Tuesday 15 November 2016

Flavour units

Flavour units are in interesting way of giving an indication of the sensory level of a particular flavour unit in beer. Paul Hughes and Denise Baxter discuss them in a chapter in their book Beer: Quality, Safety and Nutritional Aspects.

The flavour unit (FU) is a dimensionless number calculated by the concentration of the flavour compound divided by the sensory threshold. If a compound is present at less than one flavour unit it is below the sensory threshold, if it is present at one to two FUs then it is detectable and if it is present at greater than two FUs it is likely to have a major effect on flavour.

Paul Hughes was one of my lecturers at Heriot-Watt so I have come across flavour units before, though they're certainly not something widedly used in the industry. They sound very useful for describing beer flavour but I suspect that using them involves a lot more work than, for example, simply listing the lab analysis (or more likely calculated) International Bitterness Units (IBU) even though IBUs are a very poor flavour indicator.

Beer flavour is extremely complicated but the provide a handy table of the compounds which sheds some light on it.

Number of flavour units

Greater than two (substantially affect beer flavour)

Most beers

Carbon  dioxide

Niche and defective beers*

Modified iso-alpha-acids
Hydrogen sulphide
Dimethyl sulphide
 Acetic acid
Iron, copper ions

0.5-2 FUs (cause small flavour changes)

Most beers
Isoamyl acetate
Ethyl hexanoate
Isoamyl alcohol
Fatty acids (C6-C10)
Ethyl acetate
Butyric acid
3-Methylbutanoic acid
Phenylacetic acid
Non-volatile acids
Other hop compounds

Niche and defective beers
Modified iso-alpha-acids
Dimethyl sulphide
Acetic acid

0.1-0.5 FUs (changes at these levels do not affect beer flavour)

Most beers
2-Phenulethy acetate

The complexity of beer is highlighted by the note "compounds present at less than 0.1 FU are considered to account for less than 30% of total beer flavour", which to me still sounds a reasonable about for compounds you're not even close to tasting individually.

The authors conclude the chapter:
"For many beers, bitterness, alcohol and carbon dioxide dominate the sensory profile of beer, and are characters readily perceived by the consumers. Other characters, such as hoppy, fruity/estery, dimethyl sulphide and sulphury, contribute to brand differentiation. Finally, compounds such as diacetyl and aliphatic aldehydes are often indicative of defective beers."

The flavour unit ranking reminds me of George Fix's primary, secondary and tertiary effects, though with a lot more specificity. And though I fully approve of obsessing about all the details when brewing, remember some are more important than others.

* Sounds like a synonym for craft beer, doesn't it?


  1. While I was doing a little searching, this came up:

    "I plated 10ul from the 35ml pitchable vial of WLP008 East Coast Ale yeast. I see two distinct colonies and was wondering if that was normal for this culture.
    Thank you for your inquiry. It is not a dual strain, but you can see some differences when the colonies are small. It can be stress or volume size plated. 10ul is a lot of yeast to plate so there can be nutrient differences that each colony is receiving. The best way is to grow a dilute solution to giant colony size; I think there would be some protocols on the web if you want to try that. When they are bigger you will be able to better see if there are actual morphological differences."
    source: http://www.whitelabs.com/yeast-bank/wlp008-east-coast-ale-yeast

    I was wondering if perhaps this might be one of the explanations for the multiple colony shapes on some of the dried yeast you plated a few years back. I saw some cheap massive petri dishes I think I'll get myself for christmas.

  2. No, the differences in colony colour were still there after several rounds of sub-culturing, so there was definitely something going on.

  3. Got it, thanks.