Late Hop Additions and Hop Oils in Beer Brewing

by Brad Smith on January 21, 2013 · 15 comments

barley_beer_webA trend in modern brewing is the greater use of late steeped hop additions for many styles. These include steeped hop additions, whirlpool hops, use of a hop-back and even dry hopping. Surprisingly, most of these late additions are not done to increase bitterness, but instead to exploit hop oils which play a significant role in many beer styles.

Bitterness and Late Hop Additions

When designing a new beer recipe, many brewers get very focused on the bitterness level of the beer, traditionally measured in IBUs (International Bitterness Units). Bitterness comes from the alpha acids in hops (in chemistry terms these are the humulones: humulone, cohumulone, adhumulone). During the boil, these alpha acids undergo a chemical reaction called isomerization which transforms them into bitter compounds – the traditional bitterness we taste in beer.

This hop bitterness is critical for beer, as it counterbalances the sweetness provided by barley malt. However, the focus on bitterness (IBUs) ignores many of the other flavor compounds contributed by hops.

Early on, brewers recognized that just dumping hops in a pot and boiling them for 90 minutes resulted in boiling off a lot of the flavor that hops contribute, even though it did create the needed bitterness. So we started adding later additions at 30 minutes, 15 minutes, 10 minutes or 5 minutes from the end of the boil to preserve some of the flavor compounds that contribute to great beer.

Steeped and Whirlpool Hops

More recently science has helped us isolate many of the flavor compounds in hops, and understand how to preserve those flavors. Surprisingly, some of these flavor compounds vaporize at temperatures well below boiling, so the simple act of boiling hops, even for a few minutes, eliminates these oils. Also most of these oils are prone to oxidization (from exposure to air) so adding them early in the boil can also oxidize the oils from both oxygen still in the wort and also oxygen in the air during the boil, as well as when we aerate the wort before fermentation.

These volatile flavor compounds are referred to simply as “hop oils” or “essential oils”. To take full advantage of them, you can’t boil the hops. In fact even steeping them at high temperature is a bad idea – so home brewers and some commercial brewers have moved to steeping at low temperature, dry hopping, and using a hop back or whirlpool system to infuse these hops after the boil. Many have had spectacular results.

Essential Hop Oils

In addition to alpha acids in your bittering hop addition, there are four main hop oils that can add flavor late in the brewing process – each of which boils off at a different boiling point. Most also rapidly oxidize when exposed to air:

  • Myrcene – (147 F/63.9 C boil point) – The largest of the hop oils, making up 40-60% of the hop oil content in many American varieties (Cascade has 50-60%), though most noble hops are low in myrcene (Saaz: 5-13%). Mercene boils off readily, and can even volatilize in a high temperature steep (147F), so you will lose it if you boil it. It has a herbal note that can be described as green, balsamic, hoppy in small quantities. It also has a slight piney/citrus flavor. As a result of its low boiling point, it is present in much higher quantities in dry hopped or steep-hopped beers. It tends to provide a “green hop” or fresh hop aroma when used in dry hopping.
  • Humulene – (210F/99C) – Humulene is the traditional noble hop oil, providing a strong herbal component most people associate with noble hops. Humulene is actually widely used in the perfume industry for its herbal characteristic. Over long boils, it also tends to produce a slightly spicy flavor – such as that from Saaz hops in light lagers or Nugget. Because it boils just below the boiling point of water, it usually provides its best characteristics as either a late boil addition or post-boil addition. Humulene will not survive a long boil, and is also prone to oxidize.
  • Caryophellene – (262F/129C) – Caryophellene is a counterpoint to humulene – and provides a spicy, woody, earthy and even citrusy flavor. Clove and pepper contain this oil in significant quantities. While not a significant in noble hops, Caryophellene is a major aroma component in many traditional English hops such as Goldings and Northdown as well as many US hops such as Mount Hood. In beer it contributes a strong dry wood, pepper and earthy spice flavor. It may even add a citrus finish. Many hops are rated by their Humulene to Caryophellene ratio with noble hops having a high ratio of 3:1 or more. Caryophellene oxidizes rapidly, so fresh hops must be used and are often added late to preserve the flavor.
  • Farnesene – (203-257F/95-125C) – Found in the coating of apples and other fruits, it provides the “green apple” flavor as well as flowery, citrusy, woody and at the extreme end musty, woody or vegetative. Farnesene is the smallest of the hop oils – typically less than 1% of the hop oil content, but it can be higher in many noble varieties. Again because it oxidizes rapidly it is best preserved as a late or post boil hop addition.

Some Lessons on Hop Oils and Late Hop Additions

Looking at the hop oils above, we can see several common themes. First, three of the four oils oxidize easily when exposed to air. As a result hop processing and storage is really important for preserving these oils. Fortunately the microbrewery industry has been a driving force the last 10 years in improving hop processing and packaging, and also they have been driving the availability of a much larger variety of hops.

However, as a homebrewer, you need to be aware that the moment you open your vacuum sealed foil pouch of hops you are exposing your hops to air, and starting to degrade the hop oils. Oxidization also happens more rapidly with temperature, so storing your hops in the freezer until needed is important.

Another key point is that three of the four major hop oils have a boiling point below 212F/100C – so the act of merely boiling your hops for any length of time will start to boil off these flavor oils as well. So when designing a beer you may want to consider (in addition to your main bittering hop addition) adding your aroma/flavoring hops after the boil has ended. In fact if you want to preserve the Myrcene component it should be added after your wort has cooled below 147F/64C.

There are several methods for preserving these oils including steeping hops after the boil, during your cooldown, using a hop-back, whirlpool additions and of course dry hopping after fermentation. All of these have the advantage of not boiling off the oils, and also a much lower risk of oxidization as the boil or fermentation remove most of the oxygen from the wort/beer.

I’ve been experimenting with moving most of my late boil additions to post boil – usually a combination of steeping some hops while cooling the wort and also dry hopping with good results. Many small commercial brewers are also extensively using these techniques with great results. I still use boil additions, though I now use them primarily for bitterness.

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