Troubleshooting The “Reverse Step-Mash”

The old alpha and beta amylase temperature conundrum! It does indeed seem that the temperature optima for these two enzymes is reversed for the purpose of mashing. Beta amylase produces maltose by “biting” off maltose molecules from the non-reducing end of starch molecules. In the case of amylose, there is one reducing end and one non-reducing end, and in the case of amylopectin, a heavily branched molecule, there is one reducing end and multiple non-reducing ends. Amylopectin is often compared to a tree, where the trunk represents the reducing end and the tips of the branches represent the non-reducing ends. Alpha amylase randomly breaks bonds within amylose and amylopectin molecules, and in the process produces one reducing and one non-reducing end with each broken bond. The result of this activity is more sites for beta amylase to act upon, and this is the reason that brewers often lament that beta amylase activity occurs before amylase activity in a step mash.

As you suggest, this seems like an easy enough problem to solve; start in the alpha amylase range and simply cool the mash down to where beta amylase is most active. But this does not really work, and herein lies the conundrum. Enzymes are proteins with catalytic activity and are pretty resilient molecules. Change the solution pH over a pretty wide range and the surface charge of proteins change, and in the specific case of enzymes this charge change affects enzyme activity. Similarly, changing temperature results in a change in enzymatic rate. But changes in pH and temperature beyond enzyme-specific limits, cause an irreversible movement in the three dimensional structure of the protein that completely stops enzymatic activity. This structural change is known as denaturation. Fried eggs, cheese, tofu, grilled steak, and trub are all examples of food featuring denatured proteins. What this all means is that beta amylase is denatured when the mash is held at the alpha amylase optimum for any appreciable time.

Let’s take a few steps back from this discussion and consider what happens in a normal infusion mash that is held at about 149 °F (65 °C). Although this temperature is higher than the optimal temperature for beta amylase and will eventually lead to denaturation, the denaturation is not instantaneous. As temperature increases, so does the rate of denaturation. This temperature is also not the optimal temperature for alpha amylase, but alpha is active at 149 °F (65 °C) even though its optimal temperature is 158 °F (70 °C). In other words, infusion mashing is a compromise mash. And it works quite well when brewing with well-modified malts.

There is a traditional mash type that plays with the balance of the various malt enzymes, and that is the decoction mash. The classic triple decoction begins by bringing water and malt together for an initial temperature in the 104–122 °F (40–50 °C) range where beta-glucanase and proteases are active. A portion of the thick mash (that portion that settles when the mash is not mixed) is moved to a kettle and heated to a boil. Many descriptions of decoction mashing quickly skips past this step, but this step is relevant to the topic at hand, so let’s dive in a bit deeper.

When the thick mash is pumped to the kettle, much of the mashing liquid, or thin mash is left behind. This liquid contains enzymes that will hang about until the boiling mash is returned. In the meantime, the thick mash is heated up to about 158 °F (70 °C), held for about 10–15 minutes, and then heated up to a boil. The brief rest at 158 °F (70 °C) allows alpha amylase sufficient time to make a few nice whacks into the tree-like structure of amylopectin. This thins the mash out and really helps with mash pumping, and it also results in more non-reducing ends for beta amylase to act upon.

After this first boil, the mash is pumped back and mixed with the dilute, enzyme-containing, mash that was hanging about during the boil. Depending on the specifics of the brewery, the mash temperature increases to about 140 °F (60 °C) where beta amylase is most active. This process is repeated to bring the mash up to about 158 °F (70 °C) for the conversion rest, and is repeated once more to bring the mash up to about 169 °F (76 °C) for mash-off. The decoction mash method does move the system up and down in temperature, and up and down through the temperature optima of the various malt enzymes that we brewers have at our disposal. Exactly what you are asking about. A key thing with the decoction mash is that a portion of the mash, about 2⁄3 of the total, is not boiled and thus preserves enzymes.

The purpose of this answer was not to explain how to adapt principles of decoction mashing to BIAB, rather I wanted to review how decoction mashing does more than simply heat mash using traditional brewing methods. Hopefully this information is useful for you in your pursuit of mashing perfection!

Response by Ashton Lewis.