Effect of non-thermal plasma technology on microbial inactivation and total phenolic content of a model liquid food system and black pepper grains

Highlights

• Application of cold plasma technology for microbial inactivation.

• Analysis of reactive species generated by plasma using optical emission spectroscopy.

• Spoilage microorganisms can be inactivated by cold plasma.

• No significant degradation of key quality parameters observed for dried ingredients.

Abstract

The objectives of this study were to investigate the effects of cold plasma technology on the growth and survival rates of vegetative cells and spores, and total phenolic content of black pepper grains. Plasma treatment was carried out using a non-thermal plasma jet system operating at 20 kHz using atmospheric air at a flow of 11 L/min. A model liquid food system and black pepper grains were both inoculated with Bacillus subtilis vegetative cells and spores. The samples were treated at 15 and 30 kV for treatment times of 3–20 min. The plate count method was used to determine colony-forming units for selected storage times at 4 °C i.e. at 1, 24 and 48 h post treatment. The highest log reduction was observed at 24 h post treatment, i.e. 2.92 log reduction. A 1 log reduction was achieved in the case of black pepper inoculated with spores for all selected storage times. No significant differences in total phenolic content were observed between treated and non-treated black pepper samples (p > 0.05). Optical emission spectroscopy was used to detect reactive species likely to be responsible for cell death. Atomic oxygen, atomic nitrogen, hydroxyl radicals, nitrite oxide and nitrate were detected in light emitted from the plasma. Cell membrane damage caused by non-thermal plasma technology was observed using scanning electron microscopy. This study concludes that cold plasma technology has potential for industry application in food processing to reduce microbial loads in liquid food systems and dried foods with limited impact on food quality.

Introduction

Dried food ingredients including grains, spices and powders have low water activities which limitsmicrobial growth. However, pathogenic and spoilage microorganisms and spores have been found in several dried foods. For example, a Salmonella outbreak associated with the consumption of ready-to-eat salami products containing contaminated black pepper was reported in the United States (Gieraltowski, Julian, Pringle, Macdonald, Quilliam, Marsden-Haug et al., 2013). Contamination events generally occur because of poor hygiene practices during cultivation, harvesting and food manufacturing. A significant number of microorganisms are associated with dried food ingredients, pathogens of particular significance include Salmonella spp, Bacillus spp and Clostridium perfringens. In response to adverse environmental conditions, the later two of these microorganisms may produce dormant structures or bacterial spores, which are resistant to many treatments. Conventional technologies for decontamination of dried ingredients include super-heated steam, fumigation with ethylene or propylene oxide and ionizing radiation. However, there are many limitations associated with use of these technologies including nutrient degradation, health risks, legislative obstacles and consumers’ concerns. Indeed, ethylene and propylene oxide have been banned in the European Union because of carcinogenic and mutagenic concerns (Fowles, Mitchell, & McGrath, 2001; Regulation, 2008). Even though gamma-rays and x-rays have been shown to have strong potential to improve food safety with limited impacts on quality, consumers continue to have a negative perception of food irradiation treatments (Bearth & Siegrist, 2019). Additionally, conventional thermal treatments e.g. super-heated steam affect food quality due to the high processing temperatures involved. These limitations have encouraged researchers to investigate alternative approaches to ensure safety of dried food ingredients.

Plasma is defined as a partially or wholly ionized gas composed of positive and negative ions, electrons, photons, free radicals and neutrons atoms and molecules (Fridman & Kennedy, 2004). The ionization of these chemically reactive components can occur upon exposure to different energy sources, the most common being thermal, microwave and radio frequency, radioactive (gamma radiation) and x-ray electromagnetic radiation. Depending on its thermodynamic equilibrium state, plasma can be classified as thermal or non-thermal. Unlike thermal plasma treatments, the electrons and other gas components comprising cold plasma exist in a non-thermodynamic equilibrium (Shashi K Pankaj & Keener, 2017). Cold plasma has traditionally been used in the bio-medical, textile and polymer industries (Fanelli & Fracassi, 2017; Kim, Kim, Hong, & Yang, 2010; Sun & Stylios, 2004). However in the last decade it has been increasingly investigated as an alternative approach to the use of conventional technologies in the food industry. Advantages of cold plasma compared to traditional technologies include: extension of product shelf life with limited impacts of food quality, higher retention of the nutrients in the food matrix, and reduced environmental impacts due to lower energy, water and solvent requirements (Hati, Patel, & Yadav, n.d.; Shashi K; Pankaj, Wan, & Keener, 2018). Applications of cold plasma technology in the food industry investigated to date include inactivation of microorganisms in food products (Dasan, Yildirim, & Boyaci, 2018; Misra & Jo, 2017; Pasquali, Stratakos, Koidis, Berardinelli, Cevoli, Ragni, et al., 2016), modification of functional properties of food matrices (Bahrami et al., 2016; Kovačević et al., 2016; Thirumdas, Saragapani, Ajinkya, Deshmukh, & Annapure, 2016), improvement of food packaging (Oh, Roh, & Min, 2016; Shashi Kishor; Pankaj, Bueno-Ferrer, Misra, Milosavljević, O'Donnell, Bourke, et al., 2014) and hydrogenation of edible oils (Yepez & Keener, 2016). Adaptive plasma sources and designs employed these applications include: dielectric barrier discharges, corona discharge, gliding arc discharges, radio frequency, microwave and jet plasma (Ekezie, Sun, & Cheng, 2017). The energetic species generated by plasma can alter and inactivate microorganisms (Misra, Tiwari, Raghavarao, & Cullen, 2011). This study investigates the effects of a non-thermal atmospheric pressure plasma jet on microbial inactivation in an innoculated model liquid food system and black pepper grains, and on the total phenolic content of black pepper grains.