Sencha tea leaves from Shizuoka are carefully refined and aged in Seikoen's natural snow-cooled warehouse (called a "yukimuro").
While the mechanism needs more research, something about this unique aging process (perhaps due to the temperature which hovers just below zero in a state in which water is neither freezing nor liquid) causes the tea to increase multiple chemical components that make up the tea leaf's umami flavor.
Specifically, an increase was measured from 632 mg/100g to 810.7 mg/100g of 15 amino acids according to their tests
Ingredients: Green tea
Region: Leaves from Shizuoka. Aged in Niigata, Japan.