Cooking science

Syneresis

Syneresis is the name for liquid seeping out of something that has set. It is the pool under a slice of quiche, the water on top of yoghurt, the puddle in a defrosted sauce, and the sweat on an overcooked custard. In every case the network holding the liquid has tightened and squeezed some out.

What causes it

A gel holds liquid in a network of linked molecules. If those links tighten — pulling the network in on itself — the space available for liquid shrinks, and the surplus is expelled. The set looks the same from outside; the liquid simply has nowhere to be.

The commonest cause in cooking is overheating. Heat drives protein molecules to bond more tightly to one another, so an egg custard heated past its setting point contracts and weeps. The same thing happens in cheese sauces, quiche fillings and scrambled eggs.

Time causes it too. Starch gels go on reorganising for days after they are made, which is why a sauce thickened with flour separates in the fridge and why bread stales. Freezing causes it mechanically: ice crystals grow, physically puncture the network, and the damage shows on thawing.

Where you meet it

Quiche and baked custard tart

Overbaked filling. Bake lower and take it out with a wobble.

Yoghurt

Whey separating on standing. Entirely normal — stir it back in.

Cheese sauce

Overheated, or boiled after the cheese went in. The protein has tightened and squeezed out fat and water.

Defrosted sauces and stews

Ice damage to the starch network. Reheat gently and whisk, or thicken again.

Jelly left standing

Slow contraction of the gelatine network. Worse if too much gelatine was used.

Cheesecake

Overbaked, or too much egg. Beads of moisture on the surface are the visible sign.

How to prevent it

• Do not overheat protein sets. This covers most cases. Gentle heat, taken off early, cooled promptly.
• Use less setting agent, not more. A dense network is a brittle one that expels liquid. Over-gelling is a common cause of weeping.
• Include some fat or starch. Both interfere with the protein network tightening, which is why cream-rich custards and starch-stabilised ones weep less.
• Add acid at the end. Acid encourages protein to contract, so lemon juice stirred into a hot sauce can split it.
• Avoid freezing gels. If a dish will be frozen, use a waxy starch or a modified one, which tolerates it far better than flour or cornflour.
• Drain wet ingredients. A lot of "weeping" in a quiche or lasagne is simply water from the vegetables, and salting and draining them first solves it.

When it can be rescued

A weeping starch sauce usually comes back with gentle reheating and hard whisking, and a stick blender is better still. A weeping protein set generally does not — once egg has over-coagulated the change is permanent, and blending gives you a smooth liquid rather than a repaired set.

Yoghurt and soft cheeses are the easy case: stir the whey back in and nothing is lost. A quiche that has wept can be blotted and served, and most people will not notice. A jelly that has contracted can be melted and re-set with more liquid.

The general rule is that reversible gels — starch, gelatine, agar — can usually be repaired, and irreversible ones — egg, and heat-set dairy — cannot.

Common questions