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	FORUM ON BEER, HOMEBREWING, AND RELATED ISSUES
		Digest Janitor: pbabcock at hbd.org


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Contents:
  Hop Aroma Predictor ("A. J. deLange")


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Date: Wed, 03 Nov 2010 12:37:32 -0400
From: "A. J. deLange" <ajdel at cox.net>
Subject: Hop Aroma Predictor

The comparison with hops bitterness is instructive. Over the years it 
has been determined that perceived bitterness correlates well with a 
relatively easily measured quantity namely the UV absorption (at 275 nm) 
of bittering principal extracted in a particular way and that this, in 
turn, is proportional to the amount of alpha acids in the beer. Given 
this metric, the IBU, we can compare the amounts of hop alpha acid put 
into a beer (determined by measuring, also by extraction and UV 
absorption, the amount of alpha acid in the hops themselves) and the 
measured IBU as it depends on boil time (when the hop additions are 
made), wort strength etc. and come up with a model such as the Tinseth 
model. The model can be compared to measured data and its parameters 
tweaked to make it fit the data better. The model isn't perfect by any 
means. One can predict hopping levels to perhaps 5 IBU provided the 
information printed on the hops package is accurate and that one has the 
proper values for the parameters in the model. These can only be 
obtained by experiment and of course it is necessary to measure IBU on 
actual beers to get them. We are in general forced to use the parameters 
determined by someone else (Tinseth, for example) and this reduces the 
accuracy of the model somewhat because his equipment and methods are not 
the same as our equipment and methods.

Now extend this to aroma. There is no IBU equivalent WRT aroma i.e. no 
single, simply measured quantity in the beer that can be simply related 
to a simply determined property of the hops (i.e. the spectrum of aroma 
producing oils). Thus we have no basis for a model and nothing to feed a 
model with if we did i.e. no manufacturer lists the spectrum of 
essential oils on his packages intended for sale to home brewers though 
I'm sure that data would be available if you are buying in quantity 
directly from a grower or a growers cooperative.

Bitter is more or less bitter though there is coarse bitterness and fine 
bitterness which is not distinguished in the IBU metric. Aroma is, OTOH, 
multi dimensional. There are earthy, fruity, floral, spicey, citrusy, 
piney.... components to hop aroma and flavor. Each of these can, 
doubtless, be associated with particular compounds in the essential oils 
but the model itself would have to be very complex to relate the 
multiple sources to the multiple aroma note including the effects of 
processing (boil length, storage conditions...).


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