ENHANCED REMOVAL OF SALMONELLA TYPHIMURIUM AND E. COLI O157:H7 FROM BLUEBERRIES AND STRAWBERRIES BY SOLUTIONS CONTAINING SODIUM DODECYL SULFATE AND ORGANIC ACIDS OR HYDROGEN PEROXIDE by Yingying Li A thesis submitted to the Faculty of the University of Delaware in partial fulfillment of the requirements for the degree of Master of Science in Food Science Fall 2013 © 2013 Yingying Li All Rights Reserved ENHANCED REMOVAL OF SALMONELLA TYPHIMURIUM AND E. COLI O157:H7 FROM BLUEBERRIES AND STRAWBERRIES BY SOLUTIONS CONTAINING SODIUM DODECYL SULFATE AND ORGANIC ACIDS OR HYDROGEN PEROXIDE by Yingying Li Approved: __________________________________________________________ Changqing Wu, Ph.D. Professor in charge of thesis on behalf of the Advisory Committee Approved: __________________________________________________________ Jack Gelb, Ph.D. Chair of the Department of Animal and Food Sciences Approved: __________________________________________________________ Mark W. Rieger, Ph.D. Dean of the College of Agriculture and Natural Resources Approved: __________________________________________________________ James G. Richards, Ph.D. Vice Provost for Graduate and Professional Education ACKNOWLEDGMENTS My first and sincere appreciation goes to my advisor Dr. Changqing Wu for her continuous help and support in all stages of this thesis. I would like to thank my committee members Dr. Haiqiang Chen and Dr. Rolf Joerger for their interest in my work. In addition, I would like to thank my labmate, Wenqing Xu, who was always a great support in all my struggles and frustrations in my work and life in this country. I am also thankful to Melissa Ehrich, Kyle LeStrange, and Patrick Spanninger; they are wonderful labmates and friends. Finally, I would like to thank my parents for always believing in me, for their continuous love and their supports in my decisions. Without whom I could not have made it here. iii TABLE OF CONTENTS LIST OF TABLES ...................................................................................................... viii LIST OF FIGURES ....................................................................................................... ix ABSTRACT .................................................................................................................. xi Chapter 1 INTRODUCTION .............................................................................................. 1 REFERENCES ............................................................................................................... 3 2 LITERATURE REVIEW ................................................................................... 5 2.1 U.S. Disease Outbreaks Associated with Blueberries and Strawberries ... 5 2.2 Microorganisms of Concern ...................................................................... 6 2.2.1 Salmonella ..................................................................................... 6 2.2.2 Escherichia coli ............................................................................. 7 2.3 Washing Treatment ................................................................................... 8 2.3.1 Chlorine ......................................................................................... 9 2.3.2 Organic acids ................................................................................. 9 2.3.3 Hydrogen peroxide ...................................................................... 11 2.3.4 Trisodium phosphate ................................................................... 11 2.3.5 Sodium dodecyl sulfate ............................................................... 12 2.4 Frozen Fruit Safety .................................................................................. 13 REFERENCES ............................................................................................................. 15 3 ENHANCED INACTIVATION OF SALMONELLA TYPHIMURIUM FROM BLUEBERRIES BY SOLUSIONS CONTAINING SODIUM DODECYL SULFATE AND ORGANIC ACIDS OR HYDROGEN PEROXIDE ...................................................................................................... 21 iv Abstract ............................................................................................................. 21 3.1 Introduction ............................................................................................. 22 3.2 Materials and Methods ............................................................................ 24 3.2.1 Bacterial strains and inoculum preparation ................................. 24 3.2.2 Contamination of blueberry surfaces and washing procedures ... 24 3.2.3 Enumeration of Salmonella ......................................................... 25 3.2.4 Effect of washing treatments on the physical properties of blueberries ................................................................................... 26 3.2.5 Total phenolic and anthocyanins content .................................... 26 3.2.6 Impact of the two most effective treatments coupled with low temperature storage on Salmonella, yeasts and molds counts ..... 27 3.2.7 Statistical analysis ....................................................................... 28 3.3 Results and Discussion ............................................................................ 28 3.3.1 Effect of the combination of chlorine and SDS solutions on inactivation of Salmonella ........................................................... 28 3.3.2 Effect of combinations of organic acids and SDS solutions on inactivation of Salmonella ........................................................... 31 3.3.3 Effect of combinations of hydrogen peroxide and SDS solutions on inactivation of Salmonella ...................................... 33 3.3.4 Effect of combinations of trisodium phosphate and SDS solutions on inactivation of Salmonella ...................................... 35 3.3.5 Effect of washing treatments on the physical properties of blueberries ................................................................................... 36 3.3.6 Impact of treatments coupled with low temperature storage on Salmonella, yeasts and molds counts .......................................... 38 3.3.7 Comparison of Salmonella inactivation after washing for 1 and 5 min ............................................................................................ 41 REFERENCES ............................................................................................................. 43 4 SURVIVAL OF SALMONELLA TYPHIMURIUM AND E. COLI O157:H7 ON BLUEBERRIES DURING FROZEN STROAGE AFTER WASHING WITH COMBINATIONS OF SODIUM DODECYL SULFATE AND ORGANIC ACIDS OR HYDROGEN PEROXIDE ........................................ 46 Abstract ............................................................................................................. 46 4.1 Introduction ............................................................................................. 47 4.2 Materials and Methods ............................................................................ 48 v 4.2.1 Bacterial strains and inoculum preparation ................................. 48 4.2.2 Inoculation of blueberry surfaces ................................................ 49 4.2.3 Washing procedures and frozen storage ...................................... 49 4.2.4 Microbial analysis ....................................................................... 50 4.2.5 Sensory evaluation after 12-weeks of frozen storage .................. 51 4.2.6 Physical properties and total bacteria, yeasts and molds counts of blueberries during frozen storage ............................................ 51 4.2.7 Total phenolic and anthocyanins content .................................... 52 4.2.8 Statistical analysis ....................................................................... 52 4.3 Results and Discussion ............................................................................ 52 4.3.1 Effect of the combination of peroxyacetic acid and SDS on inactivation of Salmonella ........................................................... 52 4.3.2 Combination of SDS and organic acids or hydrogen peroxide on inactivation of Salmonella and E. coli during frozen storage 54 4.3.3 Sensory analysis .......................................................................... 57 4.3.4 Physical properties and total bacteria and yeasts and molds counts of blueberries during frozen storage ................................ 58 REFERENCES ............................................................................................................. 63 5 COMPARISON OF PROMISING TREATMENTS EFFICACY ON SPOT OR DIP INOCULATED BLUEBERRIES AND STRAWBERRIES ............. 66 Abstract ............................................................................................................. 66 5.1 Introduction ............................................................................................. 67 5.2 Materials and Methods ............................................................................ 69 5.2.1 Bacterial strains and inoculum preparation ................................. 69 5.2.2 Contamination of blueberries and strawberries ........................... 69 5.2.3 Washing procedures .................................................................... 70 5.2.4 Microbial analysis ....................................................................... 70 5.2.5 Statistical analysis ....................................................................... 71