I recently bought a small glass teapot. I want to use it at work. On the box there's a warning that 80 degrees Celsius is maximum for this teapot.
I know that you shouldn't brew green tea in too high temperature, as it makes tea more bitter, hiding the real taste we're after. I want to prepare black tea in my teapot. How will the taste change I infuse it in 80 degrees or less?
TFD's answer is true: the most likely change is the reduction in some astringent flavors.
I would just add that people have been brewing black tea for iced tea for many years at or near room temperature (e.g., the traditional "sun tea" method), or even in the refrigerator ("cold-brewed" tea). The flavor notes which come out are different, but a lot of people still find the tea produced to be acceptable or even preferable to "hot-brewed" tea.
Granted, these processes are used mostly for tea meant to be consumed at cooler temperatures. But my point is that those who insist that black tea must be brewed as close to 100C as possible are just brewing one possible "version" of tea from those leaves. In my own experiments, I've found that certain black teas taste good when brewed at lower temperatures for a long time, though others end up "unbalanced" in some way. (Sometimes, they simply taste "weaker" for some reason, even when still brewed longer to take into account the lower temperature. Sometimes this can be fixed by adding more tea leaves per cup; other times this still creates an imbalance.)
I could cite a number of studies which show the various amounts of certain chemical components in tea and how fast they dissolve at various temperatures. But those really won't take into account the balance of individual flavor components in a particular black tea. In this case, I think it's really a matter of personal judgment to see whether your particular tea tastes fine to you brewed at a lower temperature.
Tea brewing is a time + temp equation, and different tea types brew at different rates
Some compounds in the tea (many of the astringent ones) dissolve out much slower in cooler water, and this is why many people brew tea at 80°C to 90°C. Some people like the astringent taste and brew closer to 100°C
This is probably all unrelated to the warning sticker, which is there probably to absolve the manufacturer from any legal claim should you hurt yourself with the teapot! Normal food safe glass has no problem at these temperatures?
Iced tea will always be sweeter if brewed (steeped) cold. Use the same amount of tea as you would to make the same amount of hot tea, and let it brew for over 1 hour, but not more than 2 hours. In 2 hours it will have started to draw the astringent components of the tea. Just over 1 hour cold brew is my sweet spot for the most refreshing unsweetened tea.
