DanS wrote:That's called "Warmed -over-flavor. Seriouseats.com did a while back. Check it out https://www.seriouseats.com/2017/08/what-is-warmed-over-flavor-leftover-chicken-meat.html.
Interesting article. So it's oxidation of fats that is the culprit here, just like the rancidity process.
But their explanation of unsaturated fatty acids is way off base. Fats consist of a molecule of glycerol esterified to three molecules of fatty acid. Saturated fatty acids have the general chemical formula:
H3C-CH2-(CH2-CH2)n-CH2-COOH
Various different fatty acids have different values for n. n=12 for palmitic acid, which as the name implies is the main fatty acid in palm oil. These fatty acids are "saturated" in the sense that each carbon in the chain is bonded to the maximum number of hydrogens. Unsaturated fatty acids have one or more double-bonded carbons:
H3C-CH2-(CH2-CH2)m-CH=CH-(CH2-CH2)n-CH2-COOH
This is a mono-unsaturated fatty acid. Polyunsaturated fatty acids have more than one of these double bonds. One difference between carbon-carbon single bonds and double bonds is that single bonds can rotate whereas double bonds are rigid. a -HC=CH- group can exist in two configurations. The one most commonly found in nature has both hydrogens on the same side of the double bond. This is called the cis- configuration. The other one has one hydrogen on each side of the double bond. This is called the trans- configuration. Fats containing a fatty acid with a trans- double bond are called trans-fats and they're the bad guys implicated in heart disease.
Each of the two carbons participating in the double bond has three bonds total, and these are at 120-degree angles. The double bond thus introduces a "kink" in the long chain of the fatty acid. Saturated fats have no kinks. In the solid form, they're all lined up next to each other. Unsaturated fats don't line up as readily. This is why saturated fats (such as the depot fats in mammals and birds) solidify at higher temperatures than unsaturated fats (which remain as liquid oils at lower temperatures).
-Paul W.