Kräusen Rings & Shelf Stability: Mr. Wizard
Q
Does the dried-up kräusen ring at the top of my fermenter after primary fermentation present a bacterial contamination risk?
Daryle Sewell
Frankfurt, Germany
A
Fermenting beer can at times appear unusual with its fluffy crown of yeast that looks like gargantuan cauliflower, and some of the solids that rise to the surface and adhere to the walls of the fermenter have a dirty appearance. But all of these compounds were either present in the wort prior to fermentation or were produced during fermentation from building blocks in the wort. And this dried ring of kräusen is no more appealing to spoilage bacteria than wort, fermenting beer, or fermented beer. The key to preventing bacterial problems in beer is to prevent bacteria from entering the beer environment.
Before our modern understanding of microbiology, some philosophers and scientists believed that some forms of life could spontaneously generate. In the fourth century B.C., Aristotle pulled earlier beliefs into a general idea termed abiogenesis or spontaneous generation. Anton van Leeuwenhoek, the Dutch merchant who invented the first modern microscope in the 1670s, made observations that seemed to confirm the notion of spontaneous generation. What he observed was yeast and/or bacteria growing in a sterilized media exposed to the environment. Scientists did not know at this time that air contained living microorganisms, so when life was observed sometime after exposing the media to air, and scientists concluded this life came from abiogenesis, Aristotle’s theory from 2,000 years earlier made sense.
As microbiological research continued, scientists began to question spontaneous generation and designed experiments to address their questions. In 1859, Louis Pasteur designed a method that definitively proved that spontaneous generation does not occur. His method used a flask with a swan’s neck shaped airlock to maintain a sterile environment. Two flasks of media were sterilized by boiling the media in these special flasks, one flask was left with the swan’s neck filled with water, and the airlock from the second flask was removed. Microbiological growth was observed in the flask without the airlock, but the flask with the airlock remained sterile. Pretty cool stuff because the airlock we use to keep our homebrew isolated from the environment is the same sort of airlock Pasteur used to disprove the theory of spontaneous generation.
The point to this review of history is to stress that you have the ability to keep bacteria out of your fermenter by controlling the environment. This is the key to bacteria-free beer. However, there are reasons to remove this
brown stuff — commonly called “braun hefe” (brown yeast) by some brewers, and “brandt hefe” (burnt yeast) by others — associated with kräusen from your brew. Braun hefe is a mixture of trub, hop resins, and yeast. If you scrape off some of this residue and give it a taste you will probably agree that the flavor is less than pleasant. Brewers are a practical lot, so it is not surprising that brewers developed numerous methods to remove this unpleasant tasting and visually unappealing stuff from fermenting beer.
The simplest way to remove this “stuff” is to skim it from the surface of an open fermenter with a skimmer. This practice is still used today in commercial breweries using open fermenters. But Louis Pasteur’s research gave brewers a pretty good reason to consider using closed fermenters, which are the most common type used by home and commercial brewers today. However, this does not mean that braun hefe cannot be removed from the fermenter. There are numerous “self skimming” methods that remove braun hefe from beer using some sort of separation device that permits removal of the braun hefe without losing too much beer. Burton Unions and Yorkshire Squares from England are traditional fermentation methods that incorporate skimming into their design. Lager brewers also developed some pretty clever systems to allow kräusen from fermenting beer to foam over into a special chamber, and the beer to drain back in the fermenter.
You can skim at home by filling your fermenter high enough to ensure some blow-over. The down side to this is that you can lose a lot of beer, and if you do not use a large diameter blow-off tube you can blow the top off of your fermenter. If you ferment in a carboy with adequate headspace to contain the kräusen, a lot of the braun hefe will adhere to the walls of the carboy and not fall back into the beer after fermentation is complete.
Circling back to your question, you do not need to worry about braun hefe in relation to the microbiological stability of your beer.
Q
My beer does not last as long as commercial beers or my friend’s homebrews. I try to eliminate oxygen during transferring and bottling. Is that the main culprit or could there be another issue i haven’t thought about?
Chris Wagner
Cincinnati, Ohio
A
The primary cause of oxidized beer is the introduction of air to beer after the initial stages of fermentation. It is very easy to simply state that oxidation can be minimized by reducing air pick-up during the process, but like many things in life, this advice is much easier to offer than it is to practice. Before jumping into some of the things that can lead to oxygen pick-up in the process, I do want to touch on how different beers can be more or less sensitive to oxidation.
Many specialty beer styles use crystal malts for color and flavor. I find these malts interesting, because sometimes they can be used to terrific effect, yet other times they can muddy the flavor profile in a beer. Unless you are willing to rewrite the rule book and accept a paradigm shift, some styles seem impossible to consider without crystal malts, but when these grains are entirely removed from a recipe and replaced with a variety of higher kilned malts, such as Munich types, an “aha” moment often times follows. That is the interesting part I previously mentioned.
These malts can also be frustrating, especially when it comes to oxidation. Reading the literature leads many brewers to believe that crystal malts help ward off oxidation, but practical experience often times conflicts with this belief because beers containing more than about 5% crystal malts can really start to taste pretty unpleasant when aged. It seems to me that the crystal malts prevent beers from tasting like wet cardboard when oxidized, and instead take on an intense blackcurrant aroma, sometimes noted as Ribes or catty, coupled with more pronounced caramel flavors. It could be that you are fond of crystal malts and your malt choice is causing some issues.
Beers with lots of aroma hops can also present special problems since these pleasant aromatics have a limited life in beer. It can be very disheartening to witness the loss of hop aromas in really nice beers that do not seem to be old enough to be damaged by age. IPA brewers have come to realize that hoppy IPAs need to be consumed when fresh. Perhaps your limited shelf-life is a function of the hoppy beers you like to brew. Or maybe not?
Beer styles that are very delicate in nature, such as Pilsners, light lagers, and extra pale ales are also more likely to show the signs of oxidation. The subtle styles are like white shirts; they are great when clean, but can be really unpleasant when slightly stained. Oxidation can certainly put a stain on an otherwise excellent brew. If you don’t think your problem has to do with crystal malt, hop aroma or delicate profile, keep reading!
Oxygen in beer changes many of the compounds formed during fermentation into compounds that have different properties. The key thing to note here is that oxygen changes fermentation products, and explains why adding oxygen to wort does not ruin our beer. During fermentation, yeast produce carbon dioxide and create a positive flow of CO2 out of the fermenting liquid. This flow of carbon dioxide, even in open fermentation vessels, effectively blocks air from entering the beer; when fermenters are closed and equipped with airlocks there is effectively zero oxygen that comes into the system.
Whenever beer is racked from one vessel to another, the risk of oxidation is very real. Three things about racking that can result in oxygen pick-up are splashing, the environment of the vessel your beer is entering, and a leak in the racking hose. The simple act of splashing liquids exposes the liquid to the gas in the environment and increases the transfer of gas from the environment into the liquid. Although directing liquid flow down the side of a container is a great way to reduce splashing, it also increases liquid area and increases gas transfer . . . do not address splashing by directing beer flow to the side of the carboy.
The gas environment in the empty vessel is arguably as, or more critical, to oxygen pick-up during racking as splashing; if there is no oxygen in the empty vessel, oxygen pick-up is not possible. While completely removing oxygen from an empty vessel is difficult to do, it is pretty easy to remove almost all of the oxygen. The easiest way to remove the oxygen from a keg, is to fill the keg with water and push the water out with carbon dioxide. You can do the same thing with a carboy, minus the gas pressure; fill the carboy with water, insert a gas hose that has a slow flow of gas coming from the end, start a siphon, and displace the water with gas. You can accomplish the same thing by inserting a gas hose in an empty container and allowing a slow flow of gas into the bottom of the container. Since CO2 is heavier than air, a blanket of carbon dioxide forms in the bottom of the vessel and pushes air out of the top as more carbon dioxide is added. Regardless of the method, it is very important to change the environment of the receiving vessel.
The third thing that can cause problems when racking, a leak in the racking tube, is not always obvious. If
the hose connected to the racking cane is not sealed, air can suck into the beer flowing from the line. The same problem can occur with pumps if the pump inlet hose is not properly connected, or if you are using a pump with a leaky mechanical seal (most homebrew pumps are magnetic drive and do not have a mechanical seal). If you’re using this answer like a checklist and have not come across a suggestion that seems likely to solve your problem, you may be picking up air when packaging.
The primary sources of oxygen pick-up during bottling are the same as with racking. Ideally beer is gently filled into a carbon dioxide environment, using a filling tube without air infiltration from leaks. When carbon-
ated beer is bottled using a counter-pressure filler, the beer can be foamed or “fobbed” immediately before crowning; crowning on foam is a great way of pushing air out of the bottle headspace. If you are bottle conditioning, you probably will not have enough carbonation in the beer being bottled to generate foam (many commercial brewers that bottle condition package their beer with enough dissolved carbon dioxide to allow fobbing before crowning). Even if you are bottle conditioning, a counter-pressure filler can be used to purge air from empty bottles.
Oxidation problems are often best solved by working backwards through the process because excess air in the package is guaranteed to result in oxidized beer. If you are not sure if packaging is your problem, try doing a test where you bottle part of a batch and keg the other portion in a keg that has been filled with water and evacuated with carbon dioxide. Hopefully I have given you enough ideas to solve your shelf-life woes!
Q
I know there is a level of undermodification in dextrin malt, but can you explain why the malt will still have a high number of free amino nitrogen on the malt analysis sheet? Isn’t undermodified malt supposed to be lower in FAN?
Casper Willemse
Taichung City, Taiwan
A
Malt modification and stewing before kilning are unrelated, but are both clearly part of your question. The term modification is used to describe the changes that occur when barley is transformed to malt, and one of the key indices of modification is the ratio of soluble protein to total protein. This value is called the Kolbach Index, and is frequently listed on malt specifications as “S/T”; Kolbach values in the 40–45% range are normal for well modified malt, values of 35–40% are typical for “lightly modified,” and values less than 35% are typical for undermodified malt. Free amino nitrogen (FAN) is related to S/T, and is a measure of how many protein fragments (polypeptides and amino acids) are present in wort. Normally, modified malt has a FAN level greater than 180 mg/L. Brewers like using well-modified malts because starch conversion and extract recovery is made easier.
Crystal malts are made by adding a step to the kilning process called stewing. Some crystal malts are stewed on the germination floor by keeping the moisture content during the early stages of drying high, sometimes by covering the malt with something akin to a tarp, and other crystal malts (really the majority) are stewed in drum roasters. The stewing process is essentially mashing in the malt kernel before malt kilning, and this process creates fermentable sugars and free amino nitrogen from malt starch and malt protein. These two broad classes of compounds are the basic ingredients of the Maillard reactions. As the moisture content is reduced after stewing, and the grain temperature increases, the simple sugars and free amino acid components react; the result is color and flavor.
Pale dextrin malts are not kilned at high temperatures, and therefore have minimal color and flavor from Maillard browning. The particulars of dextrin malt production are guarded by the companies who make these special malts. Regardless of the method of production, malts with normal to high FAN levels can be produced from undermodified or well-modified malt when stewing is used since stewing is not much different than mashing. The same is true with wort; normal to high FAN levels can be achieved in wort using undermodified malt, provided that the mashing conditions favor the enzymatic degradation of proteins/polypeptides by proteolytic enzymes.
