Authoritative Data Source and Methodology
Primary references:
- White, C., & Zainasheff, J. (2010). Yeast: The Practical Guide to Beer Fermentation. Brewers Publications. ISBN: 978-0937381967. https://www.brewerspublications.com/products/yeast
- Troester, K. (Braukaiser). Yeast Propagation and Pitch Rate (archived). https://web.archive.org/web/20181201000000*/https://braukaiser.com/wiki/index.php/Yeast_Propagation_and_Pitch_Rate
- White Labs. PurePitch Next Generation Pitching Calculator – Methodology Overview. https://blog.whitelabs.com/purepitch-next-generation-yeast-pitching-calculator
Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.
The Formula Explained
SG to Plato: P = -463.37 + 668.72·SG - 205.35·SG^2
Required cells (billions): C_req = (PR × V_mL × P) / 1000
Viability (liquid yeast): v = e^{-0.009·d}
Initial cells (billions): C_init = C_src × v
Growth yield (billions per liter): Y_eff = Y_base × clamp(0.6, 1.4, P_s/9.1)
Starter size (liters): V_s = max(0, (C_req - C_init) / Y_eff)
DME (grams): DME ≈ 100 × V_s × (P_s / 9.1)
Glossary of Variables
- SG: Specific Gravity (e.g., 1.055)
- P: Degrees Plato (°P)
- PR: Pitch rate in million cells per mL per °P
- V_mL: Batch volume in milliliters
- C_req: Total cells required (billions)
- v: Viability fraction (0–1)
- C_src: Source cells before viability (billions)
- C_init: Initial viable cells (billions)
- Y_base: Base growth yield (B/L): 60 (no-stir), 120 (shaken), 180 (stir plate)
- P_s: Starter wort °P (≈ 9.1 at SG 1.037)
- V_s: Starter volume (liters)
- DME: Dry malt extract required for the starter (grams)
How It Works: A Step-by-Step Example
Scenario: 20 L ale at 1.055 with one liquid pack (100B) produced 30 days ago; stir plate; starter wort 1.037.
- Convert SG to °P: P ≈ -463.37 + 668.72×1.055 - 205.35×1.055² ≈ 13.6 °P.
- Pitch rate: PR = 0.75 M cells/mL/°P → C_req = (0.75 × 20,000 × 13.6)/1000 ≈ 204 B.
- Viability (liquid): v = e^{-0.009×30} ≈ 0.763 → C_init ≈ 100 × 0.763 = 76.3 B.
- Additional needed: 204 - 76.3 = 127.7 B.
- Yield (stir plate): Y_base = 180 B/L; starter °P ≈ 9.1 → Y_eff ≈ 180 B/L.
- Starter size: V_s = 127.7 / 180 ≈ 0.71 L. DME ≈ 100 × 0.71 ≈ 71 g.
Frequently Asked Questions (FAQ)
Do I need a starter for dry yeast?
Generally no. Pitch the correct mass of fresh dry yeast (often 20B cells/g at ~97% viability) and rehydrate per the manufacturer’s instructions.
What if my OG is very high?
Increase pitch rate by 10–20% for high gravity wort to improve attenuation and reduce stress. The “Custom” option lets you set an exact value.
Can I split the starter into steps?
Yes. If the single-step volume is too large for your flask, run a smaller first step, chill and decant, then step up again. This tool suggests a 2-step plan automatically.
How accurate is viability from pack date?
It’s an estimate based on published decay curves. Storage conditions and handling can increase variance; when possible, use a cell counter for critical batches.
What starter gravity should I use?
1.036–1.040 (≈9–10 °P) is a widely recommended range for healthy propagation. The tool adjusts yield for different starter gravities within 7–12 °P.
Does oxygenation change the result?
Proper oxygenation improves growth and yeast health but is not directly modeled here. Values assume typical aeration for each method.