Revision as of 14:47, 18 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.

Besides resulting in less than 100% extraction efficiency, a shortfall in mash conversion can also be detected though a Starch Test. This test uses the reaction between iodine and long chains of glucose (which starch is) to detect the presence of starch in the mash liquid and the spent grains.

Temperature

As shown in the Limit of attenuation experiments, the rest temperature has an affect on fermentability and extraction efficiency. Generally it can be said that lower temperatures require longer rests in order to get the mash fully converted. This is a result of increased enzymatic activity of the enzymes. How long it takes to convert a mash at a given temperature depends on the other mash parameters. Temperatures higher than 75-80 C (167-176F) may cause to much of the alpha amylase (the main starch converting enzyme) to be denatured to quickly and thus resulting in a mash that may never convert.

A conversion below the starch gelatinization temperature (60-65 C / 140-150 F for the large starch granules which represent 85-90% of the starch and 51-92 C / 125 - 200F for the small granules that represent the rest of the starch [Briggs, 2004]) conversion still takes place but at a slower pace since the enzymes only have access to the starch on the outside of the starch granules. Once the starch gelatinizes the enzymes have access to much more starch hence conversion occurs much quicker. Given a constant rest time, there will be a rest temperature at which the mash is not able to fully convert in the given time and as a result efficiency will suffer. This can be compensated by a longer rest or getting other mash parameter closer to their optimum to strengthen the diastatic power of the mash.

pH

The amylase enzymes have a pH optimum between 5.4 and 5.7 when the pH of a cooled mash sample is measured (5.4-5.6 pH for a-amylase and 5.6-5.8 pH for beta amylase [Narziss, 2005]). This was also confirmed in the limit of attenuation experiments. Outside this pH range the enzymes still work, but not as well and the mash doesn't convert as quickly or, if the rest time is not long enough, won't convert and the extraction efficiency will suffer. Because of this, and for beer quality, a brewer should pay attention to the mash pH and/or the residual alkalinity of the brewing water. The residual alkalility, which is a function of the water's calcium, magnesium and bicarbonate content, affects how low the acidity of the malt will be able to lower the pH. Besides a chemical reaction between the malt and the water's calcium and magnesium ions (see Understanding Mash pH)melanoidens present in the malt also have an acidic power which lowers the pH. As a result grists of darker malts require water with higher residual alkalinity than grists of lighter malts to create a mash pH that falls into the optimal mash pH range of 5.4-5.7 (when measured at room temperature).

Many brewers see a jump in brewhouse efficiency once they correct the mash pH. This is the result of improved extraction efficiency. On information about how to estimate and correct mash pH see Understanding Mash pH.

Time

The longer the enzymes can work, the more they can convert. Hence a longer mash time can lead to an increase in extraction efficiency. But if the mash already fully converts before the rest time is over, an increase in the rest time will not have a significant affect on the extraction efficiency since there is nothing left to be made soluble by the enzymes.

This assumes that the temperature is low enough and doesn't cause excessive denaturation of the enzymes (at least for the alpha amylase). If the rest temperature cased to many enzymes to be denatured before full conversion was reached, no increase in the length of that rest will be able to fix the conversion problem. Only the addition of fresh malt or enzyme preparations can convert the mash now.

Malt Milling

How tight the malt has been crushed can have a big impact on the extraction efficiency. If the grits are to coarse and pieces of endosperm are still (partially) enclosed by the husks, the mash needs to be more intense to reach the starch inside these grits. As a result the extraction efficiency is likely to suffer. As the crush gets tighter, the size of the endosperm pieces is reduced and more of them are separated from the husks. The amount of flour also increases. There will be a point at which the largest pieces are small enough that intensity of the chosen mashing schedule is strong enough to reach and convert all the starch. Non stirred single infusion mashes are least intense. The "intensity" is increased if a step mash is used, the mash is stirred or even boiled as it is the case for decoction mashes.

Note that the malt grain is about 1.8 - 4.5 mm (0.07 - 0.18 in) thick. If it is crushed with a mill gap spacing of 1.0 - 1.5 mm (40 - 60 mil), which is the factory setting of many mills, it cannot be expected that there will be a sufficient separation between the endosperm and the husks and small enough grits that a single infusion mash is strong enough to reach all the starch. As a result many home brewers see a jump in efficiency when they start milling the grain through a tighter roller spacing or double crushing the grain.

But there is also a lower limit to the roller spacing. As the malt is crushed ever tighter the husks are shredded more and more (although that can be mitigated though Malt Conditioning) and more flour is produced. Both impede the lauter process and a stuck mash becomes more likely. If even a mill gap spacing as low as 0.6 mm (24 mil) doesn't achieve an extraction efficiency close to 100%, attention should be paid to the other mash parameters. Most likely one or more other mash parameters are suboptimal and reduce the "strength" of the mash.

In general it is best to crush as tight as necessary for close to 100% extract efficiency (full conversion) but not tighter as to improve the run-off speed of the lauter and avoid excessive husk shedding.

Mash Schedule

As it was already alluded to in Malt Milling the intensity of the mash also affects how well a mash converts. That "intensity" is determined how the mash is performed. Triple decoction mashes are regarded as the most intense mashes since the boiling of the grain liberates starch that is still enclosed in cell walls (undermodified malts) or tucked away between husk pieces (poor malt crush) and makes it accessible to the enzymes. But on the other hand, decoction mashes also reduce the diastatic strength of the mash by denaturing enzymes during the decoction boils. Another increase in intensity is stirring and agitating of the mash. But modern well modified malts generally don't need an intensive mashing schedule and if the other mash parameters are otimal (or good enough) a mash with these malts can convert even without decoction mashing or constant stirring.

Dough-in at temperatures below the saccrificantion rest temperatures helps to preserve the diastatic power because the amylase enzymes are able to hydrate before they enter a temperature range in which they start to denature more significantly.

When starches like rice or corn, which have higher gelatinization temperatures, are used it might be necessary to gelatinize these starches before they are added to the mash. Flaked grains had that already done and can be added directly raw rice or corn need to be cooked to accomplish that. In breweries that use adjuncts the vessel used for this boil is called a cereal cooker.

Diastatic Power

Figure 2 - Diastatic power of various malts based on the malt color. (Data source Briess Malting [Briess, 2008])

Mash thickness

Dough Balls

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

[Narziss, 2005] Prof. Dr. agr. Ludwig Narziss, Prof. Dr.-Ing. habil. Werner Back, Technische Universitaet Muenchen (Fakultaet fuer Brauwesen, Weihenstephan), Abriss der Bierbrauerei. WILEY-VCH Verlags GmbH Weinheim Germany, 2005

[Briggs, 2004] Dennis E. Briggs, Chris A. Boulton, Peter A. Brookes, Roger Stevens, Brewing Science and Practice, Published by Woodhead Publishing, 2004

[Briess 2008] Briess average malt analysis data 2007/2008