Calculating Final

Gravity Before You

Ferment It

Background

• Predict final gravity to control

– Sweetness/dryness

– ABV

– Mouth-feel

– Hops dosing

• The primary effectors on these outcomes include

– Mash Temperature

– Original Gravity

– Yeast Strain

Data Set

• Requires a diverse and reliable Data Set

• “Brewing Classic Styles” by Jamil Zainasheff and John Palmer.

• Recipes for basic British and American Ales, subject to the following criteria:

– British Ales fermented with WLP002, W1968, or Fermentis S-04 (attenuation range = 63%-75%)

– American Ales fermented with WLP001, W1056, or Fermentis US-05 (attenuation range = 73%-80%)

– The recipes could not include excipients.

Data Set (Continued)

• Extracted data included:

– Starting Gravity (original gravity, OG)

– Final Gravity

– Mash Temperature

– Fermenting Yeast

• Data transformations and calculations

– Starting Gravity transformed to points/gal (ppg)

– Final Gravity to transformed to ppg

– % Attenuation = 1-(Final ppg/Starting ppg)

Yeast

Strain

British (WLP002/W1968/S04) American (WLP001/W1056/US-05

Attenuation 63%-75% 73%-80%

Mash Temp Substyle Attenuation (%) Substyle Attenuation (%)

149 American IPA 82%

151 Premium Bitter 74%

152 Strong Bitter 73% American Pale 77%

152 Ordinary Bitter 71% American Brown 1 77%

153 South English Brown 68% Robust Porter 77%

154 Mild English Brown 69% American Stout 76%

154 American Brown 2 76%

154 American Amber 75%

Abbreviated Enzymology of Mashing

α-amylase

β-amylase

limit dextrinase

Linear Regression for % Attenuation as

a Function of Mash Temperature

y = -0.019x + 3.6072

R² = 0.7145

y = -0.0114x + 2.5018

R² = 0.9283

65%

67%

69%

71%

73%

75%

77%

79%

81%

83%

148 149 150 151 152 153 154 155

Safale 04

Safale 05

Mash Temperature (F)

% A

tte

nu

ati

on

(1

-F p

pg

/S p

pg

)*1

00

Conclusions from Linear

Regression Analysis

• Attenuation by British and American Ale yeast correlates inversely with mash temperature.

– British: r2 = 0.7145

– American: r2 = 0.9283

• The crystal, toasted and roasted components of the grain bill do not significantly affect the regression.

• The separation of the curves indicates that the two yeast types have different abilities to attenuate wort.

Is The Difference in Attenuation Ability

of British and American Ale Yeast Real?

Basic Descriptive Statistics

Attenuation by N Mean

Attenuation

St. Dev Median

Safale 04 5 71% 2% 71%

Safale 05 7 77% 2% 77%

Non-parametric Tests on Medians

Indicate Yeast Differences are Real

N Median

Safale 04 5 71%

Safale 05 7 77%

Test resul;t is significant at p = 0.0058

Mann-Whitney Test

If outlier is removedTest result remains significant at

p = 0.008

Is this Difference also Seen when the

Means are Analyzed?

• Several Tests are Required to Establish if

Evaluating Means is Valid:

– Normally distributed: Kolmogorov-Smirnov test

– Have equal variances: F-test and Levine’s test

– For both, the number of observations are

sufficient to give a statistical power of 90% (i.e.

the probability of rejecting the difference if it is

not true).

• It is valid to compare the means

Two Sample Student’s T-Tests on Means

• T-Test of difference = 0 (vs not =): T-Value = -4.31, P-Value = 0.003

If the outlier is removed:The two sample no longer have equal variences and therefore the means can not be

compared

Conclusions

• Attenuation by British and American Ale yeast correlate inversely with mash temperature– British: r2 = 0.7145

– American: r2 = 0.9283

• Minor grain components (i.e crystal or toasted malts, or roasted malts and barleys) do not affect attenuation

• British and American Ale yeasts differ significantly in their abilities to attenuate wort (p=0.003)– British: 63%-75%

– American: 73%-80%

• Regression line coefficients can be used to predict final gravity, but different coefficients for different yeast strains.

A Tool for Calculating FG for Basic

British and American Ales

Estimation of Final Gravity from Mash Temperature, % Attenuation and

Yeast Strain

Safale 05

Brew Parameters Recommended Target Achieved

OG 64 64 64

Target Temp 153 153 154

% Atten 76% 76% 75%

Final Grav 15.1 15.1 16.1

m -0.015 -0.015 -0.015

b 3.089 3.089 3.089

% Attenuation vs. Mash Temp

Yeast m (slope) b (intercept)

Safale 04 -0.0190 3.6072

Safale 05 -0.0152 3.089