This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
[caption id="attachment_609" align="alignleft" width="225" caption="My Pale Ale off my back porch. Pale and caramel malts, loaded with Cascade hops, and fermented with an American Ale strain of Saccharomyces cervisiae."][/caption]
If there is one thing I enjoy more than beer, it is more beer. In fact, more beer ranks up there highly along with brewing my beer. And if I brewing more beer on top of my stash of already homebrewed beer.... well, then you can assume I'm a VERY happy boy! Beer is a simple and elegant recipe: malted barley to give body and sweetness to the beer, hops flowers to balance the sweetness with bitterness, good water and a proper yeast strain. Despite the short ingredient list, there are hundreds of variations on the theme. Brewing is truly limited only by one's imagination.
Malted barley is an amazing substance. Barley is a grass and as such is fairly nondescript. Farmers raise it to feed livestock and for use in other food products like cereals, breads and the like. What makes barley the most amazing substance on earth are its profile of highly fermentable sugars. Once these sugars are unlocked by the malting process it's sunshine, unicorns, rainbows and puppies!
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Malting (from barley's predominant sugar maltose) is a neat trick that is gets the barley seed to start germinating and then abruptly shuts it down at the right time. When seeds germinate, specific enzymes start converting the starchy "germ" of the seed into fermentable sugars, which is what we want to make beer. So brewers trick barley seeds into germinating by soaking them in water for a few days until the primary leaf shoot grows to about 75% the length of the seed. Then, BAM! Germination gets shut down by heating the grain at 50-60F, drying it out. The trick is get the enzymes to convert starch into sugars before the plant utilizes them for growth. Playing around with temperatures brings out different characteristics from the grain such as one gets when caramelizing and roasting the malts.
[caption id="attachment_617" align="alignright" width="300" caption="The malt grist for my Amber Ale, contains 6 varieties (Pale, Amber, Pilsner, Dextrine, Caramel, Black), but are all one species!"][/caption]
Without malting, yeast have no sugar to ferment and hence no beer to be had. This might be one of the saddest thoughts in the world... Many yeasts break down simple sugars (6-carbon rings) all the way to single carbon molecules, like CO2. But not Saccharomyces, the brewer's yeast, which breaks down the same sugars into 2-carbon molecules, such as ethanol. The ability of yeasts to ferment, though, goes back at least 200 million years ago as inferred by molecular phylogenetics. Geneticists have reconstructed enzymes based on inferred gene sequences, showing that while inefficient, the machinery exists to begin brewing at the end of the Cretaceous, when fruiting plants started to be more abundant. The evolution of fruit may very well have been the driver of evolution and diversification of yeasts. After several gene duplications of Alcohol Dehydrogenase - the enzyme that converts ethanol - around 100 million years later, brewer's yeast added a crucial step to its metabolic strategy known as the make-accumulate-consume model.
[caption id="attachment_612" align="alignleft" width="185" caption="Figure from Piskur et al. 2006, doi:10.1016/j.tig.2006.02.002."][/caption]
Brewer's yeast is able to accumulate ethanol because it has a novel adaptation, the ability to repress alcoholic fermentation when glucose is present. Saccharomyces will continue utilizing glucose, but breaking it down into ethanol, keeping it to the side until all the good stuff is gone and only then will it start to break down the alcohol it has accumulated. Other fermenting yeasts will inefficiently break down ethanol as a sole carbon source if it is there, but not accumulate it, or are unable to use ethanol as a sole carbon source, but as can use it inefficiently as an accessory source. This is what the graph from Piskur et al. (2006) is showing to the left. The top is Saccharomyces and the bottom is a relative Kluyveromyces which does not accumulate ethanol, though it can less efficiently use ethanol as a substrate. Once Kluyveromyces uses up all the glucose (green diamonds) it can no longer increase its biomass (blue triangles). The added benefit that ethanol accumulation and consumption (black squares) gives Saccharomyces enables the yeast to sustain its biomass at least 50% longer even though there is little to no discernible gain in biomass.
The real adaptation is not the ability to use alcohol as a metabolic substrate - that merely sustains the yeast's population and doesn't enable growth, just replacement. The unique adaptation that makes Saccharomyces so badass is regulation. It can repress alcoholic fermentation when glucose is high, accumulating ethanol for that inevitable time when the sugars are in too low of a concentration to be useful. Thus, it gets an edge from using that last-resort store of ethanol. It is this unique adaptation that brewers harness.
Once the yeast has converted sugars to ethanol, brewers stop fermentation or wait for the yeast to go dormant. Sometimes you can get a lot of off-flavors if you let your beer sit too long on the 'trub', which is the pile of cells and proteins that settle at the bottom. This is why many times, especially for heavier beers, brewers will transfer their sweet amber nectar to a secondary container for aging. Those off-flavors are the result of breaking down secondary products like amino acids and the ethanol which might result in some fouler or vinegary compounds, such as esters and aldehydes. So finding the right balance of time for the yeast to sit in the fermentor and build up ethanol is just as important as selecting the right malts and hops. While brewers like to think of themselves and the craft beer-makers, the original brewmasters have been practicing the art for over 200 million years!
Piskur J, Rozpedowska E, Polakova S, Merico A, & Compagno C (2006). How did Saccharomyces evolve to become a good brewer? Trends in genetics, 22 (4), 183-186 PMID: 16499989
For the beer geeks out there:
Bottled: Kronan's Last Stand (Pale Ale 4.2%)
Primary Fermentor 1: Baltic Amber Ale
Primary Fermentor 2: Abyssal Coffee Stout
Secondary Fermentor: Mörtfors Porter