As you will know if you want to re-create the fresh hop aroma, which some commercial brewers manage to achieve, in the beer that you brew at home we have to dry hop. If you have never heard of dry hopping or would like to find out more about this technique then you can find out more by reading our dry hopping article.
In essence dry hopping allows you to retain the fresh hop aroma in your beer by adding hops later in the brewing process. However, the question is at what stage of the process do you add your hops?
Some recent work carried out by a group of Japanese researchers perhaps helps to shed some light on this question. The researchers looked at the hydrophobic properties of hop compounds to understand the likelihood of those hop compounds surviving into the finished beer. Before we look at their research findings it is perhaps worth explaining what we mean by hydrophobic properties.
A hydrophobic compound is one that does not like to be in the presence of water in essence the compound can be viewed as water hating. The opposite, a water loving compound, is hydrophilic. The hydrophobic and hydrophilic properties of compounds are determined by their chemical structure and will affect how soluble that compound is. In reality many compounds in beer are amphipathic that is they have areas of their structure which are hydrophobic and regions which are hydrophilic. The bit that is important to us is that if you have a compound that has a hydrophobic region and it comes into contact with something else that is hydrophobic the two will stick together through a hydrophobic interaction.
So why is this so important?
Numerous components from hops are hydrophobic and flavour active and brewers would like to better understand the interaction of these components with yeast cells. The reason that brewers are so interested in interactions with yeast cells is because if you remove the yeast there is a good chance you will be removing some important flavour compounds that have stuck to the yeast cells due to hydrophobic interactions. Therefore a group of Japanese researchers looked into the yeast cell surface hydrophobicity and the interaction with different hop components. In their research they were able to prove that the content of the important hop aroma compounds, myrcene, humulene, farnesene and cohumulone increased in the extracts of yeast cells and therefore decreased in the beer. The concentrations of linalool and isohumulones were not affected. Therefore the cell surface hydrophobicity as well as the flocculation properties influenced the intensity and quality of hop aroma and hop derived bitterness.
This research therefore provides good evidence to suggest that if you are dry hopping you should not do it when your beer is actively fermenting. For me, the technique that I have utilised successfully and works best is to dry hop after primary fermentation has finished when much of the yeast has flocculated out of the beer. This should reduce the likelihood of the wonderful hop aroma compounds sticking to the yeast cells and being lost from your beer.