By Jim Busch (Brewing Techniques - Vol. 5, No.4)
This installment of Home Brewery Advancement completes a two-part exploration of step mashing. Part I described the low-temperature enzymes (glucanases and proteolytic enzymes) and their role in creating good nutrient profiles and lauterability. Part II takes a close look at the starch-degrading enzymes, the amylases.
Part I of this series (1) presented an overview of enzymes, how they are activated and deactivated, and what the various types of enzymes do. It then focused on the enzymes that are activated at low temperatures (glucanases and the proteolytic enzymes). In this second and concluding installment, I focus on the amylases, the enzymes responsible for transforming starches into fermentable sugars.
Enzymes 101 Revisited
Before delving into the amylases, it may be helpful to briefly review the basics of enzyme activity. Enzymes are merely high molecular weight proteins that act as biocatalysts to either enable or accelerate certain reactions. Enzymes play important roles throughout chemistry. In our beloved art of brewing, they are responsible for many diverse reactions that result in fermentable sugar and that affect the overall composition of our worts.
Much to our good fortune, enzymes occur naturally in cereal grains, and their abundance increases significantly when the grains are malted. The fact that enzymes are naturally present in cereal grains has been used advantageously for millennia in the production of fermented beverages. Long before any chemist produced a treatise on enzymatic activity, primitive civilizations discovered that simply mixing warm water and cereal grains altered the composition of each, resulting in a gruel that produced alcohol when fermented. While it is intriguing to delve into the chemistry of enzymes, it is also refreshing to realize just how simple this process has been for brewers throughout history. Now that science has unraveled the mysteries of enzymatic reactions, we can use this knowledge to improve our brewing methods.
In general, enzymatic activity is dictated by the quantity of enzymes present and by temperature, time, and pH. The quantity of one group of enzymes, the amylases (primarily beta-amylase), after malting is often referred to as diastatic power (DP), measured in degrees Lintner. Beta-amylase is present in raw barley, but alpha-amylase is created during the malting process. Kilning at temperatures beyond the enzymes’ limits will reduce the malt’s diastatic power. Thus, a lightly kilned Pilsener malt may have a DP of around 100 °Lintner and a pale malt may be closer to 65 °Lintner; highly kilned roasted malts will not contribute any enzymes to the mash. Malts that do have some degree of diastatic power are often referred to as base malts.*
Each enzyme tends to be most active in a narrow temperature band, often called the enzyme’s optimum. Optimum merely indicates “most active phase”; an enzyme may still retain as much as 80% of its optimum 5 °F (3 °C) off the main center point. In addition, an enzyme’s activity can remain high toward the upper limit of the optimum range, but once the limit is exceeded the activity can decrease rapidly in a logarithmic fashion. Increasing the mashing temperature speeds enzymatic activity, but at the expense of weakening the enzymes to the point of rapid denaturation.
The many enzymes active during mashing have individual temperature and pH optima. Mashing (and step mashing in particular) takes advantage of these different temperature optima to activate and then deactivate specific enzymes for specific purposes. Because each enzyme’s pH optimum also differs slightly, brewers settle on a compromise whereby mash pH is held near 5.5–5.6 — higher (more basic) than the optima for glucanases and proteolytic enzymes, but near the optima for the amylases. Brewing in general is a balancing of trade-offs of physical conditions, and enzymatic activity in mashing is one of many such trade-offs.
*DP will vary from maltster to maltster, depending on the type of barley and the malting procedure used; check the specifications provided by the maltster for accurate figures for a given lot of malt.
Introducing the Amylases
In the last issue, I explored the enzymes that are active during the lower temperature steps of mashing, namely the glucanases and proteolytic enzymes. Glucanases break down beta-glucans and therefore aid lautering by reducing the gumminess of the wort. Proteolytic enzymes — proteinases and peptidases — break down proteins and polypeptides into smaller building blocks, peptides and amino acids, which are essential components for healthy fermentation.
The remaining enzymes of interest during mashing are the amylases — beta-amylase and alpha-amylase — which break down starch to produce both fermentable sugars and those that are nonfermentable (by normal brewers yeast). Amylases are the most important enzymes to brewers because they alone are responsible for the production of sugar from malt and hence fix the potential alcohol levels of beer.
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How Did Primitive Brewers Make Beer without Malting? As mentioned in the introduction of this article, our sometimes not-so-primitive ancestors already knew how to produce fermented beverages from raw cereal grains. One may surmise that many of these cereals were not malted and hence likely contained insignificant amounts of diastatic power, which is required for enzymatic sugar production during mashing. So how did they end up with an intoxicating beverage? The answer is simple: Raw grains contain a significant amount of raw beta-amylase. Malting plays a major role in enzyme development and ultimately in the fermentability of grain-based beverages. When grains are malted, the beta-amylase levels drop during the first days of germination only to recover to roughly three times their initial concentration after day three of malting (2). Alpha-amylase, on the other hand, is not present in raw barley at all but is formed during malting. Although early brewers would likely have had inefficient mashes, the presence of beta-amylase in the raw grains could have produced low-alcohol beverages, even in the total absence of alpha-amylase. Further, some germination and enzyme formation could have occurred through chance (grains exposed to morning dew, for example), creating alpha-amylase and thus greater fermentability. Some scholars believe that bread was used as an intermediary product in the enzymatic conversion of cereal grain starch to sugar, though others argue a more direct path to the making of ancient beers. |
Beta-amylase tends to favor the production of fermentable maltose, whereas alpha-amylase tends to favor the production of maltotriose and unfermentable sugars such as dextrins. By carefully selecting the temperatures to maximize the beta-amylase activity, you can realize the highest extract per pound of malt. Alternatively, by minimizing the action of beta-amylase you can directly lower the real degree of fermentability and hence fix the beer’s final limit of attenuation, thus reducing the alcohol content and generally increasing the beer’s fullness of palate.
The low-temperature glucanase and proteinase rests are optional, depending on the raw ingredients you use and the attributes you want in the finished beer. Many step mashing programs incorporate these rests to produce specific effects in the wort. Mashing in the range of amylase activity, on the other hand, is the common element in all mashing programs. Brewers are primarily interested in ensuring that starch is converted into sugars, and the amylases are the key to that process. Further, starch conversion is desirable not only for sugar production but also for degrading the starch that, if left unconverted, can cause haze problems in the finished beer or become a food source for bacteria.
A Closer Look at Alpha- and Beta-Amylase
To understand the mechanics behind sugar production through amylase activity, we need to explore the actions and interactions of both beta- and alpha-amylase.
Beta-amylase: Beta-amylase is most active in the range between 140 and 149 °F (60–65 °C) and is rapidly denatured above 160 °F (71 °C), even though it survives to a minimal extent up to 167 °F (75 °C). Its optimal pH range is 5.4–5.5.
The primary activity of beta-amylase is to act upon larger sugar molecules to break off maltose, which, as a disaccharide, is easily metabolized by yeast. As a result, worts mashed with rests in the beta-amylase range of activity tend to be highly fermentable.
Alpha-amylase: Alpha-amylase is most active in the range between 162 and 167 °F (72–75 °C), though significant enzymatic activity still occurs as low as 149 °F (65 °C). Its optimal pH range is 5.6–5.8.
Alpha-amylase acts on malt starch to produce both fermentable and unfermentable sugars. Malt starch is composed of long chains of glucose molecules; these chains are called amylopectin (which constitutes 75–80% of the total malt) and amylose (which accounts for the remaining 20–25%) (2). Amylopectin is built of multibranched chains of up to 6,000 glucose units; amylose is built of linear chains of 200–300 of these glucose residues (2). The helical chains of amylopectin consist of multi-branched chains of linear 1–4 linkages and 1–6 linkages; amylose chains are connected by 1–4 carbon links. These chains must be broken down into smaller units (sugar molecules) to be of use to brewers.
Alpha-amylase breaks the chains of both amylopectin and amylose to form dextrins containing 7–12 glucose residues (2). From these 7–12 glucose residues, beta-amylase split off two glucose residues to form maltose, the principal wort sugar, and maltotriose and glucose. In this fashion alpha- and beta-amylases work in unison to reduce the long glucose chains in starch to ordinary fermentable sugars: maltose, maltotriose, and glucose as well as sucrose,* fructose,* and unfermentable dextrins.
The fact that these amylases work in unison may seem strange in light of the fact that the two enzymes have different ranges of activity. Beta-amylase is most active below the principal range of alpha-amylase, but enough activity remains in the overlapping range of the middle 150s °F (67–70 °C) that a synergistic effect is realized. Similarly, alpha-amylase is most active at higher temperatures, but significant enzymatic activity still occurs as low as 149 °F (65 °C). This overlap of activity ranges is yet another very fortunate feature that nature has provided brewers.
Worts mashed only in the alpha-amylase range cannot take advantage of the additional work of the betas and thus tend to have lower fermentability and more malt sweetness and mouthfeel.
*Sucrose and fructose are pre-formed through hydrolysis.
Step Mashing for Amylases
Brewers can take advantage of malt enzymes and their various activation temperatures by raising the mash to specific temperatures and holding, or resting, it there for a period of time while the enzymes do their work. The act of slowly raising the temperature (approximately 1–2 °F [1 °C]) does itself cause enzyme activity; when the progressing temperature becomes too high for a specific enzyme, it becomes denatured, ceasing activity. Step mashing describes the process of ramping a mash through various temperature rests to produce specific effects in the wort.
A typical step mash will begin with a low-temperature glucanase rest at around 113 °F (45 °C) and transition through the protein rest range of 118–135 °F (48–57 °C) before resting in the range of beta-amylase activity in the middle 140s °F (62–64 °C) (see the box on the previous page). A beta rest, or maltose rest, at these temperatures favors production of the simple, highly fermentable sugar maltose from the malt carbohydrates.
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Practical Applications of Beta- and Alpha-Amylase Rests |
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Attenuation is a measure of a beer’s fermentability and simply represents the percentage of fermentable sugars that have been converted to alcohol. A beer’s degree of attenuation can reflect both the enzyme’s efficiency in producing sugars the yeast can metabolize, and the yeast’s efficiency in doing so. The difference between original and final gravity tells the story; a beer with an O.G. of 1.050 and a final gravity of 1.000 would have achieved 100% attenuation (never actually seen and not desired in practice). An attenuation of 70–77% is considered average. To Increase Fermentability To achieve a drier character, thinner body, or higher alcohol content in your beer, you need to increase the attenuation in your fermentation (that is, you need to achieve a low final gravity). You can maximize fermentability by including separate beta and alpha rests in addition to the synergistic phase. For example, you can lengthen the duration of the beta-amylase rest (about 145 °F [63 °C]) to 30–45 minutes, then rest 15 minutes or so in the range of both beta- and alpha-amylase activity (at around 150–152 °F [66–67 °C]), with a final 30-minute rest at 158 °F (70 °C) to ensure complete conversion. An example of a beer that might benefit from such a mashing regimen would be a draught stout with a target original gravity of 1.038 S.G. (9.5 °P) and a target final gravity of 1.006–1.007 S.G. (1.8–2 °P), for an apparent degree of attenuation of 80%. To Add Body, Mouthfeel, or Sweetness If your goal is to limit attenuation to increase malt sweetness or mouthfeel in the finished beer, you will want to avoid rests in the temperature range of maximum beta-amylase activity. Rest briefly (10 minutes) in the range of both beta- and alpha-amylase activity (152–154 °F [67–68 °C]) and complete the remainder of saccharification at the high end of alpha-amylase activity (158–160 °F [70–71 °C]). A Scotch ale, for example, might have a target original gravity of 1.083 (20 °P) and a final gravity of 1.027 (7 °P), for an apparent degree of attenuation of 65%. |
After the maltose rest, the brewer holds the mash in the range of combined alpha-amylase and beta-amylase activity in the 150s °F (67–70 °C) for what is called the saccharification rest, which completes the production of maltose and generates significant glucose. (See the box, “Practical Applications of Beta- and Alpha-Amylase Rests,” for tips on using these enzymes to manipulate your beer’s final characteristics.)
Finally, some brewers perform a mash out at around 170 °F (77 °C), which rapidly denatures the remaining active enzymes, ceasing their activity and fixing the wort’s sugar composition.
The single infusion option: You can perform the entire starch conversion process at a single temperature by resting somewhere between the temperature optima of both amylases (around 152 °F [67 °C]), where both are still quite active. With single-infusion mashes, it is usually a good idea to use a malt that is highly modified because you have no means to adjust the protein content. Remember, however, that this single temperature represents a compromise, and resting at each enzyme’s optimum temperature will result in maximum fermentability.
Step up to the Best Worts Possible
Step mashing is a useful device in the brewer’s toolkit. It can be judiciously used to help break down gummy gels caused by beta-glucans, degrade large proteins and polypeptides into simpler peptides and essential nutrients such as amino acids, and (in the case of amylase activity) aid in fixing the degree of sugars produced and the ratio between fermentable and nonfermentable sugars. By carefully controlling the time and temperature of each mashing step, you can directly dictate the expected results in a mash program to optimize the wort’s composition to meet a given target beer style.
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