Understanding Efficiency

Efficiency is a commonly discussed subject among all grain brewers. But with the abundance of definitions for it, it easily becomes a matter of comparing apples with oranges. This article tries to shed some light on the various efficiency definitions that are in place and how they are defined (sometimes differently, depending on the author).

existing Definitons

In "How To Brew", John Palmer defined the brewing efficiency as the ratio between the gravity points of the wort in the kettle and the maximum potential (labratory extract) of the grain. The maximum potential of the grain is given in gravity units per pound and gallon. Based on that the gravity points of the kettle wort are [Palmer 2005]:

kettle gravity points = brewing efficiency * grain amount in pound * kettle volume * potential of the grains

When grains with different potential are used, the weighted average of their potential needs to be used in the above equation.

In "Designing Great Beers", Ray Daniels defines what John Palmer calls brewing efficiency mash efficiency [Daniels, 2000].

In "Abriss der Bierbrauerei", German brewing author Ludwig Narziss defines Sudhausausbeute (German for brew house efficiency) as the ratio between the amount of extract that made it into the boil kettle vs. the amount of grain that was used [Narziss, 2005]:

Sudhausausbeute = (kettle volume in l * kettle extract in % * kettle specific gravity) / grain mass in kg

Note that this is a different approach for defining the efficiency. The reference is not the maxium amount of extract that can be extracted from the grain, but the total weight of the grain. The latter includes the weight of the husks and other insoluble material. Because of that the the Sudhausausbeute is also affected by the potential (or laboratory extract) of the used grains. This is also the definition that German home brewers use for efficiency. Thus care needs to be taken when reading efficiency numbers from German sources. While 75% is a very good efficiency number when based on the total grain weight (most grains laboratory extract is about 80% of their weight) it is only a modest efficiency when it was based on the laboratory extract of the grain.

When asked how to calculate efficiency, the BYO Wizard replied with the same definition as was given in Narziss [BYO]. He calls that efficiency the brewhouse efficiency. But he also goes on and defines the efficiency that is based on the laboratory extract of the grain as brewhouse yield:

brewhouse yield = (kettle volume in l * kettle extract in % * kettle specific gravity) / (grain mass in kg * fine grind extract in %)

Furthermore, technical brewing articles oftentimes make mention of the Overall Brewhouse Yield (OBY). This is is defined like the BYO Wizard defined the brewhouse yield. It is affected by milling, mashing and lautering and basically indicates how close these brewhouse processes came to the fine grind laboratory extract.

Another popular set of efficiencies are the efficiency numbers given by Beersmith, a recipe design software. There 3 different types of efficiency are given: brewhouse efficiency based, efficiency into boiler, efficiency into fermenter. The brewhouse efficiency indicates how much of the extractable extract made it into a wort with the measured gravity and the the target volume that has been entered for that batch. Efficiency into boiler is the percentage of extractable extract that made it into the boil kettle. This is based on the entered pre-boil volume and pre-boil gravity. Lastly the Efficiency into fermenter is the percentage of extractable extract that ended up in the fermenter. For that the measured gravity in the fermenter and the wort volume in the fermenter is measured. The efficiency that matters for comparison with others is the Efficiency into boiler or the Brewhouse efficiency if the target volume matches the temperature corrected post boil volume. Any other efficiency measurement is not readily comparable because of losses that happened after the lautering process. One of the major losses is wort left behind in the kettle and its cause is fairly obvious.

Mash and Lauter efficiency

Despite mash efficiency being used by Ray Daniels for what is commonly referred to as brewhouse efficiency, I prefer mash efficiency being defined as the amount of extract that is brought into solution during the mash in relation to the maximum extract that can be extracted from the grain (laboratory extract). In addition to that lauter efficiency is defined as the ratio bertween the extract that made it into the boil kettle and the extract that was made solube by mashing. When adapting these defintions for mash and lauter efficiency, the brewhouse efficiency can be written as:

brewhouse efficiency = mash efficiency * lauter efficiency

Breaking down the brewhouse efficiency into its distinct parts (mash and lauter efficiency) illustrates how brewhouse efficiency can be affected. Mash efficiency is affected by mash parameters like pH, crush, diastatic power, temperature profile and mash time. It should be close to 100%. Otherwise one or more of the mash parameters wasn't in a proper range and there is an increased risk for getting unconverted starches into the boil kettle. Lauter efficiency is affected by the design of the lauter system, type of lautering (no sparge, batch sparge and fly sparge). The parameters that affect the lauter efficiency have been discussed in Batch Sparging Analysis.

Measuing Mash Efficiency

Splitting brewhouse efficiency into mash and lauter efficiency only helps in evaluating the brewhouse efficiency if one of the components can be measured separately. To determine the mash efficiency it is best to calculate the theoretical max of the first wort gravity based on the laboratory extract of the grain that was used and the volume of water that was added to the mash.

expected FW extract = grain mass * grain laboratory extract / (mash water volume + grain mass * grain laboratory extract)

• expected FW extract is the theoretical max of the extract content of the first wort in Plato (actually weight %, but that is close enough to Plato and Brix for these calculations)
• mash water volume is the volume of the strike water in liter. This means the total volume of water that was added to the mash before the FW extract is determined, in case more water is added after the mash but before the first run-off.
• grain mass is the grain weight in kg
• grain laboratory extract the (weighted) average of the laboratory extract of the grains. This comes from the malt analysis, but 0.8 is a fairly accurate estimation for most malts.

The mash efficiency is then the ratio between the expected FW extract and the actual FW extract (not exactly true, but close enough for these calculations)

mash efficiency = 100% * actual FW extract / expected FW extract

• mash efficiency is the efficiency of the mash in %
• expected FW extract is the expected first wort extract that was calculated above (in Plato, Brix or %)
• actual FW extract is the actual first wort extract that was measured (in Plato, Brix or %)

The extract content in Plato (close enough to Birx and extract % for these cases) can be estimated form specific gravity by using this formula:

Plato = (SG - 1.000) * 1000 / 4

The first wort extract can also be calculated from the mash thickness, which removes the actual grain weight and water volume from the equation:

expected FW extract = grain laboratory extract / (R + grain laboratory extract)

• R is the water to grain ratio in l/kg. If the mash thickness is known in qt/lb, multiply by 2.11 to get it in l/kg

Sources

[Palmer, 2006] John J. Palmer, How to Brew, Brewers Publications, Boulder CO, 2006

[Daniels, 2000] Ray Daniels, Designing Great Beers, Brewers Publications, Boulder CO, 2000

[BYO], Online article Gravity & Brewhouse Efficiency