Influence of pH, water activity and temperature on the inactivation of Escherichia coli and Saccharomyces cerevisiae by pulsed electric fields (2023)

Water disinfection and food pasteurization are critical to reducing waterborne and foodborne diseases, which have been a pressing public health issue globally. Electrified treatment processes are emerging and have become promising alternatives due to the low cost of electricity, independence of chemicals, and low potential to form by-products. Electric field treatment (EFT) is a physical pathogen inactivation approach, which damages cell membrane by irreversible electroporation. EFT has been studied for both water disinfection and food pasteurization. However, no study has systematically connected the two fields with an up-to-date review. In this article, we first provide a comprehensive background of microbial control in water and food, followed by the introduction of EFT. Subsequently, we summarize the recent EFT studies for pathogen inactivation from three aspects, the processing parameters, its efficacy against different pathogens, and the impact of liquid properties on the inactivation performance. We also review the development of novel configurations and materials for EFT devices to address the current challenges of EFT. This review introduces EFT from an engineering perspective and may serve as a bridge to connect the field of environmental engineering and food science.

Maltodextrin esterified to laurate (ML) was synthesized enzymatically and compared to commercial sucrose laurate (SL) for microbial growth-inhibitory properties in food systems. There was broad microbial inhibition by ML in growth media, but practical information about various foods and storage temperatures is limited with no toxicological information. This research investigated the antimicrobial property of ML against two spoilage bacteria (Bacillus subtilis and Lactobacillus plantarum) in growth media and related food models stored at different temperatures (4°C, 22°C, and 35°C). Additionally, in vitro cytotoxicity of ML was investigated in human colon adenocarcinoma HT-29cells and human normal colon epithelial CCD 841 CoN cells using a standard colorimetric assay. ML was highly inhibitive against L. plantarum in apple juice at all temperatures tested. SL could inhibit both bacteria in the food models tested (milk, apple juice, tomato ketchup, French dressing) at 4°C. ML was not observed to be cytotoxic to either cell line up to 500μg/mL, whereas SL was evidently cytotoxic with IC₅₀ ranging from 84 to 94μg/mL. In conclusion, ML can be an alternative to commercial SL as a safe food additive in certain products.

Capturing and reusing fertigation drainage is a key strategy for maximizing greenhouse production efficiency while minimizing the impact of wastewater discharge on receiving ecosystems. Fertigation drainage, in this regard, refers to irrigation water mixed with fertilizer that has once passed through soilless culture. Although an economically and environmentally prudent practice, recirculating fertigation solutions does pose an increased risk of pathogen proliferation. There are many water disinfection technologies currently available to growers, including ozone (O_3(aq)) and Advanced Oxidation Processes (AOP). Beyond currently available treatment options there are emerging technologies that have yet to be optimized for recirculating hydroponics. Electrochemical systems based on Dimensionally Stable Anodes (DSA) offer a novel method for disinfecting fertigation. Using Cyclamen persicum as a representative greenhouse floriculture crop, fertigation solutions were inoculated with Fusarium oxysporum isolated from a diseased C. persicum sample. Following inoculation, solutions were treated with one of either a DSA electrochemical system, a UV/Ozone AOP system, or ozonation. Solutions were then applied to the crop and disease progression was monitored. The positive control group (F. oxysporum added) exhibited pathogenicity following the recirculation of the fertigation solution, while the negative control (F. oxysporum absent) did not show pathogenicity in C. persicum. All water treatment systems achieved a log-4 reduction in F. oxysporum, which prevented pathogenicity in plants. Furthermore, there were no significant differences in plant physiology between the water treatment methods in comparison to the negative control group. However, all treatments performed significantly better than the positive control group which experienced pathogenicity. Solution nutrient analysis indicated stability across all treatments. Energy consumption was also monitored and demonstrated a two-fold reduction in electricity use with the electrochemical flow cell (EFC) compared to the AOP system.

Recently, the consumption of hummus has become popular in the United States, European countries, and Canada, and unfortunately, several foodborne outbreaks and recalls have been reported due to its contamination with Listeria monocytogenes and Salmonella enterica. The current study aimed to investigate the inhibitory activity of 0.5, 1.0, and 1.5% citric acid (CA) and 1.0, 2.0 and 3.0% garlic extract (GE) toward S. enterica and L. monocytogenes in hummus stored at 4, 10 and 24°C. L. monocytogenes grew well in untreated (control) hummus samples at all tested temperatures, whereas S. enterica grew only at 10 and 24°C. CA at 0.5 to 1.5% reduced L. monocytogenes numbers by 3.0–3.3 log CFU/g at 4°C, 1.7–3.9 log CFU/g at 10°C, and 0.9–1.4 log CFU/g at 24°C. Numbers of S. enterica were reduced by 0.6–1.7, 4.1–4.9 and <1.5 log CFU/g, at 4, 10 and 24°C, respectively, compared to the control during 10 d storage. GE at 1.0–3.0% also reduced numbers of L. monocytogenes at 10 d by 0.7–3.0, and 1.3–3.6 log CFU/g at 4 and 10°C, respectively, and numbers of S. enterica by 0.7–1.2, 1.8–2.6 and 0.5–1.6 log CFU/g, at 4, 10 and 24°C, respectively, compared to the control. Chromatographic analysis of GE revealed the presence of four organosulfur compounds including diallyl disulfide, diallyl trisulfide, 2-vinyl-(4H)-1,3-dithiin and 3-vinyl-(4H)-1,2-dithiin where the latter was the predominant compound with a level of 22.9mg/g which significantly contributed to the inhibitory effect of GE. CA and GE are adequate natural antimicrobials in hummus to reduce L. monocytogenes and S. enterica numbers and enhance product safety.

The paper presents the ability of coconut fibers and its modification made with Saccharomyces cerevisiae yeast cells to remove Pb(II) ions by adsorption process. Sorbents have been characterized by the SEM-EDS, FTIR, CHN, BET analysis. Maximum Pb(II) adsorption was equal to 99.32% for Pb(II) concentration of 1000mg/dm³ for unmodified adsorbent and 99.87% for coconut fiber containing yeast cells. The sorption capacity of unmodified material was equal to 73.627mg/g and 63.935mg/g for the modified one with the highest metal ion concentration (3000mg/dm³). The lead sorption on both materials has been best described by the Langmuir isotherm what suggests that monolayer adsorption has been a type of adsorption occurring in the experiment. Additionally, pseudo-first-order, pseudo-second-order, and Weber-Morris kinetic models have been used to acquire results providing information about the nature of the adsorption process. Due to the highest fit of the R² of the pseudo-second-order model, it can be supposed that adsorption has been a chemical reaction. Desorption studies showed that the best eluents are organic acids such as citric and acetic acids. Additionally, 5 cycles of sorption/desorption were carried out showing almost no reduction in the material removal efficiency.

The effects of high pressure processing (HPP; 200–600MPa for 5 or 15min) and pulsed electric field (PEF; 3kV/cm, 5–15kJ/kg) treatment on physicochemical properties (conductivity, pH and total soluble solids content), bioactive compounds (vitamin C, total phenolic (TPC), total flavonoid (TFC), total anthocyanin (TAC) and chlorogenic acid contents), antioxidant capacities (DPPH and CUPRAC assays) and polyphenol oxidase (PPO) activity of cranberrybush purée were evaluated immediately after processing. The results were compared to an untreated purée. According to the results, conductivity increased significantly after PEF (15kJ/kg) treatment. PEF and HPP treatments resulted in a better retention of bioactive compounds (increase in TPC in the range of ~4–11% and ~10–14% and TFC in the range of ~1–5% and ~6–8% after HPP and PEF, respectively) and antioxidant activity (as measured with CUPRAC method) compared to untreated sample. HPP reduced residual enzyme activity of PPO comparatively better than PEF.

Innovative Food Science & Emerging Technologies, Volume 39, 2017, pp. 179-187

International Dairy Journal, Volume 39, Issue 1, 2014, pp. 146-156

Bacterial count reduction and enzyme inactivation in bovine whole milk by pulsed electric field (PEF) processing combined with pre-heating and intermediate cooling was compared with that induced by thermal treatments (maximum of 63°C for 30min and 73°C for 15s). Milk was pre-heated to 55°C for 24s and PEF-treated in continuous mode at electric field intensities of 15.9–26.2kVcm⁻¹ for 17–101μs. PEF treatments reduced the number of inoculated (8.3logcfumL⁻¹) Escherichia coli and Listeria innocua to less than 2logcfumL⁻¹. PEF treatments at 20.7–26.2kVcm⁻¹ reduced the total plate count and indigenous alkaline phosphatase activity to a level similar to that measured following thermal treatments. Treatment at 26.1kVcm⁻¹ for 34μs, combined with pre-heating, reduced the activity of plasmin, xanthine oxidase and lipolysable fat by 12%, 32% and 82%, respectively, compared with raw milk.

Food Research International, Volume 77, Part 4, 2015, pp. 773-798

Several studies have demonstrated the feasibility of pulsed electric fields (PEF) for different applications in food industry. PEF technology is therefore a valuable tool that can improve functionality, extractability, and recovery of nutritionally valuable compounds as well as the bioavailability of micronutrients and components in a diverse variety of foods. Additionally, other studies have shown the potential of PEF treatments to reduce food processing contaminants and pesticides. This opens the doors to new PEF applications in the food industry. This review focused on some of the most renowned traditional and emerging PEF applications for improvement of osmotic dehydration, extraction by solvent diffusion, or by pressing, as well as drying and freezing processes. The impact of PEF on different products of biological origin including plant tissues, suspension of cells, by-products and wastes will be analyzed in detail. In addition, recent examples of PEF-assisted biorefinery application will be presented, and finally, the main aspects of PEF-assisted cold pasteurization of liquid foods will also be described.

Food Chemistry, Volume 210, 2016, pp. 249-261

Selected technological characteristics and bioactive compounds of juice pressed directly from the mash of whole Opuntia dillenii cactus fruits have been investigated. The impact of pulsed electric fields (PEF) for a non-thermal disintegration on the important juice characteristics has been evaluated in comparison to microwave heating and use of pectinases. Results showed that the cactus juice exhibited desirable technological characteristics. Besides, it also contained a high amount of phenolic compounds being the major contributors to the overall antioxidant activity of juice. HPLC-DAD/ESI-MSⁿ measurements in the fruits’ peel and pulp showed that isorhamnetin 3-O-rutinoside was determined as the single flavonol found only in the fruit’s peel. Treating fruit mash with a moderate electric field strength increased juice yield and improved juice characteristics. Promisingly, the highest release of isorhamnetin 3-O-rutinoside from fruit’s peel into juice was maximally achieved by PEF.

Bioelectrochemistry, Volume 103, 2015, pp. 92-97

The batch fermentation process, inoculated by Pulsed Electric Field (PEF) treated wine yeasts (Saccharomyces cerevisiae Actiflore F33), was studied. PEF treatment was applied to the aqueous yeast suspensions ([Y]=0.012g/L) at the electric field strengths of E=100 and 6000V/cm using the same treatment protocol (number of pulses n=1000, pulse duration t_i=100μs, and pulse repetition time Δt=100ms). Electrical conductivity was increasing during and after the PEF treatment, which reflected cell electroporation. Then, fermentation was run for 150h in an incubator (30°C) with synchronic agitation. Electro-stimulation was revealing itself by the improvement of fermentation characteristics, and thus increased yeast metabolism. At the end of the lag phase (t=40h), fructose consumption in samples with electrically activated inoculum exceeded that of the control samples by ≈2.33 times for E=100V/cm and by ≈3.98 for E=6000V/cm. At the end of the log phase (120h of fermentation), ≈30% mass reduction was reached in samples with PEF-treated inocula (E=6000V/cm), whereas the same mass reduction of the control sample required approximately 20 extra hours of fermentation.

LWT, Volume 77, 2017, pp. 517-524

Whole fresh blueberries were treated using a parallel pulsed electric field (PEF) treatment chamber and a sanitizer solution (60ppm peracetic acid [PAA]) as PEF treatment medium with square wave bipolar pulses at 2kV/cm electric field strength, 1μs pulse width, and 100 pulses per second for 2, 4, and 6min. The effects of PEF on native microbiota and artificially-inoculated Escherichia coli K12 and Listeria innocua populations on blueberries were determined. Color, texture, anthocyanins and total phenolic compound concentrations were also evaluated. The combination of PEF and PAA was able to achieve up to 3 log reduction of E.coli and Listeria as well as 2 log/g reduction of native microbiota. PEF treatments did not cause any changes in color and appearance of the blueberries. The treatments did, however, cause the blueberries to soften in texture. Anthocyanins and phenolic compounds in blueberries increased by 10 and 25%, respectively, after PEF treatments. The results demonstrate the potential of PEF applications to enhance the safety and improve the quality and nutritional value of fruits and their derived products.