Dear Mr. Wizard,

We have mashed at 149 ºF (65 ºC) and a starch-iodide test returns a negative result. The resulting wort should be highly fermentable and produce a dry beer if a highly attenuative yeast strain is used. If, however, the temperature is brought to 154 ºF (68 ºC) after the starch-iodide test, will there be further enzyme activity that could decrease the wort’s fermentability? Intuitively, I think not because this would involve a restructuring of the sugars present rather than the breakdown of large starch molecules.

Bill McCormack
Brisbane, Australia

Mr. Wizard replies:

Your understanding of mashing is correct — sugars are not restructured when the temperature is increased from 117 ºF (65 ºC) to 122 ºF (68 ºC) — but I know a scenario where the wort would become less fermentable when the temperature increases. There is no odd biochemistry going on, just basic solubility.

Let’s suppose that an infusion mash at 149 ºF (65 ºC) has run its course, the iodide test is negative and we are ready to begin wort collection. Before wort collection begins we quickly heat the mash to 169 ºF (76 ºC) by adding a measure of near-boiling water. This rapid mash-off step effectively denatures amylase enzymes. This step can also extract some unconverted starch from the grain kernels and decrease wort fermentability.

Many small commercial brewers have stirred mash mixers that permit multi-temperature mashing. Adding a step or two between the conversion step and mash-off is one way to deal with starch extraction late in the mash. In some of the mash profiles I have seen in German brewing texts there is a step at 162 ºF (72 ºC) that is used to allow alpha-amylase to whack up any lately extracted starch before the enzymes are rendered inactive by higher temperatures. So . . . there’s your answer.