During the mashing process of brewing, activity of the amylolytic enzymes decays due to the high temperatures used to gelatinise the starch. Because the different enzymes produce different sugars, high temperatures can be exploited to modify the fermentability of resulting worts. This is especially useful when producing low alcohol beers. The expression a.exp(b.t)-c.exp(d.t) (where t is the temperature of the mash in degrees C) provides a simple but useful description of the activity of the amylases. Combining the activities of alpha-and beta-amylases results in a prediction of the resulting fermentability. A simple modification to the expression accommodates changes in mash thickness. The error of prediction is approximately 3 degrees of fermentability. The model is not appropriate for predicting the fermentability of worts produced at the lower standard mashing temperatures. It can be used without the necessity of analytical parameters so analyses that the brewer would not normally perform are not required. If increased accuracy is needed, the results of two previous mashes can be used to modify the parameters used.