Beer Fermentation is one of the magical processes occurring in your home brewery. The process is not very well understood by beginning homebrewers, so it is usually where a lot of mistakes occur. Most will just pitch the yeast that came with the kit and hope for the best. It is an extremely important part of the brewing process. Learning about what's going on in the fermenter will make you a better brewer. Beer fermentation can be controlled, like many other parts of the brewing process. Before you learn to control it, you need to understand it.
Beer has been made since prehistoric times by people around the world without understanding the scientific processes that were occurring. Most likely the first beer fermentations were spontaneous with wild yeast. Wild yeast live everywhere. Mead was probably the first fermented liquid and could have occurred as rain water filled a cavity of honey in a tree. Wild yeasts present began fermenting the liquid into alcohol. Once the divine inebriating effects of alcohol were discovered by man, alcohol has been an important part of our lives ever since.
At first the beer fermentation process wasn't understood. Those that stumbled on ways to inoculate the beverage with yeast consistently were probably treated like magicians with divine magical powers. Various ways were discovered to save and transfer yeast from one batch to another. Whether it was by stirring the beer, wine or mead with the same paddle every time, or adding a fruit, then removing and saving the fruit for the next batch, being able to insure a batch of beer would ferment was key to improving the quality of ancient fermented beverages.
In ancient Egypt, beer was made by adding a sourdough (which contained the yeast) to damp freshly germinated barley and baking it gently until it just formed a crust. This would allow the loaves to be stored for a period without spoiling. The loaves were broken up in rainwater and allowed to ferment into beer. The first beer was probably bland or worse, flat and unfiltered. Most had to be drunk through filtering straws to keep the bitter residue out.
Beer gave ancient man a reliable way of hydrating since water supplies were mostly contaminated. Beer was given to women, and children as well as the men.
It has been speculated that civilization formed from the need to grow cereal crops for beer. Nomadic hunter-gatherers settled down to help with the planting, harvesting and storing of barley.
Once beer fermentation processes and yeast in particular were understood, the face of brewing changed forever. Sanitation became important along with yeast propagation. The resulting improvements in the quality of beer were probably impressive. Of course no one can actually know what the beer tasted like even a couple of hundred years ago. But with our scientific knowledge, we can speculate and get a pretty good idea.
In 1836 Cagniard de Latour proved that yeast in beer were living organisms and not chemicals as everyone believed. He also showed that these yeast cells were necessary if fermentation into alcohol was to take place. But saying that yeast were necessary and that they were the single cause of beer's fermentation were two different things.
In 1860, Louis Pasteur wasn't the first person to discover yeast. But he was the first to understand that beer's fermentation was caused by living organisms and that those organisms were yeast cells. Mr. Pasteur then went on to prove that yeast didn't need oxygen to live like most organisms. When in the presence of oxygen, yeast would multiply and when all the oxygen was consumed, would then begin fermentation. Louis Pasteur invented pasteurization to kill the yeast and halt further fermentation. Pasteurization was applied to beer around 1876, and the rest is history.
That's why it is so important to keep sanitization at the forefront of your mind when making beer, wine or mead. The yeast cells must have a chance to multiply and dominate the fermentation so other unwanted contaminating organisms can't have a chance to gain a foothold and spoil your beer. The faster the onset of active fermentation, the less chance of contamination. This is why we make yeast starters, to give the yeast a chance to grow faster than any other organism and thus crowd them out of the picture. They are still there, but in such minute amounts that their effects are not noticed in the flavor, aromas and appearance of your beer.
At first the processes seems pretty simple. Yeast are added to your cooled wort and they begin consuming oxygen and nutrients and multiply. They will continue to multiply until all the oxygen is consumed. Then they attack the sugars in the beer like it's their last meal. Beer fermentation can be so vigorous and so much CO2 can be produced that a foamy head can rise a foot or more above the top of the beer.
Alcohol, CO2, and other byproducts such as fusel alcohols (or fusel oils) are being produced by the yeast fermentation. Fusel oils are a mixture of organic acids, higher alcohols (propyl, butyl, and amyl), aldehydes, and esters, known collectively as congeners. It is these cogeners that are purported to cause hangovers.
This process continues until all the simple sugars are consumed in the beer. The yeast begin to settle down. Some are going dormant and falling to the bottom, while others continue to eat the more complex sugars and other byproducts in the beer. At some point, the yeast have eaten everything there is to eat. The yeast have nothing to do now but settle to the bottom of the fermenter and wait for more food. To decide if you need to transfer your beer to a secondary fermenter at this point, click here.
This beer fermentation process usually takes about a week for most beers. At higher fermentation temperatures, more of the byproducts (from fruity esters to solvent-like fusel alcohols) are formed giving the beer a fruity or phenolic flavor and aroma. At cooler temperatures, the yeast act much slower and the byproducts are minimal. This is one of the main difference between lagers (fermented cold) and ales (fermented warm).
Lagers are much cleaner tasting than ales with the flavors favoring malt and hops. Ales on the other hand are much more complex in their aromas and flavors favoring the fruity, spicy characters. There is a somewhat linear relationship with temperature though. Ales fermented at colder temperatures will have less "ale character" and more "lager character". Conversely, lagers fermented at warmer temperatures, such as the California Steam Beers, will show more of the "ale character" with more fruity/spicy ester production.
The other difference, and what usually separates the two types of yeast, is that lager yeasts ferment on or near the bottom of the beer while ale yeasts ferment at the top, then fall out. Since yeasts are everywhere, it makes sense that some would be adapted to the colder climates. This is how they were discovered. Beer that fermented in the colder climates like Germany were stored in ice caves to keep them from spoiling over the hot summer months. Normal ale yeast wouldn't be able to continue cleaning up the byproducts of primary fermentation in such a cold environment. Those yeasts that were present in the areas of Germany during the winter were already adapted for cold beer fermentations through evolution. Once yeast was understood, it became apparent that this was a different type of yeast than the ale yeasts present up until that time. The process of storing the beer in the caves is where we get the word lager, which in German means "to store".
Yeast need certain things in order to do their job. They need sugars, amino acids, fatty chemicals called lipids, and minute amounts of minerals such as zinc and copper. Fortunately these are already in most beer worts and it isn't necessary to add extra nutrients for the yeast to perform well.
The other thing they need is oxygen. They use oxygen to form the cell walls when they multiply. These cell walls are composed of sterols and unsaturated fatty acids. If there isn't enough oxygen in the beer, the yeast quit manufacturing sterols. The lack of sterols means they can't synthesize their cell walls so they must stop multiplying.
The lack of oxygen (and sterols for cell growth) is what limits the amount of yeast cell growth in your beer. A lack of oxygen will produce an under-attenuated beer (not fully fermented, leaving unfermented sugars). That's why it's so important to oxygenate your wort. Especially in higher gravity beers.
Oxygen is driven off when the wort is boiled. A little is incorporated by transferring the wort to the fermenter in a fashion that allows the wort to slosh around. You can aerate your wort by transferring back and forth from the kettle to the fermenter. A better way is to use an aquarium pump with an air stone. Since you are adding atmospheric "air" to the wort, the oxygen level can never get past a certain level, usually around 8 ppm. This is the minimum amount of oxygen needed for a good beer fermentation.
The best way to incorporate oxygen into your wort is through the use
of pure oxygen and a diffuser stone (not actually a stone but stainless
steel). You can purchase the regulator and tubing with filter at most
homebrew supply stores (support your local homebrew supply store-if they
don't carry it, he can probably order it for you). Small oxygen
bottles can be purchased at Lowes or Home Depot in the welding section. Click Here to purchase an oxygenation set-up from MoreBeer.com.
The amount of pure oxygen needed will vary depending on the specific gravity or your wort. I usually add oxygen for about 45-60 seconds for worts up to about 1.070 and about 1.5 minutes (90 seconds) for my barleywines. You can over-oxygenate your beer which will cause off flavors like excessive esters. This is a result of "oxidative stress" of the yeast. There is no exact consensus on how much pure oxygen to add so a little research is in order before you use it.
As you start to utilize the tools at your disposal as a homebrewer, such as proper pitching rates, good wort aeration, proper yeast selection and good fermentation temperature control, your beers will make a quantum leap in quality. Do these things right and you will be able to control the attenuation of your beer. To learn all about attenuation, click here. Consistent use will yield consistent results. And that's what we as homebrewers are shooting for, consistency. After all, what good does it do to make a really great beer just once? If you can't repeat it then it was just an accident. Take detailed notes of what happens during the entire brewing process from water selection to carbonation results and you will be able to repeat that award winning beer.
Beer Fermentation was adapted from: Jamil Zainasheff's MrMarly.com, John Palmer's How To Brew, Randy Mosher's Radical Brewing, and Al Korzonas' Homebrewing Vol.1.
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