This article will discuss yeast, fermentation, and techniques for manipulationg fermentation in pretty broad terms. First, we'll start with a definition, then move on to more detailed descriptions after the jump (click "read more").
Fermentation is a biological process for producing energy, by which
yeast convert simple carbohydrates into energy, alcohol and CO2.
What is Yeast?
Yeast is a fungus, classified as Saccharomyces cerevisiae. It is a ‘facultative anaerobe,’ which means that it is biologically capable of growth and respiration in the presence and absence of oxygen. Brewer’s yeast does not respire aerobically under normal brewing conditions because they’re well adapted to anaerobic fermentation. Lager yeast strains are technically a hybrid between S. cerevisiae, and a wine yeast S. bayanus.
Ale and Lager yeast strains exhibit a variety of different characteristics, though they often overlap. There are a couple of definite, if somewhat impractical tests for the home brewer that differentiate Ale and Lager yeast strains. First, Ale yeast can grow at 99° F, while Lager yeast cannot. Second, Lager yeast strains are able to utilize melibiose (a disaccharide of glucose and galactose), while Ale yeast strains are not.
It is generally accepted that Lager strains tend to operate best at cooler temperatures than Ale yeast, though exceptions can be found. Additionally, Ale yeasts tend to float on top of the yeast during fermentation, while Lager strains usually stay closer to the bottom of the fermenter. In either case, most brewing strains will sink to the bottom of the fermenter at the end of fermentation these days. Also, Lager strains tend to produce fewer esters and acetaldehyde, but more vicinal diketones (e.g. diacetyl) and sulfur compounds during fermentation. Some of this may be due to actual metabolic differences between the two types of yeast, but environmental factors (temperature and O2 levels) certainly have been shown to play a role in flavor production.
The Stages of Fermentation
Fermentation consists of 3 distinct phases: the Lag Phase, Growth Phase, and Stationary Phase. The lag phase begins immediately after pitching the yeast. During the lag phase, the majority of yeast cells are adapting to the new environment, taking up O2, and synthesizing amino acids. Very little CO2 or alcohol is produced during this phase, though many of the building blocks for off-flavors are created as byproducts of amino acid production. This phase normally lasts a couple of hours with healthy yeast.
The growth phase is characterized by heavy metabolic activity, with robust CO2 and alcohol production, and is usually broken into 3 sub-phases. During the accelerating phase, yeast reproductive rates increase rapidly. During the exponential growth phase, the growth rate levels off, with the total number of yeast cells doubling every 2 – 3 hours. The decelerating phase is characterized by a decrease in the rate of reproduction, as nutrient (necessary vitamins, O2, and minerals) sources become depleted. It is pretty difficult to distinguish between these sub-phases visually. The krausen begins to form at the beginning of the growth phase, and lasts into the stationary phase. The growth phase typically gives way to the stationary phase within about 48 hours of pitching.
During the stationary phase, the growth rate remains essentially flat as fermentation proceeds. There is plenty of sugar to consume, but few nutrients to utilize for reproduction. In this phase, all fermentable sugars and are consumed, the krausen eventually collapses, and yeast cells begin to flocculate and settle on the bottom of the fermenter.
Oxygen, pitching rate, and temperature are the most important factors that influence fermentation success, though they are certainly not the only factors.
Oxygen: 8 – 12 ppm dissolved O2 is required for healthy Ale fermentation. Lager fermentations require up to 15 ppm. At atmospheric pressure, about 8 ppm is the maximum attainable O2 level in normal wort through shaking a fermenter. Diffusion of compressed air through a sintered stone achieves higher levels, and pure O2 pushed through a sintered stone is better still. Aeration at any point after fermentation begins is generally undesirable.
Pitching Rate: Ale fermentations require about 10 million cells/ml, lagers 15 million cells/ml, for normal strength wort, though pitching rate requirements increase with gravity. In a 5 gallon batch, using a “pitchable tube” or smackpack alone pretty much guarantee that you’ll underpitch. Make a starter to increase cell counts prior to pitching. For detailed information on making starters, visit www.mrmalty.com.
Temperature: Correct fermentation temp is yeast strain dependent. Ale yeast strains are typically fermented at 60-72° F, while Lagers are usually fermented at 45-60° F. As temperature increases, off flavor production tends to increase. Fermenting below recommended temperatures can cause a stuck fermentation. Similarly, temperature fluctuation stresses yeast, leading to an increase in off flavors or stuck fermentations.
Other Byproducts of Fermentation
In addition to CO2 and ethanol, yeast produce many other flavor active compounds during the course of fermentation. Most flavor compounds are produced as byproducts of amino acid synthesis, which primarily occurs during the lag and accelerating growth phases. Virtually all off-flavors increase with fermentation temperature, thus correct temperature during these phases is critical.
One important flavor compound is Acetaldehyde, which is responsible for green apple, cidery or nail-polish remover flavors. It is a precursor to Ethanol in the glycolitic pathway (the main fermentation pathway). It is commonly found in beers that derive a lot of fermentable material from white table sugar, in warm fermented ales and lagers, and in beers lacking essential yeast nutrients.
To limit acetaldehyde production, there are a few things you can do: First, replace any table sugar in your recipe with malt extract. Second, use a yeast nutrient. Pitching rates also impact acetaldehyde production, so making a starter can also reduce these off flavors. Finally, acetaldehyde is reduced with yeast contact, so you may also be able to reduce these flavors in your finished beer by increasing time in the secondary fermenter and/or by bottle conditioning your beers.
Esters and Fusel Alcohols are two related classes of flavor compounds responsible for a broad range of flavor and aroma contributions – most notably banana and other fruit aromatics from esters and hot alcohol characteristics from fusel alcohols. They’re being discussed together because they are closely related, and are limited by the same factors. These compounds (and/or their precursors) are largely – but certainly not entirely – produced during amino acid synthesis, so once again, pitching at or below fermentation temperature is critical. Under-pitching and temperature are easily the two biggest driving factors in ester production. Thus, pitching a greater quantity of healthy yeast and proper temperature control are the most important steps that can be taken. Proper aeration of the wort is also important, so improving your aeration techniques can help limit ester and fusel alcohol production as well. On the other hand, aeration of the beer after fermentation begins can lead to increased ester production, so good beer transfer techniques can go a long way towards decreasing excessive fruity flavors in your beer.
Diacetyl is a compound that produces a buttery or butterscotch flavor/aroma in beer. It is a byproduct of amino acid synthesis, particularly of those amino acids produced during the lag and early growth phase, so taking steps to reduce the lag time will help reduce this character in the final beer. Diacetyl is also reduced naturally by healthy yeast at the end of fermentation, and this process is improved at warm temperatures. A diacetyl rest is often performed at the end of lager fermentations to reduce diacetyl. A diacetyl rest consists of simply allowing the temperature of a lager fermentation to rise to about 60-65° F at the end of fermentation, and letting it rest for 1-2 days before lowering the temperature.
There are two main types of fermentation systems used by homebrewers today – single and two vessel systems. In two vessel systems, green beer is typically racked to a secondary vessel just before – or sometimes immediately after – primary fermentation comes to a close. The beer will then condition for a few days to a week in secondary vessel before packaging. Single vessels typically consist of a cylindro-conical tank with a bottom dump valve. This system differs from two vessel systems in that after primary fermentation, instead of moving the beer from the yeast cake into another vessel, the yeast is removed from the fermenter via the bottom dump valve. This reduces chances of contamination or aeration during transfer and leaves you with one less container to clean.
Double Vessel Systems
Buckets: Your typical hardware store white buckets make excellent primary fermenters. They typically hold 5 – 6.5 g of fluid and are usually made from #1 or #2 (food-grade) plastic. Other types of plastic may or may not bee suitable for beer production. A nice bonus features can include a hole & spigot on the bottom for easy racking. These buckets make great primary fermenters. Pros: easy to clean, durable, inexpensive. Cons: Not an oxygen barrier, easy to scratch (which can make sanitization difficult), not super easy to harvest yeast.
Carboys: These glass jugs are common fare in the homebrew world. Ranging from 3 – 6.5 g in size, carboys have a narrow opening at the top, which fits a rubber stopper and airlock perfectly. They are great for primary or secondary fermentation, since glass is a good oxygen barrier, but aren’t quite as easy to clean as a bucket. When using these as a primary, be sure to allow 20-30% of headspace for krausen, so that you don’t blow off all your beer. When using carboys as a secondary, be sure to purge with CO2 before racking into the secondary, and size your secondary vessel as close to your batch size as possible to minimize aeration of the beer.
Cornelius kegs: These stainless steel cylinders are most commonly used for serving hombrew or soda, but can be used effectively for primary and/or secondary fermentation. The advantages are that they completely seal at the top, but have a wide enough opening to allow fairly easy access to the interior. Since these are most commonly found in volumes that exactly matches typical homebrew batch sizes (3, 5, or 10 gallon), they make better secondary fermenters than primaries, but they can be used for both.
Sanke Kegs: Available in 5.2, 7.5, or 15.5 gallon sizes, stainless commercial kegs can make excellent primary or secondary fermenters, with a minimum of modification. They are perfectly sized for 3, 5, or 10-12 gallon batches, but are pretty difficult to disassemble, clean and inspect. They are also pretty expensive.
Single Vessel Systems
Conical fermenters made from stainless steel or plastic are typically used in single vessel systems, though many beer styles can be produced in carboys, then racked directly to kegs or bottles. Conical fermenters work great for producing any style of beer, and are usually very easy to clean. Plastic versions should not be used to lager any beers for an extended period of time, because plastic is oxygen permeable. Some conical designs require disassembly of fittings between each use, but some do not. Though, conical fermenters have lots of advantages (and give a brewer tons of street cred to boot), they are pretty dang expensive, and pretty much require some form of temperature control.
The goals of fermentation control are pretty straightforward: to set and maintain constant fermentation temperatures to ensure healthy fermentation and tasty beer. Fermentation control can be accomplished by monitoring and adjusting either ambient air temperature or actual beer temperature. Successful fermentation control is hamstrung by the fact that ambient air temperature and actual beer temperature will never match when it matters most (during active fermentation). This is because fermentation is an exothermal process (it produces heat). During the height of yeast activity, the actual beer can be 7-15° F warmer than ambient air, though this difference will both vary over the course of a single fermentation as well as between subsequent fermentations. This variation makes it difficult to accurately and reliably control the temperature of the beer by measuring and affecting ambient air temperature.
We have several weapons at our disposal to aid us in the epic battle of yeast versus heat: thermometers are used to measure ambient air temperature, thermo-wells are used to monitor actual beer temperature, and thermostats are used to control our cooling and/or heating devices in order to maintain the desired temperature. Stick-on liquid crystal thermometer strips (fish tank thermometers) measure neither the ambient air temp nor actual beer temp, but something in between, which varies based upon the thermal conductivity of the fermenter.
Ideally, we want to directly measure and control the temperature of the beer, which is easier said than done. Direct measurement of the beer temperature is fairly straightforward: drop a thermowell and sensor into your beer and you’ve got an accurate measurement of your beer temperature. Directly affecting that temperature is another matter. Here are some common methods used to reign in fermentation temperatures:
Wet T Shirt w/ Water Bath: Not as exciting as it sounds, the wet t-shirt technique relies on the magic of evaporation to drop the ambient temperature a few degrees. Basically, put an old t-shirt on your carboy, and drop the whole thing in a water bath. The t-shirt will wick water from the bath up the neck and sides of the carboy, where it evaporates. Evaporation is and endothermic process (it absorbs heat), so it will absorb the heat produced during fermentation. It works best in a dry climate, and with good air circulation can drop the temperature of your carboy 10-15° F. Once you factor in the heat produced during fermentation, you’ve got a net gain of about 0°. So this trick really only helps you maintain whatever the ambient temperature happens to be.
Water Bath w/ Recirculation: This technique is pretty much the same as the Wet T-Shirt Classic Edition, but uses ice water to drop the temp even further. Basically, this involves dropping the carboy into the same water bath and using a recirculation pump and soaker hose to constantly shower the shoulders of the carboy with water, keeping the shirt wetter and the carboy cooler.
Ultimate Beer Fridge or Chest Freezer:
Take a standard upright fridge or chest freezer, add a Johnson Control (or similar) thermostat, and set your desired temperature. Your target temperature should be a few degrees cooler than your target fermentation temp, due to the heat produced during fermentation. The thermostat will cycle the fridge on and off in order to maintain a set temperature (within 4°F). Some temperature control devices will work with a thermowell to monitor and maintain actual beer temp, but this will make the fridge work harder. Chest freezers provide a lot of space to work with, but are more difficult to get the beer in and out of. Upright fridges are easy to move beer in and out of, but provide less usable space, though 2 carboys or buckets will usually fit easily.
Homemade Ice box (AKA "Son of Fermentation Chiller):
This is basically an old-school ice box, with a muffin fan to blow cold air from the ice chamber to the fermentation chamber. The fan is controlled by a household thermostat. Go to http://www.blackcanyonbrewers.com/BCHA-PDF-Files/chiller.pdf to see plans for building this thing.
An immersion chiller works basically the same as the ones we use for cooling wort. Take a coil of some kind of metal (copper or stainless steel), drop it in your bucket or conical fermenter (it would be difficult to get something like this into a carboy), add a thermowell to monitor the beer temperature, and use a pump to cylce a coolant through the beer as needed to lower the temperature. The advantages of this system are that this is really the only method of directly affecting the beer temperature. On the other hand, 50' of copper or stainless steel would certainly be a chore to clean and sanitize.
Raines-Casselman, MB. "Yeast Propagation and Maintenance: Principles and Practices" http://www.maltosefalcons.com/tech/MB_Raines_Guide_to_Yeast_Culturing.php
Lewis, M, Young, T. "Brewing." Aspen Publishers, 1995