Tahini: why it seizes, and how to bring it back
Anyone making tahini sauce the first time has the same experience. Smooth tahini in a bowl, a splash of water, and it turns to stiff pale cement. It looks broken. It is not.
Tahini is ground sesame: roughly half fat, solids suspended in it, almost no water. Add a little water and the solids grab it and clump, so the fat and the newly wet solids are briefly fighting each other and the whole thing stiffens and goes grainy. Keep going. As you add more and keep working it, the solids finish hydrating and it flips into a smooth emulsion, looser and much paler than what you started with. The seize is a stage, and stopping there is the mistake.
For the sauce: tahini in a bowl, then lemon juice and crushed garlic if you are using them, and expect it to seize immediately, because lemon juice is mostly water and does the same thing. Then cold water a little at a time, whisking, through the thick stage and out the other side. It loosens suddenly rather than gradually. Salt at the end, and thin it further than you think you want, because it firms as it sits.
Very cold water, or a couple of ice cubes, gives a whiter, thicker, slightly fluffy sauce — the version that goes with falafel and shawarma. Warm water gives something looser and a little duller. Neither is wrong.
If the tahini itself tastes harsh it is one of two things. Some brands are made from unhulled sesame and are more bitter by nature. Or it has gone rancid, because the fat oxidises, and faster once the jar is open and warm. Keep it cool, stir the separated oil back in rather than pouring it off, and taste before building a sauce on it — rancid tahini cannot be seasoned back into shape.
A jar separated into oil and paste has not gone off. That is what an unstabilised seed paste does. Stir it back together before measuring, because if you pour the oil away what is left is dry and your sauce will never come smooth.
