I just had some of my mom’s leftover spaghetti, and I realized that it always has a slightly more bitter taste in the morning after being reheated. What causes this kind of reaction?

Question

It happens both when stored in pyrex or tupperware.
Ingredients aren’t anything unusual; onions, garlic, hamburger, tomato paste, etc.
Tastes sweeter when fresh off of the stove, but has a more bitter taste when reheated as leftovers.

In: 159

Answer 1

Primarily, it’s due to oxidation, or where, over time, fats and proteins in the food interact with oxygen molecules in the air and become rancid – the same reason that oil or butter can get old and turn bitter, and even the same reason that wine eventually turns into vinegar. It’s also frequently identified in re-heated meats, where it’s sometimes referred to as “[warmed over flavor](https://en.wikipedia.org/wiki/Warmed-over_flavor)”. In addition, as the food goes from fridge cold to reheated hot enough to eat, it passes through a warm region where that oxidation happens faster than usual, so (ignoring bacterial growth or anything like that) it’s like you set the food out for a day or two at room temperature – more of those fats and proteins are going to go rancid than if it stayed at fridge temperature.

It’s still completely safe to eat, and one thing you can do is add salt – not enough to make it taste salty, but just a tiny pinch. Salt can block your tongue from tasting bitter flavors (same reason some people add salt to coffee!) so a little bit of it added to the dish will suppress that bitter leftover flavor.