Single infusion mashes work well when brewing with fully-modified malts. However, there are times when a step mash is more appropriate. In a step mash, the mash is initially rested at a temperature below the usual saccharification range, then raised through one or more rests at progressively higher temperatures. To raise the temperature, the mash may be directly heated, infused with hot water, or decoctions may be pulled, heated, and returned to the main mash. (Additionally, in a cereal mash, a mash that was initiated separately from the main mash may be stirred in to raise the overall temperature of the combined mash.)
Performing a step mash is beneficial when using undermodified malt or home malted grains. In home malted grains — for which the degree of modification is likely to be uneven, compared to commercial standards — a decoction mash is likely your best bet.
There are historically relevant step mashes, such as the “standard” triple decoction mash, in which a specific set of rests is called for. On the other hand, any brewer can come up with his or her own step mash by choosing to rest or not at various temperatures. Here’s a quick rundown on the common steps found in a step mash, with some final thoughts on the overall mash program.
Doughing In/Hydration Rest (Ambient Temperature)
Some historical decoction mashes call for mashing in with unheated water for a short (roughly 15 minute) rest. Sometimes called a hydration rest, the idea is give the mash solids some extra time to dissolve completely. Of course, solids dissolve into liquids more slowly at cooler temperatures and this is far below the gelatinization range of barley starch. Most homebrewers see this rest — correctly, in my opinion — as a waste of time. Stirring well at your initial rest temperature and letting the mash stand for an adequate amount of time overall should obviate any practical need for this rest. If you’re trying to mimic a historical triple decoction mash, then go right ahead. (Note that the term “doughing in” simply means to mix brewing liquor with malt — it is not associated solely with this rest.)
Acid Rest/Phytase Rest (95 °F/35 °C)
Acids rests are most closely associated with triple decoction mashes. The mash is rested, frequently for as long as 2 hours, at around 95 °F (35 °C). During this time, the enzyme phytase catalyses a reaction that slowly lowers the mash pH. The enzyme itself is active from around 86 °F (30 °C) to perhaps over 140 °F (60 °C). Sources differ on that last point, and sometimes an acid rest might be conducted at temperatures up to 113 °F (45 °C).
The key word in the previous paragraph is “slowly.” If you want any significant drop in your mash pH, you’re going to have to wait an hour or two. And, unless you are using undermodified or very lightly kilned base malt, the enzyme itself may not be present in sufficient quantities to bring about any change in pH. In more highly-kilned malts, the enzyme is destroyed by the heat during kilning. From a practical standpoint, you are far better off adjusting your water chemistry with the appropriate minerals, or adding acid, or adding some sauermalz (sour malt) or acidulated malt to your grist, or stirring in a small sour mash to adjust your mash pH.
Protein Rest/Beta Glucanase Rest (113-122 °F/45–50 °C)
Rests in this range degrade both proteins and glucans (gums). Proteins are acted on by proteinases, that cut large proteins into smaller peptide strands. In turn, peptides are degraded by peptidases that reduce peptide to their constituent amino acids. Beta-glucan is a polysaccharide that can gum up your mash. A rest in this range allows the enzyme beta glucanase to degrade the glucan and help with lautering. These enzymes all have slightly different optimal ranges — the optimal temperature for a beta glucanase rest is sometimes given as 95–113 °F (35–45 °C), a peptidase rest is given as 113–128 °F (35–53 °C), and a proteinase rest is given as 122–138 °F (50–59 °C). However, these enzymes are all active over a broad temperature range and any rest in the vicinity of the temperature range given above will yield high levels of activity from all three. (Recall that enzymes are not turned on and off when they enter or exit their optimal temperature range; they are simply most active in that range. They may still retain significant levels of activity outside of the range.)
Generally, a 15–20 minute rest will give the correct amount of activity in an undermodified malt. In a fully-modifed malt, the level of proteins and glucans should be acceptable after a single infusion mash. In fact, a rest in this range can reduce protein levels to the point that head retention is affected. Some malts, such as wheat and rye malt, have higher levels of glucans (and also protiens), and may cause problems with lautering. In this case, glucan rest may be beneficial, although generally just lautering slowly enough will usually do the trick. (The risks of a stuck mash from wheat or rye have been overstated by some sources.) These days, if a brewery is resting in this range, it is most likely to degrade glucans.
In the beta glucanase rest range, phytase is still active. However, given the short time frame, its activity (if it is even present) won’t amount to much. In this range, the amylase enzyme are also active. However, due to the temperature, their level of their activity is low compared to what it will be later in the step mash. In addition, as this rest occurs below the gelatinization temperature of starch, the enzymes can only work on starch molecules on the outside of their granules. Still, a long (two hour) mash at 131 °F (55 °C) can yield over 90% of the potential extract of the grain. So, the amylase enzymes are working at these temperatures, just more slowly (and only for 15–20 minutes.
Starch does not need to be completely gelatinized for significant amylase enzyme activity to occur. (In living barley plants, gelatinization temperature is never achieved, but the amylase enzymes still work — albeit orders of magnitude slower than they do in the mash. If a seed converted all it’s starch to sugar in 60 minutes, it would die.) However, above gelatinization temperature, the increased activity of the enzymes and increased accessibility of the starch substrate make starch conversion go much faster.
With fully-modified malts, all of these rests can be skipped. The biggest exception would be if your malts were gummy (for example, if you were using a lot of rye malt). When using undermodified malt, a rest in the 113–122 °F (45–50 °C) range is recommended. This should bring the level of gums down to reasonable range. With undermodified malt, you could try an acid rest, but there are quicker ways to achieve the proper mash pH.
Tomorrow, I will discuss the remaining two rests and give some thoughts on putting together a step mash for a few common reasons.