Here’s why: Sun tea gets warm but not hot. When you brew tea with hot water, any microorganisms in the water or on the leaves are pretty much killed. But with sun tea, you give them a nice warm bath in which they could reproduce. Usually, no big colonies form, and if they do it’s typically of benign organisms like Alcaligenes viscolactis and nobody gets hurt. Well, except for the li’l buggers once you drink ’em. (Update: While it’s not about sun tea specifically, Griffin Kelton tells a tea horror story that’s basically the same issue.)
Another reason not to make sun tea is because lots of the flavor that comes out of a tea leaf requires higher temperatures to make it into solution. Making sun tea, then, is to leave behind many of the flavors of the tea you’ve chosen.
Some like the flavor of sun tea for exactly that reason–it’s more mild than tea made the usual way. If that’s the case, make your tea using a cold-brewing process. Put the leaves in water and put it in the fridge overnight. Too cold for most microbes to flourish, but the added time allows for a similar milder tea flavor even at the lower temperature. Update: The image I’ve added shows the same tea, Dong Fang Mei Ren (aka Bai Hao Wulong, or Oriental Beauty). The cup on the left was brewed overnight in the fridge, while the cup on the right was brewed hot to ISO standards.
Now, I’ve heard that the US Centers for Disease Control have made some statement to this effect–that it’s best not to make sun tea because of the bacteria issue. However, I’ve never been able to find anything directly from the CDC about it. It’s been reported in several small-town newspapers, which gives some credence to the idea that the CDC frowns on iced tea, but I want to hear it from the horse’s mouth, so to speak.
So, if you can get me the official CDC statement/position on sun tea, I’ll give you a $15 credit in the Tea Geek store.
Rules: I have to be able to verify that what I get is really from the CDC. If I get several “correct” ones, the winner is the one I received first. Pretty much any format I can see is acceptable: link to official press release on the CDC website, scanned image of an official statement, video of a CDC official making an announcement at a press conference, whatever.