Abstract
Sour cherry juice, prized for its high antioxidant, phenolic, and natural pigment content, is a popular beverage that is highly susceptible to microbial spoilage. To assess microbial inactivation, a continuous-flow system was constructed comprising a sour cherry juice reservoir, a high-precision pump, an argon gas tank for plasma formation, and a custom-designed glass Venturi tube reactor. Plasma was generated in the liquid phase using injected argon gas under voltages ranging from 10 to 20 kV.Sour cherry juice was autoclaved at 121°C for 15 minutes to eliminate background microorganisms and then inoculated with E. coli cultured for 24 hours. Treatment parameters included voltage, flow rate (2–8 L/min), temperature (30–50°C), and time (1–5 min). Under optimized conditions voltage of 19.85 kV, flow rate of 3.52 L/min, temperature of 47.82°C, and treatment time of 4.42 min the hybrid system achieved a maximum E. coli reduction of 6.18 log cycles (99%). In addition to microbial inactivation, the hybrid system showed superior retention of phenolic compounds, anthocyanins, vitamin C, and color integrity, which are key quality markers in fruit juice processing. Energy demand remains a limiting factor, necessitating integration with energy-saving systems and optimization of plasma operating conditions.
Introduction
Sour cherry juice, prized for its high antioxidant, phenolic, and natural pigment content, is a popular beverage that is highly susceptible to microbial spoilage. This vulnerability presents significant safety and shelf-life challenges for producers. Conventional thermal pasteurization, while effective in microbial inactivation, often degrades the nutritional and sensory quality of the product. As consumer demand grows for minimally processed beverages with preserved nutritional and functional characteristics, non-thermal processing methods have become increasingly important. Among them, liquid-phase plasma and hydrodynamic cavitation stand out due to their high microbial inactivation potential without substantial heat generation. However, few studies have examined their synergistic effects in fruit juice processing.
This study addresses this gap by designing and optimizing a novel hybrid system that combines a Venturi tube-based hydrodynamic cavitation reactor with liquid-phase plasma technology. The aim is to inactivate Escherichia coli (E. coli) in sour cherry juice while minimizing the degradation of heat-sensitive compounds. The core innovation lies in the concurrent use of physical (cavitation) and chemical (reactive species) stress mechanisms. By leveraging the strengths of both methods, the system aims to deliver high microbial inactivation with minimal impact on physicochemical quality. This study also investigates the relationships among voltage, flow rate, temperature, and treatment time in determining system effectiveness using Response Surface Methodology (RSM). The results have the potential to advance sustainable, non-thermal pasteurization technologies in the beverage industry.
Material and Methods 
To assess microbial inactivation, a continuous-flow system was constructed comprising a sour cherry juice reservoir, a high-precision pump, an argon gas tank for plasma formation, and a custom-designed glass Venturi tube reactor. The reactor featured a throat diameter of 3.14 mm, length of 17.69 mm, and convergence/divergence angles of 23° and 80.81°, respectively. Tungsten electrodes were used due to their high melting point (>3000°C). Plasma was generated in the liquid phase using injected argon gas under voltages ranging from 10 to 20 kV.
Sour cherry juice was autoclaved at 121°C for 15 minutes to eliminate background microorganisms and then inoculated with E. coli cultured for 24 hours. Treatment parameters included voltage, flow rate (2–8 L/min), temperature (30–50°C), and time (1–5 min). Microbial load was determined using the plate count method on MacConkey agar. Experiments were conducted in triplicate. RSM with a Box-Behnken design was employed for optimization.
Results and Discussion 
Under optimized conditions voltage of 19.85 kV, flow rate of 3.52 L/min, temperature of 47.82°C, and treatment time of 4.42 min the hybrid system achieved a maximum E. coli reduction of 6.18 log cycles (99%). The regression model showed excellent predictive power with an R² value of 0.99, standard error of 0.059, and coefficient of variation of 2.10%. ANOVA revealed that treatment time and voltage were the most significant variables (P < 0.1), followed by temperature and flow rate. Significant interactions were found between several variables, and quadratic effects highlighted complex interdependencies.
Quality analyses compared untreated, thermally treated, and hybrid-treated samples. Total phenolic content (TPC) in untreated juice was 283.71 mg GAE/100 g, decreasing by 16.5% (to 237.11 mg GAE/100 g) with thermal treatment and 14.2% (to 243.51 mg GAE/100 g) with hybrid treatment. Total anthocyanin content (TAC) dropped from 241.23 mg C3G/L to 201.57 mg C3G/L (thermal) and 217.32 mg C3G/L (hybrid), indicating better pigment retention with the non-thermal method.
Vitamin C content showed a significant difference: 41.94 mg/L in untreated samples declined to 19.62 mg/L (thermal, 53.2% loss) and 29.67 mg/L (hybrid, 29.3% loss). Color preservation, measured via spectrophotometry, indicated a lower color difference (ΔE = 2.52) for the hybrid-treated juice compared to thermal treatment (ΔE = 3.86), preserving the natural red hue more effectively.
The combination of reactive oxygen and nitrogen species from plasma (•OH, ozone, peroxynitrite) and extreme pressure and temperature from cavitation synergistically enhanced microbial inactivation. The physical damage from bubble collapse complemented chemical attacks on cell membranes and DNA, improving penetration and efficacy.
Conclusions
This study demonstrates that integrating liquid-phase plasma and Venturi-induced hydrodynamic cavitation offers a powerful, non-thermal method for inactivating E. coli in sour cherry juice. The approach ensures microbial safety while preserving critical nutritional and sensory properties, outperforming traditional thermal treatments. The high accuracy of the RSM model validates its utility in predicting and optimizing treatment parameters for maximal efficacy.
In addition to microbial inactivation, the hybrid system showed superior retention of phenolic compounds, anthocyanins, vitamin C, and color integrity, which are key quality markers in fruit juice processing. Its scalability, however, requires further consideration. Energy demand remains a limiting factor, necessitating integration with energy-saving systems and optimization of plasma operating conditions.
Future work should focus on industrial-scale implementation, cost-benefit analysis, and application of this technology to other beverages with different physicochemical characteristics and microbial loads. Ultimately, this dual non-thermal technology represents a sustainable and efficient alternative for producing high-quality, microbiologically safe fruit juices, with strong potential for commercial adoption.
Author Contributions
The data supporting the findings of this study are available from the corresponding author upon reasonable request.
Ethical Considerations
This study did not involve any human or animal participants. All microbiological procedures involving Escherichia coli were conducted under appropriate biosafety level 1 (BSL-1) conditions. The authors affirm that the study complies with ethical standards and no data fabrication, falsification, plagiarism, or research misconduct occurred during the research process.
Conflict of Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
Funding Statement
This work was supported by Research and Technology Council of Shahrekord University .