Reduction of microbial counts during kitchen scale washing and sanitization of salad vegetables

Using ultrasonic washing combined with UV-LEDs as a novel chemical-free method to disinfect fresh ready-to-eat produce

The consumption of ready-to-eat fresh produce raises the issue of food-borne pathogen infections; thus, disinfecting ready-to-eat produce for commercial use, such as in homes and restaurants, is important to ensure food safety. Chemical sanitizers are typically used for disinfection. Ultraviolet-light emitting diodes (UV-LEDs) are a novel non-thermal disinfection technology that consumes less energy and generates less heat than traditional UV lamps, making them more appealing to consumers. In this study, we combined ultrasonic (US) washing method with UV-LEDs (US-UV-LEDs) to develop a technique for disinfecting fresh produce without using chemical sanitizers and compared its efficacy with three common household sanitizers ("84" (sodium hypochlorite) disinfectant, kettle descaler (citric acid), and vinegar (acetic acid)). In addition, we investigated the efficacy of this method in controlling pathogen numbers in the water used to wash (washing water) the produce to prevent cross-contamination between water and produce. Cherry tomatoes and lettuce were selected as produce models and Salmonella Typhimurium and Escherichia coli O157:H7 were used as the bacterial models. The results showed that US-UV-LEDs reduced the numbers of S. Typhimurium and E. coli O157:H7 on produce by 2.1-2.2 log CFU/g, consistent with the results achieved by the three household sanitizers; however, kettle descaler and vinegar had a limited effect (2.6-3.5 log CFU/mL) on residual pathogens in the washing water. Furthermore, we created washing water with low (754 mg/L) and high (1425 mg/L) chemical oxygen demand (COD) levels and determined the disinfection efficacy of "84" disinfectant and US-UV-LEDs. The results showed that US-UV-LEDs reduced the number of S. Typhimurium and E. coli O157:H7 by 2.0-2.1 and 1.8-2.1 log CFU/g under low and high COD levels, respectively, which was similar a result to that of "84" disinfectant. However, the residual pathogen numbers in the washing water were reduced to 1.4-1.9 log CFU/mL after treatment with US-UV-LED under high COD, whereas the pathogens were undetected in the washing water disinfected with "84" disinfectant. These results suggest that US-UV-LEDs have better application potential than acidic household sanitizers, but chlorine sanitizer remains the most effective disinfecting method.

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.

Biogenic silver nanoparticles and cinnamaldehyde as an effective sanitizer for fresh sweet grape tomatoes

This study evaluated the antibacterial activity of cinnamaldehyde (CIN) and biogenic silver nanoparticles (BioAgNP), alone and in combination, against Escherichia coli, Salmonella Typhimurium, and Staphylococcus aureus in vitro. Their sanitation activities on fresh sweet grape tomatoes were also evaluated. CIN and BioAgNP inhibited the growth of the tested bacteria, and at low concentrations, their combinations presented a synergistic effect. In the sanitization of fresh sweet grape tomatoes, CIN (156 µg/mL) combined with BioAgNP (31.25 µM) at subinhibitory concentrations inhibited the growth of E. coli after only 5 min of contact. Exposed samples showed no growth of E. coli during their shelf life. The combination of these compounds did not change significantly (p > 0.05) the physicochemical properties of sweet grape tomatoes and showed that CIN combined with BioAgNP could represent an effective method for decontaminating fruits and vegetables. This combination has great potential for application in the prevention of foodborne diseases.

Effect of Ultrasound Combinated with Sodium Hypochlorite Treatment on Microbial Inhibition and Quality of Fresh-Cut Cucumber

The influence of ultrasound combined with sodium hypochlorite (US-NaClO) treatment on microorganisms and quality of fresh-cut cucumber during storage were investigated. Ultrasound (400 W, 40 kHz, US: 5, 10 and 15 min) and sodium hypochlorite (NaClO: 50, 75, 100 ppm) were used to treat fresh-cut cucumber in a single or combined treatment and stored at 4 °C for 8 days and analyzed for texture, color and flavor. The results showed that US-NaClO treatment had a synergistic effect on the inhibition of microorganisms during storage. It could significantly reduce (p < 0.05) the number of microorganisms by 1.73 to 2.17 log CFU/g. In addition, US-NaClO treatment reduced the accumulation of malondialdehyde (MDA) during storage (4.42 nmol/g) and water mobility, and maintained the integrity of the cell membrane, delayed the increase of weight loss (3.21%), reduced water loss, thus slowing down the decline of firmness (9.20%) of fresh-cut cucumber during storage. The degradation of chlorophyll (6.41%) was reduced to maintain the color of freshly cut cucumbers. At the same time, US-NaClO could maintain the content of aldehydes, the main aromatic substance of cucumber, and reduced the content of alcohols and ketones during storage. Combined with the electronic nose results, it could maintain the cucumber flavor at the end of the storage period and reduce the odor produced by microorganisms. Overall, US-NaClO was helpful to inhibit the growth of microorganisms during storage, improve the quality of fresh-cut cucumber.

Uncovering the diversity and contents of gene cassettes in class 1 integrons from the endophytes of raw vegetables

Rapid spread of antibiotic resistance genes (ARGs) in pathogens is threatening human health. Integrons allow bacteria to integrate and express foreign genes, facilitating horizontal transfer of ARGs in environments. Consumption of raw vegetables represents a pathway for human exposure to environmental ARGs. However, few studies have focused on integron-associated ARGs in the endophytes of raw vegetables. Here, based on the approach of qPCR and clone library, we quantified the abundance of integrase genes and analyzed the diversity and contents of resistance gene cassettes in class 1 integrons from the endophytes of six common raw vegetables. The results revealed that integrase genes for class 1 integron were most prevalent compared with class 2 and class 3 integron integrase genes (1-2 order magnitude, P < 0.05). The cucumber endophytes harbored a higher absolute abundance of integrase genes than other vegetables, while the highest bacterial abundance was detected in cabbage and cucumber endophytes. Thirty-two unique resistance gene cassettes were detected, the majority of which were associated with the genes encoding resistance to beta-lactam and aminoglycoside. Antibiotic resistance gene cassettes accounted for 52.5 % of the functionally annotated gene cassettes, and bla_TEM-157 and aadA2 were the most frequently detected resistance cassettes. Additionally, carrot endophytes harbored the highest proportion of antibiotic resistance gene cassettes in the class 1 integrons. Collectively, these results provide an in-depth view of acquired resistance genes by integrons in the raw vegetable endophytes and highlight the potential health risk of the transmission of ARGs via the food chain.

Agricultural practices in Brazilian organic farms and microbiological characteristics of samples collected along the production chain

AIMS

To gather data on agricultural practices in organic farms in Sao Paulo, Brazil, and evaluate their relationship with the microbiological characteristics of samples collected along the production chain.

METHODS AND RESULTS

Practices data were based on field observations and interviews with farmers in 10 selected organic lettuce producing farms. Counts of Enterobacteriaceae and surveys for Salmonella were performed in samples of lettuce (before and after washing), fertilizers, irrigation and washing water, all collected in the same farm. Water samples were also tested for total coliforms and generic Escherichia coli. Isolated Enterobacteriaceae were identified by MALDI-TOF MS. Contamination of lettuce was influenced by some agricultural practices: chicken manure-based fertilization resulted in higher Enterobacteriaceae counts in lettuce when compared to other types of manure, whereas pre-washed lettuces presented lower microbial counts than non-pre-washed samples. Salmonella was detected in one lettuce sample by qPCR. Escherichia coli was detected in all irrigation water samples. All sample types contained Enterobacteriaceae species commonly reported as opportunistic human pathogens.

CONCLUSIONS

The data highlight the need for improvement in the good agricultural practices in the studied farms.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides information on agricultural practices and microbiological characteristics of organic lettuce, contributing to the development of more accurate risk assessments.

Kitchen-Scale Treatments for Reduction of Listeria monocytogenes in Prepared Produce

ABSTRACT

Listeriosis, a foodborne illness caused by Listeria monocytogenes, has relatively low incidence, but a substantial mortality rate, particularly in immunocompromised populations. Because of the known risk of L. monocytogenes and other pathogens in produce, immunocompromised individuals are often placed on neutropenic diets that exclude fresh produce. Therefore, this study aimed to evaluate several kitchen-scale treatments as potential interventions to reduce L. monocytogenes in prepared produce. Cucumbers, apples, and celery were dip inoculated with a three-strain cocktail of L. monocytogenes and dried for 24 h. Inoculated products were subjected to the following treatments as applicable: commercial sanitizer soak (90 s, with agitation), tap water rinse (15 s), tap water soak (90 s, with agitation), surface blanching (25 s), tap water rinse (15 s) followed by peeling, and surface blanching (25 s) followed by peeling. In addition, inoculum uptake in celery and the impact of two types of peelers (mechanical crank and manual) were assessed. Treated samples were plated on differential media and incubated for 48 h at 37°C. L. monocytogenes populations were then enumerated and compared with the untreated control (in log CFUs per gram). All treatments lacked efficacy for celery, with reductions significantly less (P < 0.05) than in other products, likely because of inoculum internalization. The sanitizer soak, tap water rinse, and tap water soak did not differ in efficacy (P > 0.05), which was low for cucumbers (<1.5 log CFU/g), apples (<1.3 log CFU/g), and celery (<0.7 log CFU/g). The two types of apple peelers did not differ in efficacy (P > 0.05). Surface blanching and surface blanching followed by peeling were the most effective treatments for both cucumbers and apples (P < 0.05), with average reductions of 4.2 to 5.1 and 3.5 to 5.9 log CFU/g, respectively.

HIGHLIGHTS

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.

Treated Wastewater Irrigation—A Review

As the most important resource for life, water has been a central issue on the international agenda for several decades. Yet, the world’s supply of clean freshwater is steadily decreasing due to extensive agricultural demand for irrigated lands. Therefore, water resources should be used with greater efficiency, and the use of non-traditional water resources, such as Treated Wastewater (TW), should be increased. Reusing TW could be an alternative option to increase water resources. Thus, many countries have decided to turn wastewater into an irrigation resource to help meet urban demand and address water shortages. However, because of the nature of that water, there are potential problems associated with its use in irrigation. Some of the major concerns are health hazards, salinity build-up, and toxicity hazards. The objectives of this comprehensive literature review are to illuminate the importance of using TW in irrigation as an alternative freshwater source and to assess the effects of its use on soil fertility and other soil properties, plants, and public health. The literature review reveals that TW reuse has become part of the extension program for boosting water resource utilization. However, the uncontrolled application of such waters has many unfavorable effects on both soils and plants, especially in the long-term. To reduce these unfavorable effects when using TW in irrigation, proper guidelines for wastewater reuse and management should be followed to limit negative effects significantly.

A Closer Look at Changes in High-Risk Food-Handling Behaviors and Perceptions of Primary Food Handlers at Home in South Korea across Time

Food-handling behaviors and risk perceptions among primary food handlers were investigated by consumer surveys from different subjects in 2010 (N = 609; 1st survey will be called here “Year 2010”) and 2019 (N = 605; 2nd survey will be called here “Year 2019”). Year 2010 was characterized by consumers’ risk perception-behavior gap (i.e., consumers knew safe methods for food-handling, but responses regarding the behaviors did not support their confidence in food safety): they (1) did not wash/trim foods before storage, (2) thawed frozen foods at room temperature, and (3) exposed leftovers to danger zone temperatures. These trends were not improved and the gaps in Year 2010 remained in Year 2019. Year 2010 was also characterized by other common high-risk behaviors improved during 8 years for the following aspects: (1) 70.0% of consumers divided a large portion of food into smaller pieces for storage, but few consumers (12.5%) labeled divided foods with relevant information, and (2) they excessively reused kitchen utensils. Whereas in Year 2019, more consumers (25.7%) labeled food and usage periods for kitchen utensils were shortened. Consumers usually conformed to food safety rules in both Year 2010 and 2019: (1) separate storage of foods, (2) storage of foods in the proper places/periods, (3) washing fruits/vegetables before eating, (4) washing hands after handling potentially hazardous foods, and (5) cooking foods and reheating leftovers to eat. Our findings provided resources for understanding consumers’ high-risk behaviors/perceptions at home, highlighting the importance of behavioral control.

Challenges and Prospects of Using Treated Wastewater to Manage Water Scarcity Crises in the Gulf Cooperation Council (GCC) Countries

The Gulf Cooperation Council (GCC) countries are located in the driest part of the world with an annual per capita water availability of 500 m3 compared to the world average of 6000 m3. Agricultural water demand, which is more than 80% of the total water consumption, is primarily met through the massive exploitation of groundwater. The enormous imbalance between groundwater discharge (27.8 billion m3) and recharge (5.3 billion m3) is causing the excessive lowering of groundwater levels. Therefore, GCC countries are investing heavily in the production of nonconventional water resources such as desalination of seawater and treated wastewater. Currently, 439 desalination plants are annually producing 5.75 billion m3 of desalinated water in the GCC countries. The annual wastewater collection is about 4.0 billion m3, of which 73% is treated with the help of 300 wastewater treatment plants. Despite extreme water poverty, only 39% of the treated wastewater is reused, and the remaining is discharged into the sea. The treated wastewater (TWW) is used for the landscape, forestry, and construction industries. However, its reuse to irrigate food and forage crops is restricted due to health, social, religious, and environmental concerns. Substantial research evidence exists that treated wastewater can safely be used to grow food and forage crops under the agroclimatic conditions of the GCC countries by adopting appropriate management measures. Therefore, GCC countries should work on increasing the use of TWW in the agriculture sector. Increased use of TWW in agriculture can significantly reduce the pressure on freshwater resources. For this purpose, a comprehensive awareness campaign needs to be initiated to address the social and religious concerns of farming communities and consumers. Several internal and external risks can jeopardize the sustainable use of treated wastewater in the GCC countries. These include climate change, increasing costs, technological and market-driven changes, and regional security issues. Therefore, effective response mechanisms should be developed to mitigate future risks and threats. For this purpose, an integrated approach involving all concerned local and regional stakeholders needs to be adopted.

Health risks of heavy metal exposure and microbial contamination through consumption of vegetables irrigated with treated wastewater at Dubai, UAE

The shortage of fresh water is a major problem throughout the world, but the situation is worst in the arid and semiarid regions. Therefore, reuse of nonconventional water resources such as treated wastewater (TWW) is a common practice to irrigate field crops, vegetables, and forestry sectors. The present study was conducted to evaluate the significant impact of different heavy metals such as copper (Cu), iron (Fe), chromium (Cr), and zinc (Zn) on the soil and leafy, root, and fruit vegetables following irrigation with TWW through subsurface drip irrigation. Our results indicate that iron (Fe) was highest in lettuce followed by spinach, and Zn and Cr were second and third most abundant element in the different vegetables. Eggplant and radish showed the lowest concentrations of various heavy metals. A significant difference was observed in transfer factor (TF) among vegetables, and highest TF_soil-veg was observed for Fe in lettuce and the lowest for Cr in eggplant. Estimated daily intake (EDI) was the lowest in adults and highest in children. Target hazard quotient (THQ) of Cu, Zn, and Fe being < 1.0 appears relatively safe in all the tested vegetables. Risk index (RI) values showed that heavy metals were lower than 1.0 and hence lower risk for human. The combined HI values for Cu, Zn, Cd, Cr, and Pb were substaintionaly higher 12.8 and 9.21 after consumption of lettuce and carrot. So, consumption of these vegetables should be avoided after irrigation with TWW. Spinach exhibited maximum total coliform loading, while ecological risk was negligible due to sandy nature of soil type. Health risks to human could be reduced through proper selection of suitable vegetables, time of maturity, and consumed organs (leaf, fruit, or root part). Appropriate should be followed to decontaminate the microbial load in order to avoid any risks to human health (both adults and children).

Prevalence of Antibiotic Resistome in Ready-to-Eat Salad

Ready-to-eat salad harbors microorganisms that may carry various antibiotic resistance genes (ARGs). However, few studies have focused on the prevalence of ARGs on salad, thus underestimating the risk of ARGs transferring from salad to consumers. In this small-scale study, high-throughput quantitative PCR was used to explore the presence, prevalence and abundance of ARGs associated with serving salad sourced from two restaurant types, fast-food chain and independent casual dining. A total of 156 unique ARGs and nine mobile genetic elements (MGEs) were detected on the salad items assessed. The abundance of ARGs and MGEs were significantly higher in independent casual dining than fast-food chain restaurants. Absolute copies of ARGs in salad were 1.34 × 10⁷ to 2.71 × 10⁸ and 1.90 × 10⁸ to 4.87 × 10⁸ copies per g salad in fast-food and casual dining restaurants, respectively. Proteobacteria, Bacteroidetes, Actinobacteria, and Firmicutes were the dominant bacterial phyla detected from salad samples. Pseudomonas, Acinetobacter, Exiguobacterium, Weissella, Enterobacter, Leuconostoc, Pantoea, Serratia, Erwinia, and Ewingella were the 10 most dominant bacterial genera found in salad samples. A significant positive correlation between ARGs and MGEs was detected. These results integrate knowledge about the ARGs in ready-to-eat salad and highlight the potential impact of ARGs transfer to consumers.

Disinfection of Ready-to-Eat Lettuce Using Polyhexamethylene Guanidine Hydrochloride

As a novel and safe sanitizer, polyhexamethylene guanidine hydrochloride (PHMG) has been used to inhibit the spoilage of agricultural products caused by fungi. However, little is known about its antibacterial effects on vegetables. In this study, we evaluated the disinfection efficacy of PHMG on ready-to-eat lettuce. PHMG (150-200 mg/L) treatment for 5 min was optimal for lettuce disinfection. Compared to several household sanitizers (vinegar: 1% acetic acid; kettle descaler: 1% citric acid; "84" disinfectant: 200 mg/L sodium hypochlorite), PHMG showed the greatest reductions in Escherichia coli O157:H7, Listeria monocytogenes, aerobic mesophilic counts, aerobic psychrotrophic counts and molds and yeasts. Quality analysis of color (as determined by L*, a* and b*) and determination of electrolyte leakage indicated that PHMG did not cause any additional quality loss as compared to other household sanitizers. These results provide a reference for the application of PHMG as a vegetable sanitizer at the ready-to-eat stage.

Combination of polyhexamethylene guanidine hydrochloride and potassium peroxymonosulfate to disinfect ready-to-eat lettuce

There is increasing demand for improved fresh produce disinfection technology during the ready-to-eat stage, especially in low-income developing countries. We previously reported that polyhexamethylene guanidine hydrochloride (PHMG) is an effective sanitizer using fresh-cut lettuce as a model. As a low-cost alternative, in the present study, we examined the disinfection efficacy of combining PHMG with the oxidizing sanitizer potassium peroxymonosulfate (PMS). PHMG (150 mg L⁻¹) reduced the counts of Escherichia coli O157:H7, non-O157 E. coli, Listeria monocytogenes, Salmonella typhimurium, and naturally present microbes on ready-to-eat lettuce. The disinfection efficacy of PMS was significantly lower than that of PHMG; however, the efficacy of their combination (100 mg L⁻¹ PHMG + 50 mg L⁻¹ PMS, 50 mg L⁻¹ PHMG + 100 mg L⁻¹ PMS, and 50 mg L⁻¹ PHMG + 50 mg L⁻¹ PMS) was equivalent to that of PHMG alone. Color and sensory analyses (crispness, color, flavour, and off-odor) indicated that the combination of PHMG and PMS will not lead to additional quality loss when compared with tap water treatment, and electrolyte leakage analysis showed no additional lettuce surface damage of the combination when compared with PHMG-only treatment. These results show that partial replacement of PHMG by PMS is a cost-reducing strategy, providing a theoretical foundation for its practical application.

Combination of polyhexamethylene guanidine hydrochloride and potassium peroxymonosulfate can achieve consistent disinfection effects as those obtained with polyhexamethylene guanidine hydrochloride but at a lower cost.