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I recently brewed a Malt Extract beer with WLP060. The "Attenuation" range on the White Labs site says it should be in the range of 72-80%.
I had an OG of 1.0575, and an FG of 1.0096. By my calculations the attenuation was 83% (((1.0575-1.0096)/.0575) x 100 = 83.3).
1) What could have contributed to the higher attenuation %?
2) Is the range listed on the White Labs site more of a range to try to stay within, or, is it a stating what the yeast is typically capable of?
The apparent attenuation range published by yeast manufacturers shows how the yeast generally perform, under typical fermentation conditions. The following factors will affect the attenuation delivered by the yeast:
Fermentability of the wort
Maltotriose is only partially fermentable, and dextrins, caramelized sugars, and some adjuncts, like lactose are unfermentable, so having a higher concentration of these will result in lower attenuation.
Also, keep in mind that adding highly fermentable adjuncts will increase the fermentability. Late additions of highly-fermentable sugars can drive attenuation well past the typical range.
Fermentation Temperature
Lower fermentation temperatures result in slower yeast activity, which tends to cause under-attenuated beer. Higher temperatures (within limits) increase yeast activity, helping to lower final gravity.
Oxygenation
Lack of oxygen early in the fermentation stresses the yeast and limits growth, causing lower attenuation.
Pitch Rate
Pitching the recommended amount of yeast will produce the healthiest colony, which will in turn achieve higher attenuation. Underpitching stresses the yeast, limiting growth, and overpitching creates an environment where yeast don't have enough nutrients to grow healthy.
Yeast nutrients
Providing proper nutrition supports healthy yeast. In contrast, lack of nutrients, specifically zinc, can lead to stalled fermentation.
Rousing the Yeast
This is a bigger factor with more flocculent strains, where healthy yeast will drop out of suspension early. Rousing the yeast also drives some excess CO2 from solution, helping the yeast to lower the final gravity.
Contamination
If your apparent attenuation is drastically higher than expected, the beer may have been contaminated.
Errors in Gravity Readings
While not actually a factor of attenuation, it's important to remember, as Brewchez mentioned, that there is some error inherent to your measurements.
The amount of yeast pitched and its viability has an impact on attenuation. Secondly, wort fermentability have an impact on attenuation.
The extract you used this time could be more fermentable than what White Labs is basing their #s off of. These #s are also a guideline of what to expect not a hard fast rule of what you are supposed to do as a brewer.
I wouldn't expect that 83% is much different than 80% as most of us as homebrewers aren't using equipment as carefully calibrated and dialed in as we think. Furthermore, your temperature differences when you used your hydrometer may have been off a bit, giving you a less than accurate reading. That could influence your apparent attenuation.
Fermentability of the wort is probably the greatest factor in attenuation. With extract you should get pretty close to the mid-to-upper end of the attenuation range depending on specialty grain usage, but if you used any adjunct sugars that were highly fermentable you could exceed the listed attenuation.
And yes, that is a typical range, not a set in stone rule. It's not really a target range either, but don't count on that strain producing results higher than normal all the time.
Fermentation temperature can also have an impact on attenuation, but it seems that there are no hard rules for what temperatures lead to higher attenuation. Higher temps may cause low attenuation due to yeast death, but it's been speculated to cause higher attenuation due to increased yeast activity at higher temperatures, and both have been seen in practice.
Here's a whole wealth of knowledge on the subject: http://www.homebrewtalk.com/wiki/index.php/Understanding_Attenuation
Keep in mind that the attenuation rating listed for a yeast is more a way of comparing one yeast to another using a standardized wort than it a measure of what your attenuation will be. Wort fermentability is the key factor in attenuation, followed by yeast health, amount and type. Using anything other than the very lightest extracts can lead to reduced attenuation since those extracts use malts to color them that may reduce fermentability, like various crystals. If you want to increase the fermentability of extract beers, replace part of the extract with plain old table sugar. Sugar is 100% fermentable, so using some in place of some of the extract can reduce the FG of your beer. If you're brewing all grain and want to increase your attenuation without changing the ingredients in your recipe, mash for a longer time at a lower temperature. For beers like German pils, I'll mash at 146-147F for 90-120 min. For Belgian styles like tripel, I'll do that plus add some sugar to the kettle.
