The Science of Bulk Fermentation: What's Really Happening During That Long Wait

There is a moment, usually a couple of hours into bulk fermentation, where nothing seems to be happening. The dough sits there. You poke it. It sort of springs back. You wonder if you have killed your starter, or mixed something wrong, or whether you should just go to the pub and come back later. (Honestly, that last one is usually fine.)

Bulk fermentation is the stage that confuses more home bakers than any other, mostly because it looks like nothing for a long time, and then suddenly everything at once. Understanding what is actually going on inside that dough changes how you relate to it. You stop guessing and start reading. So let us get into it.

The Players: Wild Yeast and Lactic Acid Bacteria

Your sourdough starter is not one organism. It is a small ecosystem, and the two main groups doing the work during bulk fermentation are wild yeast and lactic acid bacteria (LAB). They have been living together, competing and cooperating, since you first built that starter. By the time you mix your dough, you are essentially giving them a new, much larger environment to colonise.

The wild yeast, primarily strains of Saccharomyces cerevisiae and various Kazachstania species depending on your flour and environment, consume simple sugars and produce carbon dioxide and ethanol. That CO2 is what inflates your dough. The ethanol contributes to flavour development and partially evaporates during baking.

The LAB, which includes species like Lactobacillus and Fructilactobacillus, produce lactic acid and acetic acid as metabolic byproducts. Lactic acid gives you that smooth, yoghurty tang. Acetic acid is sharper and more vinegary. The balance between these two acids determines a huge amount of your final loaf's flavour character.

The Lag Phase: Why Nothing Seems to Be Happening

In the first hour or two after mixing, yeast and bacteria are not being lazy. They are doing something called the lag phase, a term borrowed from microbiology. The microorganisms are adapting to their new environment, absorbing water, repairing any cell damage from the mixing process, and gearing up for rapid reproduction.

Temperature has an enormous effect on how long this phase lasts. In a warm kitchen in late spring, say 24 to 26 degrees Celsius, you might see the lag phase wrap up within 90 minutes. In a cooler kitchen, it could stretch to three hours. This is not a problem. It is just physics.

The thing most bakers do wrong here is panic and add more starter, or move the dough somewhere warm too aggressively. If your starter was healthy and active when you mixed, trust the process. The biology is running on its own schedule.

The Exponential Phase: When Things Actually Kick Off

Once the microorganisms have settled in, they reproduce rapidly and metabolic activity accelerates. This is the window where your dough is doing the most visible work: rising noticeably, developing bubbles around the edges of the container, and becoming increasingly extensible and airy when you handle it.

Enzymatic activity is also running in parallel. Amylase enzymes in the flour are breaking down damaged starch into maltose, which the yeast then consume. Protease enzymes are weakening protein bonds, which is why an over-fermented dough goes slack and sticky rather than tight and strong. The gluten network and the fermentation activity are in constant dialogue during this phase.

This is also when your stretch and fold sets are most effective. The dough is extensible enough to stretch without tearing, but not so gassy that you are deflating it significantly. Each set of folds builds gluten structure and redistributes temperature and gases through the dough mass.

Reading the Signs: How to Know When Bulk Is Done

This is where things get genuinely useful. Percentage rise is the most commonly cited guide, and it is a decent starting point. Many bakers aim for around 50 to 75 percent volume increase during bulk, but this varies considerably based on your starter percentage, flour type, and hydration. High wholegrain flours ferment faster and may show less dramatic visual rise for the same level of fermentation activity.

What you are actually looking for is a combination of signals. The dough should have increased noticeably in volume. The surface should look slightly domed and have visible bubbles, particularly around the edges. When you shake the container gently, the dough should jiggle with a certain wobble, almost like a very thick jelly. When you perform your final fold, the dough should feel lighter and more airy than when you started, with a subtle resistance that tells you the gluten structure is intact.

One tip that took me ages to properly internalise: the bottom of a clear container is often your best window. Bubbles tracking up the sides and underneath the dough mass tell you the fermentation is genuinely through the whole thing, not just on the surface.

Spring Kitchens and Why Your Timings Are Shifting Right Now

If your bulk fermentation times have been creeping shorter over the last few weeks, that is not your imagination. As we move through late May, ambient kitchen temperatures in most UK homes rise noticeably, often by three to five degrees compared to winter without any heating change. That alone can cut an hour or more from your bulk fermentation time.

The practical takeaway is to stay observant this time of year rather than following winter timings. Use the dough as your guide, not the clock. If your kitchen is consistently sitting above 22 degrees Celsius, you might also consider using slightly cooler water when mixing to slow things down and give yourself more control.

What Goes Wrong and Why

Under-fermented dough tends to be dense with a tight, gummy crumb. The gluten network is there, but the gas production has not been sufficient to fully expand it. You might also get a sharp, almost metallic sourness, which is acetic acid without the balance of lactic complexity.

Over-fermented dough goes slack. The proteases have had too long and have broken down more gluten than you want. The dough spreads during shaping, loses its tension, and often bakes flat with a pale, gummy interior. If you have ever had a loaf that looked beautiful going into the fridge and came out looking defeated, over-fermentation before shaping is a common culprit.

The tricky thing is that both problems can look similar from the outside. This is exactly where having something like Doughrise Coach is genuinely useful. Being able to describe your dough, get a personalised read on what likely went wrong, and get a concrete plan for next time takes a lot of the guesswork out of diagnosing these issues.

Happy baking! Find everything you need at doughrise.store

Photo by Claudia Stucki on Unsplash