Why did my vinaigrette congeal? I was making a vinaigrette to marinate some tomatoes, and it congealed to the consistency of a pudding.  It is the first time I have tried this recipe.  I whisked it all together, then left it on the counter overnight for the flavours to meld before pouring it over the tomatoes, and this morning there was still some oil on top, with the rest of the vinagraitte a very thick mass underneath.  So I whisked again, and the rest of the oil got emulsified, but I was left with "vinaigrette pudding."  Any clues as to why it congealed like this? The ingredients list is:


1 cup Extra Virgin Olive Oil
1/4 cup Balsamic Vinegar
Squirt of Dijon Style Mustard for Emulsifier
4 Tablespoons Sugar
1 teaspoon Salt
1/2 T Freshly Ground Black Pepper (I toasted it before grinding)
2 t Chopped Fresh Oregano
18 whole Basil Leaves (chiffonade)
2 T Chopped Fresh Thyme
2 cloves Garlic, Minced Finely
It is unusual for this to happen. One possibility I see is for your mustard to contain a thickener which then also emulsifies the whole sauce. Remember, most emulsifiers will thicken even when present in tiny amounts. If the conditions are right for some other reason (maybe you mixed with an electric appliance or shaked very vigorously) it is not that strange that it can happen. 

My suggestion is to try it without the mustard and see if it thickens. If it doesn't, shop around for mustards from thickener-free brands. If it still happens with them, then they worked too well! Normally the mustard is a mediocre emulsifier and allows the vinaigrette to combine, but does not make a stiff emulsion. You could try less mustard then, or adding it at the end and gently stirring with a fork without whipping at all.