Abstract
Milk safety is traditionally ensured through thermal pasteurization, which effectively reduces microbial contamination and extends product shelf life. However, conventional heat treatment may alter heat-sensitive sensory, nutritional, and functional properties of milk. As interest in minimally processed foods continues to increase, nonthermal technologies have been explored as alternative or complementary approaches for improving microbial safety while preserving product quality. Among these technologies, nonthermal plasma has shown strong potential for microbial inactivation through reactive oxygen and nitrogen species, electric field effects, and other antimicrobial factors. However, many plasma-based food processing studies remain limited to batch systems, surface treatments, or low-throughput configurations. The application of continuous-flow liquid-phase plasma discharge (CLPD) to milk processing, particularly for microbial inactivation, quality preservation, process optimization, and reactor-based modulation of treatment performance, has not been fully established. The overall objective of this dissertation was to develop and evaluate a CLPD system as a nonthermal processing platform for milk treatment. The research focused on three connected objectives: optimizing microbial inactivation, evaluating refrigerated shelf life and physicochemical quality, and modulating treatment intensity and processing throughput using modular reactor configurations. A laboratory-scale continuous-flow plasma reactor was operated under different flow rates, applied power levels, working gas conditions, and reactor configurations. Flow rate and applied power were evaluated over ranges of 30–70 mL/min and 180–320 W, respectively. Microbial reduction, specific energy input, temperature rise, pH, electrical conductivity, titratable acidity, and color change were measured to evaluate treatment performance.The results demonstrated that CLPD achieved substantial microbial inactivation in milk under continuous-flow operation. Higher applied power and lower flow rate generally increased microbial reduction, and the optimized condition was selected as 320 W and 40 mL/min, corresponding to a specific energy input of approximately 480 J/mL. Under optimized conditions, CLPD achieved approximately 6-log or greater reductions of representative milk-associated foodborne pathogens, depending on organism and recovery medium. Refrigerated shelf-life evaluation showed that CLPD delayed microbial growth during 28 days of storage at 4°C while maintaining relatively stable pH, titratable acidity, electrical conductivity, and color. Working gas composition influenced treatment performance: CLPD-Air provided stronger spore reduction, achieving approximately 5.78-log reduction of recoverable heat-resistant Bacillus cereus populations, whereas CLPD-N₂ achieved approximately 5.54-log reduction and showed better quality retention. In comparison, HTST achieved approximately 3.86-log reduction under the tested spore-challenge condition.
Reactor modulation experiments showed that reactor configuration affected CLPD performance beyond applied power and flow rate alone. Single-reactor microbial reduction decreased as branch flow rate increased from 40 to 120 mL/min, demonstrating the throughput limitation of single-reactor operation. Two-reactor series operation enhanced treatment intensity by increasing cumulative plasma exposure for each unit volume of milk, whereas two-reactor parallel operation-maintained branch-level microbial reduction while approximately doubling total system throughput. These results indicate that modular reactor configuration can be used to separately control treatment intensity and processing capacity.
Overall, this dissertation demonstrates that CLPD is a promising nonthermal processing approach for improving milk microbial safety and quality preservation. By integrating reactor development, process optimization, refrigerated shelf-life evaluation, working gas comparison, and modular reactor modulation analysis, this work establishes a process-oriented framework for advancing plasma-based liquid food processing toward practical continuous operation.