A method for predicting late fermentation production of diacetyl in a brewing process comprises determining the ratio of FAN (free amino nitrogen content) to sugar (fermentable) to specific gravity in an initial wort composition. In addition there is provided a process for preventing incipient late fermentation production of diacetyl in a brewing process, comprising measuring and variously adjusting the initial FAN, sugar and specific gravity of the wort such that the ratio.function..function..function..function..degree. ##EQU00001## for the adjusted wort has a value of 37 or less.