Three ingredients, one of the simplest desserts there is — and yet you lifted the glass out of the fridge and it sloshed. Or worse, it had split into something the internet has lovingly nicknamed "citrus cottage cheese." The good news: a posset that won't set is one of the most fixable failures in the kitchen, because there are only a handful of things that can go wrong.
A lemon posset that stays liquid almost always used cream with too little fat (you need 36%+ — heavy or double cream), or the cream and sugar were never properly boiled. If it split into grainy curds, you added too much lemon juice or added it while the cream was still violently boiling. It also needs 4+ hours chilling to set.
There's no gelatine and no egg in a classic posset. It sets because lemon juice acidifies the hot cream, which makes its casein proteins link together into a loose network. The cream's high fat content is what keeps that from becoming a curdled mess — the fat surrounds the proteins so they thicken into a smooth, creamy gel instead of clumping. That's why fat percentage and acid amount matter so much: too little fat and there's nothing to set; too much acid (or too much heat) and the proteins clump into curds.
This is behind the majority of "my posset is soup" complaints. Posset only sets if the cream is at least 36% fat. In the US that's heavy cream (36–40%); in the UK and Australia it's double cream (around 48%). Anything lower simply doesn't have the fat to form a set.
The usual culprits: whipping cream (30–35%), single cream / light cream (18–20%), and half-and-half (10–12%). These will warm up, take the lemon, go in the fridge — and stay pourable. No amount of extra chilling fixes a fat problem.
Use heavy cream or double cream only — check the carton says at least 36% fat. If your posset is already liquid because of low-fat cream, tip it back into a pan, add 100 ml of double/heavy cream, boil gently 2 minutes, whisk in a teaspoon of fresh lemon off the heat, and re-chill.
The boil isn't optional. Bringing the cream and sugar to a full rolling boil and holding it for 2–3 minutes concentrates the cream slightly and is essential to the set — the acid alone won't do it. A gentle warm-through that never reaches a true boil (around 100 °C / 212 °F) is one of the most common reasons a posset stays runny.
Watch for: a pan that steamed but never developed big, rolling bubbles across the whole surface; using a pan so wide the cream barely covered the base, so it scorched before it boiled.
Use a medium saucepan, not a wide sauté pan. Bring the cream and sugar to a real rolling boil, then let it boil gently — stirring — for a full 2 to 3 minutes before you take it off the heat. It should look very slightly thickened and glossy.
Posset sets on a fairly narrow acid-to-fat ratio. Too little lemon juice and the casein never links up — it stays liquid. Too much and the proteins clump too aggressively, splitting the mixture (and making it taste harsh). Lemons vary a lot in how much juice they hold, so guessing "the juice of two lemons" is risky.
The reliable ratio: about 2 to 3 tablespoons (30–45 ml) of strained lemon juice per 1 cup (240 ml) of cream, with roughly ¼ cup (50 g) sugar per cup of cream. Strain and measure the juice rather than going by fruit count.
Squeeze, strain out pulp and pips, then measure with a spoon or jug. If yours came out runny and under-acidified, you can re-boil it briefly and whisk in another tablespoon of juice per cup of cream — but go slowly, because overshooting causes splitting.
This is the "citrus cottage cheese" failure — instead of thickening smoothly, the posset breaks into visible grainy curds with a thin liquid around them. It happens when the acid hits cream that's too hot and turbulent: the casein proteins seize and clump rather than forming a gentle gel.
After the 2–3 minute boil, take the pan off the heat and let the bubbles subside for 30–60 seconds. Then whisk in the strained lemon juice steadily, off the heat. If it's already lightly split, pass it through a fine sieve before pouring into glasses — a mild split can often be rescued this way. A badly curdled batch is best started again.
Posset thickens as it chills, not on the stove — it's supposed to be liquid when you pour it. If you tasted it after an hour and panicked because it was still loose, that's normal. It genuinely needs time and a cold fridge to firm up.
Refrigerate at 4 °C (40 °F) or below for at least 4 hours — ideally 6 hours or overnight. Don't cover the glasses with anything warm, and don't crowd them against the fridge's warm door shelf. If after a full overnight chill it's still soup, the cause was the cream or the boil, not the time.
Bottled lemon juice is usually diluted and carries preservatives and stabilisers that interfere with the set and leave a flat, harsh taste. Extra water — from un-strained pulpy juice, or a splash added "to loosen it" — also dilutes the acid and fat and weakens the gel.
Use freshly squeezed lemon juice, strained. Don't add any water, milk, or extra liquid flavourings. If you want to flavour it, use lemon zest infused into the cream during the boil (then strained out) rather than more liquid.
The first time we made posset for a dinner party we grabbed what we thought was double cream and it turned out to be a 30% "whipping cream." Six little glasses went into the fridge looking perfect and came out four hours later as lemon-flavoured drinks. We genuinely thought we'd ruined them — until we tipped all six back into a pan, stirred in a fresh 150 ml of proper double cream, boiled it for two minutes, whisked the lemon back in off the heat, and re-chilled overnight. They set beautifully and nobody at the table knew.
The lesson that stuck: read the fat percentage on the carton before you pour, every single time. Now we keep a thermometer by the hob, pull the pan off the heat the moment the lemon goes in, and never trust "the juice of two lemons" without measuring it into a spoon first. Three ingredients leaves nowhere to hide — but it also means there are only three things to get right.
Describe exactly what happened and paste your recipe — Recipe Doctor will identify the most likely cause and give you a step-by-step plan for next time. Free, no login.
Get a free diagnosis →| What you observed | Most likely cause | Quick fix |
|---|---|---|
| Completely liquid after overnight chill | Cream fat too low | Use 36%+ cream; re-boil with added double cream |
| Soft and loose, but cream was right | Under-boiled or under-chilled | Boil 2–3 min; chill 6 h at 4 °C |
| Thin and barely tart | Too little lemon juice | 2–3 tbsp strained juice per cup cream |
| Grainy curds in thin liquid | Lemon added too hot / too much | Off heat 60 s before adding; sieve to rescue |
| Set but harsh, flat flavour | Bottled juice or added water | Fresh strained juice only; zest for flavour |
| Skin on top, fine underneath | Uncovered, cooled too fast | Press cling film to surface while chilling |
Almost always the cream didn't have enough fat. Posset needs cream of at least 36% fat — heavy cream in the US, double cream in the UK. Whipping cream, single cream and half-and-half can't set. The second most common cause is not boiling the cream and sugar long enough to concentrate it.
If it's only soft, give it longer in the fridge — up to overnight. If it's genuinely liquid because the cream was too low in fat, tip it back into a pan, add a splash of double or heavy cream, bring to a gentle boil for 2 minutes, whisk in a little fresh lemon off the heat, then chill again. It won't be quite as silky but it will set.
Too much acid, or the lemon went in while the cream was still violently boiling. The casein proteins clump into visible curds instead of thickening smoothly. Take the pan off the heat, let it settle 30–60 seconds, then whisk in the measured juice. Stick to about 2–3 tablespoons per cup of cream. A mild split can be rescued by passing it through a fine sieve.
Heavy cream (US, ~36–40%) or double cream (UK, ~48%). The high fat is what lets it set when the acid is added — the fat surrounds the casein proteins so they thicken into a creamy gel rather than curdling. Lower-fat creams will not set.
At least 4 hours in the fridge, ideally 6 or overnight. It sets purely by chilling, so a warm fridge leaves it soft. Make sure yours is at or below 4 °C (40 °F) and don't cover it with anything warm.
It's not recommended. Bottled juice is usually diluted and contains preservatives that interfere with the set and give a flat, harsh taste. Fresh juice has the right acidity and flavour. Strain it first, then measure.
Yes. Boiling the cream with the sugar for 2–3 minutes concentrates it and is essential to the set — the acid alone is not enough. A quick warm-through that never reaches a true rolling boil is a frequent reason posset stays runny.
It contains no raw egg and the cream is boiled, so there's no undercooked-egg risk. It's a fresh dairy dessert, so keep it refrigerated and eat within 2–3 days. Don't leave it at room temperature for more than 2 hours.