How is instant coffee made?
You start by making coffee as you might usually, brewing it and straining away the grounds. Then you take the liquid coffee and, rather than drinking it, you dehydrate it.
But the dehydrating process is not as simple as it might seem.
One way to dehydrate coffee is by spray-drying, which involves spraying the coffee into tiny droplets less than half a millimeter across and letting them fall through hot dry air so that they hit the ground as dry material.
An alternative is freeze-drying, in which coffee is frozen, ground into powder, and then placed in a low pressure chamber and heated so that the ice sublimes away.
(Sublimation is a process whereby ice goes directly into vapor without turning into a liquid first.)
Coffee dried in this way (with less than 4 percent water) is quite stable and will last a long time without going bad.
It's usually sealed in containers under pressure with carbon dioxide or nitrogen since oxygen in air can react with chemicals in the coffee and spoil the taste over time.
Drying runs the risk of compounds that give the coffee flavor being lost along with the water, so in general instant coffee is never quite the same as fresh-brewed.
Caffeine is also usually lost, which is why instant coffee usually has only about 2/3 the caffeine of fresh coffee.
We may think of instant coffee as purely American, but the Japanese get credit for making the first attempts to produce it, in the early 1900s. During World War II, Nescafe was very popular.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to firstname.lastname@example.org or write to Dr. Knowledge c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819