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.
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.
Dear Mr. Wizard,
It has been a long accepted fact (myth?) in homebrewing that raising the grain bed above 168 ºF (76 ºC) for a mash-out denatures the enzymes and helps “fix” a beer’s fermentable profile. However, I’m a frequent visitor to one of the many beer forums out there and this idea was bashed. Can you please shed some light on this? It has always been my understanding that the enzymes are heat labile over a period of time. For example, take a typical single infusion mash held at 154 ºF (68 ºC) for 60 minutes, then a mash-out for 15 minutes at 170 ºF (77 ºC). What is happening to my enzymes, have they been denatured at the mash out?
Mr. Wizard replies:
Hey Joe . . . I heard you cooked your old alpha amylase down . . . how are you going to mash now? I guess if you believe the threads about mashing-off on some of the homebrew forums you’re going to keep on mashing because enzymes are actually not heat labile. I read some of the debate on this and see this as a classic case of error propagation.
There is absolutely no question that enzymes are heat labile and once denatured they permanently lose their catalytic activity. This biochemical fact is demonstrated when fruits and vegetables are blanched and when meat is grilled. Denaturation also happens at breweries around the world during mashing.
Some enzymes found in malt are extremely heat sensitive and never have a chance of surviving mashing if they make it out of kilning. These include lipoxygenase, phytase, beta-glucanse and a wide range of proteolytic enzymes. Most mashes begin no cooler than 140 ºF (60 ºF) and the listed enzymes have no activity in the mash because they are almost immediately denatured during mash-in.
The two primary enzymes of interest in mashing are beta and alpha amylase because they convert starch to fermentable sugars. In a laboratory it can be easily demonstrated that these enzymes retain activity for some time period when held above their denaturation temperature. Enzymatic re-actions are typically measured by monitoring the concentration of product over time and the change in product concentration over time indicates enzymatic rate. Usually these analyses are run at a fixed temperature to make the test conditions reproducible.
The rate of enzymatic reactions is highest when the substrate concentration is high, the product concentration is low and the temperature is at the optimum for the enzyme (there are many other conditions, but these are the most pertinent here). If malt and water are mixed together at 158 ºF (70 ºC) several things begin to happen. Starch begins to gelatinize, beta amylase begins to denature and alpha amylase begins to cleave amylose and amylopectin in smaller molecules. This temperature is well above the optimum temperature for beta amylase, but that fact does not stop beta amylase activity and maltose production is seen. Over time, the population of beta amylase enzymes denatures and those molecules that are active keep working until they denature. The time period depends on environmental conditions and enzyme concentration.
The same thing happens with alpha-amylase during mash-off. The temperature increases, the last bits of ungelatinized starch gelatinize and alpha-amylase activity continues until the population of alpha amylase enzymes has been completely denatured. Everything has a time component and these reactions are not able to occur instantaneously.
So it is logical to conclude from such laboratory experiments (and the data is out there demonstrating these phenomena) that mashing at temperatures above the denaturation point of a particular enzyme does not instantly stop enzymatic activity. To jump to the next step and assume that mashing-off does not stop enzymatic is faulty logic. It is also out of context because mash-off usually occurs long after significant changes in the carbohydrate profile of wort continue. As I mentioned earlier, enzymatic rates are highest when substrate concentration is highest and this happens in the beginning of the mash. By the time mash-off rolls around changes have slowed down considerably. What does happen during mash-off is that the wort viscosity is reduced, some ungelatinized starch is freed up and alpha-amylase activity drops off, usually after the last bits of starch are converted.