The entire brewing process involves enzymes. From before barley becomes malt, until after the beer is put into packaging, enzymes are constantly changing the chemical composition of beer. Even yeast, from a purely functional perspective can be thought of simply as bags of enzymes. What I want to focus on in this series are the main mashing enzymes of interest to brewers: alpha amylase and beta amylase. Why I believe this is so important is that many brewers (homebrewers and professional brewers alike) don't really understand how they work and why this gives them the blend of fermentable sugars and unfermentable dextrins that we call wort. The concept most brewers understand is the general rule that lower mashing temperatures lead to more fermentable wort, while higher tempuratures lead to a more dextrinous (less fermentable) wort. In a broad sense this is true, but it doesn't give brewers much of a base to trouble shoot what has gone right or wrong with their mash. There are, of course, other enzymes that play a roll in mash composition (proteases, beta-glucanase, limit dextrinase, etc.), but they are much less important. I will circle back on those later.
There will be four short segments in this series, starting with starches. By the end, I hope that anyone with interest will understand more about why mashing enzymes work as beautifully as they do. After all, without them beer would not be possible.
In order to begin understanding how alpha and beta amylases work, it is imperative to first understand the substrates they work on. Just like there are two main mashing enzymes, there are also two starches in barley: amylose and amylopectin. These represent the energy reserves of the barley grain, and it is this energy that brewers ultimately need to harness to make beer. In a very basic sense, both amylose and amylopectin are just big bunches of glucose molecules linked together. Brewers want to pry apart those links of glucose molecules using enzymes, making them small enough for yeast to digest.
The simpler of the two main starches is amylose. Amylose is just a straight chain of glucose residues linked together and makes up 20-25% of the starch mass native to barley. Given enough beta amylase action (more on this later), amylose can almost completely be reduced to maltose, the main fermentable sugar in beer wort (also more on this later).
In this diagram of amylose, each of the circles represents a glucose molecule. In reality, amylose is a much longer chain of bonded glucose molecules, but this is sufficient to understand the general structure. Also, don't worry all that much about what the reducing end means just now, but take note that it is there. Later on it will become important in determining which direction enzyme action takes place.
The much more complex and larger of the two native starches is amylopectin. This is a similar molecule to amylose making up about 75-80% of native starch. What makes amylopectin different from amylose is that it has many branches of glucose-linked chains. Amylopectin is, therefore, much larger and heavier than amylose. More or less, you can think of amylopectin as a big tree of amylose molecules. All of the glucose bonds on the branches are the same as amylose. Yet, because of the more complex structure of amylopectin near the branch points, it is the main contributor of dextrins (unfermentable sugars) in wort. It also relies more on the combined action of alpha and beta amylases to break it down into fermentable sugars than does amylose.
As with my representation of amylose, this one of amylopectin is very simplified and actual amylopectin molecules in barley are massive.