Rinse of the machines: Swetha Sivakumar on dishwashers
How does this kitchen miracle work, how does it save so much water, and what are the things it cannot do? Answers, in this week’s Sound Bites.
As someone who only grudgingly bought a dishwasher (in 2008), largely ignored it for years, and then fell in love with it in 2015, I’d like to talk about this kitchen appliance this week: how it works, who invented it, and how it manages to save so much water.
A study conducted in the UK in 2007 showed that, on average, people used 49 litres of water when hand-washing a full load of dishes, compared to 13 litres used by the machine. Modern appliances have since brought that down further, to a low of 11 litres.
The dishes emerge cleaner too. Another study found that hand-washed items had bacterial counts ranging from 1 to 6,000 colony-forming units (CFUs), whereas machine-washed items had counts below 100 CFUs. (Using a towel to dry things off adds bacteria, incidentally; it is better to simply leave them to air-dry on a rack.)
And, of course, there is the convenience.
It was a woman who invented the device. Josephine Cochrane patented her dishwasher in 1886. It had wire compartments to hold plates, cups and saucers in place, and a pressurised system that pumped hot water onto these, cleaning them.
Before Cochrane, other inventors had designed devices for this purpose, but they all involved a rotating rack, with the vessels tumbling about much like clothes do in a washing machine. Cochrane wanted far less jostling. Her love of fine bone china was what led her to her invention.
Her idea of allowing the water alone to move is still the principle on which dishwashers work (though the water is now ejected from holes in a rotating arm). And because of her design, a dishwasher loaded correctly still won’t cause a crack, even in the finest china.
But why do these devices use so much less water? Well, most models recycle water incessantly, within each wash cycle.
Most dishwashers use three cleaning cycles per load. At the start of each, water is pumped into a small pool at the bottom, and heated. As it is then sprayed about to clean the dishes, it drips back into the pool, where it is filtered, reheated and reused. And so it goes, until one cycle ends and the next one begins, with a fresh batch of water.
The high pressure involved is the reason dishwashers needs special detergents. Regular soap generates lather, and lather would smother the filtering system, hinder the recycling of water, and possibly froth and foam out of the device. Foam would also take a lot more water to wash off. For these reasons, dishwashing detergents do not foam.
But, the type of water in your region can affect the cleaning. Hard water contains high levels of calcium and magnesium, which are not very soluble. These can interact with dishwasher detergent to form a layer of scum on utensils. This is why many people add a dishwasher salt, which is basically sodium chloride in crystal form. These crystals are highly reactive and work to trap the calcium and magnesium in a resin, thus turning the hard water into soft. The detergent can now do its job unhindered. And the trapped sediments are flushed, every so often, from the resin into the drain.
At the end of a wash cycle, one may also use a rinse aid, to help dishes dry better. Since water molecules have a tendency to stick to each other, forming droplets that take a long time to evaporate, dishes can emerge relatively dry but water-stained. A rinse aid is a surfactant that interrupts the normal attraction water molecules have for each other. This makes the residual water spread out as sheets, which a hot breeze can quickly dry off.
And that, in a nutshell, is how dishwashers work. What’s your favourite part, pet peeve or unanswered question? I’d love to know.
(To reach Swetha Sivakumar with questions or feedback, email upgrademyfood@gmail.com)
As someone who only grudgingly bought a dishwasher (in 2008), largely ignored it for years, and then fell in love with it in 2015, I’d like to talk about this kitchen appliance this week: how it works, who invented it, and how it manages to save so much water.
A study conducted in the UK in 2007 showed that, on average, people used 49 litres of water when hand-washing a full load of dishes, compared to 13 litres used by the machine. Modern appliances have since brought that down further, to a low of 11 litres.
The dishes emerge cleaner too. Another study found that hand-washed items had bacterial counts ranging from 1 to 6,000 colony-forming units (CFUs), whereas machine-washed items had counts below 100 CFUs. (Using a towel to dry things off adds bacteria, incidentally; it is better to simply leave them to air-dry on a rack.)
And, of course, there is the convenience.
It was a woman who invented the device. Josephine Cochrane patented her dishwasher in 1886. It had wire compartments to hold plates, cups and saucers in place, and a pressurised system that pumped hot water onto these, cleaning them.
Before Cochrane, other inventors had designed devices for this purpose, but they all involved a rotating rack, with the vessels tumbling about much like clothes do in a washing machine. Cochrane wanted far less jostling. Her love of fine bone china was what led her to her invention.
Her idea of allowing the water alone to move is still the principle on which dishwashers work (though the water is now ejected from holes in a rotating arm). And because of her design, a dishwasher loaded correctly still won’t cause a crack, even in the finest china.
But why do these devices use so much less water? Well, most models recycle water incessantly, within each wash cycle.
Most dishwashers use three cleaning cycles per load. At the start of each, water is pumped into a small pool at the bottom, and heated. As it is then sprayed about to clean the dishes, it drips back into the pool, where it is filtered, reheated and reused. And so it goes, until one cycle ends and the next one begins, with a fresh batch of water.
The high pressure involved is the reason dishwashers needs special detergents. Regular soap generates lather, and lather would smother the filtering system, hinder the recycling of water, and possibly froth and foam out of the device. Foam would also take a lot more water to wash off. For these reasons, dishwashing detergents do not foam.
But, the type of water in your region can affect the cleaning. Hard water contains high levels of calcium and magnesium, which are not very soluble. These can interact with dishwasher detergent to form a layer of scum on utensils. This is why many people add a dishwasher salt, which is basically sodium chloride in crystal form. These crystals are highly reactive and work to trap the calcium and magnesium in a resin, thus turning the hard water into soft. The detergent can now do its job unhindered. And the trapped sediments are flushed, every so often, from the resin into the drain.
At the end of a wash cycle, one may also use a rinse aid, to help dishes dry better. Since water molecules have a tendency to stick to each other, forming droplets that take a long time to evaporate, dishes can emerge relatively dry but water-stained. A rinse aid is a surfactant that interrupts the normal attraction water molecules have for each other. This makes the residual water spread out as sheets, which a hot breeze can quickly dry off.
And that, in a nutshell, is how dishwashers work. What’s your favourite part, pet peeve or unanswered question? I’d love to know.
(To reach Swetha Sivakumar with questions or feedback, email upgrademyfood@gmail.com)