Monday, 6 April 2015

Supercritical hops

Much of the technical side of brewing can be grouped around the poles of microbiology and chemical engineering. As a microbiologist the former was pretty straightforward when I was a brewing student, but the latter was a whole new world. Things like the Reynolds number and pressure enthalpy diagrams took some getting my head around but once I had I saw how wondrous they were.

One part of the chemical engineering where we almost seemed to enter the realm of science fiction was with phase diagrams. At certain pressure and temperature combinations the liquid and gas phases of a substance will no longer be distinct, instead being a supercritcal fluid. See, I told you it was almost science fiction.

Now it may not be the sort of pressues and temperatures you'll find in a brew house but using supercritical CO2 is one way of making hop extracts. And I'd never seen it being done, until my trip to Germany.

The reason for my trip to Bavaria was to visit a hop company, and I got to have a tour of where they make the hop extracts and hop pellets. The extracts plant was first, starting with ethanol extract. In the tank pictured the ethanol extraction took place, with the ethanol percolated through the hops. I innocently asked if they fractionated the extract at all...

This is at ambient pressure so we've not got supercrital yet

...before being shown the biggest rectifier I've ever seen:

and there were some pretty hefty centrifuges too:

These separate out the solids, and divide the extract into aqueous and resin fractions.

Next we came to this large tank. I think this was a CO2 tank as we were heading to that end of things.

When it comes to engineering I like to see things on a proper industrial scale, and there was no pretence of craft here. The CO2 was at a pressure of 290 bar, so the vessels were impressively thick to cope with it:

If I've understood it right mere liquid CO2 is a non-polar solvent, but supercritial CO2 becomes polarised and its dissolving power increases. There is little flavour difference between the different types of extract, but there is some variation in the composition of the extracts, for example ethanol extracts contain more hard resins than CO2 extracts.

After the extract plant we got to see where hop pellets are made.

Bales of hops being broken up

Nascent hop pellets

There are a range of dies for the pellets. The thicker the die the harder the pellet becomes. Different ones have to be used as higher alpha acid content in the hops also makes the pellets harder.

Can't remember what this was. Probably contained hops though.

Vacuum packing the pellets
Then the packages of pellets were tested to makes sure they were properly sealed before being sent off to be boxed up. Both Type 90 pellets (said to contain 90% of the original hop material) and Type 45 pellets (said to contain 45% of the original hop as the lupulin glands are concentrated and unwanted 'leaf' material removed) are made.


  1. How did it smell in there? As glorious as one would imagine?

  2. The extract plant didn't smell but the pelletising plant certainly had its moments! Standing in a working hop kiln is really the place to be to get hopped up, I swear it has a narcotic effect!

    1. Thanks for the reply, sounds (smells) awesome.