Photo: marumaru / PIXTA
There are many things that farmers, processing factories, and finishing factories can do to create better flavor in a green tea product. In cultivation, a farmer can add more fertilizer (nitrogen) to the soil to enrich the tea plant with nutrients. The drawback is nitrogen run off hurting the environment, and increased numbers of insects due to a richer leaf forcing farmers to use pesticides.
Farmers can also grow cultivars that are known for better flavors, though some cultivars are less hardy than others, and a farmer needs to grow cultivars that are ready for harvest at different times to stagger harvests.
Fields that are higher in elevation (i.e. mountain-grown), due to the larger temperature difference between night and day, and due to slower leaf growth results in more delicate leaf than lower elevation fields in flat terrain.
Farmers can also increase the sweetness and umami content in a leaf as well as reduce its bitterness (caused by the antioxidant catechin) by shading the leaf. This helps the leaf retain its L-theanine amino acid (responsible for the umami flavor) since photosynthesis reduces the theanine to produce catechin.
The spring harvest is always regarded as better quality because crop yields are larger for a smaller, more delicate leaf than later harvests, so later harvests are allowed to grow larger creating a more bitter leaf. And of course, if picking by hand, the leaves a farmer can select higher quality leaves more carefully. Handpicking is usually done only for the farm's best teas, often submitted to competitions, and only in micro lots of a few kilograms since labor is very expensive in Japan.
At the processing factory, steaming a leaf for a longer time, while ruining the beautiful needle shape, does allow processors to extract a sweeter flavor and beautiful green color we call “fukamushi” or deep-steamed green tea (often a subcategory of sencha).
Finally, the finishing factory refines the leaves, removing smaller bits and stems. A finishing factory that is also a wholesaler may also blend leaves from different production lots together to create a unique flavor profile, for example by blending an unshaded leaf with a small amount of shaded leaf to increase the umami flavor.
They may also green-roast the leaf to impart a slightly sweet or slightly toasty flavor. This green-roasting also removes more moisture from the leaf allowing it to retain quality over a much longer period of time than unrefined tea leaves (aracha).