THE IMPACT OF WHISKY BLEND MATRICES ON THE SENSORY PERCEPTION OF PEATY FLAVOURS Tao Yang A thesis submitted for the degree of Doctor of Philosophy International Centre for Brewing and Distilling School of Life Sciences Heriot-Watt University Edinburgh September 2014 The author owns the copyright in this thesis. Any quotation from the thesis or use of any of the information contained in it must acknowledge this thesis as the source of the quotation or information. ACKNOWLEDGEMENTS The financial support for this project provided by Diageo, the Scotch whisky Research Institute and the Heriot-Watt University is gratefully acknowledged. In presenting this thesis, I would like to sincerely thank Dr John Conner and Dr Frances Jack of the Scotch whisky Research Institute (SWRI) for their tremendous and continual guidance and support, without whom none of this would have been possible. I also thank Mrs Jane Walker and Mr. Olivier Fagnen for their friendship, support and encouragement throughout this study. I would like also to thank my supervisor, Professor Paul Hughes for his friendly and continuous supervision, advice, encouragement and support provided over the course of this project. I am also very grateful to Dr Jim Beveridge, Mr. Paul Lockyer and Mr. Alan Wardlaw of Diageo Ltd., for their invaluable help, suggestions, and useful discussions throughout this project. I want to also thank them for their warm welcome during my visits to Menstrie (Europe Technical Centre), and for their sincere interest in my future I have to thank also all the wonderful people I have met in Scotland during these years that made me enjoy my life here so much since the first moment I arrived. Finally, I would like to express my immense gratitude to my family, for their constant love, support and encouragement. II CONTENTS CHAPTER 1. INTRODUCTION 1 1.1 Scotch whisky 1 1.1.1 History of Scotch whisky ............................................................................... 1 1.1.2 Definition of Scotch whisky ........................................................................... 2 1.1.3 Whisky classification by categories .............................................................. 2 1.1.4 Whisky classification by region .................................................................... 3 1.1.5 Whisky classification by aroma .................................................................... 5 1.2 Origins of Scotch whisky aroma 6 1.2.1 Raw materials ............................................................................................... 6 1.2.1.1 Air and Water ................................................................................................ 6 1.2.1.2 Cereals in whisky production........................................................................ 7 1.2.2 Malt kilning ................................................................................................... 8 1.2.2.1 Enzymatic and chemical oxidation ............................................................... 8 1.2.2.2 The Maillard reaction ................................................................................... 8 1.2.2.3 Peating .......................................................................................................... 9 1.2.3 Fermentation ............................................................................................... 11 1.2.3.1 Higher alcohols ........................................................................................... 12 1.2.3.2 Esters........................................................................................................... 12 1.2.3.3 Acids ............................................................................................................ 13 1.2.4 Distillation - separation and fractionation of aroma ................................. 14 1.2.4.1 Malt Distillation .......................................................................................... 14 1.2.4.2 Grain distillation ......................................................................................... 15 1.2.5 Maturation .................................................................................................. 18 1.2.5.1 Extraction of wood components .................................................................. 20 1.2.5.2 Chemical interactions ................................................................................. 21 1.2.5.3 Adsorption and evaporation ....................................................................... 22 1.2.6 Whisky blending .......................................................................................... 23 1.2.7 Whisky chill – filtration ............................................................................... 25 1.3 The Olfactory System 26 III 1.3.1 Odorant Receptors ...................................................................................... 26 1.3.2 Olfactory Bulb ............................................................................................. 28 1.3.3 Complex Odour Mixtures ............................................................................ 28 1.4 Aroma congener interactions 30 1.4.1 Physicochemical perspective ...................................................................... 30 1.4.2 Physiological perspective ........................................................................... 31 1.4.2.1 Spatial filtering ........................................................................................... 32 1.4.2.2 Temporal filtering ....................................................................................... 32 1.4.3 Psychological perspective ........................................................................... 33 1.5 Whisky analysis 33 1.5.1 Analytical methods ...................................................................................... 33 1.5.1.1 Phenol analysis ........................................................................................... 33 1.5.1.2 Distillate congeners analysis ...................................................................... 34 1.5.1.3 Maturation-derived compounds .................................................................. 34 1.5.1.4 Gas Chromatography – Olfactometry (GC-O) ........................................... 34 1.5.1.5 Identification of potent odorants by GC-O Aroma Extract Dilution Analysis ................................................................................................................... 35 1.5.2 Sensory analysis .......................................................................................... 37 1.5.2.1 Quantitative Descriptive Analysis............................................................... 37 1.5.2.2 Thresholds determinations .......................................................................... 38 1.5.2.3 Odour unit ................................................................................................... 40 1.5.3 Statistical analysis ...................................................................................... 40 1.5.3.1 Principal component analysis ..................................................................... 41 1.5.3.2 Analysis of Variance ................................................................................... 41 1.5.3.3 Regression analysis ..................................................................................... 41 1.6 Aims and Objectives 42 CHAPTER 2. MATERIALS AND METHODS 44 2.1 Samples and materials 44 2.1.1 Chemicals and solvents ............................................................................... 44 2.1.1.1 Phenolic compounds ................................................................................... 44 2.1.1.2 Major volatile congeners ............................................................................ 45 2.1.1.3 Trace volatile congeners ............................................................................. 46 IV 2.1.1.4 Maturation derived compounds .................................................................. 47 2.1.1.5 Solvents ....................................................................................................... 47 2.1.2 Whisky samples ........................................................................................... 48 2.1.2.1 Industry supplied whisky samples ............................................................... 48 2.1.2.2 Commercial whisky samples ....................................................................... 49 2.2 Analytical evaluation 49 2.2.1 Phenol analysis by headspace solid phase micro-extraction gas chromatography mass spectrometry ....................................................................... 49 2.2.2 Distillate congeners analysis by gas chromatography ............................... 51 2.2.2.1 Major distillate congeners analysis by gas chromatography ..................... 51 2.2.2.2 Trace distillate congener analysis by gas chromatography ....................... 53 2.2.3 Maturation derived compounds analysis by HPLC .................................... 55 2.3 Sensory analysis 56 2.3.1 Sensory panel .............................................................................................. 56 2.3.2 Aroma profiling by Quantitative Descriptive Analysis ............................... 56 2.3.2.1 Sample preparation ..................................................................................... 57 2.3.2.2 Test procedures ..........................................................................................