Yeast Growth curve in Starter

[ApresSkiBrewer]

Once active fermentation is evident in the starter, does that mean that most/all yeast growth is complete, or does the growth phase/fermentation phase overlap quite a bit?

Curious because... if I want to make two smaller beers (5gal, 1.040ish), I could buy two vials... or I could do a 1.8-2 liter starter with one vial, and split it. But can I split the starter 24 hours later (very active starter fermentation), or do I need to wait for it to complete ferment out... yada yada yada. If it is already in the 200-220 billion cells range at 24 hours... then I could get ambitious and double brew, pitching half into each small beer.

I know this is all pretty dependent on the size of the starter, the viability of the vial, etc... but in general, is an active fermentation the sign of completed yeast growth?

[Sent From My iPhone]

Depends...on the amount of oxygen the yeast has. The more oxygen, the more cell growth. Once oxygen is depleted the yeast will start fermentation, but I would guess there's some overlap. Yeast in a starter is generally done growing / expanding after 18-24 hours.

[Sent From My iPhone]

From the interweb:

The Carrying Capacity of the Environment (K)

"This graph shows the growth of a yeast population in culture. After a period of exponential growth, the size of the population begins to level off and soon reaches a stable value. This type of growth curve is called sigmoid or S-shaped.

If we add fresh culture medium to the container, exponential growth resumes until a new higher plateau is reached.

Evidently the growth rate (r) declines as the density of the population approaches a certain limiting value."

[Dubba_Brew]

nice graph.

[ApresSkiBrewer]

That graph has a little to it.... I like.

Thats what I figured... I don't have a stir plate (salvaged most of the parts to build one a while ago... and they are still just sitting around). But I had nothing else to do but to shake the starter every 2 hours or so.

Reason I asked is because I double brewed on a whim yesterday... but only had one yeast vial. Made a 1400ml (1.040) starter Monday at 7. Shook it intermittently, and it was actively fermenting by 12 tuesday. Pitched about 650-700ml's into a 1.047 english brown, and 700-750ml's into a 1.050 Pale ale. I know I underpitched, but I was hoping to pitch at least 75billion cells into each.

Goofing around on mrmalty, a 1400 ml starter with one vial would step up to 219billion cells. If yeast growth was complete, then I hopefully got 100bil into each beer at least. We'll see!

[Thirsty Boy]

your first sentiments are pretty much correct, once you see signs of active fermentation, then the growth phase is mostly done with - should happen in about 18-24hrs - after that you are pretty much just making booze, not more yeast.

note - krausen isn't necessarily a sign of active fermentation, yeast will rise to a krausen during growth phase and the krausen actually changes character when fermentation starts... more frothy less yeasty. Look for lots of C02 evolution.

[ApresSkiBrewer]

Look for lots of C02 evolution.

That's what I had when I split the starter. But it worked out, they were both a nice active/healthy looking ferment. Maybe a slight underpitch, but I'm not too worried. Still a little action from both at 67 degrees, (held steady throughout).