Why are old birds' meat and skin tough?

[TPaul87]

Every single search result that I look for on Google and specifically in this form only tells me how to cook an old bird in order to keep it tender but that's not what i am trying to learn about.

what is the biological reaction that is happening in the bird's body that causes the meat in the skin to become chewy and tough under the normal process of cooking?

I had three old laying hens i had harvested and while brining and slow cooking does wonders for the meat, i still desire that crispy skin.

So of course I cut off all sets of wings and tried to deep fry them and as soon as just chewy and I cannot bite through it it is like pure rubber..

I do NOT want an 'it is what it is' answer.
I would love to have a detailed scientific understanding of why their bodies go through this change, what is happening to their muscle and skin cells and when it begins!

Is there anyone out there that knows?

 

[Molpet]

Collagen
https://www.exploratorium.edu/cooking/meat/INT-what-makes-juicy.html

 

[Sally PB]

Thank you! That link was very interesting and informative.

 

[CNJ]

They sell the retired egg layers at my local super market. They call it stewing chicken, I tried baking it once and figured out why they call it stewing chicken. Since then, I discovered that the meat doesn't dry out in soups, and the broth is very flavorful. It makes a better tasting soup than regular chicken, in my opinion.