Fermentation Science: What Happens in Dahi, Idli, Kanji, Achaar
Dahi, idli batter, kanji and achaar all run on the same trick: friendly bacteria eat sugar and make acid, and that acid makes the food sour, safe from most spoilers and slow to go off. Once you see it that way, you can also tell a good ferment from a bad one by sight, smell and a lid.
In brief
- The workers: lactic acid bacteria, plus wild yeasts in idli and bread
- What they make: lactic acid (sourness), some gas (bubbles, rise) and new flavours
- Why food keeps: acid, salt and fewer free sugars leave little room for harmful germs
- Throw it out if: fuzzy mould, a bulging lid, a rotten or nail-polish smell, or pink, slimy growth
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What is fermentation, in plain words?
Tiny living things eat. They leave something behind.
In most desi ferments the eaters are lactic acid bacteria (the same family that sets dahi), which take the natural sugars in milk, vegetables or soaked rice and daal and turn them into lactic acid. The food goes sour, it thickens or softens, new flavours appear, and the acid rises to a level where the bacteria that rot food or make people ill mostly can’t grow.
That’s all fermentation is. Controlled spoiling, where you choose who does the spoiling.
Fermentation is spoiling on purpose, by the bacteria you invited.
Dahi: warm milk, one spoon of jaman, one night
You heat milk, cool it to warm-finger temperature (about 40 to 45 °C), and stir in a spoon of old dahi. The bacteria in that spoon multiply through the milk. Their acid makes the milk protein (casein) clump into a soft, set gel.
In summer it sets in 5 to 6 hours. In winter it can take 10 to 12, which is why mothers wrap the pot in a shawl or put it in a switched-off oven with the light on. Too hot and the bacteria die; too cold and they sleep. The full method is in our homemade dahi guide.
Idli and dosa batter: bacteria and yeast working together
Soaked rice and urad daal are ground and left overnight. Here two teams work at once. Lactic acid bacteria make the batter tangy, while yeasts and some of the bacteria give off gas that gets trapped in the sticky urad batter and makes it rise and turn airy.
Warmth matters again. At around 30 °C a batter doubles in 8 to 12 hours, but in a cold December kitchen in Islamabad or Amritsar it may need 14 hours or more, which is why so many households park the steel dabba inside the switched-off oven with only the light bulb on. A good batter smells pleasantly sour, looks bubbly and has roughly doubled. Fermenting also lowers phytates in the grain and daal, which helps a little with mineral uptake.
Kanji: salt, water, carrots and patience
Kanji is a vegetable ferment. Black carrots and a little beetroot go into water with salt and crushed rai (mustard seed), and the jar sits in the sun for 3 to 5 days.
Salt is the gatekeeper. At roughly 1 to 2 percent (about 1 to 2 tablespoons salt in 2 litres water) it holds back many spoilers, while lactic acid bacteria that ride in on the carrot skins tolerate it and take over. Mustard adds its sharp bite. When it tastes pleasantly sour and a little fizzy, it goes in the fridge, which slows everything down. Exact amounts are in our kanji recipe.
Achaar: which pickles actually ferment?
Not all achaar is fermented. Many Punjabi mango and lime pickles rely mostly on salt, oil, spices, sun and the fruit’s own acid; some lactic fermentation happens in the salted fruit, but oil and salt do most of the preserving. Brine pickles, such as salted green chillies or turnip and carrot in water, ferment far more.
What keeps achaar safe is the same set of barriers: enough salt, enough acid, dry fruit and spoons, and oil covering the top. Our achaar safety and mould guide covers the jar-by-jar checks.
How to tell good fermentation from spoilage
- Look: bubbles, cloudy brine and a thin white film (kahm yeast) on kanji or brine are usually harmless; skim the film off. Fuzzy blue, green, black or pink mould is not. Throw out the whole jar of soft or liquid food; don’t scrape and keep.
- Smell: clean sour, yeasty or pickly is right. Rotten, putrid, cheesy-foul or strong nail-polish smells mean the wrong microbes won.
- Touch: vegetables should be crisp to soft. Slimy, mushy pieces point to spoilage.
- Lid: a bulging lid on a sealed jar that isn’t an actively bubbling ferment, or a spurting jar that smells off, goes in the bin unopened if possible.
- When in doubt: throw it out. Never taste to check.
Myth: all fermented food is full of live probiotics
Only some is. Really. Dahi, kanji and raw brine pickles carry live bacteria when eaten fresh. Idli and dosa are steamed or fried, so their microbes are dead by the time they reach the plate, though the fermentation still changed flavour and texture. Oil-heavy achaar has few live cultures, and a pasteurised packet kanji or shop pickle has none. Heat is the deciding factor.
Garlic, herbs or chillies stored in oil at room temperature, and low-acid vegetables in sealed jars, can grow botulism, which gives no smell or taste warning; keep such mixes in the fridge and use within a few days, as explained in our fermented garlic jar guide. Double vision, drooping eyelids, slurred speech or difficulty swallowing after eating a home-preserved food is an emergency. Achaar and brined foods are very salty, so people with high blood pressure, heart failure or kidney disease should keep portions small and ask their doctor. Don’t ferment or store acidic food in unlined copper, brass or aluminium; use glass, glazed ceramic or food-grade steel.
For more plain-words food science, visit Kitchen Science, Honestly.
Fermentation questions people ask
What happens during fermentation of food?
Microbes eat sugars in the food and turn them into acid, gas or alcohol. In desi ferments, lactic acid bacteria make the food sour and help it keep.
Why does fermented food last longer?
The acid, the salt and the crowd of friendly bacteria leave little room or food for harmful germs to grow.
How do you know if fermented food has gone bad?
Fuzzy coloured mould, a rotten or nail-polish smell, sliminess or a bulging lid that shouldn’t bulge all mean throw it out. A thin white film on brine is usually harmless yeast.
Why is my idli batter not rising?
Usually it is too cold, the urad daal was old, or too much water went in. Keep it somewhere around 30 °C, such as an oven with only the light on, and give it more time.
Is the white layer on kanji mould?
A flat, thin white film is usually kahm yeast; skim it off. Fuzzy, raised or coloured growth is mould, and the batch should go.
Is fermented food good for gut health?
Fermented food with live cultures, like fresh dahi and raw kanji, is a good part of a varied diet. Specific health effects depend on the strains and amounts, which home ferments can’t promise.
Does idli contain probiotics?
No live ones. The batter is fermented, but steaming kills the bacteria. It is still easy to digest and a good breakfast.
Is achaar fermented?
Brine pickles are fermented. Many oil-based mango and lime achaars rely mainly on salt, oil and acid, with only a little fermentation.
Can fermented food cause food poisoning?
Rarely, when salt, acid or hygiene is off. Clean jars, correct salt, cold storage once ready, and throwing out anything suspicious keep the risk low.
How long can kanji be kept?
Once it is nicely sour, keep it in the fridge and drink within about a week. It keeps souring slowly even when cold.
Can children have fermented foods?
Dahi is fine from around 6 months, and idli is a good soft food. Keep salty achaar to a tiny taste for young children, and offer kanji only in small amounts to older children.