And now, for the exciting conclusion of Guest Week, SkiBikeJunkie wraps up his Awesome Yeast Series!
Wine consumption dates back almost to the origin of civilization, with the first known wine being produced in what is today Iran. Wine was initially made from wild grapes, but these grapes were soon cultivated and selected to produce a sweeter fruit, which would, incidentally, also yield higher alcohol content when turned into wine.
Grape cultivation originated around the Mediterranean and was a meaningful part of life in ancient Greece and Rome. Each culture had a god of wine, known as Dionysus to the Greeks, and Bacchus to the Romans. The historical wine making regions in Europe today got their start with the Romans, who first cultivated grapes there. After the fall of the Roman empire, the only real social structure in Western Europe was the Catholic church, which maintained the vineyards and wine industry, as it was a key element of the Mass.
Wine was also produced in ancient Egypt, but the preferred drink of the lower classes was beer. Beer has been produced since at least the 6th century B.C., and is again believed to have originated in the Middle East, in this case, what is today, Iraq. Virtually any beverage containing carbohydrates such as starch or sugar will naturally ferment, so it is likely various civilizations invented beer independently. If one considers that the production of beer and bread is believed by many to be responsible for humanity's ability to develop technology and civilization*, beer production is almost like an example of convergent evolution, though unlike photosynthesis, it's something that the species does without it having become part of the genetic code.
*What, you may be wondering, does beer have to do with developing technology and civilization? If folks were just drinking beer all day long, it would be hard to get any real work done, right? Not so fast. Bear in mind that safe drinking water is a relatively recent phenomenon. Beer and wine, with their moderate alcohol content, were safe from pathogens without being overly intoxicating. Wine was often diluted with water, thus providing a safe, mild drink. Beer, moreover, because of it's basis in grains, became not just a safe beverage, but a significant source of calories in many societies. Of course the beer they were drinking wasn't Budweiser, and often resembled a thick soup or gruel more than the clearish, yellow beverage** advertised so heavily during sporting events. So it was more like drinking your bread than it is today.
**Described that way, it's not so appealing.
Beer and wine making both appear to have begun with the use of wild yeasts. In the case of wine, the yeasts naturally occur on the skins of the fruit, so as the grapes are pressed, the yeast naturally becomes part of the juice, enabling the fermentation process. For beer, the process is similar to sourdough, where airborne yeasts were first used, but brewers quickly discovered that reusing the same barrels for fermentation that had been used in the last batch yielded better results. Thus they began cultivating their own yeasts, and using specific barrels (strains) for specific varieties of beer. Similar to breadmaking, the process was refined over the years to derive better, more consistent yeast cultures.
Both beer and wine can be brewed using wild yeasts and some still are today. Many European winemakers insist on using the wild yeasts even though they can cause inconsistent results and sometimes even spoilage in the resulting wine. Lambic beers, produced in Belgium in and around Brussels, are brewed using wild yeasts native to that area. Lambics are often blended with fruit juice or syrups to create a beer with a very distinctive, sweet, fruity flavor. More often than not, however, beer and wine are fermented with cultivated "sugar" yeast, most often S. cerevisiae*, the same stuff bakers are using to make their bread rise, and its cousin, S. uvarum*, the first yeast cell to be isolated back in the 1800's by Emil Hansen at the Carlsberg brewery in Denmark***.
*Almost certainly derived from Ceres, the Roman god of the harvest, which, incidentally, is the same root for the Spanish word "cerveza," meaning "beer."
**Formerly known as S. Carlsbergensis in honor of the brewery at which it was discovered.
***My ancestry is predominately Danish, which makes me feel like I should drink Carlsberg beer in honor of my heritage.
OK, so we've talked a bit about where beer and wine came from and how they came about, but enough of the history lesson already. This is supposed to be a science blog. Let's get to how yeast turns grape juice and cereal into alcohol and carbon dioxide. But since I majored in English and have been racking my brain all week* trying to figure this stuff out, we're going to do the butcher paper and crayons version.
*Seriously, after this one guest post, I have no idea how Watcher does this blog and finds time to ride his bike and finds time to read books to his kids and finds time to go to work and finds time to spend with his awesome wife** because this was seriously a hell of a lot of work***, a far cry from the lunch hour stream of consciousness missives that are the bread and butter of my (pathetic by comparison) blog that may occasionally entertain but never even begins to inform in a meaningful way.
**My family and I have spent time on a few occasions with Watcher, Awesome Wife, and the Trifecta. And as Awesome as Watcher describes AW as being in this blog, it is nowhere near as awesome as she is in real life. The Watcher family is a good bunch of folks.
***And I didn't even bother to put together a single awesome graphic. Seriously, how pathetic am I?
The simple explanation, in language that I understand and using metaphors with which I am well acquainted, is that the yeast eat the sugar in the grape juice/wort/dough/whatever else is being fermented. When they eat the sugar, it gives them a little flatulence, which is CO2, as well as makes them have to pee, which is ethanol. This is a way dumbed-down version, but puts it into terms that make sense to me.
The real, slightly more scientific description of the process is that two steps occur, glycolysis and fermentation. Glycolysis is the process by which one glucose molecule is broken down into two pyruvate molecules, in the process releasing ATP* and NADH, which the yeast cell can use for energy. Glycolysis is considered the archetype of a universal metabolic pathway--glucose as well as most monosaccharides can be converted to intermediates that follow this series of reactions.
*I learned about ATP synthesis in AP biology in 10th grade (I don't remember a thing about it). It's the only biology class I've ever taken, and I somehow got an advanced degree without ever having taken chemistry. Ever. I was able to satisfy the physical science requirement in high school by taking physics, which I should have failed but passed on a technicality. Were it not for that technicality, I would not have graduated high school. Which would have been embarrassing, since I conducted the graduation ceremony.
The pyruvate is then converted to acetylaldehyde and CO2. The NADH produced during glycolysis then reduces the acetylaldehyde to ethanol, leaving ethanol and CO2 as by-products. The ethanol remains in the beverage, which is what makes people feel relaxed when they drink it. The CO2 only dissolves in the beverage if there is an additional fermentation and the container is pressurized to ensure it doesn't escape. Regular wine does not have this second fermentation, but sparkling wine* does. If it is bubbly, the bubbles will make you happy.
*Sparkling wine, like so many culinary wonders, is the result of a happy accident, originally an error on the part of the winemaker. The wine was fermented in conditions that were too cold. It began fermenting in the fall, producing the ethanol. Then it got cold enough during the winter that fermentation ceased, but began again in the spring when temperatures rose, resulting in the second fermentation that led to the dissolved CO2. Until the invention of the wire cage to hold the corks in place, cellaring Champagne was a dangerous prospect, with corks arbitrarily popping, leading to a chain reaction that could destroy an entire cellar of product.
The bubbles in beer may come about as a result of an additional fermentation in the bottle (known as bottle conditioning) or in the cask (known as cask conditioning). They may also be added artificially by injecting CO2 into the finished beer, which is, incidentally, the most common method of making it bubbly*.
*Beer served on tap is typically served from a keg pressurized with CO2. The CO2 pressure forces the beer out of the keg and makes it bubbly at the same time. Nitrogen can also be used to pressurize the keg, so a beer served "on nitro" is using this technique. Since the beer is not cask conditioned in this case and nitrogen does not dissolve in beer, nitro beers will have a head of very fine bubbles but will not have bubbles in the beverage itself.
The same fermentation process occurs in bread dough, with the CO2 forming air pockets that become the holes in baked bread. Since the chemical process is the same, ethanol is also a by product, however, because the bread is then baked, the ethanol evaporates during baking.
Regular, commercial yeast (S. cerevisiae) does not like an acidic, or sour, environment. If bacterial activity creates too much acid, this yeast will die and make your bread taste funny. It will have an ammonia-like aftertaste and a weakened gluten structure. Wild yeasts, on the other hand, prefer an acidic environment, which is why they coexist well with the bacteria in sourdough. The bacteria create lactic and acetic acids, lending the dough its unique flavor, but this process requires about twice as long as the yeast needs to leaven the dough. A hearty yeast strain is needed to endure that long, another reason the wild varieties are preferred to the cultivated yeasts when making sourdough bread.
Punching down dough is a typical part of the bread making process and accomplishes four things: 1) it expels CO2, which would eventually otherwise choke off the yeast; 2) it allows the gluten in the dough to relax a bit; 3) it equalizes the temperature, which is typically higher inside the dough than outside; 4) it redistributes nutrients to the yeast, thereby kicking off a new feeding cycle.
Yeast are hungry critters, but then again so are we. And we wouldn't eat nearly as well as we do without them. So next time you nibble on a crust of bread, make sure and raise a glass to these little fungi that invisibly work to make eating and drinking so much better.
Wow- what a post! And a what a week! Thanks Phil, Kevin and SBJ for an awesome Guest Week!