What's the best way to clean your dishes?
What is the science behind washing your dishes? Which method leaves dishes cleaner – and crucially – more germ-free: washing with hot or cold water?
Richard and his wife Dian got in touch via the Trust Me I’m a Doctor website to ask us to investigate a question that was causing domestic disputes: which method leaves dishes cleaner and germ-free – washing with hot water or cool? Towel-dry or air-dry?
Dian prefers to use hot water which she believes gets rid of grease and germs efficiently. Once she has washed her dishes, she dries them with a dish cloth and puts them away.
Richard takes an entirely different approach. He uses cool water with washing up liquid, and then lets the dishes air dry. His logic? That the washing up liquid will kill the germs regardless of the water temperature, and if the dishes look clean, they probably are.
We got in touch with Dr Nicky Milner, a biomedical scientist from Anglia Ruskin University to devise an experiment to find out who is right. After a hearty lunch, Dr Milner took some samples from the dishes to see what microbes are present immediately after the meal. Then, our couple each washed their dishes using their own, mutually irreconcilable techniques.
We wanted to compare not just the efficiency of the washing up, but also of the drying process, so our couple left half their dishes to air-dry, and dried the other half with a tea towel that had been in use for a couple of days.
The results painted a revealing picture of this humblest of domestic chores. Straight after the meal, there were lots of bacteria on the dirty dishes, but these were mostly removed by the process of washing and drying.
According to Dr Milner: “It’s really about the scrubbing action. Research has shown that the temperature of the water doesn’t really make a difference.” So a good scrub is more important than the water temperature. And research has shown that using washing up liquid to kill bacteria is important.
When we analysed the different drying techniques, our test showed that the tea towel actually introduced extra bacteria to our clean dishes. So air-drying was better.
Richard and Dian remain set in their ways. But our small test suggests a solution to this aspect of domestic drudgery: don’t worry about the water temperature, use washing up liquid, scrub as hard as you can... then leave to let nature do the rest.