Vol. XXIV · Bread Plate 5 of 12yeast, baker's yeast, fresh yeast, active dry yeast, instant yeast, conversion, temperature, proofing yeast, doubling time, rising, Saccharomyces, 酵母MT

Yeast.

A recipe asks for 21 g of fresh yeast. You have instant. Use 7 g — and expect the same rise.

Baker's yeast is Saccharomyces cerevisiae, sold in three forms. Fresh (compressed) yeast is about 70 % water, perishes in a few weeks in the fridge and is crumbled into the dough. Active dry yeast has been dried into granules with an outer layer of dead cells, so it is usually dissolved in warm water first. Instant yeast is dried finer and faster, survives better, and is mixed straight into the flour. Gram for gram, instant is about three times as strong as fresh, and active dry about 80 % as strong as instant, so the rough conversion is fresh : active dry : instant = 3 : 1.25 : 1.

Yeast turns sugars from the flour into carbon dioxide and ethanol (XIV.2). Its rate depends steeply on temperature (V.10): it barely works in the fridge, accelerates through the twenties, peaks in the low thirties and dies above about 50 °C (which is why water for yeast is lukewarm, not hot). How much yeast to use is a choice of time: about 1 % instant yeast of the flour doubles a lean dough in an hour and a half at 26 °C; 0.2 % takes much longer and develops more flavour, as fermentation by-products and enzyme action accumulate (XXIV.6). The workshop converts and times yeast.

Below: the instant-yeast equivalent of the amount and form of yeast you choose per 500 g of flour, and the time a lean dough takes to double at the temperature you choose (course fermentation model, calibrated to a common bakery figure), with yeast activity across temperature.

What you will learn
Workshop
Conversions by weight (common bakery practice): fresh = 3 × instant, active dry = 1.25 × instant. Doubling time: the course fermentation model (gas ∝ yeast^0.8 × activity at T; 1 % instant yeast doubles a lean dough in about 1.6 h at 26 °C). Activity: cardinal-temperature curve with an optimum near 34 °C and death above about 46–50 °C.

Choose the yeast by the clock

In the course model, 1 % instant yeast doubles a lean dough in about 1.7 hours at 26 °C, 1.3 at 32 °C and 2.4 at 20 °C; cut the yeast to 0.2 % and it takes 5.9 hours at 26 °C. Bakers use that freedom: a quick dough for this afternoon, a slow one overnight for flavour, or a cold retard in the fridge that slows everything down so the bread can be baked when convenient (XXIV.6).

Handling: keep instant and active dry yeast sealed, in the fridge or freezer once opened; fresh yeast in the fridge, used within two weeks. Dissolve active dry yeast in water at about 35–40 °C, never hotter; instant yeast can go straight into the flour. Keep yeast and salt apart until mixing begins, since concentrated salt draws water from yeast cells (XX.1). If in doubt, test: stir yeast with a pinch of sugar into warm water; it should foam within ten minutes.

At the stove

Make three small lean doughs with 1 %, 0.5 % and 0.2 % instant yeast, mark each container at its starting level, and record when each doubles.

Louis Pasteur showed in 1857–60 that fermentation is the work of living yeast cells; compressed baker's yeast was produced industrially from the 1870s, active dry yeast in the 1940s for the US Army, and instant yeast in the 1970s.

Terms

Instant yeast
Fine dried yeast mixed straight into flour.
Active dry yeast
Granulated dried yeast, usually dissolved first.
Fresh yeast
Compressed moist yeast, perishable.
Doubling time
Hours for a dough to double in volume.
Fresh : active dry : instant ≈ 3 : 1.25 : 1 by weight. Yeast works fastest near 32–34 °C, crawls in the fridge, dies above about 50 °C. More yeast rises faster; less yeast, longer, tastes better.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.