Winterization is a key process in the refinement of cannabis extract to produce tetrahydrocannabinol (THC) oil. The process removes higher-melting-point components by cooling in the presence of alcohol. Raw extract is heated with ethanol before freezing. The solidified elements, i.e., plant waxes, fats and chlorophyll, are then filtered out, leaving the purified oil in solution. Subsequently, the ethanol must be removed, most commonly by rotary evaporation.