SF6018 - Practical Optimization of Ozonation Process for Enhancing Microbial Safety and Food Quality

Researcher:

Hongde Zhou, School of Engineering, University of Guelph

Objectives:

1. To expand the knowledge of ozone application for food processing industry by using fresh strawberry and green pepper grown with E. coli 0157:H7, Listeria spp. and Salmonella spp., respectively.
   
2. To quantify the inactivation kinetics for different bacterial contaminants under more realistic conditions by varying bacterial growth conditions.
   
3. To conduct semi-batch tests to that the results from the laboratory batch testing can be compared and verified further.
   
4. To evaluate different dissolution technologies that can be used for food processing.
   
5. To modify and verify the applicability of the integrated ozonation process model developed by the PI for food processing.
   
6. To provide the regulatory and design guidelines that can achieve effective controlling microbial risks while maintaining the economic competitiveness.

Expected Benefits:

• To provide a better understanding of the important fundamentals and an effective ozone contacting system for ozone to dissolve into water and attack target microorganisms. A computer program based on the integrated ozonation model will also be devised for the facility design and risk assessment.

Summary of Research Results:

With the increasing concern about the potential risks from microbial contaminants and the formation of toxic chlorination by-products, ozone is considered as a promising alternative sanitizer for food processing industry because of its powerful germicidal potential, high reactivity and spontaneous decomposition to non-toxic products.

Extensive experiments have been conducted using a batch ozonation reactor specifically designed for this project under different ozone concentration, contact time, pH and temperature. Both fresh strawberry and shredded lettuce were chosen as test samples to represent minimally-processed fruits and vegetable due to their importance to Ontario producers and consumers and susceptibility to microbial contamination. The microbial effectiveness was examined by measuring the inactivation of E. coli, Listeria and Salmonella spp. innocuated on the produce surfaces, respectively. The food quality were analyzed by determining the texture firmness, browning/decolorization, oxygen and carbon dioxide generation during packaging and the release of titratable acidity and total soluble solids content into water. To evaluate the effects of ozonation on potential impacts on produce shelf life, natural flora including mesophiles, psychotrophs, yeasts and molds were also been assayed. For the comparison purpose, additional disinfection tests were conducted using chlorine because of its extensive use in practice. The collected data were then used to fit to various disinfection kinetics. A new ozonation model was formulated by applying the principles of computational fluid dynamics to predict the hydrodynamic characteristics of ozone contactors. It was then coupled with ozone mass transfer, ozone decay and disinfection kinetics to predict the dissolved ozone concentration and disinfection efficiency.

The main conclusions include:

1. Ozone can effectively inactivate all the test microorganisms present on the surface of fresh vegetables and fruits. It is at least as effective at a dose less than 10 mg/L as chlorine at a dose of 200 mg/L to kill all test microorganisms present on the produce samples. Besides, ozone treatment will have little negative effects on food quality.
   
2. Ozone is very effective in killing the microorganisms present in water, thereby, providing a great potential to reuse the food processing water in practice.
   
3. Naturally grown microorganisms are much more difficult to be inactivated by ozone as compared to those laboratory inoculated on the surface of fresh fruits and vegetable samples. Within the ranges of ozone doses (1.5 to 5.0 mg/L) and contact times (2 to 5 minutes) used in this study, mesophiles, psychotrophs, yeasts and moulds were reduced by 0.11 to 1.08, 0.35 to 1.55 and 0.29 to 1.32-log units, respectively.
   
4. The inactivation efficiencies increased as the ozone dose increased. Furthermore, it was observed that the rate of ozone inactivation can be approximated by simple Chick-Watson kinetic model. However, the inactivation would be leveled off after the contact time became more than 5-minutes, perhaps due to the presence of subgroup of more resistant microorganisms.
   
5. The materials leached from vegetables and fruits during ozonation would significantly accelerate ozone decay in water via the occurrence of competing reactions, thereby, affecting ozone disinfection efficiency.
6. In practice, ozone disinfection will significantly be affected by contactor backmixing, highlighting the importance of ozone contactor design. As such, an integral system model is recommended that can describe the combined effects of contactor hydrodynamics, ozone mass transfer and ozone decay and disinfection kinetics for rational process evaluation and design. Because of the distinct benefits of ozone as sanitizer in food industry as compared to chlorine, a pilot-scale testing project is strongly recommended to further verify the research results obtained from this study, explore the potential of water reuse and provide additional operational experience.