🍞 Yeast, Fermentation and Dough Development

Yeast is a living single-celled organism, and everything that goes right or wrong with it follows from that one fact. You are not adding a raising agent from a tin. You are keeping a population of cells alive, feeding it, warming it and eventually killing it in the oven, and the bread is the record of how well you did that.

What yeast eats is fermentable sugar. What it produces is carbon dioxide gas and ethanol, plus acids and aroma compounds that become the flavour of bread. The gas inflates the dough. The gluten network you build by mixing is what traps that gas, so yeast and gluten are partners and neither works alone. The ethanol and the aroma compounds mostly evaporate in the oven, but what they leave behind is the difference between fermented bread and a flour-and-water biscuit.

The first practical number to learn is the food supply. A bread dough needs 3 to 3.5 percent fermentable solids for yeast activity, expressed as a percentage of flour weight. Below that, the yeast has no substrate beyond what the flour's own amylase enzymes release by breaking down starch, and fermentation is slow, the crust is pale, and the loaf is small. This is the single most common invisible fault in a lean bread formula.

Work it out for a real batch. A baker in Kisumu, Kenya, uses 50 kg of flour and adds 1 kg of sugar.

1. Sugar as baker's percentage = 1,000 divided by 50,000, times 100 = 2.0 percent.
2. That is below the 3 to 3.5 percent the yeast needs, so the yeast is short of food.
3. To reach 3.5 percent she needs 50,000 times 0.035 = 1,750 g of sugar.
4. The increase is 750 g per batch. At an indicative local sugar price she must check herself, this is a few cents spread over roughly ninety loaves.

But do not simply keep adding sugar, because there is a ceiling as well as a floor. Sugar at 5 to 10 percent of flour weight increases yeast activity. Above 10 percent, yeast activity decreases, because the sugar draws water out of the yeast cells osmotically. Above 15 percent the dough is classified as a sweet dough. So a bun dough at 18 percent sugar is actively fighting its own yeast, and it needs either a higher yeast dose or an osmotolerant yeast strain bought for the purpose. If your sweet buns proof slowly and your plain bread does not, this is why, and adding more time will not fix it as well as adding more yeast will.

The second practical set of numbers is temperature, because yeast is an organism with an operating range. At 15 to 20 degrees Celsius the reaction is slow. At 25 to 28 degrees fermentation is at its optimum. At 26 to 29 degrees it is described as normal. At 32 to 38 degrees it is fast. Then it stops being a rate and becomes a death. Yeast cells are destroyed in water at 50 degrees. Fermentation rate decreases significantly at and above 50 degrees. The terminal death point is 59 degrees, and during baking yeast dies between 50 and 60 degrees.

That oven figure is worth pausing on, because it explains oven spring. When the loaf goes in, the dough is cool inside. As it warms, gas expands, dissolved carbon dioxide comes out of solution faster, and water and ethanol vaporise. The loaf rises hard. That expansion ends when the yeast dies, between 50 and 60 degrees, and the structure locks when starch gelatinises and gluten coagulates, gluten denaturation beginning at 50 degrees and coagulation running 70 to 80 degrees. The window between those two events is the whole of oven spring. That is why an under-proofed loaf bursts at the sides, because it still had expansion left when the crust set, and why an over-proofed loaf collapses, because its gas cells had already failed before the heat arrived.

Fermentation also changes the chemistry of the dough. The optimum pH for fermentation is 4 to 6, and dough pH falls from about 5.3 when fresh to about 4.5 after fermentation. Remember that number. It is why fermented dough browns less aggressively, and it is why mould inhibitors and rope control work better in a well-fermented dough, which is Lesson 6 and is worth real money in a hot country.

There are three commercial forms of baker's yeast, and mixing them up is one of the most expensive mistakes a small bakery makes, because the error is silent. The dough simply behaves wrongly and the baker blames the flour.

Fresh yeast, also called compressed yeast, is 30 to 32 percent dry matter, which means roughly 68 percent of what you weigh out is water. It must be stored at 4 degrees Celsius. Its shelf life is 2 to 10 weeks. It should not be left unrefrigerated for more than a few minutes. In much of Africa this is the decisive fact: if you do not have reliable refrigeration and reliable cold-chain delivery, fresh yeast is not a realistic ingredient, no matter how good it is.

Active dry yeast, or ADY, is dried into granules and stored at room temperature with a shelf life of about 2 years. It must be rehydrated before use, and if you do not rehydrate it, it underperforms. Published rehydration water temperatures disagree: one technical source says 35 to 38 degrees for 10 to 15 minutes, another says 40 to 43 degrees. Do not agonise over the conflict. Work in the overlap and respect the hard ceiling: never above 50 degrees, because heat destroys yeast cells.

Instant dry yeast, or IDY, has 4 to 6 grams of moisture per 100 grams for one published product, is vacuum packed, stores at room temperature with a 2 year shelf life, and 24 months if stored below 80 degrees Fahrenheit. It does not need rehydration and can go straight into the mixer with the flour. The one caution is to avoid direct contact with ice or ice-cold water, which matters in a hot bakery where you are chilling your mix water.

Now the conversions, and here this course is going to be honest with you rather than tidy. The published factors do not agree.

1. Fresh to instant dry: one source says multiply the fresh weight by 0.35; one manufacturer says use 33 to 40 percent of the compressed weight; another says instant is dosed three to four times lower than pressed, which is a factor of 0.25 to 0.33.
2. Fresh to active dry: one source says use half the fresh weight. Another source says use the same as pressed yeast, with rehydration.
3. Active dry to instant, from one manufacturer: use three quarters of the ADY weight, which is 25 percent less.

Read point 2 again. Half, or the same. That is a genuine contradiction between two respectable sources, and this course will not pretend to resolve it by picking one. The honest and operationally correct answer is this: conversion factors vary by manufacturer. Use the factor printed on your own yeast packet. If there is no factor printed, run a test bake.

For working purposes, treat the published spread as a range: fresh to instant is roughly times 0.25 to times 0.40, and fresh to active dry is roughly times 0.5 to times 1.0.

Work it. A baker in Ibadan, Nigeria, has a formula that says 2 percent fresh yeast on 50 kg of flour, but his supplier now only has instant dry.

1. Fresh yeast required = 50,000 times 0.02 = 1,000 g.
2. Instant at the low end of the range = 1,000 times 0.25 = 250 g.
3. Instant at the high end = 1,000 times 0.40 = 400 g.
4. That is a spread of 150 g, which is a big difference in fermentation speed, so he cannot simply pick the middle and hope.
5. He starts at the middle, about 325 g, on a single test batch, and records the dough temperature out of the mixer and the time to reach his proof condition. If the batch proofs too fast he moves toward 250 g. If too slow, toward 400 g. Two or three test batches settle it permanently.

Dosage itself is form-specific and this is where formulas mislead. A general lean bread dough band is 0.5 to 2 percent yeast. Yet a published white pan bread formula specifies 3.0 percent, and does not say which form it means. Three percent is a normal fresh yeast dose and a very high instant dose. That is almost certainly the explanation for the mismatch, and it teaches the rule: a formula that says only "yeast 2 percent" is an incomplete formula. Write "instant dry yeast 0.8 percent" or "fresh yeast 2 percent" and your formula card can be handed to somebody else without a disaster.

One more figure to file away for Lesson 6: a straight dough process needs 1.0 to 1.5 percent more yeast than a sponge-and-dough process.

Two bakers can use the same formula, the same flour and the same yeast and get different bread. The usual reason is dough temperature. Fermentation rate is set by temperature, so temperature is the master variable, and it is the one a small bakery most often does not measure at all.

Recall the bands from Lesson 1. At 15 to 20 degrees Celsius, slow. At 25 to 28 degrees, optimum. At 26 to 29 degrees, normal. At 32 to 38 degrees, fast. Above 50 degrees the rate falls away sharply and cells begin to die. In a bakery in Kano or Mombasa in the hot season, room temperature alone can carry your dough into the fast band without you deciding anything, and every batch that day will be different from every batch in the cool season.

Now the target. Published target dough temperatures differ, and the differences are honest rather than contradictory, because they optimise different things.

1. For wheat-based yeast breads with the best flavour and rise: 24 to 26 degrees Celsius, which is 75 to 78 degrees Fahrenheit.
2. For a straight dough at the end of mixing in a fast commercial process: 28 to 30 degrees Celsius, which is 82 to 86 degrees Fahrenheit.
3. For the best keeping quality: 24 to 28 degrees Celsius, which is 75 to 82 degrees Fahrenheit.

Both of the first two are legitimate. The trade-off is the lesson. A higher dough temperature gives you a faster process, which means more batches per day out of one oven and one mixer. It also gives less flavour development and poorer keeping quality, and improper high dough temperature is listed in the millers' troubleshooting guides as a direct cause of poor keeping. So the question is not what temperature is correct. The question is which you are selling: turnover or quality. Decide, write it down as your house target, and then hit it every day.

Hitting it is arithmetic, and the tool is the friction factor, because mixing puts mechanical energy into dough as heat. A hand-kneaded dough gains only 1 to 3 degrees Celsius. A stand mixer adds 8 to 16 degrees Celsius. That is not a small correction; that is the difference between the optimum band and the fast band. Commercial spiral and planetary mixer friction factors were not available for this course, and every source stresses that the friction factor is unique to your mixer, your batch size and your kitchen. So you must measure your own, and Module 4 gives you the full calculation and a worked hot-climate example. For now, understand the principle: you control final dough temperature by controlling the water temperature, because water is the only ingredient whose temperature you can freely choose.

Now the time side of it, and here this course must be straight with you. A quantified table saying "bulk ferment for X hours at Y degrees" was not available from a technical source, and inventing one would be the most useless kind of lie, because it would be wrong for your flour anyway. The reliable operational rule is the old one: ferment to condition, not to the clock. Use dough temperature control to make the clock repeatable, then record your own times against your own measured temperatures until you have your own table. That is exactly what the capstone at the end of this module asks you to build, and after one week of records you will own something no textbook could have given you.

While we are correcting folklore, here is another. Every baker is taught that salt slows fermentation by stressing the yeast. The measured picture is much more limited. Gas production is retarded by only about 9 percent in a dough containing 1.5 percent salt on flour weight. At the standard 2 percent level, salt alone does not significantly alter yeast gas production or bacterial acid production, which the measuring source explicitly notes contradicts the long-held baker's assumption.

Put a number on what 9 percent means to you. Suppose your dough reaches its bulk condition in 120 minutes with 1.5 percent salt in it.

1. A 9 percent retardation means the salt-free dough would reach the same condition in roughly 120 divided by 1.09, which is about 110 minutes.
2. That is a difference of about 10 minutes on a two hour bulk.
3. Ten minutes is real but it is not the dramatic effect the folklore claims.

So what does salt actually do? It flavours, it tightens gluten, it improves gas retention and it regulates the fermentation rate modestly. Leave it out and your bread ferments somewhat faster, tastes flat and holds gas worse. It does not run riot. Knowing the difference stops you making the wrong correction when a batch goes wrong, which is the whole point of learning any of this.

Bulk fermentation is the period after mixing when the whole mass of dough ferments together, before it is divided into pieces. Knock-back, also called punching or folding, is the deliberate degassing you do during that period. Both are widely taught with confident timings, and this course is not going to give you any, because a technical source for bulk fermentation time and temperature bands, and for knock-back timing, was not available for this course. What is uncontroversial is the mechanism, and the mechanism is enough to let you design your own schedule.

Knock-back does four things.

1. It redistributes the yeast and the remaining food through the dough. Yeast cells sitting in a pocket of dough exhaust the sugar around them. Folding puts them next to fresh substrate.
2. It equalises dough temperature. A large mass of dough is warmer in the centre than at the edge, and the centre is fermenting faster. Folding brings the outside in, so the whole batch arrives at the same condition together.
3. It expels excess carbon dioxide. Accumulated carbon dioxide inhibits the yeast that produced it. Letting it out lets fermentation continue.
4. It degasses large bubbles into smaller ones, which gives a finer, more even crumb.

Read that list as a design brief. If your dough is in a shallow tub in an air-conditioned room, point 2 matters less. If it is a large mass in a hot room, point 2 matters a great deal. If you want a very open crumb for a rustic loaf, point 4 argues for folding gently and less often. Knock-back is not a ritual. It is four adjustments you make on purpose.

What is sourced about the overall shape of the process is this. A straight dough system runs about 3 to 4 hours from scaling through to packaging. A sponge-and-dough system runs 6 to 8 hours. High-speed mixing in a horizontal mixer runs 12 to 18 minutes. The straight dough system also needs 1.0 to 1.5 percent more yeast and 2 to 3 percent more water than the sponge-and-dough system, and those two figures tell you what you are paying for speed: you buy back the time with extra yeast, and you get less flavour, which is Lesson 6.

Now the faults, because this is how you read your dough. Bakers describe dough as young or old, meaning under-fermented or over-fermented, and each produces a distinct set of faults in the finished loaf.

Young dough is listed as a possible cause of: a poorly shaped loaf, a crust that is too thick, and irregular slices with blisters on the crust. Old dough is listed as a possible cause of: lack of volume, holes in the bread and coarse open crumb, a crust that is too pale, a crust that is too dark, poor keeping quality and poor flavour. Notice that old dough appears under both pale crust and dark crust, which looks like a contradiction and is not, because an over-fermented dough has had its sugar consumed by the yeast, so it browns poorly, while a dough that has gone genuinely old and acid behaves differently again. This is exactly why a troubleshooting guide lists possible causes rather than single causes. Teach yourself to diagnose by elimination, not by lookup.

Now build your own schedule with arithmetic instead of a borrowed number. A baker in Lilongwe, Malawi, wants to know how long her bulk should be.

1. She puts a marked, straight-sided clear jar next to the trough and fills it to a mark with dough from the same batch, pressed down level. The jar is her proxy for the whole mass.
2. She records the dough temperature out of the mixer, say 27 degrees Celsius, and the time.
3. She notes the time when the dough in the jar has risen by half again, and the time when it has doubled.
4. She bakes that batch and writes three words about the result.
5. She repeats for a week, always recording the dough temperature. When she has ten sheets, she sorts them by dough temperature and reads the results column, and her own time-and-temperature table appears.

One more piece of arithmetic that changes how you think about time. Fermentation and scaling losses reduce dough yield and can be up to 5 percent and more depending on the work process. On a batch of 18,000 g of dough, 5 percent is 900 g. If you scale at 900 g a loaf, that is one whole loaf gone before the oven, every batch, invisibly. Over a six-day week that is six loaves, and over a year it is roughly three hundred. This is not an argument for shortening fermentation. It is an argument for measuring the loss and building it into your scaling, which is Module 4's work.

Final proof is the last rise, after the dough has been divided, moulded and panned, and before it goes into the oven. It is the step where more small bakeries lose money than any other, because the faults it causes look like oven faults and get treated as oven faults.

The published figures are unusually clear here, so learn them properly.

1. Recommended proofing temperature: 35 to 37 degrees Celsius, which is 95 to 100 degrees Fahrenheit.
2. The general proofing range across processes: 32 to 54 degrees Celsius, which is 90 to 130 degrees Fahrenheit.
3. Relative humidity: 85 to 95 percent.
4. Standard duration: 60 to 65 minutes.
5. Short-mix doughs: up to 1 hour. Improved and intensive-mix doughs: 1 to 2 hours. Sourdough-leavened doughs: longer than 2 hours.
6. Volume increase during proof: three to four times the original volume.

That last figure is the one to build your practice on, because it is a measurement rather than a clock reading, and it works whatever your climate is doing that day.

The humidity band is worth a moment. At 85 to 95 percent relative humidity, the dough surface stays supple and can expand. In a dry room the surface dries into a skin, and a skinned dough cannot expand evenly, so it tears and shapes badly. A small bakery without a proofer can reach that humidity with a covered rack, a clean damp cloth that does not touch the dough, or a plastic-sheeted trolley with a tray of hot water underneath. What you must not do is proof uncovered in a hot dry room and then blame the oven for the crust faults that follow.

Now the faults, and these are worth memorising because they are your diagnosis.

Under-proofed bread gives: small loaf volume, shell tops, inadequate flow, and bursting at the sides. The mechanism links straight back to Lesson 1. An under-proofed loaf still has expansion left in it when the crust sets, so it bursts wherever the crust is weakest.

Over-proofed bread gives: pale crust colour, coarse grain, poor texture, and an acid overtone in the flavour. The pale crust is not an oven problem. The yeast has eaten the sugar that would have caramelised, so there is nothing left to brown with. Over-proofing is also a listed cause of a hollow bottom and of excess shredding and capping.

So before you ever touch the oven dial, ask which of those two lists your loaf belongs to.

Now, how do you know when a proof is ready? Here this course must again be honest. A poke test, or any other field test for proof readiness, was not available from a technical source for this course. The poke test is real craft practice used by good bakers everywhere, and you may absolutely use it, but you should know that it is craft convention rather than a measured standard, and you should not treat someone's confident description of it as science.

What is sourced is the volume figure, so build your test on that instead. Work it through.

1. Take a straight-sided clear jar or a straight-sided plastic cup. Straight sides matter, because in a tapered container the same volume reads at the wrong height.
2. At the start of final proof, put a small piece of the same dough into the jar, press it level, and mark the top of the dough with a marker pen. Call that mark 1 unit.
3. Measure the height of the dough from the base to that mark. Suppose it is 20 mm.
4. Three times the volume in a straight-sided container is three times the height, so mark 60 mm. Four times is 80 mm. Mark both.
5. Proof the jar alongside your pans in the same conditions. When the dough reaches the 60 to 80 mm band, your pans are ready.
6. Write down how many minutes it took, and the room temperature. After ten batches, you have your own proof schedule.

One caution on that method: a small piece in a jar warms and cools faster than a full pan, so keep them side by side in the same air and check both. The jar tells you the condition; your eye and hand confirm it on the pan.

Last, connect proof to the oven. The whole of oven spring happens between yeast death at 50 to 60 degrees Celsius and structure set at 70 to 85 degrees. A correctly proofed loaf has used most, but not all, of its expansion before the oven, and spends the rest in that window. That single sentence explains both fault lists and is worth more than any timing chart.

A preferment is a portion of the formula, usually some of the flour, some of the water and some of the yeast, fermented on its own before it is mixed into the final dough. Sponge-and-dough is the industrial version. A sourdough starter is the wild-yeast-and-bacteria version, kept alive and fed indefinitely. Both do the same commercial job: they buy you flavour, dough strength and acidity, and they cost you time.

The published comparison is clean. A straight dough system runs about 3 to 4 hours from scaling through packaging. A sponge-and-dough system runs 6 to 8 hours. To run the faster straight dough process you must add 1.0 to 1.5 percent more yeast and 2 to 3 percent more water than the sponge-and-dough version.

Read those two adjustments as a price list. Speed costs yeast. Work it on a real batch.

1. A baker in Kampala runs a sponge-and-dough process with instant yeast at 1.0 percent on 50 kg of flour, so 50,000 times 0.01 = 500 g of yeast.
2. He wants to switch to straight dough to fit two production runs into a day instead of one. He must add 1.0 to 1.5 percent more yeast, so his new dose is 2.0 to 2.5 percent, which is 1,000 to 1,250 g.
3. The extra yeast is 500 to 750 g per batch, an increase of 100 to 150 percent on his yeast line.
4. He must also add 2 to 3 percent more water, so on 50 kg of flour that is 1.0 to 1.5 kg more water per batch.
5. He should price the extra yeast at his own delivered cost per kilogram against the value of a second production run through the same oven. That comparison, not a preference for tradition, is how the decision is made.

Now the accounting, because this is where preferment formulas confuse people. The rule from baker's percentage is that flour is always 100 percent, and where a formula uses more than one flour, the flours together are 100 percent. A preferment does not break this. The flour in the sponge is part of the total flour, not extra.

Work an example. A baker takes 30 percent of her flour into a sponge on a 50 kg formula.

1. Total flour = 50,000 g, which is 100 percent.
2. Sponge flour = 50,000 times 0.30 = 15,000 g.
3. Final dough flour = 50,000 minus 15,000 = 35,000 g.
4. If her total water is 62 percent, that is 50,000 times 0.62 = 31,000 g of water for the whole formula.
5. Whatever water she puts in the sponge is subtracted from the 31,000 g, not added to it. If the sponge takes 9,000 g, the final dough takes 31,000 minus 9,000 = 22,000 g.
6. The same subtraction applies to yeast and to salt if she puts any in the sponge.

That is the whole method, and Module 4 turns it into a formula card you can hand to somebody else.

One gap to state plainly. A sourdough starter feeding ratio and schedule, meaning how much flour and water to feed and how often, was not available from a technical source for this course. Sourdough is real and widely practised, and you may certainly run one, but this course will not print a feeding ratio as though it were a measured standard. What is sourced is that sourdough-leavened doughs need a final proof longer than 2 hours, which is your planning figure, and that the friction factor calculation changes when a preferment is present: you multiply the final dough temperature by 4 instead of 3 and include the preferment's own temperature in the sum. Module 4 works that through.

Now the part that may be worth the most money to you. Fermentation acidifies dough, and dough pH falls from about 5.3 fresh to about 4.5 fermented. That acidity is a weapon.

Rope is a spoilage disease caused by spore-forming Bacillus bacteria, chiefly Bacillus subtilis. The spores arrive in the flour and survive baking, because they withstand up to 130 degrees Celsius while your loaf core only reaches 90 to 97 degrees. They germinate in the cooling loaf and digest the crumb, giving first a sweetish, fruity or rotten smell at 12 to 24 hours, then a sticky discoloured crumb that draws thread-like strands when the loaf is torn. Rope is markedly more frequent in hot seasons, which makes it a first-order risk across much of Africa.

The control is chemistry. Rope germinates optimally around pH 6.0 and can still germinate down to about pH 5.4 under heavy contamination and warm storage. Formulating to pH 5.4 or less, or below 4.6 through fermentation, controls it. Sourdough and acidifying agents containing acetic acid are named as effective, and an acidity regulator dose of 0.2 to 0.3 percent on total flour is given for hot months. A well-fermented dough at about pH 4.5 is already inside the protected range.

A second benefit follows. Calcium propionate, the standard bread mould inhibitor, works best at pH below 5.5. A fast, lightly fermented dough may not get there, and then the preservative underperforms and the baker blames the supplier. A properly fermented dough puts the preservative into its working range for free.

So the case for a preferment in a hot country is not romance about flavour. It is longer shelf life, better crumb, protection against rope, and a mould inhibitor that actually works.