Nothing starts a conversation faster than me making the statement “I brew my own beer.” People are instantly fascinated by the age-old process of turning barley and hops into the magical elixir that is called beer. And magical is exactly what beer was regarded for centuries: when yeast lay yet undiscovered, the process of fermentation was termed “Godisgood.” This process was initiated by a mysterious, unknown element that settled on vats of wort (malted and cooked barley grain) and transformed them into alcoholic and carbonated consumables called beer. Apparently, because bacteria were also undiscovered, the spoiling (souring) of beer was blamed on the action of “beer witches.”
Nowadays, we know much more about the science of beer making, what strains of yeast produce different varieties of beer, and how to protect beer from going bad. From lagers to ales, and from stouts to pilsners, we produce some of the best and most imaginative beer the world has ever tasted. How does this process work at the biological level?
It all begins with the harvested barley grain, a seed of the grass genus Hordeum. The endosperm of the barley seed is filled with variably sized starch granules that are packed into a protein matrix. Cells walls located inside the matrix are themselves composed of cellulose, polysaccharides, and protein. Such a matrix is not digestible by the sugar fungus Saccharomyces (the direct translation of the words sugar fungus), also known as yeast. In order to make the components of the barley endosperm palatable to yeast, malting must first occur.
Malting can be considered a process of controlled germination. The barley grain is first soaked in water, and then is laid out upon the ground to germinate. Such conditions mimic what might happen in nature when plant growth is initiated. At this time, the barley grain endosperm will start producing the acrospire, or the plant shoot. As this shoot grows, enzymes already contained within the aleurone (outermost) layer of the endosperm break down some of the starch/protein matrix in order to feed the growing plant. New enzymes are also produced, enabling further breakdown of the polymers. Large carbohydrates and proteins are broken down into smaller carbohydrates and amino acids, as well as lipids. A majority of the carbohydrate breakdown products are further processed into the simple sugar maltose. Some of the enzymes involved include α-amylase, β-amylase, α-glucosidase, as well as dextrinase.
Heat (at around 50°C) is used to halt germination and prevent the endosperm from becoming a full barley seedling. While many of the seed’s enzymes are denatured during the heating, others remain intact, especially the ones responsible for converting starch into sugar. This remaining potential enzymatic activity is measured and referred to as the malt’s “diastatic power”. This diastatic power is later taken advantage of by the brewer in a process called mashing.
The malt is then roasted, or kilned, in order to provide the beer with color and flavor. Malt grains (such as Vienna or Munich) that are intended for mild ales are kilned at lower temperatures and for a shorter period of time. Conversely, malts intended for darker and richer beers are kilned at higher temperatures and for a longer period of time. Kilning not only helps convert the remaining endosperm starch into maltose sugar, but when extended over time, also encourages that sugar to caramelize, resulting in a sweeter tasting malt.
After the barley grain is milled (broken up to facilitate access to water), it is ready for mashing. Mashing is the process whereby the ground malt is turned into grist, the sweet liquid that will become food for the yeast. Mashing involves cooking the barley grains for a set period of time at about 75°C, allowing the remaining plant starch to be converted into simple sugar. Proteins are also converted by proteases into simple peptides and amino acids, which are essential for yeast growth.
Because the aforementioned enzymes are being counted on to do these conversions, water is a key ingredient here and should contain traces of minerals such as calcium and magnesium, since these ions are essential cofactors for the enzymes. In fact, ions such as calcium are essential to the survival of an enzyme such as a-amylase, which is otherwise denatured at temperatures ranging from 70 – 75° C. Ions also help lower the pH of the wort, and a lower pH improves enzymatic activity and thus the conversion of the starch into sugar. The end result, of course, is that more sugar means more food for the yeast, which means more alcohol in the beer.
After the hops and adjuncts are added, the wort is ready to become beer via the action of yeast. These single-celled eukaryotes belonging to the fungi kingdom are the most important ingredient in beer brewing. Not only do the yeast convert the sugar of the wort into alcohol and carbon dioxide, they also affect the beer’s flavor. Therefore, keeping these critters happy and alive is essential to having a great beer.
Two species of yeast are typically used for beer brewing: Saccharomyces cerevisiae and Saccharomyces uvarum. S. cerevisiae is a top-fermenting yeast that is used in the production of ales such as stouts, while S. uvarum is a bottom-fermenting yeast used in the production of lagers such as Oktoberfests. Top-fermenting yeast, as the name suggests, flocculate (gather) at the surface level of the beer, while bottom-fermenting yeast grow and flocculate at the bottom of the beer. Another difference between the two yeasts is the temperature at which they optimally ferment: top-fermenting yeast have an optimal fermentation temperature range of 10-25°C, while bottom-fermenting yeast have an optimal range of 7-15°C.
Yeast require various minerals such as calcium, potassium, zinc, and magnesium, essential amino acids, and vitamins such as biotin, nicotinic acid, and pantothenic acid in order to divide and be metabolically active. Of course, they also require sugar (i.e., maltose) as their primary food source. In the process of consuming the sugar, yeast excrete two major waste products: carbon dioxide and ethanol (yes, it is the waste products of yeast that one consumes when drinking beer!). The more optimal the environment for the yeast, the healthier they will be. This in turn will translate to a higher amount of carbon dioxide and ethanol production.
Inevitably, though, even the healthiest and most vibrant yeast will be inhibited by their own waste products. This is especially true with ethanol; after an alcohol percentage of about 10-15% is reached, the yeast shut down their metabolic activity and enter a kind of “sleep” phase. At this point in time, the beer has become almost completely depleted of sugar. This beer may be artificially carbonated, or the yeast can be “woken up” and induced to carbonate the beer by the addition of a small amount of sugar. Such induction is usually only performed when one is bottling beer and carbonating it right inside of the bottle.
Of course, brewing beer is more than just a science; it is an art as well. Different styles, colors, and brewing techniques are attempted on almost a daily basis in order to achieve a better beer, or simply to make things interesting. In some cases, bacteria are introduced into the beer in order to “sour” it. Beer can be flavored and fermented with fruit, vegetables, sorghum sugar, or even pizza. In the end, there is just no telling what your next brewski will contain or how it will have been made. Yet, despite this myriad of differences, the basic science of brewing beer remains intact since it was discovered many millennia ago: the conversion of starch into sugar, followed by sugar’s transformation into the alcohol-laden sustenance called beer.
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