It’s October, and in central Virginia, that means apples. Lots of apples. Apple pie, apple butter, apple festivals, caramel apples, donuts, slushies, and of course, apple cider Hard, soft, cold hot, however you like your cider, you can find it in Virginia.
I’ve been known to get a little too excited about apple season. I’ve been known to buy more cider than I can drink before the plastic carton swells up in my fridge like bellies after too much pie a la mode. But I’m certainly not the only one to witness the miraculous transformation of innocently pressed apples into a fizzy, foaming substance that I’m not sure it’s legal for me to have around. What causes this metamorphosis? “At-Home-Brewing Kits” contain packets of yeast that you add to cider to cause a reaction. But what if it’s just sitting in my fridge? Surprisingly, there is yeast just floating around in the air. It’s actually called ‘wild yeast’, and people have been letting the free-flowing yeast cells (little members of the fungi family) work their magic on sourdough starters and sugary liquids for years. So, how do yeast buffets turn into people food? It starts as soon as you open u up that carton of cider, and expose it to the yeast in the air. It’s actually a surprisingly simple transformation overall (omitting the chemical processes that actually go on inside the yeast cell metabolism).
Many people have sweet tooths, but everyone knows that too much sugar results in a ‘sugar high’ (hello, Halloween!). Yeast like sugar too, because it also gives them lots of energy when they digest it. Fermentation happens when yeast eats, and it is the process of converting glucose (sugar) into carbon dioxide, and ethyl alcohol. Fermentation is such an old and widely used process for all kinds of things in people’s home and in major industries. It’s even the same reaction that occasionally causes overripe fruit (especially strawberries) to taste like nail polish remover. Here’s what the reaction looks like, fyi:
C6H12O6 ====> 2(CH3CH2OH) + 2(CO2) + Energy (which is stored in ATP)
Sugar ====> Alcohol + Carbon dioxide gas + Energy
(Glucose) (Ethyl alcohol)
So, last week I talked about how the carbon dioxide produced from the reactions of baking soda created bubbles in pancakes. Well, guess what the carbon dioxide made by yeast does? The same thing! This same process occurs when you use yeast to make bread, when Budweiser makes beer, or Beringer produces wine. If you’re making bread, you have to add in sugar for the yeast to eat, like in this recipe. I kept a starter for sourdough bread in my basement as a pet once when I was a kid. It was great. I only had to feed it (granulated sugar and flour) about once a week, and if it died, I could grow a new one. If you’re making alcohol, the sugar comes from the grapes (in the case of wine, no pun intended), or the grain (in the case of beer, ha ha). In grains, like rye, wheat and barley, which can all be used to produce beer, the sugars come from the breaking down of starches through a different chemical reaction (I can go into more detail about this transformation in another post if anyone wants). Of course, you can also see in the above reaction that alcohol is a by-product of the yeast chowing down, which humans have long taken advantage of. It’s a mutual relationship we have with the yeast. They can eat, and people, well, people can drink. Both species are merry.
You may be wondering why bread isn’t alcoholic. The process of making wine and beer involves allowing the liquid to ferment in barrels or kettles, meaning that most of the yeast inside doesn’t have access to as much oxygen as a bread starter does. In a dough that has access to lots and lots of oxygen, the carbon produced by the yeast is able to react until it’s all turned into carbon dioxide. All of that carbon, whether you’re making bread or beer, comes from the reaction of an intermediate compound (a molecule that’s formed before the final product) that isn’t shown in the above reaction, called pyruvate, which looks like this:
As a side note, this kind of drawing is called a line-angle drawing. And chemists use them all the time to make writing molecules quick and easy. The corners, or the ends of the middle horizontal line, represent carbon atoms. So, you can tell there are three carbon atoms in this molecule. The O’s are oxygens, and the H’s are hydrogens.
Alright! That’s a lot of chemistry! I think I need some cider….
Also, if you want to see some wild-yeast action for yourself, and enjoy some delicious bread in the process (and who wouldn’t?) this instruct able is worth checking out.
Also also, definitely watch this video featuring Alton Brown, my food science hero: