum, high acyl (HA) and HA:LA mixture gel structures and their properties upon rehydration. The water interaction with the gel structure is affected by the presence of acyl groups along the HA gellan gum polymer chain. The rehydration rate was shown to be lower for HA gellan gum and was not dependent on the polymer concentration. In the second instance, the effect of the gel pH on the dried structure is studied as well as the water re-absorption extent and rate. In this part, two drying methods, freeze- and oven-drying, were performed. Finally, a comparison of the three drying processes (freeze-, oven- and supercritical fluid drying) is reported, proposing the effect on the molecular structure by investigation of the disordered chain domains of the gel network. I As supercritical fluid drying requires a pre-treatment in alcohol, its effect on the gellan gum network was investigated. It is shown how alcohols change the gellan gum configuration, leading to a considerable variation in gel texture. A novel supercritical fluid batch rig is also proposed, characterised by high process flexibility and reduced set-up and energetic costs, and therefore particularly suitable for small/laboratory-scale use. II Acknowledgments This PhD represents a milestone in my life and my career, three unforgettable years that have given me unique skills and an incredible personal enrichment. It has been possible with the support and help of people that I consider special. Firstly, I would like to thank my supervisors, Professor Ian Norton, Dr Tom Mills and Dr Valentina Prosapio for their guidance and support throughout my PhD. They believed in me, reassuring me in difficult periods and motivated me to achieve the best results. They welcomed me to the EPSRC Centre for Innovative Manufacturing in Food, which funded my research and where I found great people. Special thanks goes to Dr Ian Hamilton and Dr Abigail Norton, whose support and mentorship have been fundamental to overcome difficulties. I will be always grateful to Lynn Draper, who welcomed me when I first arrived in Birmingham, younger but less wise then now. I would like to genuinely thank the entire Food Microstructure Group, in particular Ernesto, Domenico, Matteo, Federico, Giuseppe, Carlotta, Benedetta, Gaia, Ramona and Fabio, who made me feel a bit closer home. Thanks to Aris, Yadira, Ourania, Ioanna, Panos, Amir, Amy Ellis, Chris, John, Richard, Zoe, Kilian, Emmanuelle and all the other friends I have in Chemical Engineering. Big thanks to all my friends I found in Birmingham. To my family, who on a daily basis give me their undivided love. I should write another thesis to explain how important they are to me. Generally, the most important acknowledgment is left at the end. I leave this “special slot” to the person who loves me and always believes in me. Thank you Chiara! III Table of Contents Abstract……………………………………………………………………………………………………………………….……I Acknowledgements ……………………………………………………………………………………..…………………III Table of Contents ………………………………………………………………………………………….….……………IV List of Figures …………………………………………………………………………………………………..…….………XI List of Tables…………………………………………………………………………………………………………………XXI Chapter 1. Introduction…………………………………………………………………………………………….1 1.1 Project context………………………………………………………………………………………………..…………2 1.2 Aims…………………………………………………………………………………………………………….…….………3 1. 3 Thesis structure…………………………………………………………………………………………………..….…4 1.4 Publications and presentations………………………………………………………………………….………6 Chapter 2. Literature review……………………………………………………………………………………9 2.1 Introduction…………………………………………………………………………………….………………………10 2.2 Hydrocolloids……………………………………………………………………………………………………………10 2.2.1 Gellan gum………………………………………………………………………………………………………18 2.3 Drying mechanisms in the food industry………………………………………….………………………26 2.3.1 Freeze-drying……………………………………………………………………….…………………………30 2.3.1.1 Pre-treatments and freezing……………………………………………….………………32 2.3.1.2 Collapse temperature………………………………………………….………………………33 IV 2.3.1.3 Primary drying………………………………………………………………………….…………34 2.3.1.4 Secondary drying…………………………………………………………………………………38 2.3.2 Air-drying…………………………………………………………………………………..……………………39 2.3.2.1 Heat and mass transport mechanisms…………………………………..……………40 2.3.2.2 Drying kinetics and drying periods………………………………………………………42 2.3.2.3 Vacuum drying……………………………………………………….……………………………46 2.3.3 Supercritical fluid technology………………………………………………………….………………47 2.3.2.1 Supercritical fluid drying………………………………………………..……………………51 2.3.2.2 Supercritical fluid extraction……………………………………….………………………54 2.4 Dried gels…………………………………………………………………………………………………………………59 2.5 Rehydration………………………………………………………………………………………..……………………61 2.6 Conclusions………………………………………………………………………………………………………………62 Chapter 3. Investigation of freeze-dried gellan gum structure: effect of gel formulation……………………………………………………………………………………..…………………………64 3.1 Introduction……………………………………………………..……………………………..………………………65 3.2 Materials and methods………………………………………………………………………………….…………67 3.2.1 Gel preparation………………………………………………………………………………………67 3.2.2 Molecular interactions: μDSC and FTIR………………………………..…………………68 3.2.3 Texture analysis………………………………………………………………..……………………69 3.2.4 Freeze-drying……………………………………………………………………….…….…………70 3.2.5 Normalised Moisture Content (NMC) and water activity…………....…………71 3.2.6 Drying modelling……………………………………………………………………………………72 V 3.2.7 Microscopy…………………………………………………………………….………………………73 3.2.8 Static contact angle and wettability…………………………….…………………………74 3.2.9 Rehydration and swelling……………………………………………………………….………74 3.2.10 Rehydration modelling…………………………………………………………………………75 3.2.11 Sugar release……………………………………………………….………………………………75 3.2.12 Sugar release modelling…………………………………………………….…………………76 3.2.13 Statistical analysis……………………………………………………………..…………………77 3.3 Results and discussion……………………………………………………………………………..………………78 3.3.1 Investigation of LA/HA freeze-dried structure………………………..………………78 3.3.1.1 Freeze-drying…………………………………………………………..…………………78 3.3.1.2 Freeze-dried structure………………………………………………….……………87 3.3.1.3 Rehydration and swelling…………………………………………..………………91 3.3.1.4 HA/LA gellan mixture…………………………………………………………………99 3.3.2 Investigation of freeze-dried structure with mannitol or sucrose…………105 3.3.2.1 Gels before freeze-drying……………………………………………….……..…105 3.3.2.2 Freeze-dried gels………………………………………………………………………114 3.3.2.3 Rehydration and leaching…………………………………………………………123 3.4 Conclusions…………………………………………………………………………………….……..………………130 Chapter 4. Acidified/basified gellan gum gels: the role of the structure in drying/rehydration ………………….………………………………………………..………………………132 4.1 Introduction…………………………………………………………………………………………..………………133 4.2 Materials and methods………………………………………………….……………...………………………135 VI 4.2.2 Gel preparation……………………………………………………………………………………135 4.2.2 Zeta potential…………………………………………………..…………….……………………135 4.2.3 Texture analysis……………………………………………………………………………………136 4.2.4 Post-gelation exposure…………………………………………………………………………136 4.2.5 Molecular interactions: μDSC and FTIR……………………………..…………………136 4.2.6 Freeze-drying……………………………………………………………………………….………136 4.2.7 Oven drying……………………………………………………………………….…………………136 4.2.8 Moisture content analysis…………………………………………….………………………137 4.2.9 Micro computed microscopy (μCT) …………………………………..…………………137 4.2.10 Rehydration…………………………………………………………………………….…………137 4.2.11 Statistical analysis………………………………………………………………………………138 4.3 Results and discussion……………………………………………………………………………………………138 4.3.1 Effect of pH on gel structure and mechanical properties………………………138 4.3.2 Gel drying and rehydration ………………………………………………………….………152 4.4 Conclusions……………………………………………………………………..………………………….…………161 Chapter 5. Gellan gum dried gel structure: molecular and macroscopic investigations……………………………………………………………………………………………………………162 5.1 Introduction……………………………………………………………………………………………………..……163 5.2 Materials and methods……………………………………………………………………………………….…167 5.2.1 Gel preparation and solvent pre-treatment for sCO2-drying……………..…167 5.2.2 Freeze-drying……………………………………………………………………………….………168 5.2.3 Oven drying …………………………………………………………………………………………168