Semester

Spring

Date of Graduation

2013

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Animal and Nutritional Sciences

Committee Chair

Jacek Jaczynski

Committee Co-Chair

Gary K Bissonnette

Committee Member

Kristen E Matak

Committee Member

Janet C. Tou

Committee Member

Jianbo Yao.

Abstract

A non-thermal food processing method, Electron beam radiation efficiently inactivates foodborne pathogens. However, foodborne pathogens may develop resistance in response to sublethal stresses. Thus it is important to study the response of food microorganisms to e-beam and understand the mechanism underlying their survival abilities. The overall objective of this study was to examine the inactivation of foodborne microorganisms by electron beam radiation at sublethal levels, understand the radio-resistance development to this processing method and also to determine the effects of e-beam on chemical changes of nutrients in infant formula.;Four independent studies are included in this dissertation, in the first study, development of radio-resistant Salmonella Typhimurium in egg as a microbial response to e-beam at sublethal levels was investigated and the D10-value for S. Typhimurium was determined after repetitive processing with e-beam at sub-lethal doses. Survivors were enumerated on non-selective (TSA) and selective (XLD) media. Survivors from the highest dose were isolated and used in subsequent e-beam cycle. This process was repeated four times for a total of five e-beam cycles. D10-values for S. Typhimurium ATCC strain 14028 were 0.59+/-0.031 and 0.46+/-0.022 kGy on TSA and XLD, respectively. However, following the fifth e-beam cycle, the respective D10-values increased (P0.05) to develop radio-resistance faster on selective media, likely due to facilitated selection of radio-resistant cells within microbial population following each e-beam cycle. For all five e-beam cycles, S. Typhimurium had higher (P<0.05) D10-values on non-selective media, indicating that sub-lethal injury followed by cellular repair and recovery are important for radio-resistance and inactivation of this microorganism.;To further investigate the radio resistance development of microorganisms to repetitive e-beam sublethal doses, in the second study DNA repair deficient E.coli DH5alpha that have mutations of recA and gyrA genes was used. The objective was to determine if repetitive processing with e-beam at sub-lethal doses increases D10-value of E. coli DH5alpha in ground beef. Five cycles of e-beam were conducted. D10-values increased (P<0.05) significantly with each cycle. Following the third cycle D10-values were 0.32+/-0.006 and 0.32+/-0.002 kGy for survivors enumerated on non-selective and selective media, respectively; the fourth cycle 0.39+/-0.007 and 0.40+/-0.019 kGy; and the fifth cycle 0.46+/-0.006 and 0.46+/-0.020 kGy. The result of the study indicates E. coli DH5alpha increases radio-resistance to e-beam as a result of repetitive exposure to sub-lethal doses despite its DNA repair deficiency.;The third study investigated the role of recA and gyrA genes in E.coli DH5alpha that had become resistant to repetitive e-beam radiation in the previous study and its ability to repair the damage caused by e-beam. Genomic DNA from E.coli DH5alpha, radio-resistant A2 and A4 were extracted, and DNA fragments of the gyrA and recA genes containing the mutations were amplified, cloned and sequenced. E.coli DH5alpha and radio-resistant A2 and A4 were compared to a reference genome for identification of nucleotide polymorphisms, insertions, and deletions that may have contributed to its radio-resistance. Results of the study demonstrated that E.coli DH5alpha was able to reverse its mutation after sub lethal e-beam radiations. The study revealed that DNA repair deficient E. coli DH5alpha increased radio-resistance to e-beam as a result of repetitive exposure to sub-lethal doses despite its DNA repair deficiency due to its ability to reverse its mutation. Understanding the resistance development of microorganisms is important in designing a food process control and efficacy of the processing method.;The last study examined the effect of e-beam on chemical changes of nutrients in infant formula. Dehydrated infant milk formula was processed with e-beam at 0 (control) to 25 kGy. Amino acid, fatty acid, and mineral profiles (AAP, FAP, and MP, respectively), as well as protein degradation and lipid oxidation, were determined. Our results demonstrate proteins, lipids, and minerals in infant milk formula were stable when processed with e-beam up to 25 kGy.

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