A popular multi-rest mash schedule is the 104 – 140 – 158°F mash, using a half hour rest at each temperature, first advocated for homebrewers by George Fix. This mash schedule produces high yields and good fermentability. The time at 104°F improves the liquefaction of the mash and promotes enzyme activity. Several enzymes are at work, liquefying the mash and breaking down the starchy endosperm so the starches can dissolve. As mentioned in the previous chapter in the section on the Acid Rest, resting the mash at this temperature has been show to improve the yield, regardless of the malts used. Varying the times spent at the 140 and 158°F rests allows you to adjust the fermentable sugar profiles. For example, a 20 minute rest at 140°F, combined with a 40 minute rest at 158°F produces a sweet, heavy, dextrinous beer; while switching the times at those temperatures would produce a drier, lighter bodied, more alcoholic beer from the same grain bill.
If you use less well-modified malts, such as German Pils malt, a multi-rest mash will produce maltier tasting beers although they need a protein rest to fully realize their potential. In this case the mash schedule suggested by Fix is 122 – 140 – 158°F, again with half hour rests. The rest at 122°F takes the place of the liquefaction rest at 104°F and provides the necessary protein rest. This schedule is well suited for producing continental lager beers. These schedules are provided as guidelines. You, as the brewer, have complete control over what you can choose to do. Play with the times and temperatures and have fun.
Multi-rest mashes require you to add heat to the mash to achieve the various temperature rests. You can add the heat in a couple of ways, either by infusions or by direct heat. If you are using a kettle as a mash tun, you can heat it directly using the stove or a stand-alone hotplate. The first temperature rest is achieved by infusion as in the Single Temperature mash described above. The subsequent rest(s) are achieved by carefully adding heat from the stove and constant stirring to keep the mash from developing hotspots and scorching. The mash can be placed in a pre-warmed oven (125 – 150 °F) to keep the mash from losing heat during the rests. After the conversion, the mash is carefully poured or ladled from the mash tun into the lauter tun and lautered. The hot mash and wort is susceptible to oxidation due to hot side aeration (HSA) due to splashing at this stage, which can lead to long term flavor stability problems.
If you are using a picnic cooler for your mash tun, multi-rest mashes are a bit trickier. You need to start out with a stiff mash (e.g. .75-1 quarts per pound of grain), to leave yourself enough room in the tun for the additional water. Usually only 2 temperature rests are possible with this method because the amount of heat necessary to change the temperature of the mash increases with each addition. Reaching a third rest is possible if the change in temperature is only a few degrees. For example, raising the mash temperature for 8 lbs. of grain from 150°F to 158°F at a mash ratio of 2 quarts per pound would require approximately 2.7 quarts of boiling water.
Protein Rest
This occurs at 104 – 140 degrees. It is most active at 122-131 degrees. If you end up doing a quick protein rest for about 10-20 minutes at 131 degrees you will improve head retention and also reduce chill haze. It doesn’t work as well with American and British Malts but it really does benefit to the German and Belgium malts
b-amylase rest
This is a primary sugar-producing rest. It generates maltose, which is very fermentable. B-amylase is active at 131-150 degrees. 142-146 is the most optimal range though. Longer rest times in the lower temp ranges will get you a highly fermentable wort.
a-amylase rest
This rest gives unfermentable sugars. This gives more body and also will provide a higher finishing gravity beer. This temp range normally works best at 154-162 degrees.
Ferluic Acid Rest
This rest is one that people who make wheat beers sometimes use. It is short but its at 111-115 degrees. It develops ferulic acid. Doing this rest aids to the clove like flavors that are in wheat beers.
Acid Rest
An acid rest is used at 86 degrees to 136 degrees. Doing a rest at this point creates phytase and lowers the mash pH. Today many brewers will just add acidulated malt or different water treatments, this particular rest is really not needed.
B-glucanase rest
This rest is at 98-113 for 20 minutes. You use this rest when you are using a lot of starches in the beer. It helps break them down a bit and prevents them from getting too gummy.