The amount of fresh produce being produced across the globe is increasing, principally due to national targets promoting healthy eating and the core role that fresh fruit and vegetables play in achieving those targets.
Increasing demand is twinned with growing constraints on the use of water by manufacturers. Water scarcity has led to a renewed focus by the fresh produce industry on using water more effectively and reviewing their choice of wash water disinfectant. Reuse of water can lead to a build-up of organic material in the water, potentially resulting in higher levels of disinfection by-products (DBPs).
Zero Liquid Discharge (ZLD) is increasingly the aim of any manufacturer using water in their process. Reuse of water to achieve ZLD by manufacturers is a growing trend within the fresh produce industry which shifts emphasis onto the treatment of water both during the washing process and once the water has been used. Guidelines are now being produced to give advice to fresh produce manufacturers which may help control DBPs produced in washing fresh produce.
Testing the wash water used in the washing process is crucial to controlling the microflora that can be found in the final product. Consequently, understanding the DBP potential is crucial for interpreting the test results and making treatment decisions.
As fresh produce is generally grown outdoors, it will always contain some microflora—defined as bacteria and microscopic algae and funghi, especially those living in a particular site or habitat.
Demand for fresh produce, regardless of whether it is in season locally, leads to issues in sourcing the raw produce. The challenge is caused by the natural microbiology of the growing environment and standards of hygiene being significantly different from where the final product is finally consumed.
Hydroponics as a growth medium can circumnavigate some of the issues associated with where the product is grown, but this does not resolve the issue of standard of hygiene during harvesting.
Produce washing is one of the most common methods of reducing microbial load and no amount of washing will ever completely remove all the pathogens that may be present in the product.
The variability of log reductions is related to the type of produce being washed, the contact time and the disinfectant used. Typically, when a product or chemical is tested for effectiveness in killing germs, bacteria, viruses, etc., the term ‘log reduction’ is used.
In simple terms, log reduction provides a quantitative measurement describing what percentage of the contaminants which were present before treatment began were killed or put into a viable but nonculturable state after treatment has finished.
As an example, if we start with a microbial load of 1,000,000 cells:
- A log reduction of 3
- = 1,000,000 x 0.10 x. 0.10 x 0.10
- = 1,000 cells remain
This translates to 0.1 percent of the original load; a 99.9 percent kill rate.
If adequate disinfectant is present, all dead cells are removed via oxidation and the sanitiser should manage the remaining cells until the next scheduled purge is conducted. However, if the cells come together to form a biofilm, even with a good sanitiser level, biofilm regrowth is likely to occur quickly, particularly if the dead cells have not been removed. Chlorine dioxide is especially effective in tackling biofilms.
Historically, superchlorination of wash water was the main method of treating fresh produce and can lead to a reduction in microbial load of between 10 to 100 times as long as the contact time is sufficient and the form of chlorine present in the wash water is controlled through regular testing. Agitation and submersion of the produce during washing is an essential part of ensuring the maximum efficacy of the disinfectant.
In recent years there has been a shift to alternative forms of disinfectants due to concerns over the production of chlorination by-products when superchlorinating.
Although evidence is limited thus far, lessons learned from the drinking water industry (where testing for chlorination by-products is a legal requirement) have driven manufacturers to look at alternative disinfectants. This is especially true in the growing organic fresh produce market and in certain markets where superchlorination of fresh produce is restricted (e.g. Denmark).
Chlorine dioxide overcomes some of the disadvantages chlorine poses as it is not reliant on careful control of the pH of the wash water. As it is volatile, it is generally required to be generated on-site, but the advantages over chlorine have been clear for some time.
This does not make chlorine dioxide the perfect solution however, and with scrutiny over chlorate levels increasing, even chlorine dioxide may come under pressure in the near future.
Water Testing Considerations
On-line controllers are frequently used to monitor the level of disinfectant in wash water. Although effective at monitoring changes in disinfectant level within the wash water as they are often based on ORP (oxidation reduction potential) measurement, on-line controllers lack selectivity, meaning they cannot be solely relied upon to ensure effective disinfection is taking place.
A secondary testing method is almost always required in order to calibrate the on-line controller and provide a reserve test method in case the on-line controller malfunctions. Spot checks on on-line controller efficacy are usually carried out using a portable method such as a colorimeter.
Some of the reluctance to switching to alternative forms of disinfectant is based around difficulties associated with these secondary methods of water testing. Traditional testing methods involve using portable colorimetric methods for determining the levels of disinfectant in the wash water. However, the drawbacks of this method are known to the fresh produce industry.
These drawbacks include a lack of specificity (e.g. not being able to easily determine free chlorine as opposed to combined chlorine, specifically at superchlorination levels), the complexity of the test, and the use of glassware and chemical reagents, which are not appropriate in a food production environment.
Developments in portable testing methods such as chronoamperometric disposable sensor methods are changing the way in which portable testing is carried out in the fresh produce industry. Overcoming many of the drawbacks of colorimetric methods, the simplicity and ease of use of the sensors is the key driving force behind their adoption. They are also much more selective when multiple oxidants are present in the sample.
As the fresh produce industry grows in line with the increased globalisation of food markets, there is increased motivation for manufacturers to both consider alternative forms of disinfectant, such as chlorine dioxide, and to focus on the reuse of wash water. In doing so, the industry is adopting best practice learned from the drinking water industry.
However, it can certainly appear that as soon as a new alternative disinfectant is implemented, regulatory pressures soon follow. The dialogue between customers, regulators and manufacturers should focus on a holistic analysis of water disinfection, the need to protect water resources, consumers and, often forgotten, the need to be able to accurately measure the disinfectant residual and be sure of its efficacy.
Abandoning chlorine and, in the future possibly even chlorine dioxide based disinfectants, for alternatives that have lower efficacies and are difficult to measure accurately will only serve to expose customers to higher microbial loadings on fresh produce, as well as shorten product shelf life.
With regard to water testing, understanding the capability of the test method being used can help manage the production process. Food manufacturers, therefore, need to build closer ties with water testing equipment manufacturers in order to ensure they have the best methods of analysis for their production lines.
This is especially important when considering the potential disinfection by-products, when there are multiple oxidants present in any one sample and when evaluating new and alternative disinfectants.