Color correction

In the December 2007 issue of BYO, you gave an answer about light skunking a beer. You stated that it was UV light that was damaging the beer. I don’t see how this is possible. It must be visible light if any light is destroying a beer at all. For example, our eyes cannot see UV light, hence the name “visible light.” So, why then would the visible color of a bottle determine its ability to transmit or reflect UV light. In fact, glass itself actually absorbs most UV light that reaches it no matter what the visible color of the glass; this depends on the quality of the glass, for example the amount of silica and quartz in the glass. A brown bottle is brown because it absorbs visible “brown” light. A green bottle is green because it absorbs “green” light and so forth. So then if UV light does in fact “skunk” a beer, then the color of the glass would have no effect.

Jake Ocque
Buffalo, New York

Thanks for the great comment with respect to my answer about hops and skunkiness, Jake. I have spent several hours reading and have gained a new appreciation for glass and the intricacies of light filtration. You are indeed right that visible light causes beer to go skunky. I am also correct that UV light causes beer to go skunky. Ultraviolet light ranges from 10–400 nanometers (nm) in wavelength and visible light occupies the portion of the electromagnetic spectrum ranging from 400–700 nm. There are numerous references in the brewing literature stating that light ranging from 350–520 nm results in skunky beer. That covers the upper portion of the UV light range and visible light from violet to green, including blue.

I do want to clarify one point about color because the explanation of why objects have a given color is backwards. A green beer bottle, for example, does not absorb green light, it absorbs all visible wavelengths except green. This is an important distinction; since green bottles do not absorb green light (520–550 nm) this wavelength passes through the glass and catalyzes the reaction that results in the dreaded skunk nose! Clear and blue glass bottles produce the same result.

You are also correct that the color of an object relates to visible light and color provides no information about the absorption of ultraviolet light (and other non-visible wavelengths). High-purity, clear glass containing nearly 100% silica transmits all visible light and also transmits ultraviolet light. In contrast, the ordinary clear glass beer bottle is made from a mixture of silicon oxide, sodium bicarbonate, calcium oxide and magnesium oxide. This type of glass allows about 90% of wavelengths greater than 350 nm to pass through and blocks about 90% of wavelengths shorter than 300 nm. While most of the UV spectrum is absorbed by clear glass, they transmit all wavelengths involved in causing skunky beer.

The color brown is a mixture of red, orange and yellow wavelengths. These colors range from about 580–700 nm. This means that brown beer bottles absorb the shorter wavelengths that damage beer. Brown glass is pigmented with iron oxides, among other metal oxides, and these various forms of iron absorb UV light over a wide spectrum of wavelengths. In a nutshell, brown glass absorbs the visible and UV wavelengths of light responsible for skunkiness. Brown glass is also used to store other light-sensitive products, such as medications and film developing reagents (for those youngsters out there in homebrew land, film is what was used before digital cameras).

In the process of doing my homework, I came across some interesting patents pertaining to this subject. One patent is for a clear glass containing vanadium pentoxide and the stated application for this patent is beer bottles. Vanadium pentoxide does not impart color to glass and does absorb UV light, but does have an affect on the susceptibility of beer to light damage. Another patent was for a colorless group of UV-absorbing, Maillard reaction products extracted from roasted malt. These compounds were demonstrated to greatly reduce the rate at which beer is skunked. Interestingly, Heineken filed this patent. It has always seemed to me that dark beer has immunity to light and this paper seems to support that observation. Since neither patent addresses visible light, and both relate to skunky beer, I assume that UV light is a more potent catalyst for the reaction than visible light. Hopefully this sheds a bit more light on this stinky topic!

To blow off or not to blow off, that is the question.

Is it better to blow off during primary fermentation or use a closed system so the debris from high kräusen settles back into the carboy or conical fermenter? Does filling a carboy within one inch of the top and inserting a blow off tube lead to the expulsion of yeast that may degrade the remaining yeasts’ ability to reach the desired terminal gravity? Does blow off affect hop presence in regard to aroma or bitterness?

For a closed fermentation, a stainless steel conical fermenter with a domed lid seems like a good choice as it does not seem prone to blow off due to its large head space thereby allowing the homebrewer to completely fill the fermenter. A fermenter with a flat lid, with very little headspace, when filled to capacity, appears more likely to blow off since it has little or no headspace when filled.

Some of our primary fermentation carboys blow off very aggressively and terminal gravity is in the 20 to 30 range. If one of ours blows off less (with little or no debris leaving the carboy), it seems as if TG is lower — more in the 08 to 12 range. Would blow off cause the variability?

Most of my beers are mashed somewhere between 150 and 152 °F (65.5 and 67 °C) and initial gravities are rarely lower than 1.070. Target terminal gravity is 1.010 or less and little if any crystal malt or unfermentables which would result in a higher terminal gravity are used. My club and I use liquid yeast with a starter and visible fermentation starts in less than eight hours in all cases. We infuse oxygen in each carboy with our recipe kits from disposable oxygen tanks (20 second infusion).

Roger Swantek
Clinton Township, Michigan

This is a classic question about brewing technique. Allow me to give a little background on the technique of fermenter skimming used by some traditional brewers using open fermentation tanks. Traditional, manual skimming involves using an archaic tool resembling a giant spatula with holes. The yeast skimmer is used to remove the top brown layer of yeast from the fermenting beer. This layer, frequently called braun hefe because of its appearance, contains yeast (hefe), trub and some hop resins (the trub and hop resins are the braun part of this schmoo). Most people who have tasted braun hefe will agree that this stuff is pretty robust! It has a harsh bitter and astringent flavor that a logical brewer may think would be less than desirable in the finished beer, so the yeast skimmer was invented.

Yeast skimmers are also handy towards the end of ale fermentations and are traditionally used to harvest these top cropping yeast strains from the top of the fermenter before the beer is racked to a closed vessel. Some ale yeast strains sink into the beer after fermentation and harvesting yeast with a yeast skimmer has the advantage of minimizing yeast carry-over from fermentation into the next vessel, be it a secondary fermenter or cask. Using a yeast skimmer is somewhat labor intensive and there are methods of fermentation with built-in skimming features. Examples include Yorkshire stone squares and Burton Unions. Some of the largest US brewers have skimming systems that separate braun hefe from the fermenter. All of these methods that rely on fermenter design use the kräusen to move the braun hefe into a chamber or a separation tank where it can be removed.

So there is certainly a lot of anecdotal evidence supporting the notion that removing a portion of the kraüsen may be advantageous. At home there are a few ways to remove braun hefe. One method is to allow the fermenter to overflow or “blow off.” This method is not very well controlled and the amount of blow-off varies by batch and is influenced by the fill level of the fermenter (batch size), pitching rate, yeast strain, fermentation temperature, aeration rate, wort gravity, etc. Since most homebrewers do not brew the same beer all of the time the blow off volume is likely to vary from batch to batch. Another problem I have with intentionally blowing kräusen from the fermenter is that there can be fairly high beer losses associated with this method and I am not overly thrilled with the idea of beer loss.

I think the primary goal here is to remove braun hefe. Beer loss and yeast removal is secondary to this goal. If you have a fermenter with enough headspace to prevent blow-off but with something in it to help remove braun hefe you can achieve your primary goal without losing beer or yeast. There was a style of fermenter that was quite popular in the US during the 50’s, 60’s and 70’s called bread-loaf tanks. These tanks are still found in many breweries built during those decades. Bread-loaf fermenters are rectangular and often have a flat roof with a slight pitch where one end of the tank is slightly taller than the opposite end. Many bread-loaf tanks also have a chamber in the corner of the tall end. The chamber is open at the top and its purpose is to collect braun hefe that is pushed into the chamber as the kräusen rises and flows across the bottom side of the tank top during fermentation.

Another more contemporary design is found in cylindroconical fermenters, where a false roof with holes in it is used to retain braun hefe from kräusen that is pushed up onto this deck during fermentation. This system is really quite similar to how a Yorkshire square functions. The nice thing about this method is that braun hefe is retained on the roof and beer and yeast flow drain back into the fermentation and the loss of beer and yeast is virtually nil. Another similar system is to use a small tank to do the same thing. Kräusen blows off from the fermentation in a tank equipped with a strainer and yeast and beer return back to the fermenter. This system has some commonalities with Burton Unions. Both methods are used by large US brewers that are often mocked by beer enthusiasts.

Whether you have a fermenter with a flat top or a dished top you can prevent excessive blow-off by not overfilling the tank. I typically suggest 25% headspace or “free-board” in a fermenter for most yeast strains. Some strains are known to be very foamy and some commercial breweries have as much as 40% headspace in their tanks, although this is relatively uncommon. If you then devise a system to retain the braun hefe or separate from kräusen that blows off from the fermentation you can minimize beer and yeast losses.

You provided a lot of good information in your question to help me offer ideas I think will help. The extremely high finish gravities in some of your batches with aggressive blow-off is not a good thing. If asked “does excessive blow-off” affect final gravity I would answer “no.” In light of the information in your question, this answer is clearly wrong. It seems that you are actually removing enough yeast to cause your fermentations to finish high and I am really surprised by that fact. If you would like to continue using the blow-off technique to skim your fermentations you need to get this factor under control so that your fermentations come to completion. Most commercially brewed beer around the globe these days does not use any method specifically devised to remove braun hefe. Most cylindroconical tanks are not overfilled and do not blow-off during fermentation. There is some adhesion of braun hefe to the top and sides of the tank that form the headspace above the beer level, but this happens in almost any fermenter with enough headspace to prevent blow-off. We have this type of fermenter at Springfield Brewing Company and we have brewed many delicately flavored beers over the years that have not, in my opinion, suffered by this fermentation method.

To repeat a common theme of mine, I encourage you to evaluate the flavor of your beers. If your beer tastes better when you remove braun hefe, then by all means figure out the best way of fine tuning this technique. However, if you really cannot tell much difference between batches with blow-off from batches without blow-off then you should really question worrying about it. And if the main difference is that batches with blow-off taste more like wort than beer then any benefit realized in your brewery seems totally negated by hanging fermentations.

Brew Your Own Technical Editor Ashton Lewis has been answering homebrew questions as his alter ego Mr. Wizard for the last 12 years. A selection of his Wizard columns have been collected in “The Homebrewer’s Answer Book,” available online at brewyourownstore.com.