Clarifying with milk: how milk punch turns clear

Milk is mostly water with fat, sugar and two kinds of protein in it. One of them, casein, clumps together once the liquid around it is acidic enough — below roughly pH 4.6. In a citrus punch that happens at once. The clumps are curds, and as they form they catch fine particles, bind some of the tannins and polyphenols that make tea and young spirits harsh, and hold on to some colour.

The other protein, whey, stays in the liquid. It is part of why a clarified punch feels softer and rounder than the same punch unclarified — and why the finished drink still contains milk.

Pour the punch into the milk, not the milk into the punch. Adding the acid to the milk curdles it all at once into larger, firmer curds, which filter better; the other way round tends to give fine, cloudy curds that pass through.

Recipes cluster around four or five parts punch to one part milk. The punch needs enough acid to curdle that much milk completely, so a punch with very little citrus will not clarify well. Whole milk works best; skimmed milk clarifies but gives a thinner result. Plant milks do not have casein and will not clarify a drink this way.

Cold milk gives smaller curds and, many find, a cleaner result; hot milk curdles faster into large curds. Either works; the catalogue recipe uses cold.

After the punch goes in, stir once, gently, and leave it alone in the fridge for at least two hours and up to a day. Stirring again breaks the curds into fines that cloud the result.

Filtering is two stages. First, a coarse pass: ladle the curds into a fine strainer lined with a paper coffee filter or a clean cotton cloth, so they form a bed. Then the fine pass: pour the liquid slowly through that bed. The curds are the real filter. The first liquid through is often cloudy; pour it back through until it runs clear.

It drips slowly — one to three hours for a batch, longer if the filter clogs. Do not squeeze it. If it stops, move the curds to a fresh filter.

Colour lightens but does not vanish: clarity and colour are different things, and a clarified cognac punch is clear and pale gold, not colourless. The texture becomes silky. The acid's bite is softened and some bitterness is taken out, so a clarified punch can taste flatter or sweeter than the mix did; many recipes add a little more citrus than an ordinary punch for that reason.

Expect to lose roughly a sixth to a quarter of the liquid in the curds. The curds hold liquid at the same strength as the rest, so the finished punch is about as strong as the mix before the milk went in.

Bottle it, label it with the date and the word milk, and keep it in the fridge. Published recipes commonly suggest about a week; taste before serving anything older, and discard it if it smells sour or turns cloudy again.

Bars also clarify with agar or gelatin: the liquid is set into a gel and the clear part drained or thawed out of it, or spun in a centrifuge. They work without milk, which matters for anyone avoiding dairy, but agar needs accurate weighing and heating, gelatin is not vegetarian, and a centrifuge is professional equipment. They are noted here rather than taught, because milk does the job at home with what is in the fridge.