Revision as of 20:13, 14 September 2008

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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, how they are defined (sometimes differently, depending on the author) and how efficiency is affected.

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.

extraction and lauter efficiency

The brewhouse efficiency can be broken into two separate efficiencies that measure the performance of mashing and lautering separately:

brewhouse efficiency = exttaction efficiency * lauter efficiency

Extraction efficiency measures how well the mash extracted the extract from the grain. If all the potentially extract (laboratory fine grind extract) has been extracted, the mash efficiency is 100%. Extraction efficiency is affected my mash parameters like pH, crush, diastatic power, temperature profile and mash time and should be close to 100%.

Lauter efficiency measures how well the lautering procedure did in transferring the extract, made soluble by mashing, into the boil kettle. It is affected by the design of the lauter system, type of lautering (no sparge, batch sparge and fly sparge) and amount of sparge water used. The parameters that affect the lauter efficiency for batch sparging have been discussed in Batch Sparging Analysis.

Extraction efficiency

Measuing extraction efficiency

Splitting brewhouse efficiency into extraction and lauter efficiency only helps in evaluating the brewhouse efficiency if one of the components can be measured separately. To determine the extraction efficiency it is best to calculate the theoretical maximum of the first wort extract/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 maximum 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, including water that was added after the mash but before the first run-off. But water added to compensate for decoction boil-off should not be considered.
• 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)

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

• extraction 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 = 100 * 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

The table on the right gives the expected first wort extract/gravity readings based on the mash thickness at the time that the sample is pulled. To simplifiy the calculations this table assumes a 80% potential extract content in the grist (which is typical for most base malts) and 100% mash efficiency. Use these numbers as a benchmark to compare your measured first wort gravity against.

What affects the extraction efficiency

If the mash efficiency is significantly short of 100%, i.e. lower than 90%, the mash didn't perform as well as it should have. This is an indication that one or more of the mash parameters were suboptimal. To be precise mash parameters don't have to at their optimum for 100% extraction efficiency, they only have to be good enough. What is good enough for one mash parameter depends on the other mash parameters.

The reason for that good enough is the fact that the amount of starch to be converted is limited. And once that starch is converted and made soluble it doesn't matter how close a mash parameter was to its optimum there isn't more that can be converted. The extraction efficiency will plateau. This is illustrated in Figure 1

Figure 1 - A diagram that illustrates the dependency between the amount of starch that is converted by a mash and mashing parameters

Lets assume the mash parameter in question is temperature and the mash time is 60 min. If the temperature is to low, the enzymatic activity will not be strong enough to convert all the starch in the mash within 60 min, as a result the extraction efficiency will suffer. But if the temperature is higher, the enzymes will be active enough to convert all the starch in the mash. At this point 100% extraction efficiency can be achieved. Even if the temperature is optimal and allows enzymatic activity that could convert twice the amount of starch, the extraction efficiency will not go up as it is limited by the amount of starch in the mash. At higher temperatures there comes a point where the denaturation of the enzymes is so strong that not all the starch can be converted anymore. From this point on the extraction efficiency will suffer as the rest temperature is increased.

In addition to that, other mash parameters my be far enough from their optimum that even an optimal temperature will not be able to convert all the starch in 60 min. As a result of that 100% extraction efficiency cannot be achieved at any rest temperature.

Temperature

As shown in the Limit of attenuation experiments, the rest temperature has an affect on fermentablility and extraction efficiency.

When the crushed grain is mixed with water, the enzymes that are present in that malt will go into solution. Once the water temperature is hight enough to gelatenize the starch granules will burst and the starch gelatenizes. This is immediately the case for single infusion mashes and for step mashes once saccrification rest temperatures are reached. Once the starch is gelatinized it can be converted by the amylase enzymes create smaller dextrins and sugars from the long starch chains. These dextrins and sugars are water soluble and become part of the wort extract.

In order to come close to 100%

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