This is a great question that touches on several topics. I’ll start with a brief description of what you already know about mashing. For a given malt, wort fermentability is a function of beta amylase activity (some very pale malts do have limit dextrinase activity, but those are exceptions) and extract yield is primarily a function of alpha amylase activity. When brewers use infusion mashing, neither fermentability nor yield are maximized because a compromise temperature around 149 °F (60 °C) is used. Step, decoction, and double mash methods all provide more control where both wort fermentability and yield can be pushed beyond the limits of single-temperature mashing.
What remains after mashing where only endogenous malt enzymes are used (no enzymes from outside sources) is a mixture of fermentable sugars, unfermentable dextrins, yeast assimilable amino acids and nitrogen-containing compounds, proteins and polypeptides, beta-glucans and other bio-gums, color and flavor compounds, polyphenols, and a small amount of lipids. At first glance, this list is clear and fairly complete. However, the term “fermentable sugars” is ambiguous because not all yeast strains are able to ferment maltotriose. All brewing yeast can ferment fructose, glucose, sucrose (fructose + glucose), and maltose (glucose + glucose), but maltotriose (glucose + glucose + glucose) is left behind at varying levels by some yeast strains.
Apparent attenuation (AE) is how practical brewers gauge the progress of fermentation using a hydrometer. The “apparent” modifier is used because alcohol decreases the density of water and is a nod to how specific gravity is used to track fermentation knowing that the measurement is influenced by another variable. For example, beer has an apparent degree of attenuation of 83% when the wort value drops from 12 °Plato or 1.048 to 2.5 °Plato or 1.008 in beer. In numeric terms, this reads 12 °Plato x (1-0.83) = 2 °Plato, or 1+ [0.048 points x (1-0.83)] = 1.008. The term “real attenuation” or RE is used when alcohol is removed before the determination of beer gravity. Because this was historically impractical, brewers used AE for production measurements. Today, fancy instruments allow brewers to measure RE in process, but because of the spendy price of these devices many craft brewers and nearly all homebrewers continue to use AE.
Most of the strains known worldwide to produce crisp beers can ferment maltotriose. Examples include the Chico strain and its cousins, Kölsch yeast, and Weihenstephan 34/70, among hundreds of others. These strains typically have an apparent attenuation in the 80–85% range. Strains with lower apparent attenuation stats are either completely or partially unable to ferment maltotriose. That’s why there is a wide range of fermentability associated with many ale and lager strains known for producing fuller-
Then there are the so-called diastatic yeast strains, aka STA-1 positive strains, that produce and excrete limit dextrinase. This enzyme is capable of breaking down dextrins during fermentation resulting in very dry beers. Examples include saison and other Belgian strains, some weizen strains (although the STA-1 gene is usually not expressed in these strains), and so-called wild yeast like Brettanomyces. Diastatic yeast strains will often produce beers with 90%+ apparent attenuation.
When it comes to assigning apparent attenuation values to different yeast strains, it’s important that a standard wort is used. Although one would assume that yeast suppliers use some sort of standard wort when collecting these data for their strains, few suppliers publish any information about how attenuation data is determined. The lack of published details also means that different yeast suppliers likely use different methods to prepare wort for these tests. A quick survey of attenuation data from various suppliers reveals a range of values; the range is likely a result of the effect that wort production method and wort strength have on attenuation levels.
Few things are truly simple. The next time you brew a beer and don’t hit the final gravity cited in the recipe, consider how your mash and yeast choice may be affecting things. I am not arguing that stuff doesn’t happen, but most brews that don’t “properly finish” have finished and the deviation is just part of life. It’s been a while, but I think it’s pr