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Physicochemical Changes of Coffee Beans During Roasting

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Physicochemical Changes of Coffee Beans During Roasting Physicochemical Changes of Coffee Beans During Roasting by Niya Wang A Thesis Presented to The University of Guelph In partial fulfilment of requirements for the degree of Master of Science in Food Science Guelph, Ontario, Canada © Niya Wang, April, 2012 ABSTRACT PHYSICOCHEMICAL CHANGES OF COFFEE BEANS DURING ROASTING Niya Wang Advisor: University of Guelph, 2012 Professor Loong-Tak Lim In this research, physicochemical changes that took place during roast processing of coffee beans using fluidized air roaster were studied. The results showed that high-temperature-short-time resulted in higher moisture content, higher pH value, higher titratable acidity, higher porous structure in the bean cell tissues, and also produced more aldehydes, ketones, aliphatic acids, aromatic acids, and caffeine than those processed at low-temperature-long-time process. Fourier transform infrared (FTIR) spectroscopy and chemometric analysis showed that clusters for principal components score plots of ground coffee, extracted by a mixture of equal volume of ethyl acetate and water, were well separated. The research indicated that variations in IR-active components in the coffee extracts due to different stages of roast, roasting profiles, and geographical origins can be evaluated by the FTIR technique. ACKNOWLEDGMENTS I am most grateful to Prof. Dr. Loong-Tak Lim for giving me the opportunity to work in his group. I have always appreciated his far-sighted guidance, continued support, and constructive evaluation throughout my research and in many aspects of my life. Further, I am much indebted to my advisory committee members Dr. Lisa Duizer, and Dr. Massimo Marcone for their unlimited confidence on my research work and helps during the writing of the thesis. Special thanks to Natural Sciences and Engineering Research Council of Canada (NESRC) and Mother Parkers Tea & Coffee Inc., for their essential financial support, without which this research will not be possible. Many thanks to my Packaging and Biomaterials Group sisters and brothers: Ana Cristina Vega Lugo, Solmaz Alborzi, Suramya Minhindukulasuriya, Roc Chan, Grace Wong, Alex Jensen, Khalid Moomand, Qian Xiao, Xiuju Wang, and Ruyan Dai for their assistance, friendship, patience, and bringing colourful life for these years. Many thanks are also going to Dr. Yukio Kakuda, Dr. Sandy Smith, and Bruce Manion for their technical assistance along the way. I would like to take this opportunity to express my deepest gratefulness to my parents, my husband Dr. Yucheng Fu, my son Stanley Fu, and other family members for their infinite love, support and encouragement throughout these years of my studies at Guelph. iii TABLE OF CONTENTS ACKNOWLEDGMENTS.……………………………………………………….….......iii TABLE OF CONTENTS.…………………………………………………………...…..iv LIST OF FIGURES.………………………………………………………………...…..vi LIST OF TABLES.…..………………………..…………………………………...…..viii LIST OF ABBREVIATIONS.………………………………………………….…...…..ix 1 INTRODUCTION ............................................................................................... 1 2 LITERATURE REVIEW ..................................................................................... 4 2.1 THE GREEN COFFEE BEANS ............................................................................. 4 2.2 ROASTING OF COFFEE BEANS .......................................................................... 8 2.3 AROMA COMPOUNDS IN ROASTED COFFEE ...................................................... 14 2.4 FOURIER TRANSFORM INFRARED (FTIR) SPECTROSCOPY ................................ 19 2.5 CHEMOMETRICS ........................................................................................... 21 3 JUSTIFICATION AND OBJECTIVES .............................................................. 26 4 FEASIBILITY STUDY ON CHEMOMETRIC DISCRIMINATION OF ROASTED ARABICA COFFEES BY SOLVENT EXTRACTION AND FOURIER TRANSFORM INFRARED SPECTROSCOPY ................................................... 27 4.1 INTRODUCTION ............................................................................................. 27 4.2 MATERIALS AND METHODS ............................................................................ 29 4.2.1 Chemicals ............................................................................................ 29 4.2.2 Coffee Beans and Roasting Conditions ............................................... 29 4.2.3 Degree of Roast as Determined by Color Measurements ................... 30 4.2.4 Solvent Extraction of Ground Coffee ................................................... 30 4.2.5 ATR-FTIR Analysis .............................................................................. 31 4.2.6 Data Analysis ...................................................................................... 32 4.3 RESULTS AND DISCUSSIONS .......................................................................... 32 4.3.1 Optimization of Solvent Extraction for FTIR-ATR ................................ 33 4.3.2 Color Analysis ..................................................................................... 38 iv 4.3.3 PCA Analysis of Solvent Extracts of Coffee Beans ............................. 40 4.3.4 PCA Analysis for Coffees According to Degree of Roast .................... 47 4.3.5 SIMCA Analysis ................................................................................... 52 5 EFFECTS OF DIFFERENT TIME-TEMPERATURE PROFILES ON COFFEE PHYSICAL AND CHEMICAL PROPERTIES ...................................................... 54 5.1 INTRODUCTION ............................................................................................. 54 5.2 MATERIALS AND METHODS ............................................................................ 57 5.2.1 Chemicals and materials ..................................................................... 57 5.2.2 Green Beans and Roasting Conditions ............................................... 57 5.2.3 Degree of Roast as Determined by Color Measurements ................... 58 5.2.4 Moisture Content of Ground Coffee ..................................................... 58 5.2.5 pH Value .............................................................................................. 59 5.2.6 Titratable Acidity .................................................................................. 59 5.2.7 Solvent Extraction and ATR-FTIR Analysis of Ground Coffee ............. 59 5.2.8 Chemometric Analysis ......................................................................... 60 5.2.9 Scanning Electron Microscopy (SEM) Analysis ................................... 60 5.3 RESULTS AND DISCUSSION ........................................................................... 60 5.3.1 Evolution of physical and chemical properties during roasting ............ 60 5.3.2 Changes in coffee at various stages of roast ....................................... 66 5.3.3 Effects of roast temperature on changes in coffee .............................. 72 5.3.4 Microstructural analysis ....................................................................... 74 6 CONCLUSIONS AND FUTURE WORKS ........................................................ 78 7 REFERENCE ................................................................................................... 82 v LIST OF FIGURES Figure 1 Chemical composition of green, roasted, and brewed coffee (Barter 2004) ..................................................................................................................... 9 Figure 2 Schematic diagram of a typical FTIR spectrometer ............................. 20 Figure 3 Vibrational absorbance due to common bands .................................... 20 Figure 4 Schematic diagram of PCA analysis .................................................... 24 Figure 5 Air temperature (in roast chamber) profiles of the fluidized bed hot air coffee roaster ...................................................................................................... 30 Figure 6 Appearance of coffee extracts by dichloromethane, hexane, ethyl acetate, acetone, ethanol, and acetic acid (the right vial represent the extracts by Method #1) .......................................................................................................... 35 Figure 7 FTIR spectra of coffee extracts obtained with hexane, dichloromethane, ethyl acetate, acetone, ethanol, or acetic acid using method #1 (with water) and method #2 (no water) .......................................................................................... 38 Figure 8 Selected FTIR spectra of dark roast coffee extract obtained with dichloromethane as a solvent (using method 1#)................................................ 41 Figure 9 PCA of FTIR data for hexane, dichloromethane, ethyl acetate, and acetone extracts of medium roast coffee. Row A: Two-factor score plots. Row B: Loading plots of PC1. Row C: Corresponding FTIR raw spectra ........................ 42 Figure 10 PCA of FTIR data for hexane, dichloromethane, ethyl acetate, and acetone extracts of dark roast coffee. Row A: Two-factor score plots. Row B: Loading plots of PC1. Row C: Corresponding FTIR raw spectra ........................ 43 Figure 11 PCA of FTIR data for dichloromethane extracts of coffee (from the vi same origin) with two degrees of roast. Row A: Two factor score plots. Row B: Loading plots of PC1. Row C: Corresponding FTIR raw spectra ........................ 50 Figure 12 PCA of FTIR data for ethyl acetate extracts of coffee (from the same origin) with two degrees of roast. Row A: Two
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