Yeast, Attenuation and Priming: puzzled.

[morethanworts]

This puzzles me:

When a brew finishes it's primary fermentation, I understand it is because the yeast has 'attenuated' and will not eat up any more fermentable sugars that may remain in the beer. Our Glossary defines attenuation as "[the] Extent to which yeast consumes fermentable sugars (converting them into alcohol and carbon dioxide)." So I infer that there will be fermentable sugars (as well as unfermentables) left in the beer.

Why, then, does the yeast suddenly get a new lease of life when we add priming sugar?

and

Why does it then still only eat up exactly the 80g, 100g per 23L or whatever that we've just added, rather than ever continuing (on its second wind) into the other residual fermentables, which would make a mockery of our careful priming allowance?

 

[graysalchemy]

When the beer stops at 1012 for arguments sake it does because there are no more fermentable sugars the rest are unfermentable. It gets its second wind because you are adding more fermentables to it. It wont go past the 80g because that is all that you have given it.

 

[morethanworts]

Thanks GA. If that's the case, which is what I used to understand too until recently, then please help me with where I have misunderstood:

I read that different strains of yeast have different levels of expected attenuation. That suggests that it's not just the amount of fermentable sugar in the wort, but the 'ability' of the yeast to eat it all up as well, that has an effect - the yeast's attenuative properties.

The glossary itself suggests this to me, in that definition above, defining attenuation as the "extent to which yeast consumes fermentable sugars..." If all yeast strains ate up 100% of the fermentable sugars in the wort, then (by that definition, and others elsewhere) they would all have the same attenuation: 100%. :wha:

 

[graysalchemy]

Some yeast strains are just better at eating up all the fermentables than others

 

[morethanworts]

Some yeast strains are just better at eating up all the fermentables than others

Understood. But why do the ones that have left some fermentables untouched suddenly pull up their socks when we add priming sugar and carry on fermenting again, continuing exactly through just the priming sugar and nothing else?

 

[graysalchemy]

Fermentables and unfermentables are a very large group of molecules, some which may be fermentable to one yeast may not be to another (for example wine yeast cannot ferment Maltose). So when you add sugar it isd just consuming that energy source to the yeast anything else which is left is unfermentable as it didn't ferment it first time round.

 

[mattybabsy]

yeah, thats why you usually prime with simple sugars

 

[morethanworts]

:thumb: OK - brilliant, thanks, I think I understand now: all yeasts will eat up all the simple sugars (unless there is an excessive level of alcohol); out of the more complex sugars, different yeasts will consume different parts - and to different overall extents.

The key to it was understanding that some sugars may be fermentable to one yeast, but not another: I was working on the false assumption that sugars were either fermentable or unfermentable to all yeast strains equally, and that it was just the extent to which the yeast got through them all that varied. The wine yeast/maltose example is great.

Do we have a smiley for unpuzzled, now? Raised finger - ahh, hands in the air type thing? (party smilie is a bit OTT, perhaps.)

anyway :

 

[Dr Mike]

I've found that the beers I've done with S-04 (which is a med-low attenuator) have continued to slowly gain condition as they age, suggesting the yeast continues to slowly process the complex sugars after fermentation, whilst the ones I've done with Notty (high attenuator) don't seem to gain any more after the first couple of weeks.