Microbial Control of Produce Processing Waters: Real-Time Total Viable Counts versus Physical-Chemical Monitoring

The safety and quality of fresh and ready-to-eat produce hinge on the effectiveness of microbial control during processing. Fresh and frozen fruits, vegetables and herbs (ffFVHs) were linked to multiple outbreaks caused by Listeria monocytogenes, Salmonella spp., enterohemorrhagic E. coli and enteric viruses, with process water being the main post-harvest vector of microbial contamination. A recent comprehensive report¹ of the European Food Safety Authority analyses the characteristics of the water used during the post-harvest handling and processing operations of ffFVHs, as well as the practices followed by European food business operators to maintain process water quality. It highlights the importance of real-time microbial monitoring, specifically through total viable counts (TVC), as a more effective method for controlling microbial risks compared to solely relying on physical-chemical parameters. This article will explore the findings of the study and argue for the advantages of real-time microbial counts over physical-chemical monitoring techniques.

 

The Role of Water in Produce Processing

Water plays a pivotal role in the handling and processing of produce – it is used in multiple stages, including washing, cooling, and transporting. However, it also acts as a transfer medium for pathogens and spoilage bacteria, which can persist and grow in it if not properly managed. Large volumes of water are needed for the processing of ffFVHs, with significant environmental implications (e.g., for fresh leafy greens – up to 11 cubic meters per ton of product, 90% of which is attributed to the washing step²). Therefore, most producers are in favour of using the same water for several hours or even days of operations, particularly in the regions with limited access to potable water, resulting in microorganisms, including pathogens, accumulating in the water and causing cross-contamination and diminishing shelf-life.

The dosing of process water with sanitizers is the most widely employed microbial control measure, for some products being critical to ensuring safety (i.e., the washing operation is a Critical Control Point, CCP) and securing shelf-life, and allowing for lower water consumption rates. Thus, key consideration is given to the effective monitoring of this measure. The EFSA commissioned study discusses various disinfectants used in water treatment during produce processing, such as chlorine, peracetic acid (PAA), and hydrogen peroxide (H₂O₂). The effectiveness of these disinfectants varies depending on water conditions, such as pH, organic matter load, and temperature. The study also underscores the importance of real-time monitoring and adjustment of disinfectant levels to maintain microbial safety and quality.

 

Physical-Chemical Parameters in Process Water Monitoring

Traditionally, process water has been monitored using physical-chemical parameters such as pH, oxidation-reduction potential (ORP), conductivity, temperature, and chemical oxygen demand (COD).

These parameters provide indirect assessments of the effectiveness of disinfection processes and, ultimately, of microbial control. For example:

• Residual disinfectants like chlorine and PAA are commonly measured to ensure they remain at effective concentrations.
• pH is crucial for maintaining the efficacy of disinfectants, as chlorine works best in slightly acidic to neutral conditions, whereas PAA is effective across a broader pH range.
• COD reflects the organic load in water, which can impact disinfectant demand and water quality.
• ORP indicates the overall oxidative capacity of the water, indirectly relating to its ability to inactivate pathogens.

While these parameters provide valuable information, they do not directly assess the microbial status of the water. Instead, they give insights into factors that could influence the efficacy of the disinfection process.

 

Total Viable Counts (TVC) in the Microbial Control of Process Waters

Total viable counts (TVC) measure the number of live bacteria in a given sample. Unlike physical-chemical parameters, TVC offers a direct assessment of microbial contamination. The study's findings revealed that TVC levels varied significantly across different sectors, such as fresh-cut, fresh-whole, and frozen produce, and were influenced by the type of disinfectant used. For instance, chlorine effectively reduced TVC, whereas PAA and H2O2 showed mixed results. The use of chlorine was particularly effective in reducing overall bacterial counts, including contamination with E. coli and Salmonella, in various scenarios. In contrast, PAA and H₂O₂ did not consistently lower microbial counts and pathogen persistence to the same degree.

The key advantage of TVC measurements is that it provides a more accurate representation of water safety and quality than physical-chemical parameters. This direct microbial assessment allows for rapid decision-making, revealing when the process is out of control and thus ensuring that any microbial contamination is detected before it can affect the safety of the product. Moreover, it reduces the risk of over- or under-chlorination, which can occur when relying on indirect physical-chemical measurements.

 

Challenges of Physical-Chemical Parameters

While physical-chemical parameters are useful for maintaining adequate disinfection practices, they come with several limitations. These measurements can be influenced by factors unrelated to microbial contamination. For example, high levels of organic matter in the water can consume disinfectants, leading to a falsely low residual disinfectant reading even though the microbial load might still be high. Similarly, ORP and conductivity can be affected by the presence of non-pathogenic substances that do not pose a risk to food safety, potentially leading to misinterpretations.

The study also pointed out that while parameters such as UV254 absorbance and turbidity can provide some insights into organic matter levels, they are not always reliable indicators of microbial contamination. In many cases, these parameters require additional testing to provide a more accurate picture of process water safety and quality.

Furthermore, physical-chemical measurements typically do not account for the presence of viable but non-culturable (VBNC) bacteria, a phenomenon where bacteria are alive but unable to grow on standard culture media. The study found that some disinfectants, such as PAA and H₂O₂, were ineffective in preventing the induction of VBNC bacteria, which could still pose a food safety risk. This underscores the importance of combining physical-chemical measurements with microbiological assessments for a comprehensive understanding of process water safety and quality.

 

The Benefits of Real-Time Monitoring Systems

One of the significant advancements in the industry is the development of online monitoring systems that can track parameters in real time. In addition to established approaches, the study discussed various promising technologies, such as chronoamperometric sensors and UV absorbance measurements, which can be integrated into smart monitoring systems. These systems allow for the continuous measurement of disinfectant levels and other critical parameters, enabling real-time adjustments to the disinfection process and therefore better control over the process.

The study also highlights the importance of dynamic modelling to predict the impact of different water management strategies on microbial loads. By analysing real-time monitoring against detailed models, food business operators (FBOs) can optimize their process water treatment processes and reduce the risk of contamination.

In addition to modelling, integrating microbiological monitoring with these online systems allows for a more robust approach to ensuring process water quality. Such an approach does not only improve microbial reduction but also reduce chemical consumption, minimize environmental impact, and enhance operational efficiency by preventing overuse of disinfectants. Rapid microbiological measurements would provide a direct assessment of microbial contamination, offering an even more precise control over the quality of the water.

 

Implications for the Industry of Real-Time TVC Monitoring

Real-time TVC results would allow for immediate feedback on the water's microbiological status, enabling prompt adjustments to disinfection protocols. This would ensure a higher level of food safety, as microbial load can fluctuate rapidly due to factors such as disinfectant levels, pH, organic load, temperature, but also the introduction of new bacteria with the input of new product. Operators could make informed decisions in real time about adjusting disinfectant levels and water management practices.

Sole reliance on indirect physical-chemical measurements, which can sometimes provide inaccurate or delayed feedback, particularly if not carried out with on-line systems and integrated into comprehensive control models. For example, while parameters like COD and ORP are useful for assessing disinfectant efficacy, they may not fully capture the microbiological risks associated with water quality, as shown by the data collected from the processing sites included in the study.

 

CytoQuant^® – a Practical Real-Time TVC Monitoring Tool

If processors of ffFVHs would greatly benefit from a real-time microbial monitoring system of process waters, particularly tu ensure the control of the washing process, a practical solution has not existed until now. If over the past decades flow cytometry has been steadily becoming the preferred method to monitor public water supplies, its adoption by the food processing industry is limited by investment and operation cost – a typical flow cytometer is an expensive piece of equipment that requires skilled technicians.

CytoQuant® is a game-changer for processors of ffFVHs. This innovative handheld device brings the power of flow cytometry to the convenience of onsite use. In just 30 seconds, CytoQuant® delivers precise total viable counts in process waters, unaffected by the presence of sanitizers or by the ability of cells to develop into colonies. By providing real-time feedback on the microbial load of process waters, CytoQuant^® allows manufacturers to properly manage water safety and quality, ensuring that disinfection processes are effectively controlled.

In addition to monitoring process waters, CytoQuant^® can also be used on production surfaces. Equipment hygiene is a vital part of overall production hygiene and verifying the cleanliness of surfaces such as cutting boards, conveyors, and packaging lines is often a time-consuming process. With CytoQuant®, sanitation managers can swab these surfaces, generate an immediate count of viable bacteria, and decide whether additional cleaning measures are required before resuming production.

 

CytoQuant^® – Implications for Water Management and Waste Reduction

Efficient water management is crucial to the sustainability of ffFVHs processing. The demand for large volumes of water for processing and sanitation often leads to high costs and environmental impacts. By enabling real-time monitoring of microbial load and water quality, CytoQuant® empowers manufacturers to optimize water use, reducing the need for excessive water turnover and minimizing the discharge of wastewater.

Moreover, CytoQuant^®’s ability to provide quick and accurate microbial data supports the implementation of effective disinfection strategies, whether through the use of chlorine, electrolyzed water, or alternative sanitizers. With better control over microbial contamination, fresh-cut produce manufacturers can enhance food safety and product quality, helping to extend the shelf life of produce and reduce the volume of unsellable products.

Through the real-time monitoring capabilities of CytoQuant^®, manufacturers can make data-driven decisions on water management and sanitation, ensuring that water quality remains within safe parameters for the entire processing cycle. This not only improves food safety but also aligns with the industry's goals of sustainability and minimizing food waste.

 

Conclusion

The findings of the EFSA tendered study underscore the importance of integrating microbial monitoring data into process water management practices for produce processing. While physical-chemical parameters remain valuable for monitoring the conditions that affect disinfection efficacy, they do not provide a complete picture of microbial safety. TVC monitoring, on the other hand, offers a direct, accurate, and timely assessment of water quality, allowing for better control over microbial risks.

As the ffFVHs industry grapples with the dual challenges of ensuring food safety and minimizing environmental impact, adopting advanced solutions like CytoQuant® offers a tangible path forward. This innovative technology enables real-time monitoring of microbial contamination in process waters, helping to optimize disinfection processes, extend shelf life, and reduce waste. By ensuring proper water quality management, CytoQuant® empowers manufacturers to achieve safer, more sustainable, and cost-effective operations in the highly competitive fresh-cut produce sector.

 

References

^[1] Gil MI R. García M, Abadias M, Sánchez G, Sampers I, van Asselt E, Tudela JA, Moreno-Razo AS, Vilas C, Martínez-López N, Vanmarcke H, Hernandez N, Andujar S, Serrano V, Sabater D, Truchado P, van de Kamer D, van der Berg JP, Safitri R, Boxman I, Tuytschaever T, Vandenbussche C, Díaz-Reolid A, Anguera M and Plaza P 2025. Microbiological hazards associated with the use of water in the post-harvest handling and processing operations of fresh and frozen fruits, vegetables and herbs (ffFVHs). EFSA supporting publication 2025: 22(1):EN-8924. 504 pp. doi:10.2903/sp.efsa.2025.EN-8924

^[2] Ölmez, H. (2017). “Environmental impacts of minimally processed refrigerated fruits and vegetables' industry,” in Minimally Processed Refrigerated Fruits and Vegetables. 2nd Edn, eds F. Yildiz and R. C. Wiley (New York, NY: Springer), 747–756. doi: 10.1007/978-1-4939-7018-6_22