The invisible cooks of the kitchen: bacteria and yeasts. Why does yogurt set at 43 degrees, why does pickle brine need 2% salt, how is a sourdough starter kept alive? A science-based guide to home fermentation.
The Tatonia Editors··9 min read
Fermentation is the oldest trick in the human kitchen. The first yogurt is thought to have been discovered by chance in Central Asia around 5000 BCE; the first leavened bread rose with wild yeast in antiquity. In the modern world, industrial dairy, packaged pickles and commercial yeast have automated these processes; but the underlying biology is the same. Bacteria and yeasts break down carbohydrates, produce acid or alcohol, and create new textures and flavours. Relearning this process at home is not a hobby; it is a return to the foundations of cooking.
The biology of fermentation
Fermentation is the chemical transformation of carbohydrates by microorganisms (bacteria, yeasts, sometimes moulds) in an anaerobic or low-oxygen environment. Two main types appear in the kitchen.
Lactic acid fermentation (LAB): Lactobacillus, Leuconostoc, Streptococcus species convert lactose or glucose into lactic acid. The environment becomes acidic, the pH falls (from 6 to about 3 to 4), harmful bacteria cannot grow, and the product preserves itself. Yogurt, pickles, sauerkraut, kimchi, Turkish şalgam are all LAB fermentation.
Alcoholic fermentation: yeasts (Saccharomyces) convert sugars into ethanol and carbon dioxide. The rising of bread dough is from CO₂; wine and beer from ethanol. In sourdough, both LAB and wild yeast work together; that is why it is called .
mixed fermentation
The core principle highlighted in Sandor Katz's fermentation encyclopedia The Art of Fermentation is this: microorganisms are already present; the home cook's role is to steer them. By controlling salt, temperature, time and oxygen, the right bacteria are given the environment, and unwanted ones are suppressed.
Yogurt: temperature and the dance of two bacteria
The essence of making yogurt is the symbiotic relationship of two thermophilic (heat-loving) bacteria.
Streptococcus thermophilus starts first. Added to milk, it attacks lactose, begins producing lactic acid, and makes the environment slightly acidic (pH 6.0 to 5.0). This lower pH activates the second bacterium: Lactobacillus delbrueckii subsp. bulgaricus. L. bulgaricus accelerates acid production; the pH falls to 4.5 to 4.0. At this acidity, protein molecules denature, calcium bridges break, and the milk gels. Yogurt has formed.
These two bacteria chemically encourage each other. S. thermophilus secretes amino acids for L. bulgaricus; in return, L. bulgaricus produces formic acid and carbon dioxide to support the growth of S. thermophilus. Without one, yogurt cannot reach the right texture.
The right temperature is critical: both bacteria show optimum growth in the 40 to 44°C range. Above 46°C, L. bulgaricus dies quickly; below 38°C, both slow down. The most common reason home yogurt fails to set is the wrong temperature: the milk was overheated (the bacteria burned) or the incubation environment cooled (the bacteria could not grow).
A practical home route:
Heat the milk to 85 to 90°C (so whey proteins denature and texture strengthens), hold for 10 minutes
Cool to a temperature where a drop on the wrist feels warm but not burning (43 to 45°C)
Add 2 tablespoons of starter yogurt (with live cultures) per litre of milk, stir well
Cover, wrap in a thick towel or blanket, leave 6 to 8 hours in a warm spot
When a set layer forms on top, move to the fridge and let it rest 4 more hours
The starter (initial yogurt) must not be commercial but live cultured. Long-life pasteurised yogurt likely contains no live bacteria. Use yogurt labelled "live and active culture" or a village yogurt. Do not use raw farm milk with a village yogurt starter; raw milk's contamination risk is too high for the home kitchen.
Pickles: anaerobic brine and the balance of salt
The basis of making lactic acid pickles is to create an anaerobic environment and encourage the right bacteria.
Vegetables naturally carry Lactobacillus and Leuconostoc species on their surfaces. When submerged in salt water (brine), these bacteria multiply, converting the sugars in the vegetables into lactic acid. At the same time, salt draws moisture from the vegetable cells, firms the texture, and blocks the growth of aerobic spoilage bacteria.
According to Sandor Katz, the ideal salt ratio is 2% by weight, or about 1 tablespoon of salt per 1 cup of water (around 1.5 to 2%). Less accelerates fermentation but is risky (unwanted microbes may grow); more suppresses the healthy bacteria.
Practical rules for home pickles:
Glass jar, not a metal lid (brine corrodes metal). A plastic or enamel lid, and left slightly loose to vent gas.
The vegetables must stay submerged. Parts above the brine touch oxygen and mould. Use a glass weight or a piece of plastic lid.
Salt must be non-iodised. Iodine can slow fermentation; rock salt or sea salt is preferred.
Fermentation time: at room temperature (18 to 22°C), 2 to 4 weeks. Warmer is faster (1 to 2 weeks) but texture suffers. Cooler (10 to 15°C) is slower, 4 to 8 weeks, but texture holds better.
When is it ready? The liquid starts to cloud, light foam appears on the surface (not a problem), the taste settles into sour and salty. The acid level is in the pH 4.0 to 4.6 range (measurable with litmus paper).
If you see a white layer on the surface (kahm yeast), spoon it off; do not throw the jar away. It is a harmless yeast that does not ruin the fermentation but adds flavour and is best not stirred in. If you see green or black mould, the entire jar is thrown out; the salt was insufficient or the vegetables sat above the brine.
Sourdough: bread with wild yeast
The yeast used in modern bread is a commercial dry or fresh strain of Saccharomyces cerevisiae. Sourdough, on the other hand, is a mix of wild yeast (often Saccharomyces exiguus or Candida milleri) from the local air and from the flour itself, plus lactobacillus. This mix is called a "starter" or "levain."
Wild yeast produces CO₂ and lifts the dough. Lactobacillus produces lactic and acetic acid, gives bread its characteristic sour flavour, and partly breaks down gluten, making the bread easier to digest. The two share a symbiotic relationship: yeast converts sugars into a form LAB can use, and LAB acidifies the environment to suppress other unwanted microbes.
Building a starter is a 5 to 7 day process:
Day 1: 50 g flour (whole wheat preferred, more natural yeast), 50 ml lukewarm water; mix in a jar, cover, leave.
Days 2 to 3: first bubbles appear. Each day add 50 g flour plus 50 ml water (feed); discard half of the old mix.
Days 4 to 5: strong aroma plus bubbles plus volume rise expected. The smell should be slightly sour and fruity; acetone, vinegar or rotten smells indicate trouble (the environment is too sour; start over).
Days 6 to 7: if the starter doubles within 4 to 6 hours of feeding, it is ready to use.
A built starter can survive for years in the fridge with weekly feeding. If fed less often, alcohol (hooch) collects on the surface; pour it off and refresh.
Classics of Turkish fermented cuisine
Fermentation in Turkish cuisine is not limited to yogurt:
Şalgam suyu: an Adana classic. Carrot plus black carrot plus bulgur plus salt plus water, anaerobic fermentation for 10 to 15 days. Lactic acid dominant, slightly sour, with the pink colour from black carrot.
Kefir: of Caucasian origin. Inside kefir grains, more than 30 different bacteria and yeasts coexist. Unlike yogurt, both LAB and yeast fermentation occur (light CO₂ and alcohol form).
Tarhana: yogurt plus flour plus greens (tomato, pepper, mint) are mixed, fermented, then dried. Stored for months, ready for winter soup.
Boza: fermentation of millet (darı) or bulgur. Low alcohol (below 1%), sweetened. From Ottoman tradition to today.
Pickle: cucumber, cabbage, pepper, eggplant, carrot, all vegetables are suitable. As a Turkish preference, adding vinegar is common (it stabilises fermentation and gives a uniform taste).
These classics are the first fermented products to try at home. Instead of starting with foreign sauerkraut or kimchi, şalgam or tarhana are more familiar with the same biology.
Health: the probiotic reality
In recent years, fermented foods have been over-marketed as "probiotics." Science says something more measured:
Live-culture fermented foods (raw yogurt, raw pickle, kombucha, kefir) add diversity to gut microbiota. According to Harvard's nutrition source, regular yogurt consumption is positively associated with the immune system and digestive regularity.
Pasteurised fermented foods (most supermarket pickles, long-life yogurt) contain no live bacteria and are not functional probiotics. Taste is the same, but contribution to microbiota is near zero.
Lactose intolerance: LAB consume lactose and convert it to lactic acid, so fermented dairy is easier to digest for intolerant people (though not entirely).
Fermented food is not a magic solution. Valuable as part of an overall varied diet; it does not work miracles on its own.
Raw fermented products made at home are biologically different from the supermarket alternative. To preserve this valuable difference, do not boil at high heat (pasteurisation destroys live cultures).
Common home fermentation mistakes
Adding starter to milk that is too cold or too hot. The most common yogurt mistake. At 60°C the starter burns; at 35°C it is too slow. When you touch your wrist, the lukewarm-but-not-burning temperature is around 42 to 44°C.
Letting vegetables sit above the brine. The parts above mould. Use a glass weight or a small plate.
A metal-lidded jar. Fermentation produces acid, which corrodes metal and ruins flavour. Glass or plastic.
Insufficient salt. Below 2%, unwanted microbes can also grow. Salt both directs the bacteria and draws water from the vegetable cells.
Iodised salt in pickles. Iodine slows LAB activity; the fermentation may not take. Rock or sea salt.
Using pasteurised yogurt as a starter. No live culture, no bacterial growth, no yogurt set. Check the label; use "live cultured" or village yogurt.
Closing word
Fermentation is a patience-requiring skill, but one of the most valuable habits to gain in the kitchen. Yogurt, pickle or sourdough bread made at home is different in both taste and health from the supermarket alternative. As Sandor Katz puts it, the microbes do the work; you only prepare the environment. With a few trials, you discover what temperature, what salt, what time works in your home. After that, it becomes your own kitchen tradition.