July 6, 2008

Cleaning Methods for Fresh Produce

By Everis, Linda

As concerns grow over the use of chlorine washing, studies look into alternative ways to clean fresh produce to remove potentially threatening micro-organisms. Linda Everis reports All fresh produce contains a natural flora of micro-organisms that develops during growth in the field.

This microflora is made up of a range of different micro- organisms, which may include Pseudomonas and Enterobacteriaceae. Neither are in any way harmful, but they can spoil the harvested produce.

These micro-organisms are still present when the vegetables are minimally processed, and it is usual to find levels of more than one million organisms per gram on such products.

Depending on production, growing procedures and hygiene practices, however, human (disease-causing] pathogens have also occasionally been associated with produce. The most common and important pathogens include the bacteria E. coli 0157. Salmonella, Shigella and Listeria monocytogenes, the viruses hepatitis A and norovirus, along with the protozoa Cryptosporidium panum, Giardia and Cyclospora cayetanensis.

There are many stages during production of ready-to-eat fruits and vegetables at which microbiological contamination can occur. These include the growing stage, via contaminated seeds, soil and irrigation water and use of contaminated manure. Contamination can also occur at the harvest and post-harvest stage through handling, equipment and wash water. Effective washing techniques immediately prior to packaging may help reduce the level of pathogens and spoilage organisms on all forms of fresh produce.

Washing with chlorine

Until recently, chlorinated water was the most common form of wash agent used to reduce the microbial loading of fresh produce. The most common form of chlorine used is sodium hypochlorite, which reacts in water to form different types of chemical components such as sodium hydroxide (NaOH) and hypochlorousacid (HOCI).

The hypochlorous acid (free chlorine) is the active biocide. This free chlorine can bind with other compounds such as organic matter; this product is then termed total chlorine.

The amount of hypochlorous acid formed is pH-dependent and more hypochlorous acid remains in solution at pH 5.0 than at pH 7.0. If too much acid is present, this can corrode the washing tanks and it is therefore best practice to control chlorine solutions at pH 7.0, where 78 per cent of the hypochlorous acid remains in solution and achieves a good decontamination effect. The natural tendency of chlorine solution is to be alkaline and it is best practice to ensure that washing systems are maintained at pH 7.0 by the addition of citric acid.

It must be noted that monitoringtotal chlorine gives no information on the disinfecting power of a solution. In order to monitor this, free chlorine must be monitored. It is thought that most of the biocidal activity occurs within the first few minutes of washing, and that increasing concentration above a few hundred parts per million (ppm) does not increase the biocidal effect.

Washing fresh produce in chlorinated water reduces microbial levels by 10 to 100-fold. Washing studies at Campden & Chorleywood Food Research Association (CCFRA) have shown that a 10-fold (1-log) reduction in Salmonella, E. coli and Listeria levels can be achieved by washing inoculated lettuce in water containing 100ppm free chlorine for five minutes (se Fig 1).

Safety of chlorine

Over the past decade, there has been an increasing concern over the potential for formation of harmful by-products such as organochlorine when fresh produce is washed in chlorine-based wash waters.

However, the Committee on Toxicity of Chemicals in Foods, Consumer Products & the Environment has issued a statement regarding a commercial survey that investigated the occurrence of disinfectants and their by-products in prepared salads.

The study, published in 2006 by the Fresh Prepared Salads Producer Group, evaluated the presence of disinfectant by-products on fresh produce washed in chlorinated water. Produce was washed and 12 leachate [a product or solution formed by leaching] samples were analysed for the presence of a number of organochlorine compounds such as chloramine, chloroacetic acid and total trihalomethanes. These were then compared to UK and US drinking water standards. Results indicated that of all the tests done, only one sample exceeded the US standard for trichloroaceticacid.

The group also estimated intake levels of these compounds based on average and extreme salad consumption and concluded that a 150g bag of salad contains fewer chlorine compounds than a 250ml glass of tap water and is no cause for concern. Irrespective of this, chlorinated washes are not permitted for use on organic produce and are not permitted for use at all in certain countries, such as Germany and Denmark.

Alternatives to chlorine

Current popular alternatives to chlorine washing include chlorine dioxide and organic acids. Chlorine dioxide is considered a useful alternative to hypochlorite as it is not affected by pH and does not react with organic matter to the same degree as hypochlorite. It can be unstable and require on-site generation, but stabilised (liquid) forms are now commercially available.

Recent studies undertaken by CCFRA (unpublished data) have shown that a mean log reduction of 0.9 colony forming units (cfu) per gram and 1.3cfu/g could be achieved when iceberg lettuce or spinach was washed for two minutes in water containing 2-3ppm chlorine dioxide. This reduction was similar to that achieved with a two-minute wash in water containing 20ppm free chlorine.

There is also a trend to move towards the use of organic acids for the washing of fresh produce. Studies at CCFRA (Everis and Paish, 2005, CCFRA R&D Report 209) evaluated the efficacy of lactic and acetic acid for the reduction of Salmonella on inoculated iceberg lettuce. Washing in lactic acid gave a similar log reduction to that of chlorine washing, with acetic acid giving a slightly higher microbial kill (see Fig2).

There are some commercially available wash agents containing mixes of organic acids that are currently used to wash fresh produce. Some of these mixes, such as Citrox, Fresh Produce Wash (Drywite), Anti-bac and Aquaalive, are permitted for use on organic produce.

There are many other techniques that are being investigated as potential alternatives to chlorine. These include UV, ultrasound, ozone, irradiation, electrolysed water, peracetic acid, hot water, bio-control agents and natural compounds like essential oils. Sequential washing - where produce is washed a number of times with either the same wash agent or different agents - is also being studied.

Before using any of these alternative techniques, validation work must be carried out to ensure that the alternative is as effective as the existing techniques in terms of destruction of pathogens and spoilage organisms and the prevention of wash-water contamination.

CCFRA is able to advise on the way in which washing systems can be validated, and has a pilot scale wash facility that allows studies with pathogen inoculated samples to be carried out. For the past six years CCFRA has also run a Washing and Decontamination of Fresh Produce Forum that has focused on issues relating to washing of fresh produce. Each year, two newsletters reviewing current literature are produced and sent to forum members, while two seminars are organised featuring presentations on the latest techniques in produce washing. These seminars are free for forum members.

Washing vegetables: until recently, chlorinated water was the most common method of reducing microbial loading of fresh produce


Linda Everis joined Campden & Chorleywood Food Research Association in 1995 as senior technician in the Microbiological Analytical Services group, having graduated from the University of Wales Aberystwyth with a BSc in biology. She is currently a principal research officer in the Microbiology department's Preservation & Spoilage group.

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