How Yeast Works

[Easy Trotter]

Got a novice question: I am fermenting my second homebrew (a weissbier) and have read in several posts here that it's better to give the yeast more time to work than to bottle too soon. Guess my question is more technical on how yeast works – I know it breaks down sugar into alcohol and CO2, but what causes it to stop fermenting – when all the sugar is gone? How would the beer retain any sweetness?

[Dawg LB Steve]

It will only consume so much of the fermentable sugar chain, leaving the residual sweetness. Mash temperatures also affect the chain of fermentable sugars in All Grain and yeast Mfg's also list in the yeast's description what is called Attentution, the higher the percentage the more sugars it will eat thru. Most attentuate 75-80%. The yeast will tell you when they are done eating when you get the same Specific Gravity reading three straight days. It will not hurt to leave it go from there another couple of days for the yeast to "cleanup" in other words finish dropping itself and any other proteins out of suspension. Even though they are done, they are still very much alive and viable, which is why you need to be sure the yeast is done eating before bottling (at times the cause of bottle bombs in glass bottles), if you prime for bottle conditioning and the yeast isn't finished fermenting your wort, between the priming sugar and the sugars still left there is a very good possibility of highly over-carbonating what is in the bottle.

[mashani]

To expand/clarify, if you know your chemistry then you know there are many different types of sugars and sugar chains. Some sugars the yeast can eat. Other sugars the yeast can split into simpler sugars using an enzyme such as invertase, and then eat. And others they can't figure out how to eat. The "so much" Dawg described refers to this. If you do all grain your mash temps affect the "so much" - higher mash temps make more types of sugars the yeast can't eat. Lower mash temps make more of the sugars that the yeast can eat. Higher attenuating yeast are simply better at eating some of the bigger sugar chains that are formed or splitting them into more reasonable chunks through an enzymatic process. Most extract is produced "middle of the road" in this regards, so tend to end up in the middle of a yeasts possible attenuation range.

Most types of brewing sugar will ferment out 99+%. So brewing sugars are used to adjust the balance in some styles of beer by replacing some malt.

Now to make things fun - liquid malt extracts over time age. A process called "maliard reaction" occurs. This darkens the extract. It also changes the sugar chains, making them more complex. So older liquid malt extract will not ferment out as completely as fresher malt extract and leave you with a sweeter/more malty result.

This reaction can also occur in the boil, be it with extract or all grain - some Scottish recipes for example are boiled for 90 or 120 minutes instead of 60 to get some caramelized sugars and intentional maliard reaction going on. That also will leave you with a sweeter more malty product. Some types of decoction mashes also do this. On purpose.

On purpose is the real key here, more or less attenuation due to whatever reason - mash/boil/or yeast choice - is all about the style of beer you are making and what is appropriate for that style. Or if not to style, then what you like.

Once they run out of stuff to eat they fall asleep for lack of a better non-technical term.

What happens then depends on another term you will see listed for yeast - flocculation. If a yeast is highly flocculent, then cells become attracted to one another - think like static cling - so they clump together, get heavy, and then sink to the bottom. Yeast that is low flocculating are not attracted to each other and just float around in suspension for a much longer time, even though they aren't fermenting. So "clarity" is not always a good judge of whether your yeast is done or not.

That's why we say to use a hydrometer to test or wait.

Most of my beers are done fermenting in 7-10 days and I tend to bottle them around 14-16 days. But I am pitching 2-3 times more yeast then you get in a Mr. Beer or Brew Demon kit. Think of this as giving the yeast a big head start, they don't have to grow more cells for as long a time to achieve the "critical mass" to start up your fermentation. Yeast metabolism is a lot more complicated then simply turning sugar into alcohol and co2. That is the part we like, but to get to that part they have to do a lot of other stuff first. That other stuff part of their metabolism is why you oxygenate your wort (the whisk thing), use yeast nutrients, etc.

Hope that helped.

[Easy Trotter]

Thanks for your terrific responses, guys! I really appreciate the time you took to explain the process and now I have a better understanding of attenuation too.

Mashani, what you said about the yeast growing more cells to reach a "critical mass" was interesting. So the yeast actually multiplies during fermentation? That explains the comments I've seen about harvesting the trub.

[LouieMacGoo]

If you haven't already checked it out there is a free online version of the book How to Brew by John Palmer which is considered by many to be the Bible of home brewing. There is a complete section about yeast in that book that you may find helpful and of interest and goes into greater detail then what was covered above. It's worth checking out.

http://howtobrew.com/section1/chapter6.html

[Easy Trotter]

I will check it out, LouieMacGoo. Heard it mentioned a few times. Thanks.

[monsteroyd]

I am finding that leaving the yeast ferment for at least 3 weeks works every time, all the time, heck sometimes I can't get the batch bottled for a week or so and have gone to 5 weeks and I still get good beer. This is all in a smaller fermenter specifically the Mr Beer LBK. The yeast needs some time to clean up acetyl (I think), so I let it take it own sweet time. Then bottle and C+C for at least a month maybe more. Oh I always put the fermenter in the frig for a couple of days before I bottle to 'cold crash'. It helps with getting things to settle and thickens the trub a little so you don't get much when you bottle.

Monty

[mashani]

Thanks for your terrific responses, guys! I really appreciate the time you took to explain the process and now I have a better understanding of attenuation too.

Mashani, what you said about the yeast growing more cells to reach a "critical mass" was interesting. So the yeast actually multiplies during fermentation?

With the amount of yeast provided in those kits you are pitching at a rate where something like 5-6 or more generations of growth are occurring in the fermenter, if the yeast is older or stored at warm temps, even 8-10 or more even. The growth is nutrient limited, once they run out of oxygen or other nutrients, then they are forced to switch to active fermentation phase. But until then they will grow up to that "critical mass" as such.

Growth is exponential not linear. So you go from a "little" to a "lot" quickly.

At my pitch rates, I get 2-3 generations of growth only in the fermenter, and will end up at the same ultimate cell count (more or less considering there are billions of beasties in there) and then they are ready to go.

Simple explanation is that the "lag time" between pitching and seeing the active fermentation is where this growth is happening. Lag is longer the more generations are growing.

But different yeast strains grow at different rates, also temperature affects it, also wort composition (gravity, ph, etc.) - so it's not something you can compare batch to batch to proclaim "this is better" - unless you are using the same yeast, same temps, same ingredients, etc.

Also short lag time doesn't necessarily mean better - if you don't oxygenate, you might get short lag time because the growth can't happen. Then you get a wimpy fermentation with whatever # of beasties you got. That is more of a problem once you start to use liquid yeast. Dry yeast has "built in nutrients" that help work around that. And pitching so much that you don't get at least a couple of generations of growth tends to make beer that is not as good either. The new cells that grow in your wort are adapted better to it. Also, funny thing is that if you over oxygenate, it can actually extend the lag time because it might be enough to give the yeast a chance to grow another generation or two. That can only really happen if you use a pure oxygen system, there is no way to get that much oxygen into your wort naturally. But it can happen.

Nifty eh?

[monsteroyd]

That's why I call him mashencyclopedia.

Monty

[Easy Trotter]

Very cool...
Once again, Mashani, you've answered a question that was in the back of my head about why it takes a couple of days before I see any activity in the fermenter.

Now, if I can pick your collective brains for one more question before you tire of me. I've been using tap water to brew with and in our region it is very hard water, which is fine as far as taste, but our city adds flouride and I'm wondering if that has a negative impact on the yeast. Should I use distilled water instead?

[BigPapaG]

Hi ET,

Our municiple water has a Flouride addition...

The Federal Limit is 2.2 ppm and ours is usually detected at levels of 0.46-1.25 ppm, averaging 0.84 ppm.

Our water is also considered medium-hard.

I brew with it as is and have not experienced an deliterious effects. In fact, I have made award-winning beers with it.

Not that you could easily remove the Flouride anyway... Your average carbon filter won't do it... It would require an activated alumina filter, deionization filter, reverse osmosis or temperature distillation methods.

Or, you could dilute your tap water with distilled or spring water, or baby water (common now for keeping Flouride out of the mouths of babes), or start with distilled water and add back in the appropriate chemicals to reach your desired water profile. This assumes you are or can become versed in the level of water chemistry required... (Not alot of difficulty really, lots of online water calculators to use, forums to read etc...) A number of folks in the Borg do their own water additions.

Asside from that, I say: "Brew On!"... Thats what I do!

[Easy Trotter]

Thanks BigPapaG. It is a minuscule amount that they add so I won't sweat it.

[Dawg LB Steve]

Doug, follow this link and see if you local community water profile is listed to get an idea of your water profile. There is a forum right under yeast here for water discussion and treatment.
http://www.brewersfriend.com/homebrew/water-profiles/

[Easy Trotter]

Thanks Dawg. I did find my city on the list – pretty interesting stuff – fairly high in all categories. Once I get a few more home brews under my belt, I may experiment with the water on one just out of curiosity.

[BlackDuck]

I just started working with my water profiles too. My city wasn't on that list, so the first thing I did was send my water off to Ward Labs for a test. It's a little pricey, about $40, but if want to really know what your water is made of, this is the best way to do it. Here's the link to the Ward Labs Brewers Test. Once you know your waters profile, there a a number of good profile calculators on the web, just google Brewers Water Calculator and a few of them will come right up on the search. Play around with each one to see which you like best.

I've done two batches so far with complete profile adjustments. In both batches I've had to dilute my tap water with distilled water and have had to add back a little lactic acid, gypsum, calcium chloride and epsom salt. The calculators gave me the exact amount to add. The lactid acid, gypsum and calcium choloride were all available from my LHBS. The epsom salt is the same stuff you would get at any pharmacy type store (Walgreens, CVS, etc). So far on the two batches, I've noticed improvements to my beer, and I'll continue using the adjustments.

Adjusting water profiles can seem like a large task, but with enough reading, understanding, and playing with the calculators, it's really not all that difficult.