Ask Mr. Wizard

Mashing Enzymes

TroubleShooting

Bill McCormack • Brisbane, Australia asks,
Q

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. 

A
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
Response by Ashton Lewis.