Much of the (fairly) recent scientific work on barley starch should be of interest to advanced homebrewers or homebrewers with an interest in biology. This post is an introduction to a series of articles that will review what modern science has revealed about barley starch. Recently, I posted a series of articles on enzymes for brewers. Although it dealt with all brewing-relevant enzymes, not just starch-degrading enzymes, you can look at these starch articles as covering a lot of the same or similar ground, but from the perspective of the substrate, not the enzymes. (There will also be a few new enzyme-related topics, as well)
In this article, I will give an overview of the subject. In the subsequent articles, I will fill in all the details. In the individual articles, I will try to explain the topics so that you don’t need an extensive background in biology or chemistry to understand them.
Starch in Brewing
In brewing, starch from malted barley (and sometimes other sources) is degraded in the mash. The sugars formed in the mash from starch degradation are fermentable by brewers yeast, while some of the larger carbohydrates are not. By altering the temperature, pH, and time of the mash — or performing a step mash — a brewer can potentially alter his extract yield, wort fermentability, and influence other variables. In order to better understand how these things can be accomplished, it is helpful to know as much as possible about starch.
While barley is growing in the field, it produces the sugar glucose via photosynthesis. Late in the growing season, when the “head” of the barley plant emerges, glucose is transported to the grain kernels and they are initially filled with a milky white liquid. In the kernels, the glucose is transported into organelles called amyloplasts, where enzymes link glucose molecules together to form starch molecules. Eventually, the barley plant dies and dries in the sun, leaving the starchy endosperm of the barley kernel with only a few percentage points of water, by weight.
Starch synthesis is interesting from a biological perspective, but learning about it serves no practical end in brewing. Near the end of the series, I will give a brief overview of starch synthesis just so starch is covered from its formation to its degradation, but I will not go into very much detail.
Amylose and Amylopectin
Starch is composed of molecules of amylose and amylopectin. Amylose is a molecule composed of glucose molecules linked (mostly) in a chain. In an aqueous solution, amylose molecules form helixes that are not very soluble and tend to align themselves with other amylose molecules. Contrary to common wisdom, most amylose molecules do have a small number of side branches. Iodine binds tightly to amylose, and forms a complex with a blue or purple color.
An unbranched molecule of amylose would have one reducing end and the other, a non-reducing end. (I’ll explain that in more detail in a later post). If it contained a few branches, it would contain one reducing end and a number of non-reducing ends dependent of the number of branches. The importance of this is that beta amylase, one of the main starch-degrading enzymes, only works on the reducing end of a starch molecule.
Amylopectin is formed by chains of glucose molecules that also contain branch points. The structure of amylopectin can be described in terms of A, B, and C chains. A chains are regions of straight, (relatively) unbranched glucose molecules hanging from the B chains, with are branched. The C chain is the part of the amylopectin molecule that terminates in a single reducing end. The molecule may have hundreds of non-reducing ends. The upshot of this is that the very large molecules of amylopectin must be acted on by alpha amylase to give beta amylase more than one point to act.
Amylopectin is much more water soluble than amylose. Although there are far fewer amylopectin molecules in barley starch, they are much larger than amylose molecules and hence the most abundant component of starch by weight. Iodine binds only weakly to amylopectin, and forms a complex with a brown color.
Inside the endosperm of the barley kernel, starch is located in many small and a few larger granules. Although the small granules outnumber the larger one, the larger granules hold more of the overall weight of the starch in the endosperm. The composition of the granules differ in the percentage of amylose vs. amylopectin and these granules soften and gelatinize at different temperatures. Starch granules have internal structure, with regions of tightly packed amylose molecules and less dense, more amorphous regions of amylopectin. Both proteins and lipids are associated with starch granules and their presence can inhibit the action of amylase enzymes.
You can divide starch degradation into two steps, the preparation of the starch to be degraded (gelatinization) and the attack of the enzymes.
The major enzymes involved in starch degradation are alpha-amylase, beta-amylase, and limit dextrinase. The way these operate on starch influences extract yield and fermentability. (As you will see, newer research indicates that a better understanding of limit dextrinase may especially benefit brewers seeking to produce highly fermentable worts.) The activity of the enzymes depends on many factors including protein levels in the barley, how the malt was kilned, temperature and pH of the mash and others. However, it is also affected by things like starch granule size and the association of proteins or lipids with the starch.
Starch differs among different plants and with different cultivars of barley. The amylose to amylopectin ratio varies among starches, as does the average length of amylose strands. The sizes and distribution of starch granules varies. The amount and potentially types proteins and lipids associated with starch varies across species. As brewers, we are primarily interested in barley starch. However, it pays to keep in mind that something that applies to barley starch may not be applicable to wheat starch, maize starch, rice starch, or other starches.