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Preservation · intermediate

Fermentation

Also: khameer, souring

Letting beneficial microbes transform food, souring, leavening or preserving it.

132 recipes use it

What it is

Fermentation uses yeasts and bacteria, either wild or added, to change food. In Indian kitchens it gives idli, dosa, dhokla, appam, jalebi batter, dahi and kanji; elsewhere it makes bread, yogurt, sauerkraut, kimchi, miso and soy sauce.

Why cooks use it: It creates sourness, complex flavour, lightness (from gas) and, for lactic ferments, a more acidic and less spoilage-prone food.

The science

Lactic acid bacteria convert sugars into lactic acid, lowering pH and souring the food. Yeasts convert sugars into carbon dioxide and alcohol, which leavens dough and batter. Temperature strongly controls speed: warm batters ferment overnight, cool ones may take a day or more.

Acids and bases in cookingGluten developmentSalt and osmosisYeast fermentationLactic acid fermentationThe science of pickling

Step by step

  1. Use clean (not necessarily sterile) containers with room for the batter or dough to rise.

  2. For idli/dosa batter: mix ground urad and rice batter with salt (about 1–1.5% of batter weight) by hand.

  3. Cover loosely and keep somewhere warm, ideally 25–32 °C.

  4. Ferment until risen, bubbly and pleasantly sour (8–14 hours in warm weather; longer in cold).

  5. For dahi: heat milk to near-boiling, cool to about 40–45 °C, stir in a spoon of yogurt and keep warm and undisturbed for 4–8 hours.

  6. Refrigerate once ready to slow further fermentation.

Heat and timing

  • Fermentation 20–45 °C · Idli/dosa batter about 25–32 °C; yogurt incubation about 40–45 °C; bread dough about 24–27 °C.

Time: 4–8 hours (yogurt); 8–14 hours (idli batter); days to weeks (sauerkraut, kimchi).

How you know it’s right

  • Batter roughly doubled, with bubbles and a domed top
  • Pleasant sour smell
  • Yogurt set firm and pulls cleanly from the side

Common mistakes

  • Too cold, so nothing happens
  • Too warm or too long, so batter becomes very sour and runny
  • Over-stirring fermented batter, knocking out the air

Troubleshooting

Batter not rising
Move it somewhere warmer (an oven with only the light on) and give it more time; check that the urad dal was ground until fluffy.
Too sour
Ferment for less time or refrigerate sooner; a pinch of baking soda can soften sourness in dosa batter.
Pink, black or fuzzy mould, or a rotten smell
Discard it; this is spoilage, not fermentation.

Restaurant DNA

How professionals do it

Restaurants that serve idli and dosa ferment batter daily in large batches and refrigerate it once ripe; bakeries use proofing cabinets to control temperature and humidity.

At home

In cold weather, ferment in an oven with only the light on, or in a warm insulated box; in hot weather check the batter early.

General professional practice — not any particular restaurant’s recipe.

Food safety

  • Discard any ferment with fuzzy mould, slime or a putrid smell.
  • Keep vegetable ferments submerged under brine.
  • Refrigerate finished ferments.

Practise it

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