Efficacy of Ozone and Ultrasound for Microbial Reduction in Fruit Juice
Document Type Theses, Ph.D
Successfully submitted for the award of Doctor of Philosophy (Ph.D.) to the Dublin Institute of Technology, 2010.
Concerns have arisen regarding the microbiological safety of fruit juices due to a number of outbreaks associated with pathogens. Non-thermal technologies for the inactivation of microorganisms are of increasing interest to the food industry for the control of spoilage as well as safety concerns. The objective of this thesis was to investigate the efficacy of ultrasound and ozone treatments for control of microbial issues associated with fruit juices. Inactivation of
Escherichia coli (ATCC 25922, NCTC 12900) using power ultrasound was found to be influenced by strain, prior acid adaptation and suspension liquid, but the effect was negated at the higher amplitude levels. Power ultrasound has potential for reducing the microbial load in liquid food systems. Ozone was another non thermal technology applied to reduce microbial issues
associated with fruit juices. The optimum ozone system control parameters of flow rate, temperature and ozone concentration resulted in a 5 log reduction (t5d) in 20 min. These optimum parameters were further used to determine ozone inactivation efficacy in orange juice. The efficacy of ozone for inactivation of two strains of E. coli was evaluated as a function of different juice types. Fast ozone inactivation rates of E. coli (106 CFU/mL) in model orange juice (60 sec) and in orange juice with low pulp content (6 min) indicated that juice organic matter interferes with gaseous ozone efficacy. The
effect of prior acid (pH 5.0) exposure of E. coli strains resulted in higher inactivation times in some cases by comparison with the control cells. Ozone treatment (33-40μg/mL) of E. coli in apple juice achieved t5d within 5 min. A significant pH effect on ozone inactivation of E. coli strains in apple juice was also observed. Prior mild acid stress-habituation of Listeria strains resulted in higher ozone inactivation times in orange juice. The t5d was achieved within a 5 to 9 min range. A product specific model was developed and validated under dynamic temperature conditions for determining the growth of Saccharomyces cerevisiae in ozonated apple juice. The microbial model developed resulted in accurate predictions when compared with the independent experimental set of dynamic temperatures. In the case of ozone treated apple juice, the shelf life was increased when compared with the controls at higher static storage temperatures (8, 12 and 16ºC). Combining ultrasound and ozone treatment slightly increased the inactivation rate of E. coli ATCC 25922 in orange juice compared to ozonation alone. The effect of gene deletion (soxR, soxS, oxyR, rpoS, dnaK) towards further understanding of the ozone inactivation mechanism, indicated that mutant E. coli strains (ΔsoxR, ΔsoxS, ΔoxyR, ΔrpoS) were more susceptible to ozone treatment (6 μg/mL), signifying the important role of oxidative stress related genes in protection during
ozonation. Cell lysis was not the major mechanism of inactivation observed in this study. This study demonstrates that the use of ozone as a non-thermal technology is effective for inactivation of E. coli, Listeria strains and S. cerevisiae in fruit juice and could be used as an alternative to traditional thermal pasteurisation. However, the effect of ozone on sensory and nutritional quality retention of liquid foods such as fruit juice should be
considered before its use as a preservation technique.