Application of electro-activated potassium acetate and potassium citrate solutions combined with moderate heat treatment on the inactivation of Clostridium sporogenes PA 3679 spores

Evaluation of the additive effects of volatile fatty acids and moderate heat treatment for enhancing the inactivation of vegetative cells and spores of Clostridium perfringens by flow cytometry

OBJECTIVES

Clostridium perfringens is a well-known spore-forming bacterium that can resist the environment. A mixture of volatile fatty acids or thermal treatments can interact with these bacteria. The aim of this study was to evaluate the effects of different volatile fatty acid concentrations and moderate heat treatment on Clostridium perfringens sporulation.

METHODS

A pure culture of Clostridium perfringens type A in Duncan Strong medium was treated with a mixture of volatile fatty acids at several concentrations. A thermal treatment was also tested. To evaluate the effects, a double staining method was employed, and treatments on Clostridium perfringens were analysed by flow cytometry.

RESULTS

Moderate heat treatment destroyed vegetative forms but had no effect on sporulating forms. Volatile fatty acids combined with moderate heat treatment inhibited Clostridium perfringens sporulation.

CONCLUSIONS

The use of flow cytometry as an original method for evaluating the treatment of Clostridium perfringens is of interest because of its simplicity, short time to obtain results, and the level of information provided on the microbial population (impact on metabolism). A combination of mild treatments (moderate heat treatment + volatile fatty acids) to decrease the Clostridium perfringens concentration when these bacteria sporulate is a very promising finding for inhibiting Clostridium perfringens propagation.

Study of the Antibacterial Potency of Electroactivated Solutions of Calcium Lactate and Calcium Ascorbate on Bacillus cereus ATCC 14579 Vegetative Cells

Bacillus cereus is a pathogenic bacterium, Gram-positive, aerobic, and facultative anaerobic that can produce spores and different toxins. It is involved in serious foodborne illnesses such as the diarrheal and emetic syndromes, depending on the ingested toxin. This work is aimed to study the potency of electroactivated solutions (EAS) of calcium lactate, calcium ascorbate, and their mixture as antibacterial agents against B. cereus ATCC 14579 vegetative cells. The solutions used were electroactivated under electric current intensities of 250, 500, and 750 mA for 30 min. The obtained EAS were tested in direct contact with B. cereus (10⁷ CFU/mL) for different durations ranging from 5 s to 2 min. Moreover, standard lactic and ascorbic acids were tested as controls at equivalent titratable acidity as that of the corresponding electroactivated solutions. The obtained results showed that EAS exhibit high antibacterial efficacy against B. cereus vegetative cells. The EAS obtained after electroactivation of calcium lactate and calcium ascorbate were more efficient than those of their corresponding standard acids (lactic and ascorbic). The observed antibacterial effect of the EAS resulted in a reduction of 7 log CFU/mL after 5 s of direct contact in some specific cases. Scanning (SEM) and transmission (TEM) electron microscopic observations provided conclusive evidence of the antibacterial activity of the used EAS. These results outlined the highly antimicrobial potency of EAS against B. cereus vegetative cells and that they can be considered in an eventual strategy to ensure food safety, surface cleaning, as well as replacement of hazardous disinfecting chemicals.

Hyperspectral imaging and deep learning for quantification of Clostridium sporogenes spores in food products using 1D- convolutional neural networks and random forest model

Clostridium sporogenes spores are used as surrogates for Clostridium botulinum, to verify thermal exposure and lethality in sterilization regimes by food industries. Conventional methods to detect spores are time-consuming and labour intensive. The objectives of this study were to evaluate the feasibility of using hyperspectral imaging (HSI) and deep learning approaches, firstly to identify dead and live forms of C. sporogenes spores and secondly, to estimate the concentration of spores on culture media plates and ready-to-eat mashed potato (food matrix). C. sporogenes spores were inoculated by either spread plating or drop plating on sheep blood agar (SBA) and tryptic soy agar (TSA) plates and by spread plating on the surface of mashed potato. Reflectance in the spectral range of 547-1701 nm from the region of interest was used for principal component analysis (PCA). PCA was successful in distinguishing dead and live spores and different levels of inoculum (10² to 10⁶ CFU/ml) on both TSA and SBA plates, however, was not efficient on the mashed potato (food matrix). Hence, deep learning classification frameworks namely 1D- convolutional neural networks (CNN) and random forest (RF) model were used. CNN model outperformed the RF model and the accuracy for quantification of spores was improved by 4% and 8% in the presence and absence, respectively of dead spores. The screening system used in this study was a combination of HSI and deep learning modelling, which resulted in an overall accuracy of 90-94% when the dead/inactivated spores were present and absent, respectively. The only discrepancy detected was during the prediction of samples with low inoculum levels (<10² CFU/ml). In summary, it was evident that HSI in combination with a deep learning approach showed immense potential as a tool to detect and quantify spores on nutrient media as well as on specific food matrix (mashed potato). However, the presence of dead spores in any sample is postulated to affect the accuracy and would need replicates and confirmatory assays.

Study of the impacts of electro-activated solutions of calcium lactate, calcium ascorbate and their equimolar mixture combined with moderate heat treatments on the spores of Bacillus cereus ATCC 14579 under model conditions and in fresh salmon

Widespread in very diverse environments, the spores of Bacillus cereus are highly resistant to hostile conditions and can contaminate a huge variety of food products, posing a potential health hazard to consumers. Given this significant risk, the objective of this research work was to study the impacts of electro-activated solutions (EAS) made with calcium ascorbate, calcium lactate, and their equimolar mixture on Bacillus cereus ATCC 14579 spores in model conditions and food matrix, the fresh Atlantic salmon. The model conditions consisted of a direct application of the EAS to the spores, which avoided any interference with factors external to those of the solutions. Salmon was chosen as a food model because it is a product sensitive to bacterial spoilage and can be eaten raw. To achieve this, the solutions were prepared by electro-activation using an electric current with an intensity of 750 mA for 30 min, resulting in mean pH values of 1.94 ± 0.15-2.16 ± 0.01 and titratable acidity of 0.102 ± 0.001-0.109 ± 0.001 mol/L, depending on the type of solution. These conditions were chosen because of their excellent antibacterial efficacy previously demonstrated against vegetative cells of B. cereus. The results showed high sporicidal activities of the EAS against B. cereus with a 7 to 9 log reduction, using an initial spore population of 10⁹ CFU/mL, depending on the conditions evaluated, namely: in direct contact (2-30 min), in salmon used as a food matrix (2-7 min), and in combination with moderate heat treatments from 60 to 90 °C (0.5-2 min). In addition, it was observed that the sporicidal capacity of the EAS increased with temperature and the contact time. Otherwise, analysis of the color and lipids of the salmon have not shown any major impacts of the use of EAS as a rinsing solution for this highly perishable food. Furthermore, micrographs taken by scanning and transmission electron microscopy revealed the destructive effects of the EAS used in the vital structures/components of the spores. In general, this study has demonstrated that the electro-activation technology is effective in producing EAS capable of destroying/inactivating B. cereus spores and that they can be used for the improvement of food safety and preservation.

Study of the Electro-Activation Process of Calcium Lactate, Calcium Ascorbate Solutions, and Their Equimolar Mixture: Assessment of Their Physicochemical Properties

The aim of this study was to prepare electro-activated solutions (EAS) from calcium lactate, calcium ascorbate, and an equimolar mixture of these two salts to obtain their corresponding acids and to study their physicochemical characteristics, in particular, pH, titratable acidity, pK _a, and antioxidant activity. Indeed, the solutions were electro-activated in a reactor comprising three compartments (anodic, central, and cathodic) separated by anionic and cationic exchange membranes, respectively. The electric current intensities used were set at 250, 500, and 750 mA for a maximum period of 30 min. In general, the EAS obtained at 750 mA for 30 min showed the lowest pH (2.16, 2.08, 1.94) and pK _a (3.13, 3.07, 2.90) values and the highest titratable acidity (0.107, 0.102, 0.109 mol/L) for calcium lactate, the mixture, and calcium ascorbate, respectively. In addition, the obtained results have demonstrated that the pH, titratable acidity, and pK _a of the EAS varied proportionally and significantly (p < 0.001) with the duration of the experiment and the intensity of the electric current applied. To evaluate the migration of calcium (Ca²⁺) between the central and the cathodic compartments of the reactor, the concentration of Ca²⁺ was determined especially in the cathodic section by inductively coupled plasma optical emission spectroscopy (ICP-OES). The results showed that the migration of Ca²⁺ varied proportionally with the electric current intensity. In this context, analysis by Fourier transform infrared (FTIR) spectroscopy, high-performance liquid chromatography (HPLC), and differential scanning calorimetry (DSC) have confirmed the production of lactic acid and ascorbic acid compared to standards. In addition, analysis by the 2,2-diphenyl-1-picrylhydrazyl (DPPH) free-radical scavenging technique confirmed high antioxidant activities of >90 and >83% for calcium ascorbate and the mixture, respectively, in comparison to the standard ascorbic acid (85%). Overall, this research has clearly demonstrated the eventual potential of electro-activation to produce highly reactive organic acids from their conjugated salts. These EAS can become excellent antimicrobial and sporicidal agents in the food processing industry.

Effect of electro-activated solutions of sodium acetate and sodium propionate on geosmin producing Streptomyces avermitilis strain

Electro-activated solutions of salts of weak organic acids are defined as novel potent disinfecting agents that can be used in the agri-food industry. The aim of the present work is to study and understand the destruction mechanism of electro-activated solutions of sodium acetate (EAA) and sodium propionate (EAP) against Streptomyces avermitilis spores. The results of antibacterial activity showed high bacteriostatic effect for all the tested solutions, including sodium hypochlorite used as positive control. Under specific conditions, test on minimal inhibitory concentration demonstrated that the used electro-activated solutions have inhibition activity comparable or higher than the control solution, with the following inhibiting concentrations of 0.004, 0.002 and 0.073 mol/L, for EAA, EAP and NaOCl, respectively. The most active solutions resulted in destruction effect of more than 7 log CFU/mL. The physiological state of the S. avermitilis spores was assessed by transmission electron microscopy after treatments with the electro-activated organic solutions and NaOCl. The results displayed coreless and/or deformed cellular forms with ruptured membranes and released components of spores. The main practical importance of this study is that the targeted final objective is to develop safe and effective alternative to sodium hypochlorite to ensure microbial safety of fresh vegetables under storage conditions. In this context, we studied the potential of electro-activated solutions of sodium acetate and sodium propionate against spores of Streptomyces avermitilis and compared this activity with sodium hypochlorite, the mostly used disinfecting agent in the agri-food industry.

Selection of Surrogate Bacteria for Use in Food Safety Challenge Studies: A Review

Nonpathogenic surrogate bacteria are prevalently used in a variety of food challenge studies in place of foodborne pathogens such as Listeria monocytogenes, Salmonella, Escherichia coli O157:H7, and Clostridium botulinum because of safety and sanitary concerns. Surrogate bacteria should have growth characteristics and/or inactivation kinetics similar to those of target pathogens under given conditions in challenge studies. It is of great importance to carefully select and validate potential surrogate bacteria when verifying microbial inactivation processes. A validated surrogate responds similar to the targeted pathogen when tested for inactivation kinetics, growth parameters, or survivability under given conditions in agreement with appropriate statistical analyses. However, a considerable number of food studies involving putative surrogate bacteria lack convincing validation sources or adequate validation processes. Most of the validation information for surrogates in these studies is anecdotal and has been collected from previous publications but may not be sufficient for given conditions in the study at hand. This review is limited to an overview of select studies and discussion of the general criteria and approaches for selecting potential surrogate bacteria under given conditions. The review also includes a list of documented bacterial pathogen surrogates and their corresponding food products and treatments to provide guidance for future studies.

Study of the antibacterial activity of electro-activated solutions of salts of weak organic acids on Salmonella enterica, Staphylococcus aureus and Listeria monocytogenes

This work assessed the antibacterial activity of electro-activated solutions of salts of weak organic acids (potassium acetate, potassium citrate and calcium lactate) on Salmonella enterica, Staphylococcus aureus and Listeria monocytogenes. This activity was compared in terms of minimal inhibitory (bactericidal) concentration to the effect of commercial acetic, citric and lactic acid at equivalent titratable acidity. Staining live/dead BacLight method was used to consider physiological state of bacteria following the evaluation of pathogenic strains during exposure to the tested solutions. The results demonstrated strong inhibitory activity of all electro-activated solutions. After 10 min of exposure to electro-activated potassium acetate, a reduction of ≥6 log CFU/ml of all bacteria was observed. The electro-activated potassium citrate demonstrated the lowest minimal inhibitory concentration. Nevertheless, its inactivation power was significantly higher than that of conjugated citric acid. Although electro-activated calcium lactate was found less effective in comparison with its conjugated acid form, after 10 min of contact with the tested pathogens, it induced a population reduction of 2.23, 2.97 and 5.57 log CFU/ml of S. aureus, L. monocytogenes and S. enterica, respectively.

Electro-activation of potassium acetate, potassium citrate and calcium lactate: impact on solution acidity, Redox potential, vibrational properties of Raman spectra and antibacterial activity on E. coli O157:H7 at ambient temperature

AIMS

To study the electro-activation of potassium acetate, potassium citrate and calcium lactate aqueous solutions and to evaluate their antimicrobial effect against E. coli O157:H7 at ambient temperature.

METHODS AND RESULTS

Potassium acetate, potassium citrate and calcium lactate aqueous solutions were electrically excited in the anodic compartment of a four sectional electro-activation reactor. Different properties of the electro-activated solutions were measured such as: solutions acidity (pH and titratable), Redox potential and vibrational properties by Raman spectroscopy. Moreover, the antimicrobial activity of these solutions was evaluated against E. coli O157:H7. The results showed a pH decrease from 7.07 ± 0.08, 7.53 ± 0.12 and 6.18 ± 0.1 down to 2.82 ± 0.1, 2.13 ± 0.09 and 2.26 ± 0.15, after 180 min of electro-activation of potassium acetate, potassium citrate and calcium lactate solution, respectively. These solutions were characterized by high oxidative ORP of +1076 ± 12, +958 ± 11 and +820 ± 14 mV, respectively. Raman scattering analysis of anolytes showed stretching vibrations of the hydrogen bonds with the major changes within the region of 3410-3430 cm^-1. These solutions were used against E. coli O157:H7 and the results from antimicrobial assays showed high antibacterial effect with a population reduction of ≥6 log CFU/ml within 5 min of treatment.

CONCLUSIONS

This study demonstrated the effectiveness of the electro-activation to confer to aqueous solutions of organic salts of highly reactive properties that differ them from their conjugated commercial acids. The electro-activated solutions demonstrated significant antimicrobial activity against E. coli O157:H7.

SIGNIFICANCE AND IMPACT OF STUDY

This study opens new possibilities to use electro-activated solutions of salts of weak organic acids as food preservatives to develop safe, nutritive and low heat processed foods.