Should I run cold water over lettuce before I eat it?

Q. How many bacteria are actually displaced by this activity? Or am I just imagining that I am making my food safe to eat?

I’ll be honest with you, I rarely give my salad leaves a rinse in tap water before eating it. Most of the lettuce we buy in the shops today has already been washed before it reaches the shelves anyway, making us think that it is ready to eat. However, ‘ready washed’ doesn’t always mean ‘clean’…

Aquaponic LettuceIn her book Swallow This: Serving Up The Food Industry’s Darkest Secrets, Joanna Blythman brings to light the unpalatable truth that ‘ready washed’ salad leaves are likely to have been washed in chlorine-containing water – that could be up to 8 hours old. Chlorine is effective at killing some (but definitely not all) bacteria, but when it’s 8 hours old, the chlorinated water’s bug-killing abilities could be seriously reduced.

It gets worse though. Bacteria thrive in wet, humid conditions and if you’ve ever eaten pre-packaged salad then you’ll know just how damp it can get inside those bags (especially if you don’t eat them right away). Even at 4oC – the temperature that your fridge should be set to – bacteria can survive and flourish. Just dig out that forgotten bag of greens tucked away at the back of your fridge and you’ll see (and smell) what I mean.

But does this mean that pre-washed lettuce is dangerous? Well, not really.

As long as you wash all of your lettuce (including the kind that helpfully tells you that it’s already been washed), you shouldn’t have a problem. Research shows that washing lettuce with tap water removes more than 90% of bacteria – providing you wash it thoroughly, that is.  According to a study by Professor Martin Adams of the University of Surrey, ‘thoroughly’ means plonking the chopped up lettuce into some tap water and swishing it about for 5 minutes. This might sound a little extreme, but it seems to do the job.  At least for most bacteria.

There will always be some bacteria that aren’t removed by washing. Some bacteria can cling onto the surface of the lettuce incredibly well – especially if they are living in a community of bacteria called a biofilm. Some bacteria might also be hiding deep inside the lettuce and so removing these bacteria by washing it in cold tap water is pretty much impossible.

Salmonella Bacteria

Fret not though, because munching down on some micro-organisms may not be all that bad. The well-established ‘Hygiene Hypothesis’ states that if we’re not exposed to enough bacteria and infectious diseases, especially in early life, then we’re more likely to develop allergies. So rather than aiming to eat perfectly sterile food, it’s more important to make sure that food hasn’t been contaminated with bacteria that could do us serious harm – like some types of E. coli and Salmonella. And sadly, the Food Standards Agency say that washing is not very good at removing pathogenic bacteria from fresh produce. So however well you wash your lettuce, there’s no absolute guarantee that you’re making it safe to eat.

Mercifully, outbreaks of illness from eating lettuce and other fresh produce are pretty rare, accounting for less than 1 in 10 of all food poisoning cases in the UK. Half of all food poisoning cases are traced back to poultry and food producers nowadays do a fairly good job of keeping their crops clean of the nastier kinds of bacteria. Your chance of dying from food poisoning by Salmonella is 1 in 5.1 million per year in the UK. You’re far more likely to die by falling off a ladder (1 in 1.2 million) or drowning in the bath (1 in 2.7 million).

So does washing lettuce make a difference? Yes it does. Is it perfect at cleaning off all the nasties? Not really. But should you get worried about it? Probably not.

What you should be worrying about is whether you’re really willing to risk your life by choosing a bath over a shower tonight. Or eating that chicken for dinner. Maybe you should just play it safe and shower with your lettuce instead.

Photo Credits: Megan and NIAID via Flickr Creative Commons

Article by Kate Timms

January 28, 2016

Kate Timms

Kate is a PhD student who previously studied Biomedical Sciences (because she couldn’t decide what she wanted to specialise in) and Maternal and Fetal Health (because eventually she did decide). When not working in a science lab at the University of Manchester until an unseemly hour, she can usually be found watching women’s football (usually also at an unseemly hour). She also has a peculiar habit of trying to make other people watch also her favourite sport. Seriously, have you ever watched a game of women’s football?


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