yes....heat breaks it down....but I wouldn't toss the muffins unless they don't taste good. If the oil is good to fry in it's good to eat in muffins unless ti has begun to break down....and if so toss the oil too.
I always keep "used oil" in the refrig to slow deterioration.
Linda C

I once had a conversation I no longer recall with a nutritionist about reusing oil. So I had to go looking. From GoAskAlice:

Alice,

I am trying to convince my good friend not to save the grease she cooks with for re-use. Instead of draining grease out of a pan after frying it, she saves and stores it to use again. It seems to me that grease that is cooked once is bad enough, twice must be horrible. Am I unjustly picking on her, or am I right that there is an even greater health risk when you cook with pre-cooked oil/grease?

Dear Reader,

Reusing cooking oil has been done for ages. There really isn't a problem, if done properly. The greatest hazard is allowing the fat to become rancid (spoiled) and deteriorated to the point it produces undesirable flavors and odors. Besides ruining what would have been a perfectly good meal, rancid oils also contain free radicals that are potentially carcinogenic.

To understand how to best re-use oil, it is important to know about smoke points �" the temperatures at which oil begins to decompose. If you heat oil to a temperature that is too high, it produces smoke fumes. Acreolin, a substance that makes your eyes burn, is given off as well.

The optimal temperature to fry foods at is 190°C (375°F). At higher temperatures, the food will burn on the outside, and at lower temperatures, the food absorbs too much oil and tastes greasy. Different oils have different smoke points. Oils with higher smoke points are better for frying. The following is a list of smoke points:
Type of Oil Smoke Point Temperature
safflower oil 265°C
sunflower oil 246°C
soybean oil 241°C
canola oil 238°C
corn oil 236°C
peanut oil 231°C
sesame oil 215°C
olive oil 190°C
lard 183 - 201°C*
(*varies depending on the diet fed to hogs and the part of animal fat that is derived)

Each time the oil is re-used, the smoke point becomes lowered. This is due to:

* foreign matter in the oil (such as batter)
* salt
* the temperature to which the oil was heated
* exposure to oxygen and light
* length of time the oil remained heated
* the number of times the oil was reused

Combining different types of oils also lowers the smoke point.

When oil becomes deteriorated, it appears dark and thick (viscous). It may have an off odor, and smoke appears before it reaches 190°C. If the oil smells funky and old, better to toss it away and replace it with fresh oil.

To re-use oil safely, use these tips:

* Strain it through a few layers of cheesecloth to catch any food particles. Be careful with hot oil, though, because you can easily get burned.
* Shake off excess batter from food before frying it.
* Use a good thermometer to fry foods at 190°C.
* Turn off the heat after you are done cooking. Exposing oil to prolonged heat accelerates rancidity.
* Don't mix different types of oil.
* Store oil in a cool, dark place.
* Avoid iron or copper pots or pans for frying oil that is to be reused. These metals also accelerate rancidity.

Jo, I agree, unless the muffins taste bad and the oil was rancid or became compromised in flavor because of your prior cooking, there's not a safety issue with the muffins.

Just try one first before you give them to anyone else, LOL, or tell them it's the first time you ever got a bad recipe here. (grin) Tell them you got the recipe from me, LOL, and that you'll never trust me again.

Annie

Thanks, all. The muffins were fine. The oil looked brand new. Thanks to the thread on how to reuse coffee filters, the oil performed beautifully. Couldn't taste a difference. From now on, I will store it the used oil in the fridge.
Thanks again.
Jo