Hollandaise and mounting with butter

Why

Hollandaise is a warm emulsion: droplets of melted butterfat dispersed in the water from the yolks, the butter and the lemon juice or reduction, coated with lecithin and proteins from the yolk. The yolks thicken the sauce once they reach roughly 65 °C, but above about 70 °C their proteins curdle into grains and the emulsion falls apart – which is why it is whisked over a water bath or on very gentle heat. If the sauce cools down instead, the butter firms up and the sauce thickens until it sets, and if too much butter meets too little water, the droplets merge and the sauce splits much like an overloaded mayonnaise. Mounting a sauce with butter works on the same principle without the yolk: cold butter whisked piece by piece into a hot but not boiling reduction disperses into tiny droplets that the butter's own proteins and emulsifiers hold in place – until you overheat the sauce or bring it to the boil.

How to do it

• Whisk the yolks with a little water, lemon juice or reduction over a water bath that is barely simmering and doesn't touch the bowl.
• Whisk until the mixture thickens and the whisk leaves a trail; lift the bowl off the steam now and then.
• Add warm, not hot, melted butter drop by drop at first, then in a thin stream.
• Keep the finished sauce in a bowl over warm water off the hob and serve it as soon as you can.
• To mount a sauce, take the reduction off the heat and whisk in cold butter cube by cube, adding the next once the last has melted; then don't let it boil.
• For children, pregnant women, older people and the immunocompromised, use pasteurised yolks: a sauce kept below 70 °C won't reliably kill salmonella.

If it goes wrong

If the sauce starts to turn grainy or greasy, take it off the heat at once and whisk in a spoonful of cold water. If it has split completely, put a fresh yolk with a spoonful of warm water in a clean bowl and whisk the broken sauce into it a teaspoon at a time. Warm a sauce that has cooled and set over a water bath, whisking constantly. Badly curdled lumps of yolk can't be fixed – strain the sauce or start again.