Yeast Attenuation Question, Why?

I was recently thinking about attenuation and how very often my attenuation will be almost exactly what the manufacturer states that it will be. Almost so consistently that I can calculate OG from FG. Im a pharmacy student with some knowledge of biochemistry, and the process of fermentation. However I can't think of a reason why the yeasties would decide 75% was enough and they would go dormant. Especially in higher OG beers in which there is plenty of residual sugars remaining for fermentation. i realize that maltose is a complex sugar and therefore the yeast have more difficulty metabolizing it, but don't understand why this would lead to such precise attenuation percentages.

Really just curious as to what makes the little guys tick, and hoping someone could help me out. Thanks!

Garrett

It's possible that my personal experience is unique, but I've found that my attenuation has way more to do with mash temp than yeast strain. I'm curious as well about what limits attenuation, but I haven't found it to be true.

I've used Pacman yeast for several brews and my attenuation has ranged from 78 to 89. Pacman is rated from 72-78% attenuation. The 89 was a barley wine that went from 1.115 to 1.012. The attenuation correlated with my mash temps. Higher mash temps (154) yielded the 78% while a low mash (145) gave me 89. The others were in between, but gave me the same results. Pacman is rated from 72-78% attenuation.

I've only used Weihenstephan once, but it was with a hefeweizen at 1.064 and it ended at 1.011. That's 81. It's rated from 73-77%.

These are the only 2 strains I've used so far, but they don't seem to be following the rules.

Maltose is the principal fermentable sugar in wort, but a wort is not composed of only maltose; among other compounds there is a small percentage of simple glucose and fructose and another fraction of complex sugars (dextrins) which yeast do not ferment well if at all.

Most beer yeast ferments maltose (two glucose) well, but the higher attenuating strains can ferment maltotriose (three glucose) to a greater extent than less attenuating strains. High attenuation is also reached by yeast strains which do not flocculate well, remaining suspended in the wort and thus having extended contact with the fermentables. The rest of the gravity measurement comes mostly from the other complex sugars and proteins which contribute to the body of the beer.

The ratio of fermentable vs. unfermentable sugars can be manipulated by adjusting the temperature of the mash. Higher temperature (150-167F) favors a less fermentable extract.

In those higher OG beers, the limiting factors can be oxygen, micronutrients, and alcohol tolerance of the yeast strain.

And that higher mash temp causes partial denatueong of amylase causing lower breakdown of starches? So the amylase breaks down the dextrins less? Is that correct?

So this higher mash temp causes partial denaturing of amylase causing less breakdown of these dextrins leaving less fermentables and a higher body. Correct?

The higher mash temp causes less activity and denaturing of beta amylase which will break starches down to more simple sugars. The high temp favors alpha amylase which breaks starches down into more complex sugars. It leaves you with a less fermentable wort with a higher body.

At the higher temperatures, the activity of the amylase enzymes is increased, but beta-amylase is also denaturing at an elevated rate.

The alpha-amylase reacts with the relatively complex starch molecules (amylose and amylopectin), breaking them down into managable pieces. It attacks the starch randomly, so it produces pieces of varying size, sometimes small and fermentable, but mostly larger pieces. These pieces are then attacked by beta-amylase, which must react with the ends of these molecules, producing maltose.

If you were to denature all of the beta-amylase, while leaving alpha-amylase, you would get conversion of the starch into dextrins, but the beer would have very little fermentable sugars.

Though as others have said mash temps do have a lot to do with when the yeast quit eating. I have come up with a lot of info stating how yeast can sense there surrounds quite more than we give them credit for. It is not that they are done eating all of the possible sugar they can. They pretty much do a calculation of there surroundings and how many there are. Some decide to huddle together and face the rough time approaching while other continue cleaning up the most they can.

"Commonly, flocculation occurs only when sources of fermentable sugars are exhausted. It has been suggested (Iserentant 1996) that under such starvation conditions the ability to form flocs may represent a stress response. Thus, flocs provide a sheltered environment where the chance of survival of the population is enhanced.

Sometimes the yeast can get to stressed out and flocculate to soon, leaving to little yeast to do the job.

"Conversely , if flocculation occurs to soon, fermentation may arrest because insufficient cells remain suspended in the fermenting wort."

It is not just a matter of what mash you have done, and if it was done correctly (though this is important). Even if the correct amount of sugars are present, does not necessarily mean you will get the gravity you where expecting or stated by the manufacturer. I think pitching temp, pitching rate, and temperature all have a lot to do with when these little buggers decide enough is enough and it is time to conserve for rough times.