Why do you need to Step Up in yeast propagation?

[chipmunk]

I’ve never really understood why you need to step up when making starters. I know in Chris White’s book “yeast” he talked about limited growth rates with low inoculation rates, but Kai Troester and some others pointed out that those graphs were based on non-agitated starters where there may have been limited oxygen. So... assuming you provide enough oxygen, why can’t you make up a starter from extreme low inoculation rates? Does it have anything to do with genetic drift from too many generations?

 

[TheMadKing]

I’ve never really understood why you need to step up when making starters. I know in Chris White’s book “yeast” he talked about limited growth rates with low inoculation rates, but Kai Troester and some others pointed out that those graphs were based on non-agitated starters where there may have been limited oxygen. So... assuming you provide enough oxygen, why can’t you make up a starter from extreme low inoculation rates? Does it have anything to do with genetic drift from too many generations?

My 2 cents

If you don't step up the starter when you need more than a 10x increase in cells you're essentially asking the yeast to spend an stressful amount of time in the log phase where reproduction is very rapid before they set to work really consuming the sugars before flocculating again all fat and happy.

If we use macroorganisms as an analogy, you're starting with 2 pigs and basically breeding them to get 20 pigs as rapidly as possible, which is not going to produce the healthiest herd of 20 pigs. You'll make healthier pigs if you give them time to recover and fatten up between breeding cycles and in the end you'll have the same number of pigs either way, but the group that was given recovery time will be stronger, larger, and ready to make bacon from.

 

[chipmunk]

Thanks for the response! That’s an interesting analogy - I was thinking of the log phase as being less stressful - meaning the yeast have oxygen and all the food they can eat, although I can see how spending too much time in that log phase might be detrimental to the yeast.

I saw this video on issues with creating “sugar” starters and was wondering if you’d also get into similar issues with low inoculation rates since the first several generations of yeast might preferentially feast on the readily available sugar?

 

[Vale71]

I saw this video on issues with creating “sugar” starters and was wondering if you’d also get into similar issues with low inoculation rates since the first several generations of yeast might preferentially feast on the readily available sugar?

Yeast uses the simple sugars (fructose, glucose, saccharose) right away and since there is only a limited amount of them in wort (6-8% of total extract) they use them up really quickly and so the following generations of yeast would actually only have complex sugars available which is perfectly fine.

 

[mashpaddled]

If you can supply sufficient oxygen saturation it should not be a tremendous problem eventually reaching the desired cell count. I'm not sure that would be faster than stepping up and along the way your starter is at greater risk of infection until the ph drops and alcohol starts being produced as you move into the exponential growth phase.