Why milk and cream curdle

Why

Most of the protein in milk (about 80 %) is casein, which floats around in tiny clusters called micelles; they are kept apart by an electric charge and, above all, by a protective layer of the protein κ-casein on their surface. Acid (lemon, wine, tomatoes) and salt weaken that protection, and heat speeds the whole process up considerably: boiling alone won't curdle milk, but together with acid or salt the clusters stick together and the sauce turns into flecks and whey – and the more of these at once, the faster it happens. High-fat cream (whipping cream, from roughly 25–30 % fat) contains little protein, and what there is gets spread over the surface of the fat droplets, so it usually survives a short boil even with acid. Milk, single or cooking cream, sour cream and yogurt have more protein, and the cultured ones are already acidic, so they curdle easily in a hot sauce. Starch from flour or potato starch slows the proteins' clumping – the swollen starch granules probably keep them apart – which is why a thickened sauce holds together better.

How to do it

• Take sour cream, yogurt or milk out of the fridge 15–30 minutes in advance so they aren't ice-cold.
• Temper: stir a few spoonfuls of the hot sauce into the dairy first, then pour it into the pan.
• Once sour cream or yogurt is in, don't boil the sauce again – just heat it through.
• If the dairy has to simmer, mix it with a little flour or starch beforehand.
• Add salt and acidic ingredients (lemon, wine) at the end, tasting as you go.
• Where the recipe allows, use whipping cream – it is the most forgiving.

If it goes wrong

A curdled sauce made from fresh ingredients is edible, it just looks untidy; but if the milk curdled on its own or smells sour, it has gone off and belongs in the bin. Slight graininess can often be smoothed out with a stick blender. Large curds won't come back together – you can strain the sauce, but it won't be silky again; next time temper the dairy and don't boil the sauce after adding it.