Limit of Attenuation
The limit of attenuation is the sum of all the sugars, that the yeast is able to ferment, expressed as a percentage of the total extract content. Since ale yeasts (saccharomyces cerevisiae) can only ferment maltotriose to a third and lager yeasts (saccharomyces uvarum) can ferment that wort sugar completely, lager yeasts will show a slightly higher limit of attenuation for the same wort.
Other than this the limit of attenuation of a given wort does not depend on the yeast strain. It is solely set by the mashing process. All yeasts are able to ferment all the fermentable sugars (save for the lager - ale difference mentioned above) in a given wort. This fact is taken advantage of in a [forced ferment test]. The forced ferment test uses a sample of wort and pitches it with a large amount of yeast. To ensure complete fermentation this sample is kept warm and the yeast is regularly roused (shaking or stirring). Once the fermentation is complete the apparent extract is measured and the limit of attenuation for that wort can be calculated.
[edit] Yeast Strain Differences in Attenuation
If there are no limit-of-attenuation differences between the different yeast strains, why are there more and less attenuative yeast strains available?
Though yeast strains are able to ferment all the sugars in the beer, they usually don't get to. In contrast to a forced ferment test, beer is generally fermented at lower temperatures, with smaller pitching rates and without constant rousing. Because of that the yeast will not get a chance to ferment all fermentable sugars in the wort. Flocculation will cause it to drop to the bottom or collect on the surface where it doesn't have as much contact with the sugars anymore. Because of nutrient depletion and or high alcohol levels cells die before they get a chance to ferment every last bit of sugar in the wort. The result is left over fermentable sugars that play an important role in the character of the finished beer. The closer a beer's attenuation is to its limit the drier and less sweet it will taste. When looking at the attenuation ranges given for commercial yeast you will notice that the less flocculating a yeast is, the more attenuative it will be. This makes sense as the poorly flocculating yeasts will remain in contact with the wort for a longer time. The beech wood aging process used by Anheuser-Bush to brew Budweiser does exactly that without relying on poor flocculation alone; it maximizes the contact area between beer and yeast.
