Beer as we know it was first discovered by Sir Francis James Beer III in 1873. Beer (the man) was the court scientist to the Queen of England. After first accidentally inventing diet cola and then wine coolers, Beer succeeded on his third attempt to create the official royal beverage when he modified barley into malt, mashed it, and fermented the resulting sweet, brown liquid into beer.
Ah, if only the invention of beer had a nice, neat little story like that to accompany it. Why couldn’t it have been like the famous story of the Earl of Sandwich, Thomas Edison, or even John Crapper? Instead beer historians and writers have a muddled tale of evolution that begins with the spoiled barley of ancient Egypt, which somehow becomes the safest drink of medieval Europe, and then Louis Pasteur gets involved and suddenly we have Budweiser.
Although brewing developed over a fairly long period for a beverage, it was the Industrial Revolution that made beer into the sparkling clear, clean-tasting, consistent drink we know and love. Modern scientists now have many explanations as to why brewing works. One brewing process that scientists have elucidated for the brewers is the mashing of malt. Though the aim of mashing is the same today as it was hundreds of years ago, now chemistry and biology are used to explain how malt is made into food for yeast and ultimately beer for people.
The mash is made up of two ingredients: crushed malted barley and warm water. Adding the malt to water then heating the water creates an important biochemical reaction. How much and what types of malt (and adjuncts) you use, how much heat you add, and even the chemical make-up of the water create variations in the mash, but the essential goal is always the same.
The primary aim of mashing is to break down two types of large, complex molecules contained in barley: starches and proteins. These molecules are too big to be consumed by yeast. But their components provide essential yeast nutrients.
Starches are broken down into maltose, a type of fermentable sugar. Maltose provides the energy necessary for fermentation to occur. Proteins are broken down into organic acids called amino acids.
In any biochemical reaction the important work of breaking down molecules into their components is done by enzymes. The enzymes most important to brewers are called alpha-amylase and beta-amylase.
About 65 to 70 percent of the malted barley kernel is composed of starch. Starch is the primary energy storage mechanism of the barley seed. When the seed germinates, it draws energy from the starch for seedling growth by releasing enzymes to degrade the starch into its simplest component, sugar. Imagine starch as a long chain. Each link in the chain is a molecule of glucose (a simple sugar). However, as starch chains become larger they take on the shape of a tree limb, with many branching points from which other chains emerge. Starch molecules in the raw barley are large and reside in tightly structured units called starch granules. Brewing articles and texts often refer to the gelatinization of starch. This is the point at which a starch granule looses its ordered structure due to heat, making it accessible for enzymatic conversion.
A primary purpose of malting and mashing is to turn the barley’s large reservoir of starch into fermentable sugars. Yeast cannot consume a large starchy molecule. Like cutting a piece of meat into small pieces so a young child can eat it, the brewer must make the energy of the starch accessible to the yeast. The knife in this analogy is the group of enzymes known as the amylases. The two most important amylases are beta-amylase and alpha-amylase.
Taking It to the Limit
Beta-amylase can attack only the end of the starch chain. It clips off maltose, a sugar usable to yeast, from the starch chain. Beta-amylase is therefore responsible for producing the bulk of fermentable sugar.
Maltose is called a disaccharide because it is composed of two glucose molecules linked together (di- means two, -saccharide sugar). Beta-amylase is unable to cleave off a maltose disaccharide that is near a branching point on a starch molecule. If you start with a large, branched starch molecule and allow beta-amylase to fully react with it, the resulting starch molecule will be stripped down to the branching points. This is called a beta-limit dextrin, because beta-amylase cannot attack it any further. Since most starch molecules are large and complex, it appears that not very much sugar would be produced if beta-amylase were the only enzyme present in the mash.
In fact another enzyme called alpha-amylase can cut beta-limit dextrins, thereby yielding new ends for beta-amylase to attack. Alpha-amylase simply cuts starch molecules at a point anywhere within the chain, randomly producing two smaller starch molecules. It also can break down trisaccharides (three glucose molecules), something beta-glucans cannot do. While yeast can consume maltose, not all yeast can consume trisaccharides. Alpha-amylase makes trisaccharides available to yeast.
Both alpha- and beta-amylase enzymes are needed to conduct a successful mash. Naturally the balance of these enzymes will affect the composition and fermentability of the wort. The more beta-amylase is at work, the more fermentable the wort becomes. A lower percentage of beta-amylase results in a more dextrinous wort. (Dextrins are unfermentable complex chains. They add body to beer and affect mouthfeel.)
Beta-amylase is heat sensitive and generally functions best at 140° to 149° F. In contrast alpha-amylase can resist temperatures as high as 158° to 160° F. It makes sense then that brewers can control wort composition by effectively favoring an enzyme through mash temperature. Note that the optimum temperature of enzymes is also the point at which they are quickly deactivated. At the typical mashing temperature of 152° F, beta-amylase is almost completely inactivated within a half hour. For this reason many brewing texts don’t recommend extended conversion rests, especially ones that last more than an hour. A mash temperature of 155° F will favor alpha-amylase while beta-amylase is quickly denatured. The result is a less fermentable and more dextrinous wort.
There are many practical implications related to the heat stability of enzymes. The first among them is the importance of measuring temperature accurately. A temperature change as small as five degrees can impact the fermentability of the wort and, therefore, the flavor profile of the finished beer. Since thermometers often fall out of calibration, it is worth keeping a few reference thermometers to make sure their readings agree. If one is more than two degrees off, it should be replaced.
The brewer should alter the mash temperature to suit the style of beer being brewed. A temperature of 150° F should yield a more fermentable wort such as that desired for an English bitter, whereas a temperature of 155° F would better suit a sweeter beer like a Scotch ale.
Proteins surround the starch granules in raw barley. Protein degradation during malting frees up the starch granules, making them more available for attack by amylases. Like starches, proteins are large, complex molecules composed of simple subunits. Proteins, however, can be composed of as many as 20 different amino acids (organic acids that are the basic component of protein). A peptide is a small chain of amino acids. A protein is a fairly large molecule made of a complex arrangement of peptides.
Although starch provides the energy needed for fermentation, yeast still need other nutrients to complete a fermentation. These nutrients include a large selection of amino acids. One of the goals of malting and mashing is to provide for yeast a wort that contains all the amino acids they need in an adequate supply. Malting breaks down proteins to suply these amino acids. Protein degradation is also important to beer clarity, since proteins can cause haze problems in finished beer.
The majority of large-protein degradation occurs in the malting process rather than the mash. Endo-peptidases, the enzymes responsible for degrading proteins, attack the large molecules randomly in the chains (much like alpha-amylase and starch) and produce peptides and polypeptides. During malting, the barley kernel has an extensive arsenal of these protein-munching enzymes. In the kiln they become denatured. By the time the brewer mashes in, they are no longer active. However, exo-peptidases, enzymes that degrade peptides by cleaving single amino acids off the ends of the chain, do survive kilning and are active in the mash. The protein rest during mashing is probably not responsible for very much protein degradation. At best, some peptides will be broken down into their amino acids.
Protein decomposition is therefore more of a maltster’s responsibility than a brewer’s. Still, brewers can create worts with insufficient amino acids by using large amounts of low-protein adjuncts such as corn. To deal with this, brewers of high-adjunct beers such as American lager typically use six-row barley. Six-row’s high protein content makes up for the addition of low-protein adjuncts.
In the barley kernel, storage cells contain the proteins and starches. To make these materials available to enzymatic attack, the wall of the storage cells must be broken down. About 70 percent of the cell wall material of these storage cells is composed of beta-glucans. Like starches, beta-glucans are composed of glucose units. However, because these units are bonded together in a different manner, beta-glucans have different properties than starch.
Beta-glucans are hefty molecules that form a very viscous solution in warm water, much like combining hot water and wheat flour makes a gooey mixture. Consequently, beta-glucans play a large part in determining the viscosity of wort and beer (how “thick” or “thin” it is).
Raw barley has a large amount of beta-glucans. It is not surprising then that Irish stout, a beer renown for being “thick,” should have a dose of raw barley among its ingredients. For professional brewers who usually filter their beers, a high beta-glucan content in the beer means extended filtration times and added cost. Furthermore, it tends to slow the lauter. Consequently, most professional brewers would like to carefully control beta-glucans.
In barley, beta-glucans are held together by peptides (chains of amino acids). To break down the cell walls where beta-glucans reside, an enzyme that breaks the links between beta-glucans and peptides is needed. This enzyme is called beta-glucan solubilase, since it helps release beta-glucans into solution, making them soluble. However, the enzyme that actually breaks apart the molecule is beta-glucanase. The majority of the beta-glucanase activity must take place in the malthouse prior to kilning, since the heat of the kiln denatures this enzyme. Once again, the brewer should let out a sigh of relief, since the problem of beta-glucans is primarily dealt with during malting.
This is a benefit brewers enjoy because of advancements in malting technology. Although many brewers still practice the traditional protein rests and beta-glucan rests at lower temperatures, the benefits of the rests are in contention. Still, many brewers swear by them and site evidence of shorter lauter times to back up their case. If you really want to settle the matter of whether these rests are beneficial, then take the role of the brewing scientist yourself. Try brewing two batches of the same beer, one with a protein rest and one without or one with a beta-glucan rest and one without. Observe the difference in the final beers and decide on a mash regimen that makes the best beer.