Cold plasma technology for microbial decontamination of spices : analysis, efficacy and design

Abstract

Spices and Foods are Inseparable. Sri Lanka is renowned for its spices, with the spice trade valued at approximately USD 923,000. Spices hold universal appeal in food and beverages. However, in recent decades, food safety outbreaks caused by contaminated spices have become increasingly common. Both thermal and non-thermal technologies have been applied to reduce microbial loads, with varying levels of effectiveness. This study aimed to investigate how microbial contaminants in spice seeds or particulates can be effectively eliminated while preserving the essential qualities of the spices in real-world conditions. The specific objectives were to identify the limitations of existing cold plasma systems in spice decontamination, design a spice-specific cold plasma sterilization system, and evaluate safety and quality changes in spices after treatment with cold plasma technology. The research began by assessing microbial contamination levels in commercial spice samples and analyzing spice processing from a farm-to-plate life cycle perspective. Spices showed significant contamination by aerobic bacteria and fungi. Additionally, a novel microflora associated with black pepper seeds and red chili powder was identified, with many species reported for the first time. Considering the microbial contamination levels and homogeneity of the samples, black pepper seeds, black pepper powder and red chili powder were selected for further investigations of the research. The study evaluated cold plasma technology, a novel decontamination method gaining global attention, through in-depth process analysis. The performance of existing cold plasma technologies was tested using two systems: Gliding Arc Plasma Discharge (GAPD) at 15 kV and 50 Hz reduced the Aerobic Plate Count (APC) and Yeast and Mould (Y&M) by 73% and 93% after 15 minutes, respectively. There were minor losses of volatile oils and some piperine degradation in black pepper seeds. The process led to a temperature rise of 65.2°C. Low-Pressure Cold Plasma (LPCP) at 13.56 MHz and 0.3 mbar, operating at 250 W for 20 minutes, reduced APC and Y&M by 90% and 99%, respectively, with a temperature increase of up to 41°C. When using nitrogen and air, LPCP reduced APC by 96% and 81%, respectively, and eliminated Y&M within 10 minutes in black pepper powder. However, a 60% loss of volatile oils was also observed. For red chili powder, LPCP reduced APC by 90% and Y&M by 99% in 9 minutes, but the treatment caused significant colour loss. Key drawbacks of the tested cold plasma systems included high temperatures, limited sample capacity, inconsistent microbial inactivation, and considerable quality degradation, especially in powdered spices. To address these challenges, a Rotary Dielectric Barrier Discharge (RDBD) system powered by radio frequency was developed. This system uses a radio frequency unit to convert gas inside a glass reactor into glow-like plasma, enabling uniform spice treatment near atmospheric pressure. The reactor chamber rotates at 10 rpm, increasing spice exposure to reactive plasma species. Perforated electrodes prevent arc discharges, while the air gap between the electrodes and the glass wall acts as a dielectric barrier, ensuring a uniform glow discharge for consistent treatment. Trials were conducted using black pepper seeds, both naturally contaminated and inoculated with Bacillus cereus (ATCC 11778), comparing the performance of RDBD with LPCP. The decimal reduction times (D-values) for black pepper seeds were 4.35 minutes (GAPD), 6.41 minutes (LPCP), and 4.76 minutes (RDBD). RDBD demonstrated the least negative impact on quality attributes. Plasma characteristics for RDBD and LPCP were simulated using COMSOL Multiphysics. The time-averaged maximum electron temperature and electron density for RDBD were 1.2 eV and 1.4 × 10¹⁷ m⁻³, respectively, while the center of the LPCP chamber reached over 2.67 eV and 10¹⁴ m⁻³. Higher electron temperatures (~10 eV) can etch spice surfaces, causing physicochemical changes. Thus, RDBD’s lower electron temperature (1.2 eV) ensures minimal surface damage during decontamination. Meanwhile, its high electron density enhances interaction with the spice surface, increasing microbial inactivation. Both LPCP and RDBD systems reached electric potentials capable of rupturing bacterial cell walls (26–110 V), supporting their effectiveness in microbial reduction.