Wet Hop Aroma (or why the hell does my beer smell like grass!)

lawn-cuttingHop harvest is upon us and the aroma of freshly picked cones is heavy.  Along with this special time comes requests for “wet hops” or cones fresh from the field with no drying.

Brewers and beer geeks alike seem gaga for wet hops (soon to be pushed aside for pumpkin atrocities…) so what’s the deal and why are they so unique?  Honestly, to me they all smell like lawnmower and here’s why.

What’s so Unique about Fresh Hops?

Besides the obvious (i.e. 75% water vs 8% in a dry cone) freshly harvested hops have a few unique aroma chemistries that vanish upon drying, regardless of temperature.  Most folks pronounce the aroma to be “fresh” or “sweet”, neither of which are aromas but we get their meaning.  So what’s in these beauties that gives that impression?

Bear with me as we take a dive into the deep end of the hop chemistry pool.  Remember my earlier rants about aroma compounds and their volatility: Some molecules turn to vapor faster than others.  This is important since the compounds that small “fresh” are mostly associated with plant stress like, say, mowing a lawn.

Fresh is a Cry for Help

when plants are wounded, stressed, or otherwise mplant screaman-handled they produce chemicals that easily vaporize to warn other plants and predators that something untoward is happening.  Plants have stores of these compounds at the ready just in case.  When we smell an intact plant leaf we may be able to detect a slight “vegetal” aroma.  We’re not smelling chlorophyll folks.

When the plant tissue is crushed (like in a hop rub) these compounds are released and we perceive the aroma.  Now really…how often are we exposed to crushed and mangled plant leaves?  If you live in suburbia or the country it’s around you all the time from April through October:  Lawns.

It Smells so, so Grassy!

open coneYes, grass emits these compounds in bucket loads and for me is a deep-rooted fragrance of my childhood.  I love the aroma of cut grass…just not in my beer.  So what’s going on in hops, grass, tomatoes, palms, etc and how do we keep it or lose it?  Simple.  It’s all about heat.

The Culprits

Our primary culprit is a frequent offender by the name of (Z)-3-hexenal.  This chemical is very volatile and has an extremely low odor threshold of 0.25 parts per billion (ppb).  Doesn’t take much to notice for sure but it doesn’t stick around long.  Soon after release it transforms into a more stable but higher odor threshold chemical called (E)-2-hexenal.  This is the compound synthesized for the fragrance industry on a large scale.  It’s called “leaf aldehyde” so read through your cologne and perfume ingredients because it’s in there.

Secondary to these aldehydes is an alcohol molecule called (Z)-3-hexen-1-ol.  But more importantly they are both soluble in alcohol and in wort but not water.  This is important because it tells us something about how to use them in the brewhouse.

Take It or Leave It

So if we want to keep this aroma strong and center stage the wet hops MUST be added no earlier than whirlpool.  The hot wort will drive off the chemicals and the essence is gone.  Hot hot is too hot?  Anything over 110F and goodbye hexy!

But that’s not all bad.  There are many, many more volatile aroma compounds buried in the wet cone that can be overwhelmed by the hexenal brothers.  Reducing the grassy aroma by adding very late boil to knockout can allow these other more floral components like geraniol and nerol to shine through.

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Posted on August 25, 2016, in Hop Chemistry in Beer, Wet Hop Chemistry. Bookmark the permalink. Leave a comment.

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