black tea extracts tested in the foregoing manner generally will be found have a turbidity of about 26 ntu a portion of about 0 35 kg of a sample from a batch of dry japanese green tea leaves are moistened in a vessel to a moisture content of about 23 by weight dry tea solids by spraying them with water while agitating them after adding the water agitation is continued for enabling the moisture to equilibrate throughout the leaves a jacketed pressure vessel having a void volume of about 2 8 l is preheated to a temperature of about 90 u00b0 c and then the moist leaves are placed in the vessel the vessel then is closed to the atmosphere and heated to about 115 u00b0 c to heat the moist leaves to about 115 u00b0 c as determined by a temperature probe in the vessel while agitating the leaves by stirring with paddles which extend through the vessel while heating and agitating the moist leaves in the vessel oxygen is introduced into another vessel which has a void volume of about 2 0 l to purge the vessel of atmospheric air so that the vessel will contain substantially only oxygen and be under a pressure which is about 10 kg cm 2 greater than the pressure in the jacketed vessel containing the heated moist leaves when the moist leaves attain a temperature of about 115 u00b0 c the pressurized oxygen is directed through a water bath heated to about 115 u00b0 c for heating and moistening the oxygen the heated moist oxygen then is introduced into the vessel containing the heated moist leaves such that about 0 7 moles 0 2 kg.