# Free energy change in collagen hydrolysis

In now-deleted comments on this answer, a user with a self-proclaimed chemical engineering background claimed that the reactions that take place when slow-cooking meat are endothermic and require far more energy than actually heating the meat. My intuition says this can't be - the meat would rapidly cool itself down if removed from the heat source, the sous vide circulator would have to work hard the whole time not just during initial heating, the bonds in the collagen are presumably not terribly strong, and, well, heating mostly-water things takes a lot of energy! But I don't want to assume too much.

I believe the primary interesting reaction here is collagen conversion, hydrolizing it into gelatin, and the collagen is something like a few percent of the meat by mass. (When cooking something like this, the rapid reactions have already taken place: the meat looks cooked, and it gets slowly more tender over time as the collagen breaks down.)

So, how much energy does that actually require?

(1) Peptides do hydrolize in presence of enzymes. It means that Gibbs energy of hydrolysis is negative. However, Gibbs energy consists of $\Delta$ H and $\Delta$ S*T. $\Delta$ S of hydrolysis is favorable (more disorder), but $\Delta$H can be either lower or higher. $\Delta$ H is what we are discussing here (the enthalpy of the reaction).
(2) Some endothermic reactions are really very endothermic (like dissolving NH4NO3 in water). Other reactions are mildly endothermic. Hydrolysis of meat doesn't have a large $\Delta$ H change. "HEATS OF HYDROLYSIS OF AMIDE AND PEPTIDE BONDS" paper has exact value for the reaction: -2.5 Kcal/mol of peptide. A negative value indicates the reactants have greater enthalpy, or that it is an exothermic reaction. So the reaction is slightly exothermic.