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What Is Extraction Efficiency?
What Is Extraction Efficiency?
Revision as of 13:34, 19 July 2009
What Is Extraction Efficiency?
For someone who is just starting out in all-grain brewing, the concept of extraction efficiency may seem a bit confusing -- what the heck do all those numbers mean, anyway? I hope this page will help "demystify" extraction efficiency, for brewers who are making the transition from extract to all-grain brewing.
In extract brewing, the malt extracts that are used to provide fermentables always yield a predictable amount of sugar. A pound of LME in a gallon of water will yield 37 points of specific gravity, and a pound of DME will yield 45 points (give or take a point or two, depending on the brand of extract).
For grain mashing, the story is different. Various types of malt yield different amounts of sugars, and there are many other variables that also influence how efficiently your mashing and sparging procedures extract sugar from malt. Because of these factors, the amount of sugar obtained from a pound of malt can vary over a fairly wide range. The good news is that as long as your procedures remain reasonably consistent from batch to batch, it is possible to predict how much sugar you will get from your malt, provided you know what your extraction efficiency is. Extraction efficiency is the ratio of the amount of sugars you actually obtain, to the theoretical maximum amount of sugars available. Because of the large number of variables involved, the only way to really determine the extraction efficiency of your brewing system is by trial and error.
Typical extraction efficiency in a homebrew setting is in the 60-80% range.
A table giving the theoretical maximum extract for some commonly available types of malt is available here.
A Simple Example
The theoretical maximum yield for 2-row Pale malt is approximately 37 points per pound of malt, per gallon of wort. So, with perfect (100%) efficiency, mashing 8 lbs. of Pale malt to produce 5 gallons of wort would give you ( 8 x 37 ) / 5 = 59.2 points (or in other words, a specific gravity of 1.059). If the actual specific gravity (as measured by your hydrometer) is 1.041, then your efficiency is ( 41 / 59 ) = 0.69 (i.e. 69%).
A Real-World Example
In a real mash, you would rarely use only a single type of malt. In a mash containing more than one type of malt, the contribution of each type of malt must be calculated individually.
The following grains were mashed:
* 4 lbs. Pilsner malt, theoretical yield 37 points/lb-gal * 2.5 lbs. Munich malt, theoretical yield 33 points/lb-gal * 14 oz (0.86 lb) Cara-Vienne, theoretical yield 34 points/lb-gal * 2 oz (0.13 lb) Cara-Munich, theoretical yield 34 points/lb-gal
The final volume was 3.5 gallons, at a specific gravity was 1.058. So what was our efficiency?
Well, if we had perfect (100%) extraction, our total gravity points would have been:
( 4 x 37 ) + ( 2.5 x 33 ) + ( 0.86 x 34 ) + ( 0.13 x 34 ) = 264 points
Dividing by the volume of wort gives us 264 / 3.5 = 75 points; this means that -- in a perfect world -- we would have expected to get a specific gravity of 1.075. But our actual gravity was 1.058, so our efficiency is 58 / 75 = 0.77, or 77%.
Adjusting Your Grain Bill
For your first couple of mashes, I would actually recommend that you assume an efficiency of only 50%. It is better to err on the safe side: a high OG is easier to correct than a low one -- just add more water!
Once you've done a few all-grain batches, you should start to get some idea of what to expect in terms of extraction efficiency from your system. If you're using one of the popular recipe calculation programs, you can plug this efficiency number into the program, and it will be taken into account automatically when you formulate recipes (or when you check an existing recipe you've gotten from someone else).
If you're doing recipes "by hand", calculate the total gravity points, and divide by the volume of wort (as shown in the "real world" example above). Then multiply by your expected extraction efficiency. This should give you an estimate of what your specific gravity will be. If the estimate seems off, tweak the grain bill up or down as needed, to bring the estimate into line.
Some Factors Which Can Affect Extraction Efficiency
There are many variables which can affect your extraction efficiency. The following list summarizes the important ones:
* Quality of crush. Too coarse of a crush will reduce efficiency, since any sugars in the interior of the grain particles will be less accessible during the sparge. But too fine of a crush can cause lautering problems (stuck sparge). * Mash temperature and duration. If the mash is not held at the proper temperature, or is held for too short a period of time, some of the starches will not be converted to sugars, reducing your extraction efficiency. * Sparge time. If you sparge too quickly, the sugars will not be rinsed from the grain effectively, reducing your efficiency. For good extraction efficiency, the flow rate should be adjusted such that the sparge lasts at least 30 minutes (some people even recommend an hour). * Sparge volume. Sparging with a larger volume of water (then boiling the runnings down) will give you better extraction efficiency, since you will extract more of the available sugars. The danger of over-sparging is that you will tend to extract astringent tannins from the grain husks, as the pH rises throughout the sparge. The pH rise can be offset by adding a small amount of food grade lactic or phosphoric acid to the sparge water. * Sparge temperature. Sugars are less soluble at low temperatures; you should try to maintain the temperature of your grain bed at 150F or above during sparging. But there is a tradeoff here as well -- sparging too hot can lead to excess tannin extraction, just as over-sparging can. * Mash size. If you always collect approximately the same volume of runnings, you will see significantly lower extraction efficiency when doing a large mash (for high gravity beers). This is because you are trying to rinse a larger amount of sugar (from a larger volume of grain) using the same volume of water. (My efficiency for "normal" gravity beers tends to run around 75%; for high gravity beers, it is lower -- more like 60-65%.) * Mash pH. A mash pH that is drastically too high or low will adversely affect extraction efficiency, by inhibiting the enzymes that convert starches to sugars. But unless your tap water is very alkaline or very acidic, you should not need to worry about this -- the chemical reactions which occur during mashing tend to push the pH into the proper range automatically.
Efficiency and Consistency
In a homebrew setting, getting the highest possible extraction efficiency is not the goal. The important thing is to be reasonably consistent from batch to batch, so that you can predict -- and hit -- your target gravity. You can always compensate for low efficiency by using extra malt -- an extra pound or two of malt isn't going to break your budget.
Large commercial breweries need to worry more about absolute efficiency, because even a slight difference in efficiency can easily translate into hundreds, or even thousands of dollars.
There are a number of people who claim that under-sparging -- that is, intentionally collecting less runnings, at the expense of efficiency -- will result in better tasting beer. The rationale for this is that the later runnings tend to contain a higher percentage of astringent tannins, which can negatively affect beer flavor.
Mash Efficiency Vs. Brewhouse Efficiency
Ok, if you're still with me this far, it's time to split a few hairs.
Some people calculate their efficiency based on the volume and gravity of pre-boil runnings. This gives the efficiency of the mash and sparge alone, which I refer to as mash efficiency.
Other people calculate their efficiency based on the volume and gravity of wort that actually goes into the fermenter. This gives the overall efficiency of the entire brewing process -- the brewhouse efficiency.
What's the difference? Well, the decrease in volume which occurs during the boil actually has no effect, since the decrease in volume is accompanied by an increase in specific gravity -- i.e., the total amount of sugar remains the same. What does have an effect is the fact that some wort is typically left behind in the boiling kettle, with the hops. This means that the brewhouse efficiency will always be a few points lower than the mash efficiency, because some sugar is left behind.
Which method is the "right" one? They both are; either one will work. Just pick one method, and stick with it, so that your records are consistent. Or, if you tend to be anal on the recordkeeping side of things, just record both sets of numbers!
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Copyright © 2000 by Michael Uchima, All Rights Reserved
(Posted to Web January 11, 2000, last updated January 18, 2000)