Measuring IBU in a small brewery lab

The method in one paragraph

The standard method, published by both the ASBC and the EBC, works like this. Take a measured volume of cold, degassed beer, usually 10 millilitres. Acidify it with a small amount of hydrochloric acid so the hop acids move out of the beer. Add 20 millilitres of iso-octane (2,2,4-trimethylpentane) and a drop of octanol to stop foaming. Shake hard for 15 minutes. Separate the layers, centrifuging if needed. Read the absorbance of the iso-octane layer at 275 nanometres against a pure iso-octane blank. Multiply by 50. That number is the bitterness units, or IBU.

The kit you need

• A UV-capable spectrophotometer. It must read at 275 nanometres. Many cheap visible-range instruments start at 340 nanometres and cannot do this test.
• Quartz cuvettes. Glass and most plastics absorb ultraviolet light and will give nonsense readings.
• A mechanical shaker set to a fixed speed, so every sample gets the same extraction.
• A centrifuge for samples that form an emulsion between the layers.
• Accurate pipettes and spectroscopy grade iso-octane.

Iso-octane is flammable and its vapour should not be breathed. Work in a fume hood or at least a well ventilated bench away from flames, wear gloves and goggles and keep a solvent waste container. Our guide to using a spectrophotometer for colour and bitterness covers instrument set-up.

Where small labs get the wrong number

Most errors come from preparation rather than the instrument:

• Foam and gas. Undegassed beer carries carbon dioxide that makes volumes inaccurate. Degas by pouring between beakers or gentle stirring. Keep the beer cold.
• Inconsistent shaking. Hand shaking gives different extraction each time. Use a shaker and a timer.
• Emulsions. A cloudy layer between beer and solvent traps hop acids. Centrifuge rather than guess.
• Dirty cuvettes. A fingerprint absorbs ultraviolet light. Handle cuvettes by the frosted sides only.
• Readings out of range. Very bitter beers can push absorbance high enough to lose accuracy. Dilute and correct rather than reading off the top of the scale.

The floor detail we would insist on: run one control beer every time you test, a commercial beer or a frozen batch of your own with a known value. If the control drifts, the day's results are suspect.

IBU is not the same as iso-alpha-acids

The 275 nanometre reading picks up iso-alpha-acids, but also other compounds that absorb there: unisomerised alpha acids, oxidised hop acids and some polyphenols. In a beer bittered only in the kettle, IBU tracks iso-alpha-acids reasonably well. In a heavily dry-hopped beer, it does not. Dry hopping adds compounds such as humulinones that raise the reading. A hazy IPA can measure higher than its taste suggests or the other way round.

For a true iso-alpha-acid figure you need HPLC, which separates the compounds. Most small breweries send a few samples a year to an outside lab for that and use the IBU method for routine work.

Using IBU to improve the brewhouse

One number on a finished beer is trivia, however carefully you measured it. Measured consistently, IBU becomes a tool:

1. Measure kettle-knockout wort and finished beer for each batch of a core brand to see your losses through fermentation.
2. Compare calculated IBU from your recipe software with what you measure. The gap is your real hop utilisation, which depends on your kettle, boil vigour and hop stand.
3. Track the same brand over months. A sudden fall points to a new hop lot, a change in alpha acid or a process slip.
4. Measure retained samples during storage to see how much your beer loses on the shelf.

Linking those measurements to process decisions is where lab work pays for itself.

Start with one brand and one month

Do not try to test everything at once. Pick your best selling hoppy beer and measure it at knockout and at packaging for every batch over a month. Our guide to how much bitterness beer loses in storage explains what to do next with retained samples.

Your first step: check whether your spectrophotometer reads at 275 nanometres and whether you have quartz cuvettes. If either answer is no, that is the purchase to plan before any other lab kit.