Wednesday, February 8, 2012

Brewing Basics - Fermentation 101

For today's post, I will be stepping away from the brew-day portion and will be focusing on the basics of fermentation.  I'll be describing what happens during the fermentation process, basic ways to track the progress of fermentation, and what steps we can take during the fermentation to create the best possible end product.

Some Terms to Clarify:

  • Aerobic: Bio-chemical process that require oxygen
  • Anaerobic:  Bio-chemical process that do not require oxygen
  • Fermentation: Anaerobic process where yeast metabolizes sugars and produces alcohol and C02
  • Priming: The addition of a specific amount of fermntable sugars to fermented beer prior to bottling.  These sugars are used by yeast while in the bottle to add carbonation.
  • Rack\Racking:  The act of transferring, usually by siphoning, fermented beer from one vessel to another, whether fermenter to fermenter or to bottle\ botteling bucket. 
  • Respiration: The first stage of yeast after pitching.  Aerobic process where yeast consumes oxygen for    reproduction, cell maintenance and fermentation
  • Secondary\Secondary Fermenter: The act of using a two stage fermenting system
  • Sedimentation: Life cycle of yeast after fermentation.  Yeast begins to flocculate (clump) and drop out of solution.

What is Fermentation?
Fermentation is an anaerobic process (one that does not require oxygen) that converts simple sugars into two main products, alcohol (ethanol) and CO2.  This is achieved by introducing healthy active brewers yeast to sterilized, room temperature beer wort.  Often with brewing, fermentation is done in two stages, Primary and Secondary fermentation.  The fermentation (ignoring bottle conditioning and aging) process usually takes about two weeks, but is dependent on factors such as temperature, available nutrients, yeast strain and sanitation.  These items will also effect the final product.

The chemical reaction of actual fermentation is:
C6H12O6 --> 2CO2 + 2C2H5OH

An Important Note About Fermentation:
Sanitation. Everything I'll be talking about here must be sanitary.  Keep your fermenter sealed at all possible times, only opening when necessary.  Anything that touches the inside of the fermenter must be sterilized.  Keep airlocks and rubber stoppers clean and sanitized. Poor sanitation is almost always the cause of the following problems in the final product:

  1. Over Carbonation: While this can happen due to improper priming of your beer prior to bottling, introduction of bacteria or wild yeast can kick in a fermentation in bottle, either fermenting leftover sugars, or converting starches that brewers yeast is unable to
  2. Sour Beer: Caused by bacterial infections
  3. Moldy Beer:  There are a ton of molds that can live on the surface of your beer.
  4. Cloudy Beer:  There are a ton of infections that result in a haze in your brew
In fact, I'd say almost all problems with finished products are due to sanitation, and the fermenting time is when your brew is very vulnerable.  One good sign of a bacterial infection is if you see a film build up where the surface of your beer is in contact with the bottle. Note, there are no known pathogens that can live in fermented wort.  So that infected beer won't make you sick, well minus the stomach ache from drinking too much vinegar. 

Variables We Are Worried About During Fermentation
  1. As mentioned, sanitation
  2. Ensuring enough oxygen for the first cycles of yeast (respiration)
  3. Ensuring no additional oxygen is added
  4. Available nutrients
  5. Average temperature
  6. Consistent Temperature
So I've talked about item #1.  Item 2 I have gone through in the Ingredients 101 Post, on the section regarding yeast.  Basically, after boiling wort we need to re-introduce oxygen into the wort prior to pitching. This is because during respiration, they need to eat up oxygen.  This helps them store energy for the next few weeks of fermentation.  

But! Item #3 above mentions we don't want to add anymore oxygen, but you'd think that the yeast would like more available oxygen during fermentation.  The fact that the fermentation process is anaerobic means we can use the lack of oxygen to our advantage.  For a newly introduced colony of wild yeast to survive after your pitched brewers yeast has taken off, it needs oxygen.  No oxygen available means it can't colonize the wort.  Same with most bacteria.  So try hard to keep you fermenter closed, and try not to agitate the brew.  Keep it stationary

Available nutrients are very important to make sure your yeast is able to colonize your wort quickly before any wild strains can get in on the action.  Obviously we need to make sure we have properly converted our starches into fermentable sugars for yeast to metabolize. We also need to make sure that there are the proper amount of amino acids available.  If using fully modified grains, a normal mash will give us enough amino acids.  If we are using under modified grains or a lot of adjuncts (like wheat), then we need to preform a "Protein Rest".  Essentially, prior to raising our grains to our strike mash temperature, we raise the grains to 120°F, and hold for 20 minutes.  This breaks down the proteins in the grain into usable amino acids.

Average temperature is another important factor.  Choosing a yeast that works within the temperature range your fermenter will be sitting at is crucial. If your fermenting environment is too warm, you'll be getting odd, undesirable flavors in your beer.  Also, you're less likely to hit the target body style of your brew.  If it is too cold, fermentation will be slow and likely to not ferment fully.  This will result in an overly sweet, malty beer. This can also result in fermentables left that may kick into fermentation in the bottle, creating over carbonation.

Finally, keeping a consistent temperature is an important factor in getting the highest quality brew.  Fluctuation of temperature will cause off flavors, or could shock yeast.  Although it is hard for the homebrewer to keep their fermenters at consistent temps (especially if it's in a garage!), there are little things you can do to help mediate this.  Wrap your fermenter in something that's an insulator.  Also, there are products you can buy that help keep your temperature constant, like the electric carbouy warmers that wrap around the outside of your fermenter.
Tracking Your Fermentation Process:
So, how can we tell how far along we are in our fermentation?  The key here is to pay attention from the beginning and make sure your keep good notes.  The two easiest ways to monitor the progress of fermentation are Gravity readings and airlock bubble rate (quite a technical term!)

We want to make sure that prior to pitching our yeast we take a specific gravity reading and record it.  Using this number as a starting point, we can track the progress of our fermentation.  As our wort ferments, chains of sugars are metabolized into Alcohol and CO2.  As those sugars are converted, the overall density and therefore Specific Gravity drops.  By taking gravity readings during the fermentation process, we can watch the gravity drop.  As we move forward through the fermentation, the rate of change will slow.  We should also have an estimated final gravity, the Specific Gravity we think we will hit after fermentation has completed (there will always be non-fermentables plus some fermentable left, see the Attenuation portion of the ingredients post).  Once this has slowed to almost a stop, and we're pretty much at our estimatedFG we can say that primary fermentation has completed.

The other way to track the progress is to look at our airlock.  Since one of the products of fermentation is CO2, we can draw a parallel to the production of CO2 to the rate of fermentation.  Once fermentation commences, CO2 fills the head space of the fermenter, then starts getting pushed up and through the airlock.  The airlock will start to bubble at constant rate.   In my experience, after  about 12 hrs of pitching, I'll get about 1 bubble per second. As fermentation slows, so does the rate of bubbling.  Usually the bubbling will slow to 1 every minute or so, and that's when we're pretty much done with Primary fermentation. 

Sedimentation and Secondary Fermentation:
After the first 3-7 days from pitching, fermentation will slow.  During this period, the yeast begins the process of sedimentation.  Once sedimentation occurs, very little fermentation happens. Here the yeast is getting ready to go into dormancy due to the lack of fermentables available.  If you want to culture yeast, this is the time to grab it.

So, once sedimentation has occurred, we could pretty much call our beer ready for bottling, and some people do stop here.  But we can add another step that will help clear up our final product, reduce sediment in the bottle and clean up some of the flavors. The yeast now will start to go through the process of autolysis, or the act of deterioration.  This will impart an undesirable flavor on our final product. Now this step is more of an aging type of a step, but since it is referred to as "Secondary Fermentation" I'll be talking about it here.  Note, this step is often called just "the secondary".

After sedimentation, we want to carefully transfer our beer to a new sanitized fermenter. We usually do this with a siphon.  Make sure you have the proper siphon equipment, or you are just going to waste your time.  Get a good racking cane and the proper length of tube.  Take care if you need to move your fermenter prior to racking.  If your fermenter is on the ground you'll need to raise it as we normally use gravity to move the beer.  Allow your wort to settle prior to racking.

Go slow and take your time transferring.  You'll either need to purchase some sort of racking cane holder or hold it yourself.  Start your siphon correctly.  The key here is to keep the flow moving at all times, stopping can make liquid backup into the fermenter you are attempting to transfer out of, kicking up sediment. Keep the tip of the racking cane in between the sediment and the top of the liquid level.  You'll have to decide when to stop.  It's a game, the more you pull out the more and more sediment you'll pull up.  You will need to leave some liquid behind. We don't want to pick up the yeast cake on the bottom, or any leftover foam on top.  While we need to have yeast in our secondary, there is still a ton of it in suspension in the beer.

The secondary is a great place to add hops for dry hopping.  Also a great place to add other herbs or spices to add mellow aromatic flavors.

After transfer is completed, seal up the fermenter.  Usually secondary last about another week. During this time, more yeast and sediment will fall out. There might be a slight up-tick in the rate of fermentation.  Once everything is settled, it's time to bottle.

No comments:

Post a Comment