How do I blend hot soup without it exploding?

Why a sealed jug of hot soup behaves like a pressure vessel, and the three ways to blend without one.

Difficulty: beginner
Time: 10 min
Read: 3 min
Safety: warning

Short answer

Use a stick blender in the pan — it removes the problem entirely. If you must use a jug blender, fill it no more than half, remove the centre cap and cover the hole with a folded tea towel, and start on the slowest speed.

Hot soup in a closed blender jug releases steam the moment it is agitated. The steam has nowhere to go, pressure builds in a second or two, and the lid leaves at speed. It is one of the more common serious burns in a domestic kitchen.

Safety

Blending hot liquid in a sealed jug builds steam pressure that can blow the lid off and throw scalding soup over your hands and face. Never fill a blender more than half full with hot liquid, never clamp the lid shut without a vent, and always start on the lowest speed.

Step by step

Use a stick blender if you have one.Straight into the pan, off the heat, blender head fully submerged before you switch it on. No sealed container, no pressure, nothing to go wrong.
Let the soup cool for a few minutes first.Off the heat for five or ten minutes. Below a rolling boil it produces far less steam and everything becomes safer.
Never fill a jug blender more than half.The headspace is what absorbs the expansion. A three-quarters-full jug of hot soup has nowhere for the steam to go.
Vent the lid.Take the centre cap out and cover the hole with a folded tea towel held down by your hand. Steam escapes through the cloth and soup does not.
Hold the lid down firmly.A flat hand on a folded towel over the lid, arm straight. Do not put your face over it.
Start on the lowest speed.Let it run for a few seconds before increasing. Starting on high is what launches the lid.
Blend in batches.Two half-jugs is quicker than cleaning soup off the ceiling. Pour each batch into a second pan.
Take the same care with a food processor.The same pressure applies, and processor bowls seal less well. Cool the soup first and fill it no more than half.
Watch out for starchy soups.Potato, parsnip and cauliflower soups can go gluey if overblended. Blend them in short bursts and stop as soon as they are smooth.

Tips

A stick blender removes the risk entirely and is the reason to own one.
Half full, vented, lowest speed. All three, every time.
Overblending starchy soups makes them gluey — stop as soon as they are smooth.

Common mistakes

Filling the blender to the top with boiling soup and clamping the lid on — The soup releases steam the instant the blade turns, pressure builds within seconds and the lid is forced off. The soup that follows it is at or near boiling point and it goes upwards and outwards.
Starting on maximum speed — The sudden agitation flashes a large volume of steam at once, which is exactly the surge the lid cannot hold. Ramping up gives the steam time to vent.

If it doesn't work

Lid lifted while blending

Cause: Too full, or sealed — Fix: Stop. Cool the soup, halve the quantity in the jug and vent the lid.

Soup gluey after blending

Cause: Starchy vegetables overblended — Fix: Blend in short bursts. Stir in stock or cream to loosen what you have.

Still lumpy

Cause: Not enough liquid, or vegetables undercooked — Fix: Make sure the vegetables are completely soft before blending, and add liquid to help the blades circulate.

Stick blender spitting

Cause: Head not submerged — Fix: Submerge it fully before switching on and keep it below the surface.

Questions people ask

Can I blend soup in a jug blender at all?

Yes, safely, if you cool it a little, fill no more than half, remove the centre cap and cover the hole with a folded cloth, and start slow. The risk comes from doing none of those, not from the blender itself.

Why does my soup go gluey when blended?

Starchy vegetables such as potato release starch when the cells are ruptured by fast blades. Blend them briefly, or mash rather than blend, and use a stick blender in short bursts.