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Preparing salads

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    Posted: 22 Jul 2016 at 6:06pm
Thank you for this topical and interesting question. Your question is not as simple as you might expect and I apologise if I take a little longer than usual to answer it.

We coexist with microorganisms and for most people the normal microbial flora on vegetables and fruits cause few if any problems. Most people can even tolerate high levels of certain pathogens. Evolution in people has resulted in the development of good defences against most microorganisms e.g. high acidity in the stomach, protective gut bacteria combined with an effective immune system enables most humans to tolerate quite high microbial loads in food.

Unfortunately, there are a number of pathogenic viruses and bacteria that if present in food at even low concentration can make even normal, healthy people ill, and perhaps even kill them. Many of these organisms are of faecal origin and have very low infectious doses. While some people can ingest millions of some salmonella or listeria without apparent problems, foods containing as little as 1 particular type of  salmonella or E.coli cell can cause illness or even death in a vulnerable individual. Some viruses can also cause disease when present at very low concentrations in foods e.g. 1 infectious particle.

Pathogens gain access to vegetables and fruits mainly during growing and harvesting. To ensure that vegetables and fruit in supermarkets are safe to eat, controls should be in place to reduce contamination during these stages. The major supermarkets in the UK and Ireland have contracts with processors / growers to minimise the risks of contamination in fruits and vegetables.

Washed and ready to eat salads from British and Irish supermarkets have an excellent safety record and I am not aware of any medical or governmental advice against their consumption that applies to any group within the population.

The salads sold in ready to eat form in UK and Irish supermarkets have generally been carefully and extensively washed and then sanitized using hypochlorite or chlorine.

If the vegetables have been irrigated with water containing un-treated sewerage or contaminated with manure or grown in contaminated compost then any salads containing these vegetables may contain high concentrations of pathogens. Under these circumstances normal washing and chlorine or hypochlorite treatment may not render them safe to eat. Washing and sanitising will only reduce microbial numbers by a certain extent e.g. 90%. So the lower the initial numbers, the lower the final residual number of organisms. There are big advantages to eating fruits and vegetables that have been produced by British and Irish farmers.

So what do you do with your garden salad vegetables? Grow them first of all in a way that minimises exposure to faecal contamination. There are guideline for doing this that you can access On Line. Providing that they are not contaminated with large concentrations of pathogens, normal washing as recommended by the FDA, Food Standards Agency etc. should be sufficient for most people.

What if you have an immunocompromised person to feed? I am not aware of any concerns in giving these people prepared ready to eat salads from the major supermarkets in the UK and Ireland. The retailers will have a contract with the processor that ensures safe-growing, and appropriate processing, storage and distribution. Obviously the products must be refrigerated and used within the time guidelines.

If you want to further reduce the microbial load on your vegetables you could consider using a very dilute solution of hypochlorite prepared using Milton tablets (essentiallysource of hypochlorite). The solution must be prepared in accordance with the directions supplied by Milton. The vegetables must be thoroughly clean before you immerse them in the solution and you need a minimum contact time of several minutes. Providing you use the correct concentration of hypochorite it is not essential to rise off the Milton solution. However, most people will probably want to rinse with clean mains water.

if you do use Milton remember that its effectiveness against heavily contaminated pathogens is limited. Also the Milton will destroy most of the natural flora of the vegetables, meaning that they may be more susceptible to the growth of any surviving pathogens. The natural flora would be expected to restrict their growth. So it would be desirable to refrigerate and eat without too much delay.

There are some downsides to using chlorine (or any halogen) based sanitisers. They can react with material in the environment producing "disinfection by products" including in the case of chlorine based compounds traces of chloroform. These products are not desirable in food.


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