Watery gravy now

[Clarissa]

My gravy, which was thickened with a slurry of cornstarch/water, was reheated and covered for later. It suddenly turned back to water, and I have no idea why this happened. Do I add more cornstarch/water? It already has more than the recipe called for.

Thanks for any suggestions. (I posted below about sugar snaps that also failed. Apparently tonight's dinner is going to be a dud)

[Clarissa]

Thank you for the replies, I'm only now able to get back online. I wound up following mrsmegawatt's advice, and made a roux and then added the gravy. It turned out perfectly thick, and even reheated nicely today for leftovers. I'd had no idea that cornstarch was so delicate. I appreciate all of your feedback.

[Clarissa]

Just got back on the computer and saw your helpful replies. Yes, I think I shouldn't have brought it to a full boil, because it turned to water. I wound up following mrsmegawatt's suggestion and made a roux, and then added the gravy. It turned out perfectly thick and even reheated beautifully for today's leftovers. I'd had no idea that the cornstarch thing was so delicate. Thank you to all for the feedback.

[rmoskow]

Things thickened with cornstarch can't be reheated and maintain their thickness. Some food-chem person could probably explain why, but I think it just has to do with how and when the starch molecules absorb liquid. This phenomenon also occurs with the sauces on chinese takeout/leftovers- a sauce that is thick and glossy when you get the food becomes watery as it sits in the fridge and doesn't rethicken if you bother to nuke the stuff before chowing down 2 hours later.
Solutions: add more cornstarch and thicken the gravy again; use instant flour instead of cornstarch if you are avoiding the roux step, or just take the extra 3 minutes to make it with flour in the first place.

[Tartinet]

I've learned (but have not actually experienced) that the one drawback with thickening with cornstarch is that you cannot boil it after it has thickened. It is a strictly last-minute, no-leftovers method.

My understanding, anyway.

[mrsmegawatt]

yes you can still do that. It should be fine. It sounds like you cornstarch slurry was weak or maybe not brought to full boil?

[mrsmegawatt]

How did you make the gravy? Are they from pan drippings or straight stock? You can always try to make a roux, put the gravy back in the pan and whisk it, cooking till thickened.

[Clarissa]

From stock. By roux, do you mean mixing butter with flour and cooking it? Will that be okay, considering that the gravy already contains the cornstarch thing?

Oops -- am about to lose the computer. If I don't get a chance, thank you for your suggestion.