A PREDICTIVE MODEL FOR THE INACTIVATION OF SALMONELLA IN DRY FOODS By LISA MICHELLE TRIMBLE (Under the Direction of JOSEPH F. FRANK) ABSTRACT Low water activity (aw) foods and ingredients such as milk powder, almond butter, spices, and cake mix are naturally dry or have been dried through processing to aw=<0.85. These products typically have a long shelf life, do not require a lethality step and do not require heating before consumption. Salmonella will not grow in such foods, but can survive for extended periods of time if introduced into the food or the process. Little is known about the influence of fat content on Salmonella survival in low aw foods. The aim of this study was to determine the influence of fat content on the survival of Salmonella in dry foods and use this information to modify an existing predictive model so that it is valid for low and high fat foods. Survival data was obtained in whey protein powder supplemented with peanut oil which was equilibrated to various water activities below 0.60 and held at 21-80ºC for 0-168 days. The Weibull model was selected to describe the data and a previously developed secondary model was modified to include foods with a fat content ≤50% (w/w) based on the influence of temperature, aw and fat content on survival. Predictions were validated in 4 foods within the range of the modeled data. The model was useful in predicting survival in low and high fat dry foods with a prediction performance of 66% and improved % bias and % accuracy compared to the unmodified model. Fat content (20% and 50% w/w) provided a protective effect toward survival at temperatures ≥50ºC and additional fat content did not increase this effect. The presence of an emulsifier did not influence survival. Fat and sugar content did not influence the relative survival of 3 out of 4 serovars during storage. S.Tennessee was the most prevalent serovar and S. Typhimurium was not detected after 6 months of storage at 37ºC. The revised model can be used as a quantitative support tool in risk mitigation strategies for low aw foods containing fat. INDEX WORDS: inactivation; fat; dry food; serotype; Weibull model; peanut flour; Salmonella; predictive model; monoglyceride; water activity A PREDICTIVE MODEL FOR THE INACTIVATION OF SALMONELLA IN DRY FOODS By LISA MICHELLE TRIMBLE B.S., Biology, University of West Georgia, 2003 M.S., Food Science, University of Georgia, 2012 A Dissertation Submitted to the Graduate Faculty of The University of Georgia in Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY ATHENS, GEORGIA 2015 © 2015 LISA MICHELLE TRIMBLE All Rights Reserved A PREDICTIVE MODEL FOR THE INACTIVATION OF SALMONELLA IN DRY FOODS by LISA MICHELLE TRIMBLE Major Professor: JOSEPH F. FRANK Committee: DONALD W. SCHAFFNER MARY ALICE SMITH XIANG-YU DENG Electronic Version Approved: Suzanne Barbour Dean of the Graduate School The University of Georgia December 2015 DEDICATION To my parents and my grandparents iv ACKNOWLEDGEMENTS I would like to thank my advisor, Dr. Joseph F. Frank for his mentorship, guidance and financial support during my Ph.D. program. It has been such an honor to be your graduate student. I also extend my sincere gratitude to Dr. Donald Schaffner, Dr. Mary Alice Smith, Dr. Xiang-yu Deng and Dr. Walid Alali for challenging me and providing me with thoughtful feedback as my advisory committee members. Without a doubt, I have grown as a researcher and as a person during my doctoral studies because of your influence. I gratefully acknowledge Stephanie Barnes for her support and friendship during our time as lab mates. I would also like to thank our undergraduate student workers; Sheena Patel, Jennine Jarrett, Anthony Chiu, Meredith Meyer, David Rospond, Kim Cramer, Morgan Crandall, Austin Bernard, and Walker Whitley. I will always think of this group as my graduate school “lab family”. I also thank Dr. Antonio Lourenco, Dr. Angela Rincon Villegas, Dr. Chi Ching Lee, Sue Jee Park and Lauren Hudson for encouraging me and believing in me. A special thanks to Jessica Hofstetter for her assistance in the lab. Thank you to Carl Ruiz his friendship, wisdom and sense of humor. Finally, I want to thank my family and friends for their patience, strength and support during my time in graduate school. v TABLE OF CONTENTS Page ACKNOWLEDGEMENTS......................................................................................................... v LIST OF TABLES .................................................................................................................. viii LIST OF FIGURES ................................................................................................................... ix CHAPTER 1 INTRODUCTION .................................................................................................... 1 References ........................................................................................................... 4 2 LITERATURE REVIEW .......................................................................................... 5 References ......................................................................................................... 21 3 A REVISED MODEL FOR THE INFLUENCE OF TEMPERATURE, FAT CONTENT AND WATER ACTIVITY ON THE PERSISTENCE OF SALMONELLA IN LOW-WATER ACTIVITY FOODS .................................................................. 27 Abstract ............................................................................................................. 28 Introduction ....................................................................................................... 29 Materials and Methods ....................................................................................... 30 Results and Discussion ....................................................................................... 38 Conclusions ........................................................................................................ 47 References .......................................................................................................... 49 vi 4 VALIDATION OF SECONDARY MODEL PREDICTIONS OF SALMONELLA SURVIVAL IN A LOW-WATER ACTIVITY, HIGH FAT PEANUT FLOUR MODEL FOOD SYSTEM CONTAINING AN EMULSIFIER HELD AT 37ºC OR 70ºC ........................................................................................................................ 69 Abstract ............................................................................................................. 70 Introduction ....................................................................................................... 71 Materials and Methods ....................................................................................... 72 Results and Discussion ....................................................................................... 75 References ......................................................................................................... 78 5 THE INFLUENCE OF SUGAR AND FAT CONTENT ON THE RELATIVE PERSISTENCE OF FOUR SALMONELLA ENTERICA SEROVARS IN A LOW- WATER ACTIVITY MODEL FOOD SYSTEM HELD AT 36ºC ........................... 84 Abstract ............................................................................................................. 85 Introduction ....................................................................................................... 86 Materials and Methods ....................................................................................... 87 Results and Discussion ....................................................................................... 89 References ......................................................................................................... 91 6 CONCLUSIONS ..................................................................................................... 94 vii LIST OF TABLES Page Table 3.1: Statistical parameter fit data used to determine the best equation to describe Salmonella inactivation data in whey protein isolate powders with varying fat contents (20-50% w/w) and 5 water activity ranges (aw<0.60) at 6 T (°C). .................................. 63 Table 3.2: Fitted Weibull model parameters (δ, β) for Salmonella inactivation in whey protein isolate powders containing added fat (20% and 50% w/w)) equilibrated to 5 aw ranges <0.60 and held at 21-80ºC ........................................................................................ 66 Table 3.3: Model performance indices for a revised model predicting Salmonella inactivation in low aw (aw<0.60) foods with fat levels (0-50% w/w) held at 21-80ºC. ......................... 68 Table 4.1: Statistical parameter fit data and fitted Weibull model parameters for peanut flours held at 37ºC and 70ºC ................................................................................................... 81 Table 4.2: Secondary model performance indices for predictions in peanut flour held at for 28 days at 37ºC or 48 hours at 70ºC .................................................................................... 82 Table 5.1: Prevalence of Salmonella serovars in fat or sugar supplemented WPI held at 36ºC for 168 days ........................................................................................................................ 93 viii LIST OF FIGURES Page Figure 3.1: Salmonella inactivation at 22°C during 6 months (168 days) of storage in whey protein isolate powders with various fat contents (non-fat, 20%, 50% (w/w)) equilibrated to