Yeast Propagation

[norrisk66]

I saw an article on the homepage and it reminded me of another article I had read a while back in BYO.

Interesting article-

Explains the “Crabtree” effect where yeast will choose to intake glucose (if it is present over .4%) instead of take in Oxygen (which yeast needs to replicate). Parkes, S. (2002)

Makes you wonder how effective adding DME (glucose) to a yeast starter really is, if what the yeast really need to multiply is O2 (where a stir plate comes in...)

http://byo.com/stories/issue/item/1498-the-science-of-yeast-homebrew-science

Thoughts?

Keith

 

[Weezy]

Quic cell phone response. There's a lot of good threads on here about yeast life cycle, aerating starters vs. The wort, and more. I'm assuming you're suggesting starters just need o2 and not the sugars? yeast, fully fortified with nutrients, don't need o2 to reproduce (like dry yeast which doesn't need aerated wort). They just need healthy cell walls to support budding (where healthy cell walls come from yeast working in the presence of o2 and food and nutrients to build strong cell walls).

In a starter, the daughter cells needs the food and nutrients to fortify themselves.

 

[Chino_Brews]

> Makes you wonder how effective adding DME (glucose) to a yeast starter

DME is not glucose. You may be thinking of dextrose, aka corn sugar, which is just glucose. Dried Malt Extract contains all of substances in beer wort, except water, including maltose, maltotriose, dextrins, and minerals.

 

[F_R_O_G]

wow! this is why we need more articles, to many people push bad information.

"Makes you wonder how effective adding DME (glucose) to a yeast starter really is, if what the yeast really need to multiply is O2"

"like dry yeast which doesn't need aerated wort"

 

[Warthaug]

You've got it backwards; because of the crabtree effect, saccharomyces will not engage in respiration (breakdown of sugars using oxygen, producing water, CO2 and ~36 units of energy [ATP]) and instead engage in fermentation (breakdown of sugars without oxygen, producing ethanol, CO2 and ~2 units of energy) regardless of the amounts of O2 present.

To avoid the crabtree effect and get maximal levels of respiration (and therefore maximum growth) you need to provide sugars at a very slow rate (i.e. keep them less that ~0.2% total concentration) in the presence of lots of O2. This is called batch-feeding, is beyond the abilities of most of us to implement in our homes, and based on statements by Chris White, apparently produces yeasts that are near-useless for fermentation.

In the case of the starter you & I can make at home, the amount of O2 required to get a good number of healthy yeast is pretty small - roughly the same amount that would be pre-dissolved in a starter if you aerated it well before throwing in the yeast. While the stirring of starters does add in some O2, its main advantage in terms of yeast growth is the removal of CO2. High CO2 levels impair fermentation, and thus slow growth and limit yield. A lot of this work was done in the 1960's, but at least one study (R. H. De Deken (J Gen Micro, 1966)) can be found be found on the net that looks at these factors.

Because our yeast are relying on fermentation for growth, the amount of sugar present is key in determining yield. In Christ White's yeast book there are graphs showing the yeast yield relative to starting gravity - with yields generally increasing with increasing sugar concentrations. Somewhere over 1.040 the benefit of more sugar drops, which is why most of us use worts in the range of 1..035 - 1.040; in that range you get the most growth per mass of sugar added.

Bryan

 

[helibrewer]

Lower pitch rates also increase propagation rates. Dr. Smith (BeerSmith), recommends using a starter size/pitch rate combination that keeps your growth factor less than 3. This makes sense since trying to achieve growth factors in excess of 3 leads to starters the size of your actual batch of beer...impractical. You can find his discussion on the BeerSmith website blog.

 

[norrisk66]

Thanks Bryan-

This was the quote from the article in question I was trying to better understand and relate it to our "yeast starters" at home.

"During fermentation, glucose is always present at greater than 0.4%, so brewing yeast are not able to respire. As a consequence, their mitochondria are small and serve little function. If brewing yeast are exposed to oxygen in low glucose conditions, they will overcome the Crabtree effect and their mitochondria will proliferate within six to eight hours. This fact is used to grow yeast rapidly in commercial yeast propogators." (Parkes, 2002).

~Keith

 

[Warthaug]

I can see why that would be confusing. The part that was missing from the article is that the amount of growth possible is pretty much directly related to the total amount of sugars (i.e. energy) available. So if you made a starter with 0.4% glucose you'd have a starter with a S.G. 1.0015; you'd get oxidative metabolism, but only minimal growth as the yeast would tear through the sugars pretty fast.

To get larger growth rates aerobically you need to batch-feed, which means you continually monitor the sugar levels, and when they start getting too low you inject some additional glucose. For yeast, this usually means holding the glucose concentration around 0.2% (plus lots of oxygenation and agitation). This requires some pretty sophisticated equipment that we homebrewers are unlikely to be able to make/buy for ourselves, but its a pretty common technology in industrial microbiology.

Bryan

 

[norrisk66]

The part that was missing from the article is that the amount of growth possible is pretty much directly related to the total amount of sugars (i.e. energy) available.

Exactly-

Vitality of yeast needs to be taken into account. Sugar is needed for healthy yeast to be able to replicate more healthy yeast cells. This goes back to your statement from Chris White in stating that neglecting the yeast of necessary sugars would create yeasts that are "near-useless" for fermentation.

~Keith