Keg Hopping, Residual Sweetness: Mr. Wizard
Q
Some questions came up in our homebrew club about keg hopping. First, will you get the same effects in a cold keg as you would a warm secondary? and if it does work, what is a good method (quantity, temperature, time frame)?
Josh Jayne
West Haven, Connecticut
A
My general rule, and one that is shared by many a brewer, is that most changes to a process variable will likely change something that can be detected in the finished beer. Sometimes the change can only be detected with a laboratory instrument and in other cases small changes can have very dramatic effects. This does not mean that the same end result cannot be obtained using different methods and the real challenge some brewers face is brewing the same beer in different breweries with very different tools.
When it comes to dry hopping, there are more opinions and hotly debated explanations of what is happening than there is science. This can drive a brewer crazy, especially for those of us who really want to know why!
If you want to end up with a nice dry hop character (carboy or keg is not so critical), give the following a whirl. Add 1 to 11⁄2 ounces (28 to 42 g) of good aroma hops (I will come back to this later) to a 5-gallon (19-L) batch towards the end of fermentation. When using a keg, I suggest racking beer from your primary to the keg before fermentation has ended. The reason for this recommendation is that hops, whether pellets or cones, are little air sponges and can lead to oxidation. Adding hops to beer with active yeast helps guard against detectable oxidation. Personally, I would rather dry hop in my primary because limited contact is something that I believe in . . . this is that opinion stuff coming into play!
There has been some very helpful research coming out of Dr. Tom Shellhammer’s group at Oregon State University and one of the practical, take-home messages is that not much time is required to maximize the aroma yield from dry hopping; two days is plenty of time. And since extended dry hopping can lead to the extraction of vegetal flavors from the hop matter there is a good argument to limit the contact time. Racking your beer off of the dry hops is a good way to simply minimize these flavors. This is why dry hopping in the primary is convenient; just rack your beer into your keg and move forward as usual. Pretty simple.
You can certainly dry hop cold beer, and with the change in method you should expect some changes. One of the more notable differences is that it takes longer to extract the hop aroma oils with cold beer. Time is something that most homebrewers can accept, and the added time is not a major drawback. But there is one difference that does concern me and that is the reduced yeast activity with the cooler temperatures. Since active yeast help mop up oxygen, and dry hopping does add oxygen to beer, it logically follows that dry hopping warm beer with active yeast is less likely to result in beer oxidation than dry hopping cold beer with much lower yeast activity. This is the reason that I prefer dry hopping warm beer.
There are many new dry hopping methods being used by commercial breweries, which have been developed to address the challenges of adding large masses of hops to large fermenters. I do not believe there is any reason to use these sorts of methods when brewing at home. But there is one thing that the best hoppy beers all share that is as important to a 1-gallon (4-L) batch of beer as it is to a 50,000-gallon (190,000-L) batch of beer; hop quality.
Hop quality is subjective and not all brewers and beer consumers prefer the same sorts of hop aromas. Hop quality is also objective in the sense that hops devoid of tropical fruit and citrus aromas will not yield these aromas in a dry-hopped IPA, regardless of the hop variety name that is printed on the bag. Great hop aroma happens when you use great hops. This is truly a topic fit for an entire book, with much of the information yet to be defined.
What all brewers can do is evaluate hops before purchase and before use. If you know where hops are grown and the year the hops were harvested you can do research about the crop and the crop year. Knowing how the hops have been packaged and stored is also valuable information because great hops can be ruined if not packaged and stored properly. Smelling those hops before use is the last line of defense to prevent adding subpar hops to an otherwise great batch of beer.
Q
How and why do hot alcohol (solvent) smells get produced and is there a way to eliminate them in brewing?
Matt Stich
Bellwood, Pennsylvania
A
The “hot” and solvent-like aromas you describe are associated with higher alcohols. The term “higher alcohol” is used to describe alcohols that have a higher molecular weight than ethanol, and grammatically, the term “higher molecular weight alcohols” makes more sense to me! These compounds are also known as fusel alcohols and fusel oils; fusel translates from German as “bad liquor” and is notably associated with poorly made distilled spirits.
Name aside, these compounds are always found in beer because they are secreted by yeast during fermentation. Anything related to amino acid metabolism will effect higher alcohol production since the amino acid backbone can be converted to higher alcohols depending on the myriad factors that push biochemical reactions. What is known for certain is that increased higher alcohol production positively correlates with the following factors: Wort amino acid content, wort original gravity, yeast growth, fermentation temperature, and excessive wort aeration. Higher alcohols are negatively correlated with pitching rate, meaning that a low pitching rate increases higher alcohol production. Yeast strain has a major influence with higher alcohol production, where different yeast strains produce different concentrations of higher alcohols from the same wort.
If your goal is to produce a very clean beer with a low level of higher alcohols, select a yeast strain with a squeaky clean reputation, make sure you don’t under-pitch, control the fermentation temperature, and keep your wort original gravity less than about 15 °Plato/1.060. Elimination of higher alcohols is not possible because yeast produce a range of alcohols during fermentation and that is just a fact. The thing to keep in mind with this topic is that higher alcohols are not always bad as they contribute to the overall aroma profile of beer. Pushing the original gravity higher usually results in detectable higher alcohol notes, so if you don’t like these aromas stay away from the bocks, barleywines, and other beefy brews of the beer world.
There are a few tricks used to keep higher alcohols in check when brewing bigger beers. Wort amino acid levels can be diluted using amino acid-free adjuncts like rice, corn/maize, and brewing sugars. Yeast nutrients can also be used to reduce higher alcohol levels by helping to minimize metabolic speed bumps that influence higher alcohol production. Breweries producing clean lagers utilizing high gravity brewing methods (high alcohol beer is brewed and later diluted with water to normal strength) are often very adept with these tricks.
Finally, aging reduces higher alcohol levels as alcohols react with organic acids to yield esters. This is a very slow process and is not a remedy to turn bad to good, but it does explain why big beers age with grace and develop over time.
Q
Is it possible to stop fermentation to a target final gravity by chilling it to near freezing, kegging it, and never letting the temperature rise? For instance, if brewing a style that is not supposed to finish dry at near 1.000.
John Zybko
Crystal Lake, Illinois
A
Halting a fermentation is certainly possible and some brewers do indeed use this technique to produce beer with residual sweetness. I don’t think you are looking for residual sweetness, but the topic all relates to the same thing. When fermentation is stopped by cooling, pasteurizing, or adding sulfites, residual sugars are almost always left behind because the basic idea is to arrest progress at a point that is known to be higher than the terminal gravity. An easy way to know the terminal gravity is to perform a forced or accelerated fermentation of a small sample.
If you want to make a malty lager, perhaps with some diacetyl notes, a sweet cider, or a sweeter fruit beer, stopping fermentation may be the ticket. Commercial operations often use a combination of methods to insure the fermentation does not start back up after packaging. The first step is often to chill the fermentation to halt activity or, in the case of cider and wine, sulfites are used to stop the fermentation. Yeast is then removed by filtration or a combination of centrifugation and filtration and pasteurization or another dose of sulfite is used to preserve this state. These products almost always have a detectable sweet note that is desired.
All brewing yeast strains can ferment glucose and maltose and a healthy, normal fermentation ends with no residual glucose or maltose. However, maltotriose, the second most abundant fermentable sugar in wort next to maltose, is often times not completely fermented by brewing yeast and the variability of attenuation rate by yeast strain is largely due to how well different strains ferment maltotriose. For instance, although some saison strains have irritating tendencies to stall during fermentation, most saison strains are able to produce very dry beers, which is one of the key traits of the style. A general answer to your question is to select yeast strains that have normal or below normal attenuation rates if you do not want to end up with very dry beer. Some yeast, most notably certain Brettanomyces strains, are so-called super attenuators because they produce debranching enzymes that allow for the metabolism of limit dextrins that are not fermentable by typical brewing yeast strains; limit dextrins are the primary source of residual extract in beer.
For as long as I have brewed I have been fascinated with mashing and how malt enzymes are used to convert the malt, and other, starches into a blend of fermentable sugars. The fact that beta and alpha amylase have temperature optima that are sufficiently different gives brewers a fair degree of control over the mashing process and the resultant wort. When I think about designing a beer with either a relatively high or relatively low final gravity, mash profile is always at the top of my mind. The malt bill is closely related to this thought process because some malts, most notably crystal and higher kilned malts, contain carbohydrates that are not fermentable and are not converted to fermentable sugars during mashing.
There are four very important and distinctly different things that happen during mashing that relate to wort fermentability. The first is starch gelatinization; not so related to fermentability when brewing all-malt beers, but very relevant when brewing with adjuncts that require high temperature gelatinization, usually by boiling. Starch crystals must be gelatinized, or hydrated and swollen to the point of bursting, to permit enzymatic activity. The handy thing about malted barley, malted wheat, and raw wheat starch is that these materials all contain starches that gelatinize in the temperature range of beta and alpha amylase activity. Rice and corn/maize starches have higher gelatinization temperatures and are almost always boiled before mashing. Flaked rice and flaked corn/maize contain gelatinized starch and can be added to the mash without boiling.
When hot water and gelatinized starch are mixed together in the 140 °F–158 °F (60 °C –70 °C) range, two changes can be detected using one’s eyes and a spoon. The appearance of the liquid changes from milky to clear and the thickness changes from a heavy porridge to soupy. These changes are generally called conversion and can be crudely monitored using the reaction between iodine and starch. Conversion is just one factor related to mashing and simply knowing that conversion is complete does not reveal much about wort fermentability. Alpha amylase is the enzyme that is responsible for conversion and is most active in the 158 °F–162 °F (70 °C–72 °C) range.
Fermentability is primarily a function of beta amylase activity and this enzyme is most active in the
144 °F–149 °F (62 °C–65 °C) range. The practical problem with alpha and beta amylase is that beta amylase can only completely do its job if alpha amylase does its job, and beta amylase works better at cooler temperatures. This is where multi-temperature mashing methods give brewers a lot of control over mash enzymes. Decoction mashing and double mashing techniques both expose a significant portion of mash starches to alpha amylase activity before bringing beta amylase to the party. Step mashing can also be used to control the interplay between beta and alpha amylase.
Depending on the brewing objective, these methods can be used to increase fermentability or to limit fermentability. The latter can be achieved by rapidly adding hot mash or hot water with the bulk mash to very quickly raise mash temperature from the lower end of beta amylase activity (140 °F/60 °C) to the upper end of alpha amylase activity (167 °F/75 °C). If you want an Oktoberfest lager with a nice dose of malty richness and full body, this is one of those methods that may be very helpful.
The fourth important thing that oftentimes happens during mashing is increasing mash temperature to about 168 °F/76 °C for mash-off (also called mash out). This step effectively halts enzymatic activity (although alpha amylase retains some activity up to 176 °F/80 °C and this temperature allows unconverted starch to convert during wort collection) and decreases wort viscosity. The former is useful because it allows the brewer to control mash time and temperature and then hit the stop button and the latter is handy because lower wort viscosity during wort separation improves yield and allows for faster wort collection. Modern brewing malts are quite different from brewing malts used as recently as 20 years ago, tending toward greater degrees of modification and enzymatic power. Adding a mash-off step is an easy way of preventing enzyme activity to continue during wort collection and wort heating prior to the boil. The easiest way for infusion mash brewers to mash-off is simply by adding an aliquot of very hot water to the mash while stirring to increase the temperature to 168 °F (76 °C).
Mashing is clearly a very relevant topic to your question. But don’t forget malt selection! Malt is more than a source of starch and enzymes. Color and flavor compounds come from malt, and these are especially important for brewing styles like Oktoberfest. I am a big fan of the nutty, toasty, and bready notes found in higher-kilned, enzymatic malts like Munich and Vienna. The same compounds responsible for these wonderful colors and flavors also provide a boost to residual extract in your finished beer.
I am a modern brewer, but often times ask myself how brewers managed without our modern tools and understanding of brewing science. Your question about limiting wort fermentability is really a perfect example of such a dilemma. And the things that were done by brewers of yesteryear are the same sorts of things that are still relevant today; malt selection, mashing technique, and yeast selection. You can certainly use refrigeration and pasteurization, but consider some of the basic tools in your brewer’s toolbox before adding to the complexity of your process.
Q
I’ve got a vial of White labs WLP670 (Farmhouse Blend), which contains a saison strain and a Brettanomyces strain and the “best before” date is slightly expired (less than a month). I guess I’d better make a starter to wake up my culture, but as the two strains have pretty different reproduction speeds and metabolisms, wouldn’t a “classic” starter change the balance between the strains?
Simon Ryr
Woluwé-Saint-Pierre, Belgium
A
If I had to choose between using an old mixed yeast culture that has the “correct” ratio of yeast cells that the yeast supplier used in the blend or using a vital culture with a different ratio of cells made from that old culture, I would always choose the vital culture. The thing about this question is that the first option is probably not quite what it appears to be because not all yeast strains lose viability
and vitality to the same degree during storage. But like you state, these yeasts have different growth and fermentation rates and these differences are true during propagation and fermentation.
In mixed culture fermentations, especially when the organisms are not merely different yeast strains, there tends to be several phases of the fermentation where the various populations rise and fall in waves. In the case of a mixed Saccharomyces and Brettanomyces fermentation, the Saccharomyces fermentation rate will be faster than the Brettanomyces fermentation. The first wave of activity will result in the base beer and the second wave will see the development of the phenolic aromatics associated with Brettanomyces and further reduction in apparent extract as limit dextrins are consumed.
I really would not worry too much about changing the balance of the strains. By making a starter you will insure that you have a healthy cell population and you should expect to see a rapid first fermentation followed by a slow secondary that brings with it funkiness and dryness.
