Fermentation 101 Techniques

On brewday, you make your wort. Then you unleash an army of yeast cells to turn the wort into beer. The biochemical details of fermentation are the same for all brewers, whether professional or homebrewer. However, homebrewers approach their fermentations with a different set of equipment and operate at a much smaller scale than professional brewers do. In this article, I will discuss the homebrew-specific aspects of fermentation, including fermentation options, equipment and what to do if you experience a stuck fermentation.

Open vs. Closed Fermentation

Homebrewers have the choice of performing an open or closed fermentation. As the name implies, open fermentations take place in a container that is open to the environment. Traditionally, most English ale breweries used open fermenters and some still do. One advantage to an open fermentation is that the yeast have access to environmental oxygen during fermentation. Breweries that use open fermentation typically have wide, shallow fermenters that expose much of the surface area of the wort to air. The fermenter shape itself may also influence the development of beer flavor. Most homebrew fermenters, in contrast, are relatively tall and deep. The absolute depth of most homebrew fermenters, however, is not that great.

The obvious problem with open fermentation is that the wort can easily become contaminated by airborne microorganisms, leading to sour or other off-flavors or aromas. Some breweries with open fermenters use positive pressure in the fermentation room to reduce the chance of contamination. Filtered air is pumped into the room, resulting in a net flow of air — and things floating in the air — out of the room.

Closed fermentations take place in sealed containers. The carbon dioxide gas (CO2) produced is vented through some sort of pressure release mechanism — homebrewers use a fermentation lock or blow-off tube. Most commercial breweries use closed fermentations. Likewise, the vast majority of homebrewers use closed fermentations to lower the risk of contamination.

If you do want to try an open fermentation, select a top-fermenting strain of ale yeast and the widest fermenter you can find. You can use a stainless-steel kettle, if you’d like. Place the fermenter in the cleanest room (or closet) in your house and make sure it won’t be disturbed by kids or pets. Make and pitch a yeast starter so you get the fermentation started as soon as possible. Consider putting a lid on the fermenter until there are visible signs of fermentation. Then, remove the lid and let the beer undergo primary fermentation. Rack the beer to a closed secondary fermenter or package it when primary fermentation stops.

Secondary Fermentation

Many homebrewers perform a “secondary fermentation” of their beers. Secondary fermentation simply means the beer is racked to another vessel (a secondary fermenter) and allowed to age prior to bottling or kegging. Despite the name, the beer does not begin a second phase of fermentation. For this reason, some people call this phase conditioning or maturation instead of secondary fermentation.

The main advantage to secondary fermentation is that the beer is racked off the trub and yeast from the primary fermentation. If the beer remains in contact with these materials for too long, the beer can pick up “meaty” or “brothy” off-flavors. How long a beer can remain in primary is a debated point among homebrewers. Some rack their beer as soon as primary fermentation has stopped, or slowed substantially. Others report batches that have sat in primary for months and still tasted fine.

Two disadvantages of racking a beer to a secondary fermenter are the potential to expose the beer to microorganisms or oxygen during the transfer. If you practice good cleaning and sanitation procedures, the first disadvantage is negligible. However, this still leaves the problem of oxygen pickup. To minimize this, rack your beer as quietly as possible. Keep the outflow end of the racking tube under the beer in your secondary fermenter. Some homebrewers use secondary fermenters that are smaller than their primary fermenters to minimize the headspace in the secondary fermenter. In addition, they may top up their secondary fermenters with water that has been boiled, then cooled.

If possible, you should lower the temperature of your beer during secondary fermentation. Both ales and lagers benefit from a period of cold bulk conditioning. One or two weeks of cold conditioning, below 40 °F (4.4 °C), is sufficient for most ales. Lagers are typically aged longer (up to three months) around freezing.

To perform a secondary fermentation, clean and sanitize a fermenter and racking cane. Rack the beer from primary to the secondary fermenter once primary fermentation has ceased. Wait until the beer clears, then move the beer to bottles or a keg.


Once the wort has been cooled, aerated and the yeast has been pitched, the primary task for the homebrewer is to maintain the temperature at a suitable level. Homebrewers employ many diverse methods for temperature control and these have been discussed in detail elsewhere. See my article “Make Me Sweat” (Summer 2001) and Glenn BurnSilver’s “Chill Out” (January-February 2001) for suggestions on cooling and heating your wort, respectively.

Fermentation Equipment

The two most popular fermenters for homebrewers are buckets and carboys. For homebrewers with some spare change, cylindro-conical fermenters also are gaining in popularity.

Buckets: Like most homebrewers, I fermented my first batches of beer in a bucket. A bucket fermenter should be made of a food-grade plastic that is not affected by alcohol. Most homebrew bucket fermenters are made from HDPE (High-Density Polyethylene) plastic, which can easily withstand any of the substances found in a normal batch of homebrew. Five to seven-gallon (19–26.5 L) buckets are the most common sizes.

As a fermenter, buckets have several advantages. Buckets are cheap and, for all practical purposes, non-breakable. Unless you place them in direct sunlight, most will shield your wort from harmful levels of light. Light can cause skunking and perhaps cause mutations in the yeast. They are easy to clean because, unlike carboys, you can reach all the surfaces without a brush. A bucket can also withstand the temperature extremes of homebrewing, from boiling to cool liquids. In addition, buckets have handles and are stackable, which saves storage space — so what’s not to love?

Well, over time plastic buckets will absorb flavors and odors from the materials in contact with them and some sanitizers — iodine, for example — can discolor the plastic. Bleach will also leach into the plastic. Many homebrewers believe that plastic buckets become progressively more susceptible to harboring contaminants the longer they are used. They argue that small scratches in the plastic provide hiding places for bacteria. Finally, plastic is oxygen permeable. Over time, oxygen can diffuse into your beer.

The point about sanitizing is debatable, but the fact that buckets gradually become discolored and take on an aroma isn’t. I still have my very first homebrew bucket. I can detect a vaguely beer-like smell coming from it from a couple feet away. I’d brew a batch of beer in it if I had to . . . as long as the beer was a ginger-raspberry Imperial stout lambic.

Of course, buckets are cheap enough that you can simply replace them when they become unusable. Like tubing, they are something that should be replaced periodically. A brand-new bucket fermenter will last for many batches and have no adverse impact on your beer. As long as it cleans up and doesn’t harbor any aromas, you can keep using it. As for oxygen permeability, this shouldn’t be a problem during active fermentation. There will be plenty of yeast cells to scavenge any oxygen while the batch is still fermenting. However, to be safe, you should never let a batch sit for too long in a plastic bucket once fermentation has subsided. For this reason, many homebrewers use a bucket for primary and a carboy for secondary.

Carboys: Glass carboys are available in a variety of volumes, with 2, 3, 5 and 7 gallon (7.6, 11, 19 and 26.5 L) volumes being common. Carboys are cheap, but have many disadvantages compared to buckets. They are slippery when wet and breakable if dropped, making handling them potentially messy and dangerous. Carboys do nothing to block light and you need a brush to clean them. You can crack a carboy if the temperature changes too quickly in it, for example if you pour hot wort into room temperature glass. Carboys also take up a lot of storage space, as they aren’t stackable.

Despite these shortcomings, most homebrewers prefer carboys to buckets. The primary reason for this is that glass will never absorb any flavors or odors and can be completely cleaned after use. Handled properly, the carboy will remain flavor-neutral forever. In addition, oxygen cannot permeate glass. So, you can let a batch of beer age indefinitely without worrying about oxygen getting to the beer through the glass. For many homebrewers, these two advantages outweigh all of the disadvantages.

In addition, the shortcomings of carboys — except for the extra effort of cleaning them — are easily overcome. If you make sure to siphon cool wort to your fermenter, wipe off any liquid on the outside before moving it and cover it with a dark T-shirt, you will be fine. If you are overly worried about flying glass from a breaking carboy, you can wrap the carboy in a criss-cross pattern with duct tape. This won’t keep a carboy from breaking, but it will ensure that most of the glass shards will be contained if it does break.

Glass can develop beer stone, however. Beer stone is a deposit of minerals that often builds up on beer equipment, especially when hard water is used. If your carboy develops beer stone, you can remove it with vinegar or a commercial scale-removing solutions such as CLR.

Cylindro-conical fermenters: If you can afford one, stainless-steel fermenters are a great choice. Cylindro-conical fermenters are available in 7-gallon (26.5 L) and larger sizes. Cylindro-conical fermenters allow open or closed fermentation, are accessible for cleaning and light proof. Stainless steel won’t absorb flavors or aromas. It can handle homebrew-relevant amounts of temperature stress and is not fragile, like glass.

Cylindro-conical fermenters also have features that buckets and carboys lack. A valve at the bottom of the fermenter allows you to draw off yeast and trub that has sedimented. So, you can perform both primary fermentation and conditioning (secondary fermentation) in it. Many include a racking arm for transferring beer to Corny kegs. In addition, some have glycol jackets for controlling the temperature during fermentation.

Stainless steel fermenters start at around $400 and proceed upwards. Plastic models, starting around $100, are also widely available and offer a good, affordable alternative.

Fermentation locks: Homebrewers performing closed fermentations seal their fermenters with either a fermentation lock or a blow-off tube. Fermentation locks sit on top of your fermenter and seal the wort from the outside with a layer of water. As carbon dioxide is produced, it must bubble through the water to escape. Your fermentation lock should be thoroughly cleaned and sanitized before placing it on your fermenter.

The two most popular fermentation locks are the “S” type tube and the “jiggle hat” locks. Either works fine under most circumstances. If your fermenter will be cooled after the lock is placed, the “S” type of lock can prevent “suckback” — the condition where lock liquid is drawn into the fermenter by the lowering pressure in the fermenter brought about by cooling. If you fill the “S” lock with the minimal amount of water it will bubble backwards and not be sucked into the fermenter. If you add too much water to an “S” type lock, the whole column of water in the lock can be deposited into your beer. The water itself wouldn’t be a problem, but without the water the barrier to contaminants would be gone. Plus, if your water wasn’t sterile, it could contaminate your wort. Another way to avoid suckback is not to affix the fermentation lock until the carboy is at a stable temperature and fermentation has begun. You can fill the fermentation lock with water or some form of sanitizing solution. Adding water to the lock allows you to monitor your fermentation by smelling the gas escaping from the carboy without having its aroma tainted by the sanitizing solution. And, if suckback seems likely, you will avoid getting sanitizing solution in your wort.

Some homebrewers, however, add sanitizing solution to their locks as an added protection against contamination. For this, you can add dilute solutions as you would use for sanitizing your equipment. Alternately, you can use alcohol, such as cheap vodka. Be aware that some types of plastics can be clouded or cracked by some chemicals, so watch that your solution isn’t destroying your lock.

Blow-off tubes: During early fermentation, a blow-off tube is an alternative to a fermentation lock. A blow- off tube is a tube that extends from the fermenter to a small bucket of water. During early fermentation, kraeusen is pushed through the tube and out of the fermenter. Kraeusen, and the gunk that floats in it, does not taste good and using a blow-off tube is a way of separating this material from your wort.

Most homebrewers using blow-off tubes ferment in carboys. They fit a large-diameter Tygon tube over the neck of the carboy and place the other end in a large glass or small bucket. The carboy must be sufficiently full to allow the kraeusen to reach the tube. However, if the carboy is too full, you will lose some beer. The reservoir should be filled with a sanitizing solution. Otherwise, the kraeusen floating in the water and coating the inside of the blow-off tube can be a pathway for contaminating organisms to enter your wort. After early vigorous fermentation, the blow-off tube can be replaced with a fermentation lock.

Stuck Fermentation

One problem that homebrewers face is stuck fermentations — fermentations that don’t start or stop before attaining a reasonable final gravity.

The most common cause of a stuck fermentation is an inadequate amount of healthy yeast in the wort. If you pitch a vial of White Labs liquid yeast without making a starter, you have no idea how many of the yeast cells in the package are alive or healthy. The package may have been frozen or overheated in transit or at the store. Making a yeast starter will ensure that you have healthy yeast when it is time to pitch.

You can tell if the yeast inside Wyeast “smack-packs” are not healthy — the package won’t swell. If a package does swell quickly, you’ll know that you have some healthy yeast cells in the package. However, you can’t see inside Wyeast packs and I have found that it is relatively easy to pour the liquid out of a package and leave the bulk of the yeast behind. If you do use smack packs, be sure to shake well before pouring off the yeast.

White Labs tubes and Wyeast XL smack-packs are billed as “ready to pitch.” This is no doubt accurate, but I still recommend making a starter. Starting with plenty of healthy yeast, it’s still possible to end up with an underpitched wort if some of the yeast are stunned or killed when pitched. If your wort is too hot — over 100 °F (38 °C) or so — or, if the temperature difference between your wort and yeast is too great, you can end up with too few healthy cells in the wort.

When faced with a potentially stuck fermentation, you should first check to see if the fermentation is really stuck. One possibility is that the fermentation has simply run its course. Under optimal brewing conditions, a normal-strength ale will complete most of its fermentation in three to five days. At higher temperatures, however, the fermentation can go faster. Check and see if there is a ring of kraeusen, which looks like brown crud, around the inside of the fermenter. This is a sign that fermentation has occurred. Second, measure the specific gravity of the beer with a hydrometer.

If you do have a stuck fermentation, you need to correct the situation as quickly as possible. The best remedy is to immediately pitch an adequate amount of yeast. Keeping a couple packages of dried yeast in your refrigerator is a handy way to always have some backup yeast on hand. Pitching two packages of dry yeast should quickly kick-start a stuck fermentation.

Along with the added yeast, you may want to aerate the wort again. Late aerations can cause problems in a beer, however. If the fermentation has progressed less than one-third of the way to the final gravity, aerate the wort as you normally would on brew day. If the wort has fermented more than this before quitting, you are probably better off adding the yeast only.

Some homebrewers attempt to restart fermentation by adding yeast nutrients. However, lack of yeast nutrition is rarely the problem. My advice is to always make a yeast starter and pitch more yeast if necessary.

Issue: January-February 2003