Starch, a large-molecule carbohydrate used in making beer. To make any alcoholic beverage, there must be sugar present for yeast to ferment. Plants make glucose, a sugar, during photosynthesis but need to store it until it is needed. Because glucose is a highly soluble and fairly small molecule, it attracts a lot of water into the plant cells. By joining several glucose molecules into fewer larger molecules, the amount of water drawn into the cell is much reduced, which makes storing it much less demanding. The larger molecule in question is called starch. Grain starch will be broken down into sugars to create wort, which will then be fermented into beer.
Starch is a carbohydrate, meaning that it is built up from carbon, oxygen, and hydrogen, literally, carbon and water. Because starch contains many molecules of sugar, it is called a polysaccharide. Starch exists in two slightly differing forms, one a linear molecule and the other branched. The straight chain form is called amylose and is usually about 10% to 30% of the starch present. The branched chain, called amylopectin, makes up 70% to 90%. Corn starch, for example, is 25% amylose and 75% amylopectin.
The starch is packaged as starch grains inside special storage cells until such time as it is required. The starch grains must be gelatinized before the starch can be enzymically attacked and converted into sugars in the brewhouse. Gelatinization occurs at around 65oC for barley starch, which dictates the typical conversion temperatures used by brewers. However, gelatinization occurs at higher temperatures for rice and corn, demanding that they be cooked separately and added later to the main barley malt mash.
To check for conversion of starches into sugars, brewers will often perform a simple starch test in which iodine (as potassium iodide) is added to starch, producing a characteristic blue–black color. The different structure of amylopectin produces a red–violet color with iodine. As starches are hydrolyzed into smaller molecules, this reaction no longer occurs.
Starch is the primary material from which fermentable sugars used in brewing are derived. Yeast is unable to use large and complicated starch molecules, so the starch must first be hydrolyzed into smaller carbohydrates. During the malting, roasting, and then mashing of barley, many of the enzymes normally present in the grain are destroyed. However, two enzymes persist, at least well into the mash. These are alpha- and beta-amylase, which, between them, are capable of hydrolyzing much of the starch present into sugars no larger than three glucose units long. Hydrolysis means literally “to add water,” resulting in the starch being broken up and turned back into sugars.
Alpha-amylase attacks the bonds between glucose subunits in starch at any point in the chain, producing random-length carbohydrates. Left long enough, alpha-amylase will reduce amylose into a soup of glucose, maltose (a disaccharide with two glucose rings), and maltotriose (a trisaccharide with three glucose rings).
Beta-amylase starts at one end of the starch molecule and chops it up into pairs of glucose molecules, called maltose. Beta-amylase can only start its work from one end of the starch molecule, which slows things down.
The two enzymes work in concert in the mash tun, the alpha-amylase producing more ends for the beta-amylase and between them producing approximately 80% fermentable sugars in the form of 10% to 15% glucose, 50% to 60% maltose, and 10% to 15% maltotriose from the original starch. The remaining 20% consists of the nonfermentable fragments of amylopectin containing the branch points because neither enzyme is capable of breaking them down. These fragments of the amylopectin molecules are called limit dextrins. Endogenous industrial enzymes such as glucoamylase are able to hydrolyze even branch points and so can produce 100% fermentable sugars from amylopectin.
Scientific Psychic. www.scientificpsychic.com/fitness/carbohydrates1.html/ (accessed January 4, 2011).