When you steep tea leaves in hot water, water-soluble materials are extracted to produce the tea. “Changes in brewing temperature affect extraction of color- and flavor-responsible chemicals exponentially,” says Segall, explaining that reaction rates generally double for every rise in temperature of 18 degrees Fahrenheit and conversely decrease by the same factor when the temperature is lowered.
So though milk won’t stop the brewing process, it can still affect the flavor if you add enough to significantly cool the tea down before it’s done steeping. In addition to the temperature factor, Segall says that the presence of other components in the brewing water, such as minerals and proteins from milk, affect the rate of extraction, and therefore the flavor. For most people this isn’t a concern, though, because they judge the strength of the brewed tea based on its color and pour in the milk after removing the bag, according to Dr. William C. Franke, associate director of the Center for Advanced Food Technology at Rutgers University. (And, he says, only about 5 percent of American tea drinkers even add milk.)
Tea experts, like David Wong of Tillerman Tea, confirm that the temperature of the water is important: Wong recommends about 180 degrees Fahrenheit. And for the best-tasting cup, you’ll want to put fresh water in your kettle before you brew.
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