Tristen Wang, THURJ Writer

Along the ancient Silk Roads in western Iran, a 5500-year-old pottery jar was found to contain a beige, crystal-like substance, now known to be calcium oxalate (1). To the common passerby, this scum can easily resemble thousand-year old dust. To the brewer, it is beerstone: the familiar chemical residue of beer-making. Languages, religions and stories are often touted as overlapping factors across civilizations. Brewing may not be as old, but it is surely just as interesting and widespread.

Beer owes its ubiquity to the simple ingredients and processes required to create it. At its heart, beer is purely grain, hops (a plant added for flavor), yeast, and water tied together through the chemical process of fermentation. Beer owes its longevity, however, to the finesse employed in creating the perfect drink. The distinct permutations of beer from lagers to ales and malts are countless, not to mention the differences in color, flavor, and aroma, which have provided a challenge to always create a better beer.  These numerous suds hint to the fundamentals of what beer really is: yeast and fermentation.

Yeasts

When beer is created, sugar-infused water along with soluble starches called dextrins create a mixture known as wort (1), which serves as the food supply for the yeast. After the nutrient mixture is boiled in a kettle with hops, it is placed into a sealed vessel to fuel yeast fermentation (1).

Yeasts are fungal microorganisms that belong to several large phyla of fungi including the Ascomycota and Basidiomycota. Typically, these critters are unicellular, but they can also become lumped together through hair-like structures called pseudohyphae (2). In nature, yeasts serve as saprobes by breaking down dead organic matter to receive energy, often with an association to an animal or plant (2). More rarely do yeasts serve as plant diseases (2).

“True yeasts,” which include those used for beer making, refer specifically to the class called Saccharomycetes within Ascomycota (2). This fungal division includes about 1000 known species, many of which hold high scientific and recreational value (2). One species in particular, Saccharomyces cerevisiae, serves as an integral model system in biological experimentation, helping scientists to understand the association among genes, protein production, and cellular function (3).

Just as importantly, S. cerevisiae also serves as one of the more important yeasts used in fermentation. Different strains, genetically distinctive but not species-specific, help to add to the distinctiveness of beers. For example, the species S. cerevisiae is used in the top fermentation of ale while S. uvarum is used in bottom fermentation of lager, although both species have been recently categorized as different strains of the same species S. cerevisiae (4).

Fermentation

One famous byproduct of yeast fermentation is the alcohol ethanol, the potent substance that gives beer its seductive qualities. From the ecological standpoint, fermentation serves as one possible way of breaking down sugars into usable forms of energy. Fermentation is analogous to cellular respiration in that both are metabolic processes. The main difference is that while cellular respiration occurs in the presence of oxygen, fermentation does not, which is why fermenters must be sealed off to prevent the entry of air (1). While fermentation, which can occur with or without the presence of oxygen, is a much more flexible process, it is vastly less efficient than cellular respiration and cannot provide the necessary compounds for regular growth of yeast cells as well as respiration can (1). As a result, yeast cells only use fermentation as a secondary option.

 

C6H12O6 (sugar) –> 2CO2 (carbon dioxide) + 2C2H5OH (ethanol) and energy

 

Fermentation begins with a process called glycolysis, which is also the first step in cellular respiration. In this process, yeasts turn sugar into adenosine triphosphate (ATP), a ubiquitous molecule that provides energy throughout the organism, and the byproduct pyruvic acid (1). First, two phosphate groups from ATP attach onto two different points of the sugar molecule (1). This six-carbon sugar breaks into two three-carbon sugars and another two free phosphates attach to each three-carbon sugar piece to produce 4 free hydrogens (1). The four phosphates then attach to adenosine diphosphate (ADP), an ATP molecule with one less phosphate group, to create four new ATP molecules, a net gain of two ATP (1).  Each of the remaining pyruvic acid molecules releases a carbon dioxide molecule to make a two-carbon compound called acetaldehyde (1). Hydrogen atoms are then added to the carbon-oxygen double bond of acetaldehyde to create ethanol (1).

 

Variations in the beer fermentation process results in different combinations of flavor and visual appeal. For example, the sweeter of the beers called ale is fermented in warm environments and derives most of its fruity flavors from the esters (another type of organic compound) formed during fermentation (1). Meanwhile lager-style beers, like many of the commercial canned beers we see today, are fermented using a different strain of yeast in lower temperatures, producing a beer without that fruity taste (1). Many people who try beer for the first time often notice a bitter sensation upon ingestion. This bitterness is due to the presence of alpha acids, a class of chemical compounds often derived from the flowers of hops. These compounds are isomerized (structurally changed) during the boiling process into a bitter form so the longer the boiling time, the more bitter the beer (1).

Because of the commercial importance of beer fermentation, much research has gone into improving the process. One of the more promising approaches to fermentation lies in yeast immobilization. Whereas conventional beer fermentation suspends yeast cells in a mixture, immobilization entails the suspension of yeast cells in porous beads. Practically, these beads can be used in future fermentation attempts, making the separation of yeast and beer simpler. This matrix barrier also seems to provide some protection for the yeasts from changes in acidity, nutrients and inhibitory substances that may arise from fermentation (5). Several studies have also shown that using this continuous fermentation system may yield a higher fermentation rate due to the increased cell density per unit volume of beer, as well as a more consistent product (5). Although research in this field has progressed steadily, , the beer-making industry has been slow to incorporate immobilization  (6).

Conclusion

Thousands of years have been spent in search of the right method of fermenting beer. Even to this day, people are trying to brew the perfect batch. As much as scientists try, the complexities of beer fermentation cannot be understood simply through species identification and complex chemical equations. Studying a glass of beer may lead to some of the most curious observations about yeast and fermentation, but ultimately, knowing everything about how beer is made doesn’t quite measure up to the grand feat of drinking it. So until then, drink up!

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