Japanese green tea easily change its nature if you don’t keep it correctly. Japanese green tea is produced by steaming and not fermentation. It easily oxidises. While Oolong tea and black tea are produced by half-fermentation or full fermentation, they keep more easily. The changing factors are oxygen , humidity, light, temperature, and smell. Here is the best tip to keep your Japanese tea correctly.
Keep your tea in an air tight container. Put the container in a cold( 5℃-10℃）, dark place.
Preferably put the container in a zipped plastic bag in fridge. This is one of the good ideas but when you use the tea, don’t open the container immediately. Try to open when it reaches room temperature to avoid the liquid from condensation.