Understanding the effect of soluble fibres on the hydrolysis of starch and the diffusion of glucose during simulated human digestion by Hrvoje Fabek A Thesis Presented to The University of Guelph In partial fulfilment of requirements for the degree of Doctor of Philosophy in Food Science Guelph, Ontario, Canada © Hrvoje Fabek, April, 2015 ABSTRACT UNDERSTANDING THE EFFECT OF SOLUBLE FIBRES ON THE HYDROLYSIS OF STARCH AND THE DIFFUSION OF GLUCOSE DURING SIMULATED HUMAN DIGESTION Hrvoje Fabek Advisor: University of Guelph, 2015 Professor H.D. Goff Dietary fibre consumption is associated with a wide range of health benefits including reductions in postprandial glycemia. It is widely accepted among researchers that the effect is due to an increase in viscosity that fibres exert along the gastrointestinal tract; however, the mechanism remains to be elucidated. In this study, protein-starch solutions were fortified with four types of soluble fibre – xanthan gum (XG), guar gum (GG), soluble flaxseed gum (SFG) and soy soluble polysaccharide (SSPS). Subsequently, all solutions were passed through a three-stage in vitro digestion model. Rheological investigations showed that all solutions behaved as pseudoplastic shear-thinning materials and exhibited solid-like behaviour. Quantitative analyses of reducing sugar release (RSR) demonstrated an inverse trend between RSR and digesta viscosity. Glucose release measurements revealed that even low viscosity systems were effective at lowering the hydrolysis of starch. Analysis of glucose diffusion using a dialysis system indicated a lesser effect and only the fibres with a measurable G’ and higher viscous component inside the simulated small intestinal stage were able to attenuate glucose diffusion. Native (uncooked) tapioca starch granules were separated, purified and dried at different times of digestion and analyzed for changes in particle size using light scattering and starch morphology using scanning electron microscopy (SEM) and light microscopy. Most granules were in the range of 5-50µm, with an apparent reduction in particle size as hydrolysis continued. SEM images of starch granules revealed smooth surfaces in solution and substantial degradation (exo- and endo-corrosion, with some granules becoming completely hydrolysed) as the treatments progressed through the in vitro digestion. The progression of morphological changes was attenuated in granules extracted from the digesta of XG and GG. Light scattering and microscopy also revealed an aggregation effect between granules extracted from the XG and GG treatments. The results of the study indicated that the mechanism by which soluble fibres are able to moderate the in vitro glycemic response may be multi-faceted. Moreover, they may not be due to viscosity alone as other effects may be important, including the ability of fibres to create and maintain gel networks inside the lumen and allow starch granule aggregation. Acknowledgements I would like to express my appreciation to my advisor Dr. Douglas Goff, without whom this project would not have been possible. The experience, knowledge, and skills that I have gained through the completion of this project are invaluable. Thank you for the guidance and incessant support you have offered throughout my graduate degrees. Special thanks to my two advisory committee members, Dr. Alison Duncan and Dr. Amanda Wright. Your knowledge and contribution to this project are greatly appreciated. Thank you for offering me your time, feedback and expertise throughout this project. Thank you to Dr. Sandy Smith who helped me with all the microscopy analyses and providing training on the SEM, CLSM and light mic. Thanks to all my labmates and friends who have helped make the time spent on this project enjoyable and a special thanks to Dr. Fernanda Peyronel for her efforts in maintaining the laboratory where I conducted most of my research. Also, thank you to Ms. Tricia Anderson, Ms. Anne Ingram and Ms. Leona Varga-Lowes for their help on the administrative side of the project. Finally, I would like to thank and dedicate this thesis to my family who have been supportive of me every step of the way. To my beautiful wife Danielle and our two beautiful children - our son Luka and daughter Arwyn (born 4 days before my defence) – thank you for making every day the happiest day of my life, I love you guys so much!! Thank you to my parents for the never-ending support offered to me throughout my life - hvala na svemu! iv Table of Contents 1. Introduction ........................................................................................................................ 1 1.1. Importance of dietary fibre and research focus .......................................................... 1 1.2. Overall Objectives ..................................................................................................... 5 2. Literature Review ............................................................................................................... 7 2.1. Digestion of foods ...................................................................................................... 7 2.1.1. Gastrointestinal physiology...................................................................................... 7 2.1.2. Carbohydrate metabolism ...................................................................................... 12 2.1.3. Glucose absorption ................................................................................................. 14 2.1.4. Review of in vitro digestion ................................................................................... 16 2.1.5. In vitro digestion methods related to starch digestion ........................................... 23 2.2. Dietary Fibre ............................................................................................................ 29 2.2.1. Dietary fibre definition .......................................................................................... 29 2.2.2. Health benefits of dietary fibre .............................................................................. 31 2.3. Soluble fibres employed in the study ....................................................................... 42 2.3.1. Guar Gum............................................................................................................... 42 2.3.2. Xanthan Gum ......................................................................................................... 46 2.3.3. Flaxseed Gum ........................................................................................................ 50 2.3.4. Soy soluble polysaccharide .................................................................................... 52 2.3.5. Summary Comment ............................................................................................... 55 3. The effect of in vitro digestive processes on the viscosity of dietary fibres and their influence on glucose diffusion ................................................................................................. 56 3.1. Introduction .............................................................................................................. 57 3.2. Materials and methods ............................................................................................. 60 3.2.1. Materials ................................................................................................................ 60 3.2.2. Soluble flaxseed gum extraction ............................................................................ 61 3.2.3. Food matrix preparation ........................................................................................ 61 3.2.4. Viscosity measurements ......................................................................................... 62 3.2.5. In vitro digestion .................................................................................................... 63 3.2.6. Glucose release measurements .............................................................................. 63 3.2.7. Statistical Analysis ................................................................................................. 64 3.3. Results and discussion ............................................................................................. 64 3.3.1. Effect of in vitro digestion on solution viscosity .................................................... 64 v 3.3.2. Glucose diffusion during in vitro digestion ............................................................ 71 3.4. Conclusion ............................................................................................................... 75 4. Effect of soluble fibre inclusion on starch hydrolysis and glucose mobility during simulated small intestinal digestion ......................................................................................... 77 4.1. Introduction .............................................................................................................. 78 4.2. Materials and methods ............................................................................................. 81 4.2.1. Materials ................................................................................................................ 81 4.2.2. Food matrix preparation ........................................................................................ 82 4.2.3. Dynamic viscoelasticity and viscosity measurements ............................................ 83 4.2.4. In vitro digestion