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I have just started a transition to making yeast starters and also using a homemade stir plate and am curious how to estimate the viability of the yeast when using this method. Normally, there are definite, visible signs when using an airlock, but how does one judge the level of attenuation when the yeast and wort are constantly moving? Is it primarily by color, viscosity, foam or some combination of all of them?

I've prepared a 1000ml 1030-1040 starter with Wyeast 1057 in a room ~ 72F. After 12 hours, I see a 1/4" foam ring at the top of the wort. Is this what I should be seeing and is there a more systematic way to gauge when it's ready to pitch. I have read a lot of conflicting advice about the length required on the stir plate and would like to be able to understand what guidelines others typically follow.

-bill

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I'd like to see some questions asked around the conflicting advice out there. –  brewchez Sep 10 '11 at 18:46
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4 Answers

up vote 1 down vote accepted

I know what a standard starter looks like when I pour in one vial of yeast. When I come back to it 24 hrs later, it is usually creamier in appearance. This indicates that cell growth has occurred and there are more cells present than before.

The estimate is >90% viable. The "how" is based on experience and knowing the biology of yeast. But if you are looking for a way to know that anything has happened in the starter (because you can't count bubbles) then you should check the gravity, pre and post.

I am not sure if viability is what you are actually interested in. If you are trying to understand or estimate the cell count, then you need a microscope to count cell #s.

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Understand totally about the gravity, but I was really looking for some reasonably reliable "eyeball it" method. It's definitely very creamy in both color and texture and the foam ring is growing increasingly taller in height so I'm guessing that all is well. I suppose I'll get better at judging the progress with more experience. For now, I'll use the popular 18-36 hour range as a baseline. Thanks. –  Bill Craun Sep 10 '11 at 19:43
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I always start with a "smack pack" of yeast, so I know it's viable because it inflates. When I pitch it to the starter flask I just shake it up good to aerate (I don't use a stir plate) and give it 24 hours before I pitch it to my wort. As Brewchez said, it will look milky or cloudy, and mine also always has a clearly visible sediment of yeast in the bottom (more than what you originally put in) and will form a foamy "head" when agitated.

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If you want to get a feeling for what the right quantity of yeast looks like, you could pitch a new dry sachet of yeast into a cold starter. An 11g sachet dry yeast has 150-200 billion cells - the right quantity for a 5 gallon batch of 1.040 ale. (0.75-1 mill. cells per ml.) The cold starter will not start to ferment and the yeast will not stay in suspension, falling to the bottom in a few hours. The amount of yeast at the bottom can serve as a rough guide for the right quantity of yeast to be looking for in your starters.

Having said that, you don't need to worry about cell density too much - cell health is more important than cell density (within reason!). Under the right conditions, healthy yeast can propagate 16x in 24 hours, so any underpitching is quickly made up.

If the starter is active and you see a milky color change, you know the yeast are viable and have propagated and you're good to go.

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There is nothing stopping you from putting an airlock on top of your starter so you can see some bubbles... give that a shot.

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