Understanding IBU Predictions

IBU (International Bitterness Unit) is a laboratory measurement of bitter compounds in beer using a UV-Vis spectrophotometric method (e.g., ASBC Beer-23A). Prediction models like Tinseth and Rager estimate IBUs from recipe variables (alpha acids, time, gravity, volume), but lab values can differ due to losses, polyphenols, and measurement scope.

Formulas Used (explicit)

Tinseth:

Utilization factor: \( b(G) = 1.65 \times 0.000125^{(G - 1)} \)
Time factor: \( f(t) = \dfrac{1 - e^{-0.04 t}}{4.15} \)
Overall: \( U = b(G)\, f(t) \)
IBUs (metric): \( \mathrm{IBU} = \dfrac{U \times \alpha \times W_g \times 1000}{V_L} \)

Rager (metric adaptation):

Time utilization \(U_t\) from Rager table (piecewise by minutes).
Gravity correction \(F_G = 1 + \max(0, \dfrac{G - 1.050}{0.2})\).
IBUs (metric): \( \mathrm{IBU} = \dfrac{U_t \times \alpha \times W_g \times 1000}{V_L \times F_G} \)

Adjustments implemented:

  • Pellet vs Whole: pellet factor ×1.10.
  • Altitude: boiling temperature estimated via barometric pressure; slight reduction to utilization as boiling point drops.
  • Whirlpool: if ≥ 80 °C, we approximate continued isomerization with a slowed rate; below 80 °C, contribution trends toward zero.

Glossary of Inputs/Outputs

  • Boil Gravity (G): Specific gravity during the boil (not necessarily the final OG).
  • Alpha Acid (α): Hop alpha percentage as decimal (e.g., 10% → 0.10).
  • Weight (Wg): Hop charge in grams.
  • Volume (VL): Final volume into fermenter, in liters.
  • Utilization (U): Fraction of α-acids converted to iso-α and retained in wort/beer under the model.
  • IBU: Predicted bitterness; may differ from lab (ASBC Beer-23A) due to losses/compounds outside the model.

Worked Example

Target: 20 L batch, boil gravity 1.050. Hops: 20 g @ 60 min (12% AA, pellet), 30 g @ 10 min (7% AA, pellet), 50 g whirlpool 20 min at 85 °C (6% AA, pellet). Altitude 0 m.

  1. Compute b(G) and f(t) (Tinseth) for each timed addition; multiply to get U.
  2. Apply pellet factor (×1.10) per addition.
  3. Whirlpool: use reduced time factor with temperature scaling (≥80 °C contributes; lower temps taper).
  4. Compute each addition’s IBU and sum for total.

Load the values via “Reset to Example” and compare Tinseth vs Rager.

Citations (Authoritative Sources)

  • ASBC Beer-23A — spectrophotometric bitterness method: American Society of Brewing Chemists. (Lab gold standard). :contentReference[oaicite:0]{index=0}
  • Tinseth model — widely adopted gravity/time utilization function. :contentReference[oaicite:1]{index=1}
  • Rager model — utilization table + gravity correction (original Zymurgy article; widely summarized). :contentReference[oaicite:2]{index=2}
  • Comparisons & modern context — model behavior vs measured IBUs; whirlpool considerations. :contentReference[oaicite:3]{index=3}
  • Dry hopping & IBUs — adsorption/measurement nuances (MBAA proceedings). :contentReference[oaicite:4]{index=4}

FAQs

Is “IBU” the same as perceived bitterness?
No. Perception depends on residual sweetness, sulfate/chloride ratio, polyphenols, and other compounds. IBU is a lab metric, not a taste score. :contentReference[oaicite:5]{index=5}
Why do calculators disagree?
Different utilization curves and gravity corrections. Rager tends to run higher; Tinseth is smoother vs time/gravity. Process losses, equipment, and whirlpool practices also vary. :contentReference[oaicite:6]{index=6}
Should I include hopstand/whirlpool IBUs?
Yes when ≥80 °C for extended periods. This tool includes a tempered isomerization model for whirlpool additions. :contentReference[oaicite:7]{index=7}
Do pellet hops really add ~10% more IBUs?
Pellets typically show slightly higher utilization than whole cones; many pro calculators assume ~10% more. Adjust per your system if you record lab or sensory deltas. :contentReference[oaicite:8]{index=8}