Why learn how to do a cereal mash? Because when you brew American lagers or other beers requiring large amounts of adjuncts, you may need to perform a separate mash on the
adjuncts to break them down and convert the starches to fermentable sugars.
Both rice and corn are made mostly of starch and lack any enzymes which can convert the starch to fermentable sugars. What the adjuncts do is add physical stability (such as reducing chill haze) to your beer by diluting the proteins and polyphenols in the wort. Rice adds almost no flavor to your beer and corn may add a slightly sweet flavor.
How to do a Cereal Mash The "Hard Way"
If you want to do it the hard way, and for some the "fun" way, begin with whole rice or corn grits instead of flaked rice or flaked maize. A mash to gelatinize the grains usually isn't required for unmalted barley, wheat, or rye since the gelatization temperatures for these grains is within the same temperature range as the saccharification range of malted barley.
You will basically be performing a double mash, one for the cereal and the other for the rest of the grist. To gelatinize the cereal grains:
First mill your adjuncts, either rice or corn, to a very fine grist
(think grits). For rice it's probably better to use a coffee mill than a
grain mill. To the milled adjunct you will be adding about 10-20% of
the total malted barley in the recipe. The enzymes in this addition
will help break down the starches. It's best to use high-enzyme malts
such as domestic 6-row or domestic 2-row because the adjuncts will be
diluting the total enzyme concentrations of your mash. British ale
malts just don't have enough enzymes to produce acceptable American
lagers and aren't recommended.
Next, add heated brewing water at a ratio of 2-3 quarts per
pound (4-6 liters per kilogram) to reach a mash temperature of about
158° F (70° C) which represents a temperature that will mash both corn
and rice, and hold it there for about 15 minutes. If you are using
adjuncts other than corn or rice, or want to be more exact, here are the
gelatinization temperatures of the usual adjuncts you will want to use
(choose your own mash temperature):
Barley 140-150° F (60-65° C)
Wheat 136-147° F (58-64° C)
Rye 135-158° F (57-70° C)
Oats 127-138° F (53-59° C)
Corn (Maize) 143-165°F (62-74° C)
Rice 154-172° F (68-78° C)
Now, bring the mash up to a gentle boil and boil for about 20
minutes or up to 30 minutes (until the mash breaks down and coats the
back of your spoon) stirring constantly so the mash doesn't scorch.
Now the question is, how to add the cereal back to the main mash.
There are several options here. Normally when doing a double mash,
you would start your main mash and be at the protein rest when the
cereal is through. Adding the cereal and more hot water should get the
main mash up to the saccharification rest temperature. You will have to
use the decoction formulas to determine the amount of water to add or
just wing it and stop adding water when you reach your target temp. Or,
you can do both mashes in the main mash tun. First do your cereal
mash. Add cool water to cool the cereal mash to the first temperature
rest (if doing a multiple-step infusion mash or decoction) or to ambient
temperature, then add the rest of your grist and water to bring the
mash up to saccharification rest temperature. Evidently some
experimentation or adjustments on the fly will be necessary for the
first few cereal mashes you do.
If you don't think this is much fun, do what I do, buy flaked grains
that are pre-gelatinized and don't require the separate cereal mash.
References: Information for the article on cereal mashing was adapted from How to Brew by John J. Palmer and The Home Brewer's Answer Book-solutions to every problem, answers to every question by Ashton Lewis.