Crème Fraîche

Pour the heavy cream into a clean glass jar or bowl. Add the buttermilk and stir gently with a clean spoon for 20 seconds — just enough to distribute the cultures evenly throughout the cream. Do not whip or aerate; you are introducing live bacteria, not incorporating air. Ultra-pasteurised cream has been heated to a point that destroys the lipids needed for the cultures to grip, which is why it must be avoided.

Cover the jar loosely — not airtight, as the cultures need to breathe — and leave it in a warm spot at room temperature, ideally 20 to 24 C / 68 to 75 F. Check it after 12 hours. In warm kitchens it may be done in 12 hours; in cooler kitchens it can take up to 24. You are waiting for the cream to thicken to a spoonable, semi-solid consistency and develop a mild, pleasant tang. Do not stir during fermentation.

Transfer the crème fraîche to the refrigerator, loosely covered. It will firm further over the next 2 to 4 hours as the cold halts the bacterial activity and the fat crystallises slightly. The finished product keeps for 1 to 2 weeks in the refrigerator. To make the next batch, reserve a tablespoon as your starter in place of the buttermilk — the cultures regenerate indefinitely with each batch.