Take the guessing out of baking: Calculate the right amount of yeast

Balances with a 0.001 g readout are are not so expensive and quite useful in the kitchen. A 12 g sachet of instant dry yeast can last for a long time if you use less yeast and allow the dough to leaven a little longer.

Never before have I seen so many references to exponential growth as during the last year. Regrettably this has been in a negative context related to the spread of the SARS CoV-2 virus. Exponential growth is very counterintuitive (as the wheat/rice chess board problem illustrates). It looks like nothing is happening for a long time. And then all of a sudden a lot happens. This is part of the explanation why COVID-19 cases suddenly seem to skyrocket out of nowhere. On the positive side, many have been left with evenings and weekends free during lockdown, leading to a steep rise in baking with yeast. In Norway where I live, yeast was sold out for weeks and months on end. But I was lucky to never run out of instant dry yeast, due to the exponential growth of yeast.

Exponential growth is very counterintuitive. It looks like nothing is happening for a long time, and then all of a sudden a lot happens. The graph shows how a single yeast cell can grow into 1 000 000 000 000 (a trillion) yeast cells in less than 3 days if conditions are right.

Exponential growth means that something grows with a fixed percentage over a given period of time. Under ideal conditions with access to nutrients, air and the right temperature a single yeast cell can grow and split into two cells in the course of about 100 minutes – that’s a full life cycle in less than 2 hours. After another 100 minutes the two yeast cells have become 4, and so it continues. After 24 hours there are about 16 000 cells, and after 48 hours we are counting 268 million cells. Before 3 days are gone there are on the order of a trillion yeast cells (1 000 000 000 000). That’s about as many cells as there are in a 50 g piece of compressed yeast. As long as conditions are ideal the growth continues. This can be achieved in commercial production of yeast for a limited amount of time, but in an actual bread dough the process will be a lot slower.

To determine the amount of yeast needed to leaven a dough made with 1 kg wheat flour, decide how long you want to leaven/proof and read out amount of yeast for the given temperature. Data is taken from the app PizzApp+ (Android)) – download the App to determine amount of yeast for other time/weight/temperature combinations.

Like everyone else I have also spent more time in the kitchen baking. In order to get more mileage out of each sachet with instant dry yeast I have used a very small amount and rather let time do the work. To help determine the right amount of yeast for a simple bulk fermentation (aka. a direct dough or straight dough, so no preferment such as a poolish, biga, levain etc.) I found the pizza baking app PizzApp+ (for Android) to be of great help. For instance, a dough made with 1 kg of flour needs only 0.48 g of instant dry yeast if left to rise for 20 hours at 20 °C. Small balances that can measure such minute quantities are not too expensive as long as you don’t need laboratory grade equipment. The long bulk fermentation allows some lactic acid bacteria to develop as well, which improves the flavor. The problem with running out of yeast can of course be completely eliminated if you go all in on sourdough, but sometimes the convenience of instant dry yeast is simply too alluring.

PizzApp+ (Android) is a useful tool to calculate the amount of yeast needed to leaven a dough in a given time. You can choose between fresh compressed yeast, active dry yeast, instant dry yeast and even liquid and firm sour doughs!

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