Dr. Martin Rundkvist is a Swedish archaeologist, journal editor, public speaker, skeptic, atheist, lefty liberal, bookworm, and father of two.
Dear Reader, usually the deal here on Aard is that I tell you what to think and you reply, zombielike, "Yes... Master... Kill... Kill...". But today, let's turn the tables. I'm going to ask a question about a simple scientific-culinary matter that has baffled me for decades. And I hope someone out there knows enough about yeast to enlighten me.
- When starved of oxygen, yeast turns sugar into alcohol.
- When germinated, barley grains, by means of the enzyme amylase, turn some of their constituent starch into sugar. This process is called malting.
- In order to make beer, you must malt the barley. This suggests that yeast cannot make alcohol out of starch.
- But Swedish vodka is made from potatoes, which are very high in starch but cannot be malted. This suggests that yeast can make alcohol out of starch.
So here's my question: if yeast can make alcohol directly out of starch, why bother malting the barley before making beer? Couldn't you just mix barley flour with water and yeast and put a lid on the slop?
Update same evening: Dale P and other Dear Readers solved the conundrum för me. Yeast cannot in fact ferment starch. To ferment potato mash, you add enzyme-containing barley malt to it!
[More blog entries about yeast, beer, brewing, alcohol; jäst, öl, bryggerier, alkohol.]
Life Science
Comments
I did some quick digging on the interwebs and found two tidbits for you...
"...germination creates enzymes which are required to convert starch to fermentable sugar later in the brewing process..." from http://www.leeners.com/beer-about-barley.html
and
"...Malting is a process of bringing grain to its highest point of possible soluble starch content... It is at this point that the seed is rich in the starch it needs to use as food for growth." from http://www.alabev.com/ingredie.htm
So it's either to generate enzymes, or make more of the barley's starch soluble. Or both (I'm suspecting both).
Posted by: Sam Wise | November 21, 2009 9:18 AM
So how can you make vodka from potatoes?
Posted by: Martin R | November 21, 2009 9:21 AM
Potato starch still requires conversion to sugar for fermentation to occur. Typically, this is a thermal conversion by a long cooking step, rather than an enzymatic conversion as takes place in barley malt. It's worth noting that in beermaking, the partial conversion during malting is taken to completion during the brewing process (mashing). Potato vodka skips the malting stage, but still requires the mash step to hydrolyze the starch into fermentable sugars.
So no, you can't just ferment a pot of barley flour - you could heat it, and get some fermentable sugar out of it, but it'd be inefficient. Since vodka is a distilled spirit, inefficiency can be compensated for by the distillation process.
Posted by: Eric | November 21, 2009 9:25 AM
Aha! Now we're definitely onto something. Starch can be broken down into its constituent sugars by extended boiling?
Posted by: Martin R | November 21, 2009 9:27 AM
From
http://www.howtodothings.com/food-drink/how-to-make-potato-vodka
"2. Cook the potatoes. Place the potato cubes into the pressure cooker. Then, pour water over them until the potatoes are completely covered, plus about an inch of water. ... Cook until the potatoes have completely combined with the water, or the potatoes have nearly liquefied.
"...
"4. Add malt or enzymes. Wait until your potatoes have cooled to about 150 degrees before adding malt or enzymes. The malt converts the starch in potatoes to fermentable sugars.'
Posted by: DaleP | November 21, 2009 9:37 AM
How cool! You actually use the amylase from germinated grain to break down the potato starch?
Posted by: Martin R | November 21, 2009 9:46 AM
Amylase is amylase, for the most part. In Chicha production, it's human saliva that provides the amylase to break down cassava starch. And yes, thermal hydrolysis is a way of breaking down starch into fermentable sugars, but it's reasonably energy-intensive and requires high heat and pressure to be efficient compared to enzymatic hydrolysis.
Posted by: Eric | November 21, 2009 9:59 AM
When baking sourdough bread, a common technique is to proof your dough in the fridge overnight. This slows down the yeast activity, while leaving the amylase enzymes free reins converting the starch. As the dough is removed from the fridge, the yeast wakes up and starts feasting on the resulting sugars.
I believe that the addition of malted barley to the dough has a similar effect, but saves you time, probably at the expense of flavours.
Not sure if this has anything to do with anything, really. Your #3, perhaps.
Posted by: Pär | November 21, 2009 10:00 AM
Nope,
Yeast cannot ferment starch, they only take in simple sugars (glucose, fructose, maltose, etc). In making beer or vodka, the first step is to convert the starch to sugars via enzymes. Traditionally in European production malted barley was the source of the enzymes.
When barley germinates (the first step in malting) it starts producing large quantities of amylases (there are several kinds) and other enzymes that together breakdown starch. Total starch & starch breakdown products don't increase. The heating step that follows kills off one set of enzymes but leaves others working. It also adds flavor.
Boiling potatoes doesn't break down starch. It does melt starch crystals so they are more easily digested by the enzymes
Brought to you by a Certified Biochemist!
http://recipes.howstuffworks.com/beer.htm
http://en.wikipedia.org/wiki/Fermentation_(food)
Posted by: Ex Biochem Prof & occasional brewer | November 21, 2009 10:56 AM
As always, oxygen is as much of a killer as it is an enabler. Oxygen will kill the enzymes given half a chance, but they can survive with the help of enough sugar to feed the engines as it were. In order to make fermented brews with Stone Age technology which could not be completely anaerobic you needed a lot more sugar than you do with modern or even historical technologies. Malting and adding sugar in the form of honey helped a lot.
Posted by: ArchAsa | November 21, 2009 11:19 AM
Another, and important reason, for malting the barely is by maximizing the starch --> sugar process, you can get a greater diversity of flavour when you roast the malted barley, which is why you can brew very light lagers up to very dark stouts from the same two row malted barley - roast it longer to caramelize the sugars more.
Posted by: Doug Alder | November 21, 2009 12:42 PM
Thanks everybody, finally I understand!
Posted by: Martin R | November 21, 2009 4:05 PM
Martin: I am glad to see that you have been enlightened on the core question (starch needs to be broken up into its constituent sugars for these to be available for fermentation, and using enzymes is the most efficient way to do this).
However, I have a nit to pick, which may enlighten you further:
While that is true for many Swedish vodkas, it is not always so. Absolut, for example, is a grain vodka.
/nitpick
Posted by: Thinker | November 23, 2009 4:06 AM
If I showed that I know that, then I would lose tee-totaller street cred.
Posted by: Martin R | November 23, 2009 4:15 AM
Posted by: Bayesian Bouffant, FCD | November 23, 2009 11:06 AM
http://en.wikipedia.org/wiki/Malted_milk
Posted by: Martin R | November 23, 2009 11:18 AM