Citric acid as alternative to sodium hypochlorite for washing and disinfection of experimentally-infected spinach leaves

Effects of washing with boric acid solutions on residual boric acid content, microbiological load, and quality of fresh-cut spinach

Insufficient disinfection of fresh-cut spinach poses significant health risks, along with potential issues like odor, color changes, and softening during short-term storage. To address these challenges, boric acid solutions were explored as an alternative to chlorine washes, which are known to produce toxic compounds. Among various concentrations, 1 % boric acid exhibited the most effective microbial inactivation, leading to substantial reductions in total mesophilic aerobic bacteria, total yeast and mold, and Enterobacteriaceae counts, with reductions of 1.64, 1.38, and 1.77 logs, respectively. Additionally, washing spinach leaves with this solution for 1 min maintained quality parameters, with enhanced antioxidant activity (55.26 mg kg-1 Trolox equivalent), increased total phenolic content (1214.06 mg kg-1 gallic acid equivalent), retention of chlorophyll a (839.16 mg kg-1), chlorophyll b (539.61 mg kg-1) and ascorbic acid content (264.72 mg kg-1). Mechanical properties such as puncture strength (1.81 N) and puncture distance (52.78 mm) also showed favorable outcomes, alongside optimal moisture content at 89.81 %. Notably, residual boric acid content was lowest in spinach leaves (1252.49 mg kg-1) and highest in the wash water (53.88 mg kg-1) after treatment. Scanning electron microscopy images demonstrated maintained tissue integrity, while Hunter Lab readings indicated minimal color changes post-washing. Additionally, sensory evaluations and various physicochemical analyses further supported the efficacy of boric acid washing. Consequently, washing spinach leaves with a 1 % boric acid solution for 1 min yielded favorable results across multiple quality parameters. These findings suggest the potential of boric acid as a safe and effective alternative disinfectant in the fresh-cut produce industry, highlighting its practical implications for food safety and quality. Future research should focus on exploring long-term effects and optimizing washing protocols for broader applications.

The Use of Ultraviolet Irradiation to Improve the Efficacy of Acids That Are Generally Recognized as Safe for Disinfecting Fresh Produce in the Ready-to-Eat Stage

Fresh-cut produce is usually produced under standardized disinfection processes, which are unavailable at the ready-to-eat stage. Currently, chemical sanitizers are used for washing, but their disinfection efficacy is limited. In this study, UV-C (1.03 kJ/m2) was combined with organic acids that are generally recognized as safe (GRAS), including citric, malic, acetic, and lactic acids (LAs), to wash lettuce and cherry tomatoes that are contaminated with Escherichia coli O157:H7 and Salmonella Typhimurium. The results showed that LA was the most effective treatment among the single treatments, with a pathogen reduction and cross-contamination incidence of 2.0-2.3 log CFU/g and 28-35%, respectively. After combining with UV-C, the disinfection efficacy and cross-contamination prevention capacity of the four GRAS acids significantly improved. Among the combination treatments, the highest pathogen reduction (2.5-2.7 log CFU/g) and the lowest cross-contamination incidence (11-15%) were achieved by LA-UV. The analyses of ascorbic acid, chlorophyll, lycopene, antioxidant capacity, and ΔE indicated that neither the single nor combination treatments negatively affected the quality properties. These results provide a potential hurdle technology for fresh produce safety improvement at the ready-to-eat stage.

Antibacterial mechanism of ultrasound combined with sodium hypochlorite and their application in pakchoi (Brassica campestris L. ssp. chinensis)

BACKGROUND

In order to prolong the storage and inhibit microorganisms of pakchoi, the antibacterial activity and mechanism of ultrasound combined with sodium hypochlorite (NaClO-US), the efficiency of NaClO-US in reducing Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa as well as preserving quality of pakchoi were investigated.

RESULTS

Ultrasound treatment could significantly reduce the usage of NaClO solution from 800 ppm to 500 ppm. NaClO-US decreased the counts of E. coli, S. aureus and P. aeruginosa, which disrupted the bacterial cell membrane with cytoplasmic leakage. In addition, NaClO-US significantly increased cell membrane permeability, while cell membrane integrity decreased, the secondary structure of bacterial proteins showed several obvious changes, such as the increase of random coil content, as well as the decrease of α-helix content. The bacterial counts, E. coli, S. aureus and P. aeruginosa population in pakchoi treated with NaClO-US reduced by 1.89, 1.40, 1.60, 1.72 log CFU g^-1 , respectively compared to control sample after storage for 15 days. NaClO-US resulted in minimum chlorophyll depletion, flavor and sensory deterioration.

CONCLUSION

NaClO-US solution treatment inhibited microorganisms and prolonged storage of pakchoi. © 2022 Society of Chemical Industry.

Use of Acetic Acid to Partially Replace Lactic Acid for Decontamination against Escherichia coli O157:H7 in Fresh Produce and Mechanism of Action

Escherichia coli O157:H7 is frequently detected in ready-to-eat produce and causes serious food-borne diseases. The decontamination efficacy of lactic acid (LA) is clearly established. In this study, LA was mixed with acetic acid (AA) to reduce costs while achieving consistent or better inhibitory effects. Time-kill curves and inoculation experiments using fresh-cut spinach and arugula indicated that 0.8%LA+0.2%AA shows similar antibacterial effects to those of 1%LA. To determine whether 1%LA and 0.8%LA+0.2%AA exert antibacterial effects by similar mechanisms, proteomics analysis was used. The proteins related to macromolecule localization, cellular localization, and protein unfolding were uniquely altered after the treatment with 1%LA, and the proteins related to taxis, response to stress, catabolic process, and the regulation of molecular function were uniquely altered after the treatment with 0.8%LA+0.2%AA. Based on these findings, combined with the results of a network clustering analysis, we speculate that cell membrane damage is greater in response to LA than to 0.8%LA+0.2%AA. This prediction was supported by cell membrane permeability experiments (analyses of protein, nucleotide, ATP, and alkaline phosphatase leakage), which showed that LA causes greater membrane damage than 0.8%LA+0.2%AA. These results provide a theoretical basis for the application of an acid mixture to replace LA for produce decontamination.

Combination of ozone and ultrasonic-assisted aerosolization sanitizer as a sanitizing process to disinfect fresh-cut lettuce

Reduction of sanitizer dosage and development of non-immersion disinfection methods have become major focuses of research. Here, we examined the disinfection efficacy of combining gaseous ozone (4 and 8 ppm) with aerosolized oxidizing sanitizer [sodium hypochlorite (SH, 100 and 200 ppm)] and aerosolized organic acid [acetic acid (AA, 1% and 2%) and lactic acid (LA, 1% and 2%)]. Notably, 1% AA and 4 ppm gaseous ozone were ineffective for disinfecting Salmonella Typhimurium, and treatment with 1% AA + 8 ppm ozone caused browning of lettuce leaves and stimulated increases in aerobic mesophilic count (AMC), aerobic psychrotrophic count (APC), S. Typhimurium, and Escherichia coli O157:H7. Treatment with 2% LA + 8 ppm ozone resulted in the lowest S. Typhimurium, E. coli O157:H7, Listeria monocytogenes, AMC, APC, and molds and yeasts during storage (0-7 days at 4 °C). Quality analysis indicates that LA + 8 ppm ozone and SH + 8 ppm ozone did not negatively affect L*, a*, b*, polyphenolic content, weight loss, and sensory properties; however, the levels of two individual phenolic compounds (3,4-dihydroxybenzoic acid and vanillin), responsible for phenylpropanoid synthesis, were significantly increased after treatment with 2% LA + 8 ppm ozone. These findings provided insights into the use of LA combined with gaseous ozone for application in disinfecting fresh produce.

Transfer and bioaccumulation of mercury from soil in cowpea in gold mining sites

In this study, we evaluated the phytoremediation ability of three different genotypes of cowpea (Vigna unguiculata L. Walp) grown on mercury-contaminated soils from gold mining areas. In particular we compared a native genotype with two commercial lines L-019 and L-042. The plants were cultivated in soils amended at different concentrations of Hg (i.e. 0.2, 1, 2, 5 and 8 mg kg^-1). After three months exposure, we determined plant growth, seed production, and Hg accumulation in different plant tissues (root, leaf, seed and stem). Indices of soil-plant metal transfer such as translocation, bioconcentration and bioaccumulation factors were calculated. Results showed that the native variety presented the highest seed production (3.8 g), however the highest plant biomass (7.9 g) was observed in line L-019, both on Hg-contaminated soil of 1 mg kg^-1. The different plant tissues differed in terms of Hg concentration (root > leaf > stem). In the highest treated soil, the line L-042 accumulates higher Hg in both roots and leaves, while line L-019 accumulates more metal in stems. In line L-019, Hg concentrations in the fruit showed significant differences being higher in the valves than in the seeds. The transfer factors were generally lower than 1 and indicates the low accumulation of Hg by cowpeas. The estimated daily Hg intake through cowpea consumption showed values far below the threshold of 0.57 μg kg^-1 dw day^-1 recommended by the World Health Organization. Our results show cowpea V. unguiculata as a good protein-rich food substitute of Hg-contaminated fish for populations living near gold mining sites.

The effect of neutral electrolyzed water as a disinfectant of eggshells artificially contaminated with Listeria monocytogenes

Neutral electrolyzed water (NEW) was tested as a disinfectant against Listeria monocytogenes on the surface of table eggs. Eggs were collected from a single Bovans White flock and were exposed to L. monocytogenes. Artificially contaminated eggs were divided into three different treatment groups: NEW, 2% citric acid solution (CAS), and saline solution (SS). To evaluate the bactericidal effect, the Mexican norm for antimicrobial activity determination protocol was performed. The observed bactericidal effect was compared against those obtained from CAS and SS. Bacterial cells present on the eggshells were quantified. NEW exhibited a significantly higher bactericidal effect than CAS when evaluated on the surfaces of chicken eggshells (6.11 log10CFU/ml reduction in vitro and a 2.18 log10 CFU/egg reduction on eggs vs. 1.06 log10CFU/ml in vitro reduction and 1.74 log10CFU/egg). Additionally, CAS was found to react with the carbonate egg shield, resulting in a loss of cuticle integrity. Mineral content of NEW-treated eggshells was similar to SS-treated eggshells; however, CAS-treated eggshells showed a significant decrease in phosphorous concentration compared to NEW treatment. In this study, we demonstrated the effect of NEW and CAS on the integrity of the L. monocytogenes wall using transmission electron microscopy. To the best of our knowledge, this is the first report of the effect of NEW against L. monocytogenes on eggshells. Our results show that NEW is a viable alternative solution for the disinfection of table eggs that does not affect the cuticle or shell.

Reduction of Escherichia coli O157:H7 and naturally present microbes on fresh-cut lettuce using lactic acid and aqueous ozone

Lactic acid (LA) is an effective sanitizer for disinfection of fresh produce. Tap water is generally used to wash disinfected fresh produce because sanitizer residues negatively affect the quality and organoleptic properties of the produce. However, tap water is ineffective for secondary disinfection compared with sanitizers. Thus, we propose a disinfection method using LA plus aqueous ozone (AO), an oxidizing sanitizer that does not lead to secondary residue. We compared the proposed method of 1% LA (90 s) plus 1 mg L⁻¹ AO (30 s) or 2 mg L⁻¹ AO (30 s) with the traditional method of 100 ppm chlorine (120 s) or 1% LA (120 s) plus tap water (30 s) and 2 mg L⁻¹ AO (150 s). Microbial analysis showed that LA plus AO led to the greatest reductions in microbes (Escherichia coli O157:H7, aerobic mesophilic counts, aerobic psychrophilic counts, moulds, and yeasts) during storage (0–5 days at 5 °C). Quality analysis (colour, sensory qualities, electrolyte leakage, polyphenolic content, and weight loss) showed that LA + AO did not cause additional quality loss compared with tap water treatment. These results indicate that the hurdle technology proposed (LA plus AO) has a good potential for use in fresh produce disinfection.

Lactic acid plus aqueous ozone is an effective hurdle technology for fresh produce disinfection.