Crema

Steps

1. Temper5 min
   gently, to about 30°C — warm to the wrist, no more
   • cream
   • buttermilk
   • cream
   cream warmed in a clean jar. The bacteria in buttermilk work fastest around body temperature and stall in the refrigerator; above roughly 45°C they start to die, which is the one way to fail this at the beginning rather than the end
2. Mix2 min
   clean spoon, jar not sealed tight
   + buttermilk, stirred through. Use buttermilk that lists live cultures — ultra-pasteurised or long-dated buttermilk has none left, and nothing will happen overnight. Cover with a cloth or set the lid on loosely; the culture produces carbon dioxide and a sealed jar builds pressure
3. Ferment840 min
   room temperature, 20-24°C, out of the sun
   for twelve to sixteen hours, until it has thickened to the consistency of pouring cream and smells faintly sour and clean. The bacteria are converting lactose to lactic acid, and the falling pH makes the casein proteins clump loosely — that clumping is the thickening. Cooler rooms simply take longer; a kitchen at 15°C may need a full day
4. Season3 min
   after culturing, never before
   • lime
   + salt + lime juice, stirred in. Salt and acid both inhibit the culture, so adding them early stops the fermentation before it has done its work. Taste now: it should be sour enough to cut fat but not sharp
5. Store50 min
   sealed, refrigerated
   at least an hour before use, which both stops the fermentation and firms it further as the fat crystallises. Keeps about a week; it will continue to sour slowly and thicken, which is not spoilage. Discard it if it smells yeasty or bitter rather than cleanly sour, or if it separates into curds and clear whey