Iced tea

I like to make iced tea during the summer months. Not that vile powdered stuff, but real tea or herbal infusions. To make it quickly–so one doesn’t have to wait for near-boiling tea to cool all the way down to an icy-cold temperature, I prefer to brew double-strength tea and pour it over ice, such that most of the ice melts, and the near-boiling tea cools, together making an appropriately strong chilled drink.

How much ice does one need? Well, to cool 1 gram of boiling water down to the freezing point, 100 calories¹ have to be extracted from it. Melting 1 gram of ice takes about 80 calories of heat. So a mixture of 56% (by weight) ice and 44% boiling tea will melt all the ice and leave the final mixture at 32°F.

How do you measure this amount of ice? Well, you could weigh it, but that’s not always convenient. Here’s a bit of mathematics to justify a simple approximation: The density of ice is approximately 92% that of liquid water. If you fill a container with ice cubes–or with any solid particles, for that matter–there is a fair amount of air space between the grains. If ice cubes were spherical, then only about 64% of the volume would be ice, and the rest air–this is known as the random close-packed fraction. Ice cubes aren’t spheres, but the fraction should be roughly the same. Which means that if you fill a container up with ice cubes, they would melt to a volume about 59% of that of the container. If you add 50% of the volume of the container of boiling water, the ice would represent about 54% of the total mass of water and ice, and mixing the two together you’d end up with a volume of liquid equal to 109% the volume of the container, at 32°F. To avoid overflow, you’d need to use slightly less ice and boiling water.

So, my iced-tea algorithm:

1. Fill a container most of the way up with ice cubes
2. Measure out as much tea as you need for the full volume of the container
3. Brew the tea using a volume of water that’s slightly less than half that of the volume of the container
4. Pour the brewed double-strength tea (through a strainer, if necessary) into the ice-filled container
5. Stir to cool the tea and melt the ice; most of the ice will melt. Since the brewed tea will have cooled off a bit while steeping, it won’t have enough heat to melt all the ice and so there will still be some ice left.

The tea leaves will absorb some of the water, and many containers hold (slightly) more than their nominal volume, so using (say) exactly 1 quart of water to make tea in a 2-quart container shouldn’t present any problems.

To brew, I’ve adopted the Cook’s Illustrated technique of mixing the tea and cold water in a saucepan, heating over medium heat to 190°F, then shutting off the heat to let steep for 3 or so more minutes: all total, the brewing should take about 15 minutes.

I’m fond of a mint infusion: for a 2 quart container, use 2 Tablespoons dried mint. I also like minted iced tea, for which I use a mixture of 4 teaspoons loose tea plus 3 teaspoons mint for a 2 quart container.

1. Thermodynamic calories, not food Calories. A food Calories, spelled with a capital C, is 1000 thermodynamic calories. [↩]