The differences between them
Liquid cream: Naturally fluid, with a fat content generally around 30-35%.
It is homogenized, meaning that the fat particles are evenly distributed in the liquid.
This structure makes it stable to cooking: it incorporates easily without breaking down.
Thick cream: In addition to fat (similar to liquid cream), it often contains lactic ferments, which give it its tangy taste and denser consistency.
These ferments slightly modify the structure of the proteins.
Cooking behavior
Liquid cream: It retains its fluid texture when heated, as it does not contain the lactic ferments that could interact with heat.
It is ideal for sauces, gratins or dishes requiring a stable emulsion.
Thick cream: When cooked, heat can break down the interactions between proteins and fats stabilized by lactic ferments.
This releases the water contained in the cream, giving it a runnier consistency.
The little trap
It's this cooking behavior of thick cream that's a bit of a trap: when you're looking for a thick sauce texture, you naturally think of using thick cream, but that's a mistake, as it will liquefy very quickly.
Contrary to its name, thick cream doesn't thicken, quite the opposite, so it's best avoided for thick sauces and preparations.
Don't worry either, both will cook well, but if you're looking for a slightly thicker texture, not too liquid, you'll have to go for the liquid cream.
To sum up: if you want a thick texture in a recipe with cream, especially a sauce, use liquid cream, not thick cream.