If you read about malted grain, you probably keep coming across the word "diastatic." Diastatic power and its source, diastatic enzymes, play a key role in the conversion of malted grain's starch into sugar.
All barley begins with a large amount of diastatic enzymes. These are seeds, after all, and the starch inside the seeds is meant to be converted by diastatic enzymes into sugar to feed the plant as it grows.
In as much as that, our goal is the same as the barley plant's.
Unlike the barley, however, we're also interested in color and flavor. The processes of kilning and roasting malted grains produce the huge range of variety in malt that then leads to the differences in beer all the way from the palest lager to the blackest stout.
The trade off for most of those flavors and colors is a loss of diastatic power. As a basic rule of thumb, the darker a malt is, the longer and hotter it has been heated, and the more its diastatic enzymes have been destroyed.
That's why we use base malts. Base malts are kilned very lightly, preserving most of their diastatic enzymes. Including a large amount of base malts in your grain bill means the starches of your other grains, like your kilned and kilned and roasted malts, will also be converted to fermentable sugars during the mash.
This is one of the main differences between two-row and six-row barley.
While both are used frequently in North America as base malts, six-row tends to have greater diastatic power than two-row. That's why adjunct-heavy beers tend to be brewed with six-row base malts.
But what do diastatic enzymes do, exactly?
When we talk about "diastatic enzymes," we're really talking about three different enzymes: alpha-amylase, beta-amylase, and limit dextrinase.
Each one has its own job, converting different types of starch into different types of sugar. (There is a fourth, alpha-glucosidase, but it doesn't really help with the brewing process).
These enzymes need moisture and a specific temperature in order to do their work, which is why the mash needs to be held at a fixed temperature for the duration of the mashing process - too cool and the enzymes won't be kicked into gear, too hot and they'll burn away. Really each enzyme has a slightly different temperature at which it works best, but somewhere between 150 F and 155 F is the compromise that is most often used.
When you're preparing your grain bill, it's important to make sure you have enough diastatic power to convert the entire mash. If you don't, your beer will wind up being too sweet and weak.
A malt's diastatic power is usually measured using a unit called "degrees Lintner." This number can range anywhere from 0, in things like black malts and unmalted adjuncts, to 180 in some base malts. Basically, a malt needs at least 30 degrees Lintner to be able to convert all its own sugars.
By the same token, your entire grain bill should have an average of 30 degrees Lintner to ensure that the mash will result in a successful conversion.
It's very easy to figure this out. Simply multiply each malt's diastatic power (degrees Lintner) by its weight in the grain bill (pounds). Add each malt's number, then divide that number by the grain bill's total weight in pounds. If this number is over 30, you should be fine.
For example, let's take a look at a recipe:
7 lbs. pale malt, 160 degrees L
1 lb. Munich malt, 25 degrees L
0.5 lb. amber malt, 0 degrees L
First, we multiply the weight of each malt by its diastatic power.
Pale = 7 x 160 = 1120
Munich = 1 x 25 = 25
Amber = 0.5 x 0 = 0
Now we add those three numbers together.
1120 + 25 + 0 = 1145
And we divide that by the number of pounds in the grain bill
1145 / 8 = 143.125
That is way over 30, so we're in good shape! Basically if you're brewing an all-grain batch and you include a base malt, you're going to be in the clear.
Try to brew without base malt, though, and you'll be in trouble. Look at a grain bill like this one:
5 lbs. Munich malt, 25 degrees L
2 lbs. amber malt, 0 degrees L
1 lb. crystal malt, 0 degrees L
1 lb. chocolate malt, 0 degrees L
0.5 lb. black malt, 0 degrees L
Do the math, and you'll come out to 13 degrees L for the whole grain bill. This mash won't convert properly, and you'll wind up with a strange, sweet beer that's very low in alcohol.
This is a problem most brewers run into when they're brewing a partial mash. To brew a partial mash, you begin brewing as you would would with an all-grain batch, but you add extra malt extract before the boil. This gives you much more control over a wider range of flavors and colors than extract brewing, without the hassle and extra equipment of all-grain brewing.
The problem with partial mash brewing, however, is diastatic power. You can't add just any grains to a partial mash, or you run the risk of them not fermenting at all. Adding two pounds of amber malted grain in your beer may give it a beautiful color, but with a diastatic power of 0 degrees L, it'll also give your beer an overly sweet flavor you did not intend.
Even when you're partial mashing, include a base malt to make sure your beer has enough diastatic power to convert its starches to fermentable sugars.