The Chinese learned to process tea in a different way in the mid-13th century. Tea leaves were roasted and then crumbled rather than steamed. By the Yuan and Ming dynasties, unfermented tea leaves were first pan-fried, then rolled and dried. This stops the oxidation process which turns the leaves dark and allows tea to remain green. In the 15th century, oolong tea, where the tea leaves were allowed to partially ferment before pan-frying, was developed. Western taste, however, preferred the fully oxidized black tea, and the leaves were allowed to ferment further. Yellow tea was an accidental discovery in the production of green tea during the Ming dynasty, when apparently sloppy practices allowed the leaves to turn yellow, which yielded a different flavour as a result.