Collagen and gelatin
I went on a pork broth spree last weekend, making a couple different batches to compare starting with roasted and non-roasted bones. I expected the flavor and color differences, but the texture threw me: the broth from non-roasted bones set up into a loose jello, while the broth from roasted bones stayed a thin liquid. What gives?
After my own investigation and also asking all of my coworkers (sometimes my new job really pays off), I’m working my way toward an answer. Let’s start with the basics. Things we know:
-The broth set up because it contains gelatin, a protein that cross-links with itself to form a water-trapping network.
-The gelatin comes from the breakdown of collagen, a triple helix made of three strands of gelatin that forms connective tissue in animals.
-When collagen hits 140 degrees F, its weakly bonded strands of gelatin start to separate from each other and become water-soluble as individual proteins.
So we know pretty well what happened in the broth from non-roasted bones. The meat and bones hit the collagen-breakdown temperature while in water, allowing the gelatin to dissolve. When the broth cools, the dispersed gelatin molecules tangle with each other in a loose network, preventing the water from moving so easily and transforming it into a semi-solid mass.
That much we know. Since the broth from roasted bones didn’t follow this same path, something about the high temperature must have changed the collagen. Heat and proteins don’t always play nicely (ever had an overcooked steak?) so it’s not entirely surprising, but the difference here lies in the fact that collagen forms gelatin by literally breaking into pieces. Sounds like it shouldn’t be that hard.
My working theory holds that the high temperature must destroy the collagen, forcing it above its breakdown temperature without water around to dissolve the free gelatin. The increasing heat probably goes on to break the gelatin into smaller pieces that can no longer form the network to set the broth. Without the gelatin, the broth stays liquid. Mystery (mostly) solved.