Fading Hop Character

I think the most difficult thing about trying to troubleshoot brewing problems in my column is not being able to taste the beers that I am being asked about. I sometimes flash to hearing car owners attempt to describe the sounds their cars make to Click and Clack on Car Talk. I just happen to be down the street from Monk’s Café in Philly and did have a pint of Pliny to help me imagine what may be happening! Based on your description, I think you either have an oxidation issue and/or hops that are not the best quality. Low-quality carbon dioxide is a possibility, but I would rank that pretty low on the scale. Another thing that may be happening is headspace scrubbing from the O-ring in your Corny keg, but this is another low odds explanation in my opinion.

So let’s talk about oxidation a little bit. These super hoppy beers do have aromas that are very sensitive to oxygen and the fresh hop aroma can quickly fade when these beers sit in a serving tank/keg for relatively short time periods when oxygen is present. While a small amount of oxygen may enter an airlock during cooling, the volume of gas sucked into the headspace is relatively small.

You did bring up this topic and it is something worth discussing. I personally have never liked the practice of allowing liquid in an airlock to suck into a fermenter when the fermenter is cooled because it simply seems inexact and it does not do anything constructive. Assuming the headspace volume is 1 quart (1 L) and the fermenter is cooled from 68 °F to 32 °F (20 °C to 0 °C), the ideal gas law tells us that 68 mL of gas must flow into the headspace to maintain a constant pressure. This means that there is a vacuum created in the headspace that must be balanced by 68 mL of gas being pulled into the airlock from the atmosphere and that the alcohol you put in the airlock will always be sucked into the carboy to allow gas to flow.

I would rather plug the top with cotton and simply allow the gas to enter without adding a shot of booze to my beer. The cotton ball plug is something that has a long history of use in microbiology labs and does present a handy alternate use if you really are worried about oxygen from cooling. You could insert a small hose in the headspace of your carboy, stuff in the cotton plug and very, very slowly flow carbon dioxide into the headspace during cooling. This would create a flow of gas out of the carboy and preserve the integrity of the headspace. I do not believe that this is going to solve your issues, however because this is not much gas.

The bigger possibility is within your secondary vessel. When a keg is flushed with carbon dioxide the contents of the keg still contain oxygen. A common misnomer with gas purging is that carbon dioxide displaces air as it “pistons” up the keg from the bottom; this argument looks good on paper but is not how things really happen in a keg. Temperature differential in the keg and gas flow into the keg both cause gas mixing to occur and this ends in a dilution effect versus what many envision as a blanket pushing up from the bottom of the keg.

And depending on how long the keg is flushed the oxygen content is likely to be high enough to pose significant oxidation risks; especially if the keg is shaken to speed up the carbonation process because this will also speed up the dissolution of oxygen. Brewers who have measured the oxygen content of gas flowing out of vessels being purged with carbon dioxide know that it can take a long time to achieve very low levels of oxygen in the tank. A much easier way to purge a vessel is to fill it up with water and then displace the water with carbon dioxide. Not only is this method fairly rapid, but it also consumes less carbon dioxide than prolonged purging. I do not like the effects of oxygen on beer and at Springfield Brewing Company we use several techniques to minimize oxygen pick-up. Filling our bright tanks with water and blowing them down with carbon dioxide is one and it is a very effective technique.

This was a long segment about keg flushing, but I think there are many brewers who think they are doing everything they can to reduce oxidation, when in fact they are barely scratching the surface of this topic. If you do everything possible to reduce oxygen in your beer and are convinced that you have oxidation issues, then looking at your carbon dioxide quality is something to consider.

If you ask your supplier for a carbon dioxide specification and they either do not understand the question or cannot get the information from their supplier, you may be using a low-grade supply. Many suppliers sell gas that is at least 99.95% and believe it is really pure carbon dioxide. The way I look at this is that the gas may contain 0.05% oxygen and this is enough oxygen to cause beer oxidation, especially if beer is force-carbonated with gas of this quality.

Hop quality is extremely important for brewers who are making the best hop bombs. These brewers are really serious about hop selection and doing everything possible to preserve hop freshness from field to the brewery and into their beer. I wrote about the Hop Quality Group in 2012 following a talk given by John Mallett (Bell’s Brewery) at the Craft Brewer’s Conference in San Diego. This group works with hop breeding programs, growers and processors to help their brewery-members further hop quality. Not all hops are created equally. Hops of the same variety differ based on numerous factors including where they are grown, the crop year (climate), when the hops are harvested, how the hops are kilned, die temperature during the pelletizing process, gas barrier properties of the package and hop storage temperature.

It is very difficult for small commercial brewers and homebrewers to select our ingredients because the likely truth is that the selection was done by others before the hops were processed and packaged. I am not at all suggesting that small brewers do not have access to good hops. It’s just that some of the really great hop bombs have been brewed with hops specifically selected for use in these beers and this selection goes way beyond knowing the hop variety.

One other thing that may be happening to your hop aroma is scalping by elastomers exposed to the beer. The liners on bottle caps are well known to adsorb hop aroma compounds and result in a decline in hop aroma during bottle storage. The same thing is possible with the large O-ring used to seal a Corny keg, although I have not read or heard any reports of this actually happening. There is no doubt that some aroma compounds are adsorbed by these gaskets because they smell hoppy. Good luck with trying to put the hop Genie back into your brew!

Response by Ashton Lewis.