February 26, 2009 - 3:30pm

## Retarding poolish,levain

I'm relatively new to baking and I want to try use prefermented dough(poolish into my ciabatta bread.my book says to let it rest in room temperature( 18C- 21C) for about 12-16 hours. What I really want to know is if its possible to slow down the process to it takes about 24-28 hours instead.

I know that addings some salt and putting in the fridge would slow it down but I have no clue since I have absolutely no experience with prefermented dough.

If you are using a poolish - which is nominally defined as a 100% hydration preferment leavened with commercial yeast - the first thing I would try is simply use less yeast in the poolish. Use less yeast, it will take longer to mature.

The next thing I would try is keeping it in a cool environment. Maybe even the refrigerator.

Only as an act of desperation would I add salt. I would hear "my teacher" chuckling half to him/herself and half at me in my head. I don't think I could do it...

If I were using a sourdough to make a levain, this of course, would be a different story.

Hope this helps (Sorry - I went into some kind of flashback on this and had to reply...)

If you want to slow your poolish down and let it ferment for 24-48 hours, cut your yeast back by half (for 24) to 3/4 (for 48).

=== If you want to slow your poolish down and let it ferment for 24-48 hours, cut your yeast back by half (for 24) to 3/4 (for 48). ===Interestingly, that turns out not to be the case. Yeast grows exponentially not in a straight line. While the full amount gets an initial head start, the one-half amount does not take twice the time to match the full amount.

You can demonstrate this with an Excel/Open Office Calc spreadsheet. Open a new sheet and enter the following:

A1: 2

B1: 4

A2: =power(a1,1.05)

B2: =power(b1,1.05)

C2: =ROUND((ROW()*15)/60,0)

Select cells A2-C2 and copy them down from A3 to A85. You will have to reformat columns A and B to no decimal places and widen them out, because they will get very big very fast.

2 and 4 are the number of yeast cells to start. 1.05 is the growth factor. Column C is the number of hours since start. We will assume that the dough it done when it reaches 1 trillion yeasts and all the flour is eaten up.

I arbitrarily chose a growth factor of 1.05 and assumed that doubling time was 15 minutes so the example would be simple. The real numbers would have to come from a biology textbook, but interestingly I think the elapsed hours are about right.

Note that the dough starting with 4 yeast cells goes over 1 trillion at 14 hours. Its little brother, starting with 2 yeast cells, passes the 1 trillion mark around 18 hours: only a 4 hour difference. Not the 14 hour difference we would expect from linear growth.

Exponents grow fast!

sPh

Funnily enough I was considering exactly the same thing in another thread yesterday.

Norm cited a formula which essentially stated that the ratio between fermentation times in a given recipe using different amounts of yeast is the same as the actual ratio between the differing initial amounts of yeast.

This holds true for doubling and halving amounts but if yeast growth is truly exponential then I wonder if it Norm's formula works for scale factors greater than 2?

example:

Let's say 4g yeast in a given formula has a doubling time of 1 hour

it's reasonable to assume 2g yeast will double in 2 hours

but 1g of yeast, according to Norm's formula would double in 4 hours

However I suspect the reality is closer to 3 hours.

I did a bit of reading around and found that while initial yeast growth is indeed exponential does then switch to linear growth apparently following a 'malthusian model'. The growth of yeast in bread dough turns out to be pretty complex with all sorts of limiting factors...not to mention 'mass effect' when dealing with large quantities.

My guess is that Norm's formula for linear growth is a useful rule of thumb - which holds for small scaling factors but not extreme ones.

FP