Zero Liquid Discharge (ZLD) is increasingly becoming a target of manufacturers using water.  Reuse and recirculation of water is therefore increasing in the fresh produce industry and this shifts the focus from purely considering the dosing and monitoring of water during the washing process to considerations about additional dosing to water that is being recirculated.

Guidelines¹ are now being produced to give advice to fresh produce manufacturers which may help control DBPs produced in washing fresh produce. Testing is crucial to controlling the microflora that can be found in the product and understanding the potential for DBPs is crucial in understanding the meaning of those test results.

As fresh produce is generally grown outdoors it will always contain some microflora, (defined as bacteria and microscopic algae and fungi).

The 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/or standards of hygiene being significantly different from where the final product is to be consumed. Hydroponics as a growth medium can circumnavigate some of these issues doesn’t resolve the issues of uncertain standards of hygiene during harvesting.

In simple terms, log reduction describes the “order of magnitude” reduction in the microbiol population in the wash water.  (A tenfold reduction is Log1, a hundred fold reduction is log 2 etc).

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 (0.1%); a 99.9% kill rate. The table below shows CT-values for the deactivation of viruses by various disinfectants*:

*CT values from the AWWA, 1991
1 – values based on temperature of 10°C, pH range of 6 – 9 and a free chlorine residual of 0.2 – 0.5mg/L
2- values based on temperature of 10°C, pH of 8
3- values based on temperature of 10°C, pH range of 6 – 9

If adequate disinfectant is present, there will be a sufficient reduction in viable cells via oxidation and the sanitizer 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 sanitizer 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 predominant method of treating fresh produce and can lead to a reduction in microbial load by 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 increasing 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 of using chlorine to disinfect as it 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 are becoming clear.

In-line controllers are frequently used to monitor the level of disinfectant within wash water. They are often based on ORP measurement (oxidation-reduction potential) and lack selectivity, so cannot really be solely relied upon for ensuring effective disinfection is taking place. A secondary testing method is almost always required in order to calibrate the on-line controller and provide a secondary test method for if the on line controller malfunctions. Spot checks on in-line controller efficacy is usually carried out using a portable method such as a colorimeter.

Some of the reluctance in switching to alternative forms of disinfectant is due to difficulties associated with these secondary methods of water testing. Traditional testing methods involve using portable colorimetric methods but some have well known drawbacks with in the wash water

They 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 food production environment.  Sometimes high levels of dilution are used prior to the test and these can introduce low levels of organics which are then multiplied by the dilution factor and reported as if they existed originally in the wash water.

Developments in portable testing methods such as chronoamperometric disposable sensor methods are changing the way in which portable testing is carried out within 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 highly selective when multiple oxidants are present in the sample.