Tubs of ready dosa and idli batter now sit in refrigerated cases in American grocery stores. The product is unusual because what is being sold is still biologically active.
The batter is a live culture in a tub
Fermentation is driven by bacteria and wild yeast already present on the rice and lentils, which produce acid and gas as they consume the starch and sugars.
That activity does not stop when the batter is packed. It continues at whatever rate the temperature allows, which makes the product closer to fresh dough than to a shelf item.
Batter that ferments too far turns sharply sour and loses its rise, so the entire distribution problem is holding it at the right point.
Cold slows the culture without killing it
Refrigeration reduces microbial activity enough to buy days rather than hours, which is what makes retail distribution feasible at all.
Pasteurizing would extend life much further but would end the fermentation, and a dead batter does not develop the flavor or the aeration the dish depends on.
The compromise is a short dated window, an unbroken cold chain from producer to case, and packaging that vents the carbon dioxide the culture keeps producing.
Why home fermentation is hard outside India
The traditional process relies on warm ambient temperatures to ferment overnight, and most American kitchens are far cooler than the climate the method assumes.
Cooks work around this with an oven light, a proofing setting or a multicooker's yogurt function, all of which supply the steady warmth the culture needs.
Water treatment adds another variable, since chlorinated tap water can slow the wild culture, which is why many recipes suggest filtered water for the soak.
The commercial version has tradeoffs
Producers standardize the rice and lentil ratio and the grind, so the batter behaves predictably and every tub cooks the same way.
What is lost is the flavor a household culture develops over repeated batches, since a commercial ferment is short, controlled and deliberately mild.
Mildness is a commercial requirement rather than an oversight, because a batter shipped at its ideal point would arrive past it and a sour tub is returned.
Handling a tub at home
A tub taken straight from the case is cold and sluggish, and batter poured cold spreads badly and sticks, so it is left out until it comes to room temperature.
The batter thickens as it sits and continues to sour, so the last portion of a tub usually needs thinning with water and suits idli less well than dosa.
Signs that a tub has gone too far are a sharply alcoholic smell and a collapse in volume, at which point the culture has exhausted what it had to work with.