Imagine what happens when you over-boil your spinach; the leaves become extremely soft, breaking apart easily, and the water you boil them in is dyed green with the essence of the spinach leaves.
This is exactly what happens to tea leaves when you steam them for a long time. Perhaps the most popular tea in Japan today, fukamushicha is a variant of sencha in which the leaves are steamed for 60 seconds or longer (vs 30-40 seconds for normal sencha). Of course, with younger more tender leaves, a shorter amount of time will result in a fukamushicha while 60 seconds may not be enough for larger leaves to create the deep green effect.
As a result of the long steaming, the leaves become very soft, and break apart during rolling. The dry leaf is very powdery, and when steeped, the water is dyed a deep, opaque green with the leaf particles, so you are actually drinking much more of the leaf and its components - catechin, cholorophyll, theanine, vitamins, minerals (see the Nutrition Chart for more details).
FLAVOR - Deep steaming seems to sweeten the tea leaf, though this is entirely dependent on the base leaf. For higher grade spring tea leaves, you'll find a much richer umami flavor than you would otherwise for a similar, normal sencha.
Like other sencha, steeping at a lower temperature (60C/140F) will results in a sweeter tea, while steeping at a hot temperature results in a much stronger, bitter brew.Time is the factor your should watch for as the leaf will steep more quickly than leaves that are more intact. At a higher temperature 30 seconds should be enough to create a decent strength while 45-60 seconds may be needed at a lower temperature (everyone has their own strength preference, so do experiment!)We also recommend cold-steeping with this tea. A good starting recipe is to use 10g per 500 ml of cold water, and let it steep in your refrigerator for 3-4 hours.
Sencha was invented 250 years ago in 1768 by Nagatani Soen in Kyoto. More specifically, he invented the method of steaming tea leaves instead of pan-firing them. Deep-steaming tea leaves, though, probably began in the area now known as the Makinohara Plateau, Shizuoka (modern day Shimada, Makinohara, and Kikugawa Cities), and the story is an interesting one.
During the feudal period in Japan, or the Edo Period ruled by the Tokugawa Shogunate from 1600-1868, travel in this area required employing ferrymen to cross the Oigawa River. When a bridge was built over the river by the Meiji government that took power from the Tokugawa Shogunate, the entire ferrying industry turned to farming.
However, the area itself had soil that was quite poor for farming rice and other food crops, so at the start of the new Meiji era in 1872, a 500 hectare site was set aside for farming tea. The tea leaves though grew too fast in the flat plain of Makinohara, and became thick and stiff. Deep-steaming was employed to soften the leaves creating what we know today as fukamushicha. The initial site is now the famous Makinohara tea fields, some 6000 hectares farmed by hundreds of families. Gaining popularity in the 60's, this method is now utilized all over Japan, but is most famous from its origin in Shizuoka.
Map below centered on the Ocha no Sato Tea Museum, click for Google Map