In classical cooking, a liaison is a mixture of egg yolks and cream, used to enrich and lightly thicken a sauce or other liquid. Egg yolks have the power to thicken a sauce slightly due to the coagulation of egg proteins when heated.
Caution must be used when thickening with egg yolks because of the danger of curdling. This happens when the proteins coagulate too much and separate from the liquid.
Pure egg yolks coagulate at 140° to 158°F (60° to 70°C). For this reason, they are beaten with heavy cream before use. This raises their curdling temperature to 180°–185°F (82°–85°C). (Note this is still well below the boiling point.) The heavy cream also adds thickness and flavor to the sauce.