DRYING AND PRETREATMENTS AFFECT THE NUTRITIONAL AND SENSORY QUALITY OF OYSTER MUSHROOMS A Dissertation Submitted to the Graduate Faculty of the North Dakota State University of Agriculture and Applied Science By Itai Mutukwa In Partial Fulfillment for the Degree of DOCTOR OF PHILOSOPHY Major Program: Natural Resources Management May 2014 Fargo, North Dakota North Dakota State University Graduate School Title DRYING AND PRETREATMENTS AFFECT THE NUTRITIONAL AND SENSORY QUALITY OF OYSTER MUSHROOMS By Itai Mutukwa The Supervisory Committee certifies that this disquisition complies with North Dakota State University’s regulations and meets the accepted standards for the degree of DOCTOR OF PHILOSOPHY SUPERVISORY COMMITTEE: Dr. Chiwon W. Lee Chair Dr. Clifford Hall Dr. Charlene Wolf-Hall Dr. Larry Cihacek Dr. Garry Goreham Dr. Edward DeKeyser Approved: 07/14/2014 Dr. Edward DeKeyser Date Department Chair ABSTRACT The effect two drying treatments (solar and oven), three blanching treatments (no blanching, water and steam), and four chemical treatments (no chemical, lemon juice, vinegar and potassium bisulfite) on oyster mushroom quality was investigated. Sensory quality, total phenolics, total flavonoids, ergothioneine, oxygen radical absorbance capacity, moisture, mold infestation, mineral content and protein were evaluated. Among the un-blanched samples, those that were treated with lemon juice and those without any chemical pretreatment before drying had better appearance, flavor and were more generally acceptable than those with vinegar and potassium bisulfite treatments. However, when blanching was done, samples treated with potassium bisulfite had superior sensory quality when compared to lemon juice, vinegar and the control. Solar drying caused more browning when compared to oven drying. The combination of water blanching with either lemon juice or vinegar treatments before drying resulted in higher flavonoid content. Lower ergothioneine and total phenolic compounds were observed in blanched mushrooms compared to the un-blanched ones. Total flavonoids were highest in the water blanched samples and least in the un-blanched ones. Among the chemical pretreatments, higher total phenolic compounds were observed in vinegar and potassium bisulfite treated samples. Blanching resulted in lower K, Mg, Na, S and P content when compared to the control. Mineral nutrients varied with chemical pre-treatments. Blanching followed by either lemon juice or no chemical treatment resulted in high mold infestation. Among the un-blanched samples, those treated with vinegar had the least mold infestation. Drying method, blanching, and chemical pretreatments affected oyster mushroom quality hence a need to carefully select preservation methods so as to minimize quality compromise. iii ACKNOWLEDGEMENTS I would like to express heartfelt gratitude to my major advisor Dr. Chiwon Lee and co- advisor Dr. Carolyn Grygiel for all their help and support during the course of my studies. I would also like to thank my committee members Dr. Clifford Hall, Dr. Charlene Wolf- Hall, Dr. Larry Cihacek, Dr. Garry Goreham and Dr. Edward DeKyser for all their help and support. I would like to extend a special thank you to Dr. Clifford Hall. Without your expertise and lab resources this research would not have been a success. I would also like to thank Mary Niehaus for all the help with the sensory study and lab work. I am grateful to Tom Tolman, Matt Ironroad, Jim Flaherty, Brad Hall and Dr. Clifford Hall for helping me with solar drier construction. I would like to say a big thank you to Dr. Jawahar Jyoti and Dr. James Hammond for all the help with data analysis. To the faculty and staff in the Natural Resources Management and Plant Sciences departments, thank you for all your help and support. I would like to thank Super valu, Fargo, ND, for donating all the oyster mushroom used for my research. I am so grateful for the funding and support I received through the Fulbright program. I would like to thank my husband Courage Mudzongo for his love and support. I am also grateful for our daughter Matida who has been such a joy and inspiration. I thank my parents Costen and Margaret Mutukwa for all their support and I am also grateful for the support of all my friends and family. I have had a unique opportunity to cross paths with so many wonderful people at NDSU as well as within the Fargo-Moorhead community during the course of my studies, too many to mention but, all so special in their own way. It has truly been an amazing experience and for that I am grateful. Above all, I thank the almighty God for this opportunity to study and for all the other blessings I have been bestowed with in this life. iv TABLE OF CONTENTS ABSTRACT ................................................................................................................................... iii ACKNOWLEDGEMENTS ........................................................................................................... iv LIST OF TABLES .......................................................................................................................... x LIST OF FIGURES ...................................................................................................................... xii LIST OF APPENDIX TABLES .................................................................................................. xiii LIST OF APPENDIX FIGURES.................................................................................................. xv CHAPTER 1. GENERAL INTRODUCTION ............................................................................... 1 1.1. Overall Objectives ................................................................................................................ 3 1.2. Hypothesis ............................................................................................................................ 3 1.3. References ............................................................................................................................ 3 CHAPTER 2. LITERATURE REVIEW ........................................................................................ 6 2.1. Mushroom Classification ..................................................................................................... 6 2.2. World Overview ................................................................................................................... 6 2.3. Oyster Mushroom Nutrition ................................................................................................. 7 2.3.1. Protein ............................................................................................................................ 7 2.3.2. Mineral composition ...................................................................................................... 8 2.3.3. Carbohydrates, fiber, and fat ......................................................................................... 9 2.3.4. Vitamins....................................................................................................................... 10 2.3.5. Antioxidants................................................................................................................. 11 2.3.5.1. Phenolic compounds ............................................................................................. 11 v 2.3.5.2. Ergothioneine ........................................................................................................ 13 2.4. Mushroom Sensory Quality ............................................................................................... 15 2.5. Mushroom Preservation ..................................................................................................... 17 2.5.1. Dehydration ................................................................................................................. 17 2.5.2. Pretreatments ............................................................................................................... 18 2.5.2.1. Blanching .............................................................................................................. 18 2.5.2.2. Chemical pretreatments ........................................................................................ 18 2.6. References .......................................................................................................................... 20 CHAPTER 3. DRYING AND PRETREATMENTS AFFECT NUTRITIONAL AND ANTIOXIDANT PROPERTIES OF OYSTER MUSHROOM ................................................... 29 3.1. Abstract .............................................................................................................................. 29 3.2. Introduction ........................................................................................................................ 29 3.3. Objectives ........................................................................................................................... 30 3.4. Materials and Methods ....................................................................................................... 31 3.4.1. Experimental design .................................................................................................... 31 3.4.2. Mushroom sample preparation .................................................................................... 31 3.4.2.1. Chemicals and mushroom ..................................................................................... 31 3.4.2.2. Chemical