Making Sense of Efficiency DefinitionsFriday, November 30th, 2012
When using brewing calculators like Brewer’s Friend brewers are inadvertently confronted with the concept of efficiency. While efficiency is an important tool in making the brewing process more predictable, there exists a lot of confusion around various definitions for efficiency. Let me clear up most of them.
In brewing, just like in any other field, efficiency is the ratio between output and input. This is where all efficiency formulas have to agree. The disagreement tends to lie in what is considered the input and what is the output.
Efficiency in brewing cares about soluble extract in wort, commonly referred to as sugars, even though what’s dissolved is more than just sugars. The amount of extract is determined by both the volume and gravity (extract content) of the wort. Gravity point based calculations express this extract as the product of gravity points (1000*(sg-1)) and wort volume in gallon. The volume and specific gravity need to be corrected to a standard temperature (generally 68 F / 20 C) and the result is simply referred to as points:
points in wort = 1000*(sg-1) * volume in gallon
In metric calculations the extract content can be calculated slightly more accurately by multiplying the wort volume with its specific gravity and extract content in Plato (see blog post about the relation between specific gravity and Plato). Since Plato is a percent number the result has to be divided by 100.
extract weight in kg = wort volume in liters * sg * Plato / 100
1 kg extract is worth about 100 points.
The input side of the efficiency calculation is commonly the extract potential of the grist. Extract potential is a key malt analysis parameter which is tested and reported for each batch of malt. The maltster’s lab grinds a malt sample very fine and uses a congress mass (special mashing schedule that is used by convention) to extract the malt’s soluble extract. That dissolved extract is then measured and reported as weight percent of the malt’s dry weight. The resulting number is referred to as DBFG, dry basis fine grind. Batch to batch variability tends to be low and working with default values for various grain types works just fine.
When determining the extract potential for a given grist we need to multiply the grist weight with the average extract potential in points or percent. The latter is the DBFG number multiplied by (1-MC/100) where MC is the moisture content of the grain. Considering the moisture content (generally around 4%) is important since it does lead to an inaccuracy of a few percentage points. In gravity point based calculations extract potential is expressed as points per pound per gallon (pppg or sometimes called ppg). An ingredient with an extract potential of 100% (table sugar, for example) has 46 pppg. This number can be determined through a thought experiment that dissolves sugar in water and calculates the resulting gravity and volume. Note that the volume, which matters, is not the volume of the water but the volume of the sugar solution.
Now that we established the input and output side of efficiency let’s look at different efficiency definitions:
Efficiency into the kettle or mash efficiency (Palmer) is the ratio between extract in the kettle at start of the boil and extract potential of the grist. This is the most useful efficiency definition for all grain brewers since it considers losses during mash conversion and lautering. Since no (significant) extract is lost between the beginning and the end of the boil it can be calculated with kettle full volume and pre-boil gravity or cast-out volume and post boil gravity.
Conversion efficiency has been introduced to asses the performance of mash conversion before lauter losses take effect. Since it is difficult to measure the volume of wort in the mash, this efficiency is best assessed by calculating the highest possible mash wort gravity based on grist extract potential and mash water amount and then comparing the actual mash gravity to it. The mash gravity test for conversion efficiency allows brewers to troubleshoot low efficiency by determining if significant efficiency is lost during the mashing step.
Lauter efficiency builds on the fact that efficiency into the kettle is the product of conversion and lauter efficiency. It can be calculated if both conversion efficiency and efficiency into the kettle are known. Lauter efficiency only counts losses due to dissolved extract being held back in the grain. i.e. it is the percentage of extract dissolved during mashing that was transferred into the kettle. To avoid oversparging and resulting negative beer quality effects, lauter efficiency in home brewing should be kept below 90%.
Efficiency into the fermenter is calculated using the wort volume collected in the fermenter. It is lower than the efficiency into the kettle due to wort losses in the hop trub. Many brewers rely on this efficiency for planning.
The definition for Brewhouse efficiency in literature and practice is rather ambiguous. Some brewers define it using cast-out gravity and cast-out kettle volume while others consider the volume collected in the fermenter. Brewer’s Friend notion of brewhouse efficiency is based on batch size in the fermeter.
This diagram illustrates where extract (i.e. efficiency) is lost starting with the grain and ending with the dissolved extract in the fermenter.
As a side note, German brewers generally consider the total grist weight as the input for their efficiency calculations. As a result the reported efficiencies tend to be much lower since in that calculation a ~80% number (the extract potential of the grain) represents the maxim.
More technical detail on efficiency calculations can be found in this article: Understanding Efficiency.
Post by Kaiser