May 4, 2020 - 5:52am
Yeast doubling time
I've stumbled upon to some discussions about increasing yeast amount to decrease fermentation time. From what I've understand fermentation is mainly about the yeast growth in the dough. So I've check doubling time of the yeast bacteria. It appears that doubling time is 90-120 mins.
So I'm guessing if I double the yeast amount I only decrease the fermentation time by 90 mins. And if I decrease yeast amount by half fermentation time increases by 90 mins.What do you think about that?
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(I'm assuming commercial yeast discussion, not sourdough)
Yes, fermentation time is decreased in proportion to the quantity of yeast. Double the yeast, halve the time. And vice versa. (I aim to reduce my yeast, ferment for longer, and so have tastier bread.)
However the yeast does not reproduce in normal conditions. My understanding is that yeast reproduction only occurs in aerobic conditions. In a typical dough, the little oxygen is consumed very quickly and so the yeast does not reproduce.
Firstly, reproduction is the wrong term. Yeasts replicate through mitosis and that will happen under anaerobiosis. Indeed Oxygen is an important factor but in general terms you don't need to worry about that.
Cell growth is exponential and so double the yeast doesn't equal halve the time.
Mitosis is one form of reproduction. Mitosis is a more specific term, but reproduction is not the wrong term.
I guess for me when I hear "reproduction" I think sexual reproduction (meiosis), which yeast very rarely do.
Thank you for the feedback.
When I started looking into growing / propagating commercial bakers' yeast (due to shortages) the answers I got were "you can't (at home), it needs oxygenated growth environments."
If commercial bakers' yeast can grow anaerobically can someone please explain why is there such a push for sourdough starters, based on bacteria and wild yeasts, rather than propagating a tiny pinch of bakers' yeast? (I'm not anti sourdough, I simply want to learn about commercial bakers' yeast (instant dried, etc)).
Similarly, when I make a poolish over 12+ hours (8 lots of 90 minutes), the starting amount of yeast will have doubled 8 times, producing circa 256 times the starting amount. Even allowing for extensive cell deaths, there should be plenty of yeast remaining to not require the addition of extra yeast when combining the poolish with the rest of the ingredients. However that does not seem to be the case; extra yeast is (typically?) added. (I hesitate to say "typically added" because I only have a few sources of info - "Bread" by Hamelman and "BBA" by Reinhart).
Hoping to learn.
Bakers yeasts originally derived from the brewing industry represent strains of Saccharomyces cerevisiae that been have selected for their high CO2 output and ability to leaven bread dough. Strains grown industrially on a large scale use molasses and high levels of oxygen to promote biomass production and limit ethanol production which is not desired.
Flour is not sterile and it's not sugar rich either. LAB will dominate overtime where continuous feeding occurs and the environment will become unfavourable for bakers yeast which AFAIK is quite intolerant of acetic acid, low pH and ethanol.
I once had a starter that soured but did not rise/leaven. I added some yeast and it chugged along happily for another year, until I replaced it.
IIRC there is a point at which the yeast will continue respiration (so contribute CO2 to the dough) but will not reproduce.
What's the switch over point between yeast activities (whether reproduction or just CO2 production) and LAB taking over? i.e. how quickly do the conditions become toxic and/or unfavourable to mitosis?
In case it makes a difference, for the purpose of discussion I'm thinking of domestic environments and quantities of ingredients.
Cell division will occur concurrently with CO2 production.
If the aim is to grow solely bakers yeast you might look to replicate industrial practices. However, you would need to precisely manage a number of factors including; the substrate, nutrition and environmental factors. But I'm sure you'll agree this is not practical.
A flour and water medium will grow a bacteria component under normal conditions and unless you're doing something very different it will be difficult to successfully limit that. One option might be the use of PMS since LAB are much more sensitive to SO2 than yeast.
Without the appropriate technology attempting to grow pure cultures in a domestic setting is most likely futile.
Just a few small points.
There are plenty of ways to spin out very tiny amounts of commercial yeast, including using a bit of the dough from the last batch. Some of the recent interest in sourdough was because most people had zero yeast on hand and couldn't buy any. Some of it is because people like sourdough.
I've never heard of people propagating commercial yeast at home, though, probably because there is no obvious way to dry it and return it to inactivity. Plus IDY is just so incredibly convenient. A pound of the stuff lasts me over a year.
There's clearly some limiting factor on yeast growth in a poolish. I mix them up, let them rise and put them in the fridge and use them whenever, and I can't say there's an obvious difference between one and four days.
You actually don't need to add more yeast when you make up a dough with a poolish -- if you wait the yeast in the poolish will do the job. But depending on temperatures and whatever, that can be some hours. Hamelman's first audience is commercial bakers who have timetables; Reinhart also has in mind novice bakers who may get impatient. Once you've baked a while you get more comfortable with letting the dough tell you what it needs.
Thank you, interesting insights, very helpful.
My interest is not to produce IDY per se, just to keep some form of yeast going without going down the sourdough route. At this stage that's now (thankfully) reduced to a thought exercise; my wife found some dry active yeast locally (without being gouged on price).
Good points on the book audiences; hadn't thought about that. I enjoy both books but am happy to buy others. Any recommendations (not sourdough biased)? Am guessing I'll be looking for something aimed at "artisan" bakers? :-)
I will try producing dough from my next poolish without adding extra IDY. Any idea *roughly* how long I should expect the bulk fermentation to take? Assuming 500g strong white flour total, 64% hydration. 12hours? 24?
I'm still confused by the thought that the yeast is doubling ~ every 90 minutes. The 0.3 - 0.4g of IDY in my poolish would, after 12+ hours, turn into way more than the 5g one may put into a basic direct dough. So everything should happen much faster once the poolish is added to the remaining ingredients.