Hop Oils

I consider my knowledge on this topic typical for the average craft brewer and will answer your question from my perspective and comment on some of the topics brewers and hop researchers are looking at related to this very deep subject. In general terms, many brewers approach hops with very little numerical precision.

Hop additions continue to be primarily based upon alpha acid concentration measured shortly after hop processing. Over time, alpha and beta acids oxidize and the concentration is reduced. However, these oxidized compounds do not negate the bittering value of hops. Oxidized beta acids are bitter and some argue that these compounds offset the loss of alpha acids during storage. For this reason, many brewers simply use the post-harvest alpha acid analysis result in their brewing calculations. Brewers know that hops that have been stored in a warm environment and/or exposed to oxygen because of opened bags of pelletized hops or loose bales of whole, compressed hops lose their bittering value due to excessive oxidation and loss of hop aroma. So hop storage temperature and duration is something that most brewers control. Alpha acid loss is variety dependent and a common method used to assess storage qualities of different varieties is the hop storage index (HSI). High HSI values indicate lower alpha acid loss compared to low HSI values.

You mention cohumulone in your question. Cohumulone is one of five types of alpha acids, the other four being humulone, adhumulone, posthumulone and prehumulone. Hop varieties with high cohumulone concentrations are considered by many to have a harsh bittering quality. Indeed, one of the commonalities of “noble” hops and prized bittering varieties is their low cohumulone levels, which is usually less than 20% of the total alpha concentration. Brewers do use this measure when selecting hops.

For many brewers, the most tantalizing feature of hops is their wonderful array of aromas, and many of the styles of beer being brewed by craft brewers around the world feature hop aromas. So it only makes sense that hop aroma is somehow quantified and used by hop merchants and brewers when hops are bought and sold. Unfortunately, quantifying hop aroma is very far from easy and the only thing that most brewers know about the hops they purchase is the alpha acid and total oil content. It is the total oil component of the analysis that relates to aroma, but simply knowing the total oil content of hops is really not very useful. This is why brewers heavily rely on old-fashioned sensory methods to select varieties and specific lots of varieties for use in brewing. Many brewers annually travel to hop growing regions to participate in hop selection. During selection these brewers evaluate hops with their eyes and noses to determine which lots they want to purchase. Brewers also base their purchasing decision on variety, where the hops were grown, how they were processed and when they were harvested. All of these factors affect the brewing qualities of hops.

The problem with basing decisions on these hands-on methods is that the method depends on the skill and knowledge of the person conducting the evaluation. Since these methods are not quantitative, it is not possible to compare various lots without having the evaluator check out the sample. This is an obvious problem and scientists have been studying hops using a variety of chemical methods for nearly a century. Gas chromatography (GC) is the most effective tool to separate hop aroma compounds. Since GCs simply separate compounds from one another, various detectors are used to determine the identity of the various fractions that flow from the column. The most common GC detector is the mass spectrophotometer (MS). The GC-MS is used to produce a “finger print” of sorts of the aroma compounds present in a sample. Some GCs are equipped with a sniff port that allows a person to smell and describe aromas during the runtime of the sample, adding important information to the “finger print” produced by the mass spectrophotometer. Scientists are able to use these types of analytical methods to compare varieties, evaluate and describe new varieties, help brewers better understand why we prefer certain varieties and to quantify the hundreds of aroma compounds found in hops.

The practical side of this can be seen when a new variety is being evaluated for use in the brewery. Using GC-MS data is one way to determine similarities between varieties. If you know some of the key aroma compounds you like to see in aroma varieties, GC-MS data can be used to find the varieties that seem to have the properties you desire. You asked a question about a very broad topic and my objective with this answer was to give a small peak through the laboratory door into the types of things that scientist are evaluating. I hope this information begins to answer your question.

 

Response by Ashton Lewis.