Grains, Malt, and Sugars

Overview

The earliest beer was probably stored grain that accidentally got wet and the resulting grain-water was fermented by wild yeast. After thousands of years of brewing tradition, gradual refinement, and eventually scientific inquiry we now have a delicious beverage in a number of distinct regional and historical styles. Fundamentally beer is a solution of grain derived sugars dissolved in water and fermented by yeast to produce alcohol.

Barley

Barley contributes the bulk of fermentable sugars in most beers, but mixtures of barley and other grains are also common. Wheat, rye, and oats are all popular grains with their own subtle character; they are used in various styles of wheat beer, rye IPA, oatmeal stout, etc. There are also fermented grain beverages made using other grains such as traditional African beers made from sorghum, sake made from rice, and chicha made from maize.

Each grain of barley is a seed that, if planted under the right conditions, would sprout and produce a new barley plant. In preparation for this goal each grain is stocked with a large amount of stored energy in the form of starch – comprising about 90 percent of the grain by weight. When a grain sprouts or germinates the plant embryo uses its starch reserves for energy to grow until it has enough leaves above the soil to produce energy by photosynthesis. Enzymes in the germinating seed break starch apart into simple sugars that can be readily used for energy while the embryo grows into a plant.

Malting

Malting is a process that mobilizes enzymes and begins converting starch into fermentable sugar by allowing the grain to partially germinate. The maltster begins the process of making malt by wetting the grains, and keeping them warm and moist for a period of days. This encourages the grain to start germinating and produce enzymes for starch conversion. Next the maltster toasts the grain to remove most of the water. This eliminates the aqueous solution needed for further enzymatic activity; thus halting germination in its tracks. A wide range of toasting temperatures can produce malt of various colors and flavors. When the brewer mashes the malt, by mixing it into warm water, the enzymes and starch are again in an aqueous solution, and the starch conversion resumes. Next the brewer will rinse these sugars off the grains to produce wort in a process called sparging or lautering.

Chemistry of Sugars and Starches

All sugars and starches are made of just three types of atoms: carbon (C), hydrogen (H), and oxygen (O). Owing to this composition sugars and starches can be collectively referred to as carbohydrates (the same as the item listed on nutrition labels). Sugars are sweet tasting molecules that are easily digested into a quick source of energy for plants, animals, and yeast. Starches are larger more complex carbohydrates made from long chains of simple sugars, and since these chains need to be broken apart before the component sugars can be used they take longer to digest. In humans this process of breaking apart starch chains begins as soon as food enters the mouth. Enzymes in our saliva partially digest starches resulting in a slightly sweet taste.

All carbohydrates are made of building block units called monosaccharides. There are three monosaccharides of interest to brewers: glucose, fructose, and galactose. On their own, glucose and fructose are very sweet tasting and very readily fermented by yeast. Galactose on the other hand is almost never found as a single unit sugar. Glucose is sometimes made from corn into a purified powder and sold under the name corn sugar. Some home brewers use this as priming-sugar for bottle carbonation. Fructose is found naturally in many fruits, and it is also made from corn into the ubiquitous high fructose corn syrup. All monosaccharaides have the same chemical formula (C6H12O6). They are differentiated only by the spatial configuration of their atoms (i.e. they are chemical isomers).

Disaccharides are made of two monosaccharaides bonded together in a chain. Maltose is a disaccharide composed of two glucoses. It is by far the most important disaccharide for brewers since it makes up about fifty percent of the sugars in typical all-malt wort. Not surprisingly, maltose is named for malt, and brewers’ yeast ferments maltose without issue. Malt and malt extract have the added benefit of providing nutrients needed by yeast as well as some less fermentable sugars that allow finished beers to be slightly sweet and full-bodied.

Sucrose is another disaccharide made of one glucose with one fructose. In its most refined form sucrose is what is commonly known as table sugar, and generally it is derived from either sugar cane or sugar beets. Sucrose is well fermented by yeast, but there is some speculation that sucrose can lead to cidery off-flavors in beer. These off-flavors are the result of yeast stress during fermentation that probably have more to do with low yeast nutrients in typical sources of sucrose than it does with the fermentability of sucrose itself. These off-flavors can be avoided by using sucrose in moderation, ensuring yeast cell health, and the addition of yeast nutrients.

Lactose is a disaccharide found prominently in milk – composed of galactose and glucose. It is responsible for the sweet flavor of milk. Infant mammals (and some adult humans) can easily digest lactose, but it is not fermentable by yeast. Brewers sometimes take advantage of this fact to add sweetness and body to beer that will survive fermentation. This effect is seen in the style called milk stout or sweet stout.

Three glucoses bonded together are known as the trisaccharide maltotriose. It is found in wort in smaller quantities than maltose. Starches are very large collections of chained glucose units. Malodextrins, also known as dextrins, are molecules composed of glucose that are intermediate in size between maltotriose and starches. Although dextrins are flavorless and unfermentable they are important to brewers because provide body and mouthfeel to beer. Brewers have a number of options for controlling the concentration of dextrins in beer. in as seen in dextrin malt and crystal malts) dextrin production is favored by higher mash temperatures, or dextrins can be added to wort directly in the form of maltodextrin powder.

Starches are found in two configurations the branched molecule amylopectin and the linear or helical molecule amylose. Amylopectin typically has up to 3000 units of glucose with approximately 24 to 30 units between branches. Amylose has a linear configuration typically 300 to 3000 units long. Amylopectin accounts for around 75 percent of all starches found in most grains including barley. The highly branched structure of amylopectin means that it has many more chain ends for enzymes to digest. Starch that is a higher percentage amylopectin will have a waxy character since amylopectin is more easily gelatinized than amylose. Gelatinization is a process where the crystal structure of dry starch molecules packed together is disrupted so that the molecules can interact with water. Amylose is relatively insoluble in water and accounts for the remaining 25 percent of all starches.

Enzymes in Brewing

Enzymes are biological catalysts that accelerate biological reactions so that they complete quickly enough to be useful for life. They do this by lowering activation energy for a given reaction and without being consumed themselves by the reaction. The molecules enzymes act on are known as substrates. Enzymes are composed of long chains of amino acid units folded into complex three-dimensional shapes. This shape includes an active site, a cleft or indentation. The substrates fit into the active site and bind there while they are reacting. The geometry and chemical properties of the active site are very important for the enzyme to work effectively.

The rate of enzymatic reactions is determined primarily by a few factors. First, there must be sufficient water for enzymes and the substrate molecules to float around and get together. Second, the concentrations of enzymes and substrates will affect how likely molecules of substrate are to encounter an available enzyme. Third, each enzyme has an ideal temperature range. Higher temperatures generally accelerate chemical reactions. As the temperature of the solution rises, enzymes and other molecules have more energy and move faster. This increased entropy increases the likelihood that enzymes will collide with substrates and a reaction will take place. However each enzyme has a particular temperature at which it begins lose integrity and beyond which effectiveness steeply declines. Denaturation is a term for deformation and possibly permanent destruction of protein structure. If the active site is deformed by heat the enzyme will no longer function. Fourth, each enzyme has an ideal pH above or below which it will be less effective. Each amino acid in an enzyme can act as an acid or a base depending on the pH of the solution it is in. Under the Brønsted-Lowry definition an acid is a proton (H+) donor and a base is a proton receiver. The affinity of amino acids for protons depends on the concentration of protons which is measured as pH. Changing amino acids by adding or removing protons can also deform or block the active site.

There are a number of enzymes found in grains and active during mashing. The activity of each enzyme can be favored by holding the mash at a particular temperature and pH. Holding the mash at a particular temperature is called a temperature rest. At least one rest is needed at temperature favoring starch conversion enzymes, but multiple rests may be used to produce other effects. By controlling these mash variables the brewer has some control over the sugar composition of the resulting wort. For example if dextrin production is favored this will lead to a less fermentable wort and a full bodied beer with substantial mouth-feel. Alternatively if simple sugar production is favored the wort will be more fermentable and produce a thinner beer with a dry finish. The most important mash enzymes are the starch converting enzymes alpha-amylase and beta-amylase. Other less important starch converting enzymes include limit dextrinase. A mash rest can also be used to favor enzymes that acidify the mash (lower pH) to benefit the starch converting enzymes in a later rest. Sometimes for particularly gummy grains it is beneficial to use a rest favoring enzymes that break down gummy compounds to prevent the mashed grains from turning into a ball of dough.

Beta-amylase is an enzyme found in barley that converts starch to units of maltose. It can only work on an exposed end of glucose chains. Each glucose-glucose bond has a particular directionality and the chain in turn has the same directionality. In this way only one end of the chain is a valid target for beta-amylase. Beta-amylase removes two-glucose units at a time to make a maltose. After many repetitions this can completely convert an amylose molecule into maltose molecules. Beta-amylase is unable to work on portions of amylopectin near the branching bonds. Other starch digesting enzymes such as limit dextrinase and alpha-amylase are needed to expose new ends for beta-amylase to continue working on amylopectin. Beta-amylase cannot remove the last two glucoses before a branch therefore a dextrin with two glucoses past the branch point is a beta-amylase limit dextrin. The ideal temperature range for beta-amylase is between 131 and 150°F. Beta-amylase is most active in the pH range between 5 and 5.5. Favoring beta-amylase activity with some alpha-amylase activity during a starch conversion rest in the range 150 to 154°F will produce a more fermentable wort.

Alpha-amylase is another starch digesting enzyme found in barley. Unlike beta-amylase it is not restricted to working from the end of starch chains, and it is able to cleave amylose and amylopectin in the middle of a chain of glucose units. In this way alpha-amylase accelerates the activity of beta-amylase by exposing additional chain ends as well as creating a mechanism for beta-amylase to get past branching links in amylopectin. Alpha-amylase breaks glucose-glucose bonds in more random way than beta-amylase sometimes creating sugars other than maltose such as maltotriose and glucose. Alpha-amylase cannot break branching bonds in amylopectin but it can act within one glucose of branch points. Therefore a dextrin with one glucose past the branch point is an alpha-amylase limit dextrin. The ideal temperature range for alpha-amylase activity is 154 to 162°F although it still usefully active at lower temperatures. Meaning that mash temperatures somewhat below this range can still have full starch conversion. The ideal pH range for alpha amylase is 5.3 to 5.7. Favoring alpha-amylase with a mash rest between 154 and 156°F (or even slightly warmer) will produce a less fermentable wort.

Limit dextrinase is an enzyme that cleaves branching bonds found in amylopectin and limit dextrins. It has maximum activity in the temperature range 133 to 140°F. A long mash at this temperature range can produce wort with substantially reduced dextrins. This may be desirable when using unmalted grain or minimally modified malt in particular malts historically used in continental Europe. Modern malts from that area now have a higher degree of modification while still being less modified than British and American malts. Generally modern malts will not benefit from a limit dextrinase rest –the low dextrin wort instead producing an overly thin and unappealing beer.

References

Palmer, John. "Chapter 14 - How the Mash Works" How to Brew. Retrieved 2012-12-19. http://www.howtobrew.com/section3/chapter14.html
Wyeast Laboratories. "Yeast Fundamentals." Retrieved 2012-11-29. http://www.wyeastlab.com/he-yeast-fundamentals.cfm
Daniels, Ray. 2000. Designing Great Beers: The Ultimate Guide to Brewing Classic Beer Styles. Boulder: Brewer's Publications.
Ophardt, Charles. 2003. "Denaturation of Proteins." Virtual Chembook: Elmhurst College. Retrieved 2012-12-19. http://www.elmhurst.edu/~chm/vchembook/568denaturation.html