7 3 volumetric ratio cooling the solution to 30 u00b0 c under strong stifling cooling the precipitate to 0 u00b0 c and keeping the suspension under stirring at 0 u00b0 c for a time between 6 and 24 hours the suspension is then filtered the solid is washed with water and dried to a water content lower than 2 0 thereby obtaining rifaximin u03b3 the process for the transformation of the form u03b1 into the form u03b2 consists in keeping powder rifaximin u03b1 in an ambient having relative humidity higher than 50 for the time required to obtain a water content in the powder higher than 4 5 which time is usually between 12 and 48 hours the process for the transformation of the form u03b2 into the form u03b1 consists in drying powder rifaximin u03b2 under vacuum or under conditions of normal pressure optionally in the presence of a drying agent at a temperature between the room temperature and 105 u00b0 c for a time between 2 and 72 hours to obtain a water content in the powder lower than 4 5 preferably between 2 0 and 3 0 it is evident from what stated above that during preservation of the product particular care should be taken so that the ambient conditions do not affect the.