Revision as of 03:35, 16 August 2008

Work in progress

Rather than listing all the factors that can affect the brewhouse efficiency, this article is intended to provide a systematic approach to indentify and fix brewhouse efficiency shortcomings.

Calculating Efficiency

Before we can look for efficiency shortcomings and their causes we need to determine the efficiency. Since a number of different definitions for efficiency exist, it is possible that the actual efficiency is in a acceptable range but was calculated based on a different efficiency formula that returned a lower result. Other error factors are incorrect gravity or volume measurements.

In this analysis we will use the brew house efficiency into the boil kettle. That efficiency is the same at the beginning and end of the boil since no extract is lost during the boil. Efficiency shortcomings after the boil (i.e. transfer to the fermenter) are obvious as they are proportional to the amount of wort that is left behind in the boil kettle. For a list of various efficiency definitions see the article Understanding Efficiency.

Measuring

Before the efficiency can be calculated, 3 things need to be measured on brew day:

• amount of grain used
• specific gravity of the collected or finished wort
• volume of the collected or finished wort

It is important to be reasonably accurate when measuring these values. The more precise the measurements are. the more precise the calculated efficiency will be. As an example: If the grain weight has an error of 5% (which is +/- 200g for a 4.0kg grist or 0.5 lb for 10lb), the calculated efficiency will also have a 5% error, i.e. a calculated efficiency of 70% could actually be between 65% and 75%. The same applies to volume and gravity measurements.

The following measurement errors can be seen as reasonable:

• 1% error for the grain weight. This means +/- 40g for a 4.00 kg grist or 0.1 lb for 10 lb
• 2% error for the specific gravity. This means +/- 0.25 for 12 Plato or +/- 1 point for 1.050 SG
• 2% error on the volume measurement. This means +/- 0.5 on 25l or +/- 0.5 qt on 6 gal

and with them the efficiency can be calculated with a 5% accuracy.

Check the hydrometer

If you have already checked the calibration of the hydrometer you can proceed to calculating efficiency

To check the hydrometer use it in water at the calibration temperature that is noted on the scale. In water the hydrometer should read 1.000 or 0.0 Bailling/Brix/Plato. If another reference point is desired, dissolve 40g of table sugar in 160g of water. The resulting solution has a extract content of 20 Plato or 1.083 SG. But such a 2 point calibration is generally not necessary for hydrometers.

If the hydrometer doesn't read 1.000 SG or 0.0 Plato in water, it doesn't have to be thrown out. Simply remember to always subtract the difference to 1.000 SG or 0.0 Plato from every hydrometer reading.

Examples:

• If the hydrometer reads 1.002 in water, subtract 2 gravity points (i.e. 0.002) from every hydrometer reading
• If the hydrometer reads 0.998 in water, add 2 gravity points to every hydrometer reading

Measure Volume

When measuring volumes on brewday, the temperature based expansion of water should be taken into account and all measured volumes should be corrected such that they represent the volume at room temperature. And since wort is mostly water, what is true for water is also true for wort. At 100 C or 212 F a given amount of water/wort has about 4% more volume than at room temperature. This means that 25 liter or quart 100C/212F hot wort is actually 24 liter or quarts. This is a difference of 1 liter or quart and fairly significant for precise efficiency calculations. But because it results in more volume than actual, it results in a higher than actual calculated efficiency.

At mash and thus lauter temperatures ~ 70C / 160F the water/wort volume is only by about 3% larger than at room temperature.

Also, take the time to calibrate or check the volume mesurement for the boil kettle. Be it a dip-stick or markings on the kettle. To be off by 1 qt for a 6 gal is a 4% error in the calculated efficiency.

Calculate efficiency

To calculate this efficiency two distinct approaches exist. The weight based approach calculates the weight of the extract that is present in the collected wort and puts it in relation to the total extract weight that was available in the grist. The gravity potential based approach puts the specific gravity that was achieved in relation to the maximum gravity potential that the grist has. Both approaches are explained below and either one can be chosen.

Weight based efficiency calculation=

The weight based efficiency calculation is best done with metric measurements. If your measurements are not yet metric, please convert them using these formulae

• weight in kg = weight in lb * 0.45
• volume in liter = volume in gal * 3.78 = volume in quart * 0.95
• gravity in Plato = gravity points * 4 (this is simplified, but good enough for average specific gravities)

To calculate the efficiency, we need to calculate the amount of extract that is present in the collected wort. Because this amount doesn't change during the boil, unless additional malt extract or sugar is added during the boil, it doesn't matter if it is calculated based on pre or post boil measurements. Just make sure that the volumes are corrected for temperature (see above):

kettle extract weight in kg = volume in liter * gravity in SG * gravity in Plato

Multiplying the volume with the specific gravity of the wort, which is basically the kg/liter for the wort, with the volume gives the weight of the wort. That is then multiplied by the specific gravity given in Plato which expresses how much of that wort weight comes from extract (sugars, proteins, etc.).

The other weight that is needed is the weight of extract that can theoretically be extracted from the grist. When a maltster tests the malty quality one of the properties determined is the fine grind extract. That is determined by grinding the malt to a flour and mashing it with a step mash program called Congress Mash. After that the dry matter is separated from the mash through multiple sparge steps such that 100%, or very close to that, of the extract that was made soluble ends up in the collected wort. The amount of extract is then determined and expressed as percent of the original grain weight. Luckily for us, the amount of extract present in the various grains is fairly constant and we don't need to acquire a analysis sheet for all the malts that we used for our mash. Here is a table that lists commonly used types of malt and average fine grind extract percentages.

Average extract ratings for commonly used malts

The weight of extract in the grist is calculated as

extract in grist in kg = weight of malt 1 in kg * fine grind extract grain 1 in % / 100 + weight of malt 2 in kg * fine grind extract grain 2 in % / 100 ...

If only little (<5%) specialty malts (crystal, roasted malts and Carfa) were used, the formula can be simplified to

extract in grist in kg = weight of grist 1 in kg * 0.8

The factor of 0.8 represents the average 80% extract content of most base malts.

Now that the weight of the extract in the collected wort and the weight of the extract initially present in the grain are known we can calculate the brewhouse efficiency as

brewhouse efficiency in % = 100% * kettle extract weight in kg / extract in grist in kg

Dealing with kettle additions of sugars and/or extract

Some brewers add sugars or additional extract in the brew kettle. These additions can distort the calculated brewhouse efficiency if that efficiency is based on post boil measurements. To compensate for that, simply subtract the weight of the added sugar/extract from the calculated kettle extract weight before calculating the brewhouse efficiency. In case of liquid malt extract, subtract only 80% of the extract weight that was added. Or work with gravity and volume measurements that were done before the sugar/extract was added.

Continue to "Is my efficiency good or bad?" to see how this efficiency stacks up against commonly achieved brewhouse efficiencies.

Example

grist weight: 4.5 kg Pilsner malt
cold post boil volume: 21 liter
post boil gravity: 1.048 / 12 Plato

kettle extract weight = 21 l * 1.048 * 12% / 100 = 2.64 kg
extract weight in grist = 4.5 kg * 80% / 100 = 3.6 kg
brewhouse efficiency = 100% * 2.64 kg / 3.6 kg = 73 %

Gravity potential based efficiency calculation=

This type of calculation is best done with US units. If the measurements are not already available as such, used the followling fomulas to convert the numbers:

• volume in galons = volume in liter / 3.78
• gravity in gravity points = extract in Plato * 4
• weight in lb = weight in kg / 0.45

This calculation takes a different approach. It determines how many gravity points (the digits after the 1. in the specific gravity value) could one get from the given grist in the measured volume with 100% brewhouse efficiency. The actual brewhouse efficiency is then the actually achieved gravity points expressed as a percentage of that theoretical maximum. To calculate the theoretical maximum, we need to know the wort volume, the weight of the used grain and the gravity potential (i.e. how many gravity points can be contributed) of the used grains. The latter is expressed as points per pound and gallon (ppg) and can easily be derived from the fine grind extract percentage that is listed for malts. Pure table sugar is completely soluble and adding one pound of it to a volume of water that results in 1 gal of sugar solution will result in a solution with a specific gravity of 1.046. This means that sugar has a potential of 46 ppg. Malt and many adjuncts used in mashing are not completely soluble. Only their extractable weight (Fine grind extract) can me made soluble in mashing. To determine their gravity potential we simply multiply the gravity potential of sugar (46 ppg) with the fine grind extract percentage from the malt spec. This has been done in the following table

Finally the theoretical maximum for the wort gravity is:

maximum gravity points = (weight grain 1 in lb * potential grain 1 in ppg + weight grain 2 in lb * potential grain 2 in ppg ...) / volume in gal

If only a small amount of specialty grains are used, this simplified equation can be used

maximum gravity points = weight of grist in lb * 37 ppg / volume in gal

To get the efficiency we put the actually measured gravity points in relation to the maximum gravity points

brewhouse efficiency = 100% * actual gravity points / maximum gravity points

Dealing with kettle additions of sugars and/or extract

Some brewers add sugars or additional extract in the brew kettle. These additions can distort the calculated brewhouse efficiency if that efficiency is based on post boil measurements. To compensate for that, simply subtract the gravity points gained from the added sugar/extract from the actual gravity points before calculating the brewhouse efficiency. The gravity points to subtract can be calculated as:

gravity points from dry kettle extracts = extract weight in lb * 46 ppg / kettle voulme in gal

in case of liquid extract use 37 ppg since liquid extract is 80% extract and 20% water. Or work with gravity and volume measurements that were done before the sugar/extract was added.

Example

grist weight = 10 lb (Pilsner malt)
cold post boil volume = 5.5 gal
post boil gravity = 1.048 (48 gravity points)

maximum gravity points from grist = 10 lb * 37 ppg / 5.5 gal = 67.3
brewhouse efficiency = 100% * 48 / 67.3 = 71 %

Is my efficiency good or bad?

Though a lot of opinions exist on what efficiency numbers are good and which ones are bad, let me try to give some guidelines in assessing the brewhouse efficiency. The following table lists a range of brewhouse efficiencies that can be expected from various brewing systems:

Another factor for the achievable efficiency is the amount of grain and the preboil voulme. The larger the grist in relation to the batch size, the more wort gets trapped in that grist after running off. As a result larger beers generally show a lower brewhouse efficiency. If more water is used for sparging, more of the extract can be washed out of the grain. The result is a higher brewhouse efficiency but also a higher preboil volume which requires longer or more intensive boiling. That and the increased sparging can be detrimental to the beer quality and should not be used to fix an efficiency problem.

I would consider an efficiency (assuming a regular gravity beer: up to 1.060 / 16 Plato) that is below the ranges given above as problematic and worth investigating. Unless it is known where in the process the efficiency is lost a decision can be made if and how the efficiency should be improved. While a low efficiency can be an indication of a suboptimal mashing and lautering process, A very high efficiency does not necessarily mean the best beer possible. The latter statement is targeted at oversparging which can lead to the excessive extraction of unwanted grain compounds (in particular tannins from the grain husks).

Sources

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

[HUPPMANN] brosure for the Hupmann Lauterstar lauter tun design PDF

[WEYERMANN] Malzfabrik Weyermann malt specifications ZIP

[BEST MALZ] Best Malz Malzqualitaeten