Why step up yeast starter in stages?

[Ellis]

I am reading through information on harvesting commercial yeast from bottles, and also looking through "Slanting yeast":
https://www.homebrewtalk.com/f163/slanting-yeast-133103/

There is always talk of doing several stages of yeast starter. For example, the above talks about a 1 cup starter for a day or two, then a 1 liter starter. Is there any reason to do this in two steps, as opposed to dropping the yeast colony in a 1 liter starter to begin with?

For that matter, what happens if you put a very small (but viable) yeast colony into 5 gallons of wort? What would prevent that yeast colony from propagating and fully fermenting the 5 gallons, albeit over a longer period of time?

 

[erikpete18]

There's a pretty good article here (http://www.maltosefalcons.com/tech/yeast-propagation-and-maintenance-principles-and-practices about half way down) that goes over a lot of the science behind it. If you started right off the bat with a large starter, your main concern would be that you've got a whole lot of media there and very few yeast. If your sanitation wasn't perfect and the media was slightly contaminated, that contamination (bacteria, wild yeast, etc.) doesn't have very many good yeast to combat and can quickly take over the culture. Remember that most of our stuff is never sterile and there's always a few random bacteria and wild yeast that make their way into our beer/starters. If you've got a small volume you've got a larger yeast:volume ratio, meaning its gonna be harder for any contaminant to take hold. The article above also argues that you could also run into problems with mutations over the course of several passages of yeast as you'd be selecting for yeast that can grow faster over those that can ferment well (since you only ferment once the oxygen is all used up). I feel like the mutation concern is more for large breweries and not homebrewers, but the contamination issue is probably more prevalent.

For the second question as to a tiny colony in your beer, now you've got the same problems as above, but you've also got the fact that you've got a lot longer growth phase before you can start to ferment. This growth phase is when the yeast are producing a lot of off-flavors like diacetyl and esters. The longer the growth phase, the more off-flavors you'll get. In some styles (like Belgians) you can underpitch a little bit to get a higher production of esters, but you also run the risk of the yeast getting tired and getting a stuck fermentation. As the yeast start making alcohol, some of them aren't going to be able to handle it and die off. The more yeast you get before fermentation starts, the more you've got surviving through the whole course of fermentation which also means the more attenuation you'll get.

 

[GodsStepBrother]

"A solute may exert a specific biochemical influence on the physiology of yeast. Neutral solutes exert a general osmotic stress, where as charged species introduce an additional electrochemical factor. The way in which the stress is applied is also important. A very rapid change in the water activity of the medium has the greatest potential for damage."

This was taken from Brewing Science and Practice. There are better quotes where the author explains but I do not have he book near me. Anyways, the pressure exerted on yeast by sugar is very important to the health of the yeast. What the previous posters have posted is very true, but even if you hypothetically did have perfect sterilization. You still would not want a lot of stater medium to yeast population. You also would not want high gravity wort because the sugar content in the wort can apply to much osmotic stress to the cell wall and cause unhealthy yeast as well as stressed out yeast, and if there is enough sugar in the starter wort it could even kill the yeast ( kind of like how pure honey is pretty much unfermentable).

edit:
It is also important to note that yeast can survive in low water mediums, but that doesn't mean that it will give good results, and most of these yeast are labeled osmophilic or xerophilic and are mainly wild yeast.

If you under-pitch your starter, you will force your yeasties to multiply to fast and all those mothers will have there cell walls rittled with scar tissue that they can no longer eat through, hence more stress for the yeast! By starting with a little bit of starter wort you A. minimize the chance of infection by having your yeast take over the environment before anything else can. B Yeast can sense there surroundings by doing so they can relax eat slowly let there daughter cells become mothers and help out with the budding of new cells instead of expecting the mom's in the bunch to be octo moms leading to poor fermentations. C. You can greatly improve the health of the yeast by putting them in an low gravity wort they feel comfortable swimming in and does not exert to much pressure on them.

Hope this helped some.

 

[Paramecium]

I don't have the book with me but when I read "Yeast" a while back I remember there being a chart with cell count/ starter size needed. Long story short when talking about starters up to 2 liters with a vial or smack pack there really wasn't much benefit to stepping up. When going larger then a step or 2 may be needed but it would be really hard to under pitch 2 liters.

 

[Paramecium]

To clarify, you could go straight to 2 liters, you would need to step up if for example you were brewing a high gravity beer and wanted 300 billion cells or something like that. You would make the first 2 liters with 1 vial giving you about 200 billion cells, then decant off and add 2 liters of fresh wort to get to 300 billion cells. Stepping up smaller than that though doesn't seem to be necessary.

 

[Calder]

The amount of new yeast reproduced depends on the concentration of yeast (millions of cells per ml) being pitched. There is an optimal concentration to get the best yield; if the concentration is too high or too low, the amount of new cells will be low. That is why you step up a starter.

Pitching a very low cell count into 5 gallons could result in the ferment starting slow (allowing contamination to take hold), the yeast being stressed and creating fusels and strange flavors, and the yeast dying before the ferment is done.

 

[Clementine]

The amount of new yeast reproduced depends on the concentration of yeast (millions of cells per ml) being pitched.

I would disagree with this, from what I have read and my trails with it thus far the quantity of yeast produced from my starters (stepped from slants) seems to be reliant on

1) Aeration method (you really notice the rate increase when I move from vial to stir plate)

2) Volume of starter

3) OG and nutrient level (held constant)

Although when i start with a larger initial cell count the starter is finished (flocked out) quicker it does not seem to produce more cells (size of cake). The only reason you don't pitch one cell into the final starter volume and just wait a long time on the stir plate is infection and mutation risks.

Clem

 

[iaefebs]

great question...I rehydrate dry yeast in my kitchen where I am sure there is a lot of yeast spores from the wife's bread making, pizza dough and who knows what from the garden.I have used yeast harvested from bottles and from slurry, I try to be sterile but not anal... As for slanting or freezing I plan to freeze and will be doing my first attempt Monday. I will be sterile as hell through the whole process and step up as recommended..... For about a month, ........then I'll get sloppy in my procedure and know how to answer your question