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French Thickening for Soups
The favorite liaison or thickening for cream soups and purees is a mixture of raw egg yolks and cream, the real proportion being two yolks well beaten with one-half of a cupful of cream and stirred into the soup when it is off the boil to prevent separation or curdling, and a moment or so before taking from the fire.
This is a component rather than a dish: the classic egg-and-cream liaison that finishes a cream soup or purée. The proportions are sound, but the method as written skips the step that keeps it from scrambling.
- Temper the yolks firstThe hinge of the whole thing. Beat the two yolks with the half cup of cream, then whisk a ladleful of hot soup into that mixture before returning it to the pot. Pouring cold yolks straight into hot liquid, even off the boil, is a common cause of curdling.
- "Off the boil" means a number"Off the boil" -- The original gives no temperature, only the instruction to add the liaison off the boil and a moment before removing the pot from the fire. Modern practice puts the working window at roughly 170-180F; much above that the yolk proteins can tighten into visible curds. The soup should not be brought back to a boil afterwards, including on reheating.
- How much soup this thickensThe recipe gives no yield. Two yolks and half a cup of cream is broadly in line with the usual modern liaison for about a quart of soup. More yolk enriches the body as much as it thickens it.
- Cream, then and nowPeriod "cream" was not a standardised grade and varied a good deal in fat; recipes of the era distinguish thin from thick cream. Heavy or whipping cream is the safest substitute here; light cream and milk both work but give a thinner, less stable result.
- Strain if you want it flawlessPass the finished soup through a fine sieve before serving. It catches any chalazae from the yolks and any small bits of egg that set, and costs nothing in texture.
- It is a finish, not a baseA liaison adds sheen and richness but very little structural thickening. If the purée itself is thin, thicken it with roux, potato or the vegetable puree before the yolks go in, not with more yolks.
- Serving and holdingServe at once. A soup finished with yolks does not hold on a burner or reheat well, and it will not freeze cleanly, so add the liaison only to the portion going to the table.
A liaison is simply a binding, and the yolk-and-cream version is one of them, the finish that turns a purée into a velouté-style soup. Naming the technique "French thickening" and specifying the exact ratio brings a restaurant cook's habit to a household reader. The warning to add it off the boil is the one piece of the instruction that has never needed revising, since it addresses the same egg chemistry cooks still fight with today.