A bulletin for the Australian Food Industry August 2002
Contents: US Emerging Infections Program | Evaluation of Codex Alimentarius program | Pasteurisation of fruit juice | Methods for disinfection of fresh produce
Methods for disinfection of fresh produce
We have discussed previously the challenge posed to food processors by the microbial contamination of fresh produce (Food Safety & Hygiene September 2001, November 2001, May 2002). Seeds for sprouting and the sprouted seeds represent a special problem (Food Safety & Hygiene November 2000).
A concise summary of the present knowledge in the area is contained in a review (Food Technology and Biotechnology 39 2001 305-311) by USDA research worker Gerald Sapers.
He concludes that new washing technologies using sanitising agents of greater lethality are needed to contact and kill microorganisms that survive conventional washing and sanitising methods. In other words, more of the same is unlikely to produce the desired results.
Sapers also notes that new technologies must not only be superior in efficacy, they must also be approved by regulatory authorities, safe to apply, compatible with existing industry practices, and affordable.
The review indicates that conventional, time honoured methods are not capable of reducing microbial populations on produce by more than 90 to 99 percent. While such population reductions are useful and not to be discounted, they are insufficient to assure microbiological safety. It must be realized that conventional washing technology was developed primarily to remove soil from produce, not microorganisms. Even with newer sanitising agents such as chlorine dioxide, ozone and peracetic acid, improvements in efficacy have been incremental. Because of these limitations, it is preferable wherever possible to avoid microbial contamination of fruits and vegetables by following good agricultural and manufacturing practices.
Sapers reports that chlorine is the most widely used sanitising agent for fresh produce. Published data, however, indicates that at permitted concentrations, and those that are commercially practical, population reductions on produce surfaces are in the range of 90-99 percent. This is due in part to the rapid loss of efficacy of chlorine in the presence of organic matter in soil and on product surfaces. Some improvement in efficacy can be obtained by adding a wetting agent.
Alternative sanitising agents including ozone, chlorine dioxide and peracetic acid (an equilibrium mixture of hydrogen peroxide and aceticacid) have been studied. These too are limited in their ability to kill bacteria attached to produce surfaces when realistic inoculation and treatment conditions are used. It should be noted however that chlorine and these alternatives are highly effective against microorganisms suspended in water. They can therefore play on important role in preventing or reducing the risk of produce cross-contamination from flume and wash water.
Sapers believes that hydrogen peroxide, which his group has worked with extensively (Food Safety & Hygiene September 2001), shows potential as an effective sanitising agent for produce. It is not approved as an anti microbial wash in the US but it is an approved processing aid (for washing) in Australia. He also sees promise in novel means of applying sanitising agents, e.g. vacuum infiltration and vapour phase treatments (Food Safety & Hygiene November 2001).
In a paper published since Saper's review, workers at Washington State University have reported a study which compares the efficacies of chlorous acid (268 mg/L), sodium hypochlorite (200 mg/L) and lactic acid (2 percent) as sanitisers (Journal of Food Protection 65 2002 1088-1092). These workers measured the effectiveness of the sanitisers in eliminating total mesophilic microorganisms, Salmonella typhimuruim and Listeria monocytogenes on commercial mung bean sprouts. Counts were taken immediately after treatment and during post-treatment refrigerated storage.
Chlorous acid (H Cl O2) is a recently introduced sanitising agent approved by the Food and Drug Administration in the US for use on raw agricultural commodities. It is not a permitted processing aid in this country.
Treatment with the chlorous acid reduced the inoculated pathogens by 5-log (100,000 fold) but it was much less effective against the native mesophilic microflora. The reason for this is not clear. The authors suggest that the native microflora may have more resistance to the chemical agents than the pathogens cultured in the laboratory. However it is possible that the inoculated organisms were more accessible to the sanitizing agents than the native microflora (Food Technology and Biotechnology 39 2001 305-311). Certainly this report does not agree with earlier studies using chlorous acid or its salts (Journal of Food Protection 62 1999 318-324; Journal of Food Protection 63 2000 1475-1482). These differences emphasize the need to study the effectiveness of sanitisers under conditions as close as possible to industrial requirements.
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