Should I put a lid on top of the mug when brewing green tea? I do it for other herbal teas to capture organic compounds with a boiling point lower than water, and to stop heat loss, but with green tea I don’t boil the water.
I also use a strainer, which is great for multiple infusions.
Here is my response:
A lid would help keep the tea warm while it brews and, after you've removed the strainer, while you are drinking it. But it depends on personal preference: I use a mug and strainer, too, but I don't use a lid because I don't mind tea that's a little on the cool side. I especially like the last half of a mug of tea to be cool enough to gulp down. (And a big, cool mug of green tea gulped down is incredibly thirst-quenching, especially after exercising or on a hot summer's day.)
If you consider the traditional way of brewing—a large teapot and small tea cups—what that setup does is keep the tea warm in the pot while it's brewing or just sitting. The closed teapot also keeps all the flavorful, aromatic, and healthy compounds inside. Drinking the tea by the small cupful ensures that every sip is nice and hot. And if you're drinking quality, authentic green tea like ours (excuse my modesty), its flavor and aroma are best enjoyed at higher temperatures.
Incidentally, at tea-judging competitions in Japan, judges brew the tea with a very low (for tea) water temperature—only 70 degrees Celcius (158 degrees Fahrenheit). They say that avoids "cooking" the tea so that they can tell its true taste and aroma. (Most tea-drinkers in Japan, however, use hotter water and would probably even be surprised to learn what low water temperatures are used by tea judges, so it's by no means common.)
I hope this helps. Please let me know if I can answer any other questions, or if you have any comments or suggestions.
By the way, if you want to try using a lid, you don't have to buy one of those teacups that comes with its own lid (although some of them are very cool looking). Instead, you can use an ordinary saucer. Remember one of Mellow Monk's Teachings: "Focus on the tea, not the accoutrements."