Compounds
Catechins
The polyphenols behind astringency. They dissolve far better in hot water, which is why a hot brew tastes brisk. The more sun the leaf gets, the more it accumulates. Roasting transforms some of them, which is why hojicha is so much softer.
In more detail
The source of green tea's astringency, and the compound usually meant when tea's antioxidant properties come up. The hotter the water, the more readily the astringent catechins extract — which is why a ten-degree change rewrites the cup. The more sun the leaf gets the more it holds, so shade growing is in effect a technique for suppressing it. Roasting transforms part of it, which is where hojicha's softness comes from.