Meat – Why is the meat still tough after hours of cooking

The answer depends on the type of cut. If you have a tender cut of meat then there's no reason to cook it any longer than then desired doneness. If you are using a tough cut then there's lots of collagen that needs to be broken down, and that requires moisture and time. You want to cook it until all the collagen is broken down as that will make the meat tender. That may take 2 hours, it may take 6, it depends on the cut and the thickness.

I have a similar experience with my slow cooker, it goes way too fast. To get around this I've ended up putting in big chunks of meat and veggies because anything else is completely nuked after 8 hours! I've also thought about using a light timer to have it start 3 hours after I leave home.

I was unable to find information that might tell what happened to your dish, but found some extremely interesting and detailed sources of information on collagen and gelatin which I think are worth sharing.

I did find that the pH is unlikely to be the major contributing factor, as there are both acid and alkaline processes for gelatin formation--see the Science of Cooking, Gelatin.co.za, and Hydrolization references below.

Since you have read On Food and Cooking, you almost certainly already know the basics of what affects the conversion rate. To paraphrase one of the articles, it is a high energy stochastic process so:

• Gelatin conversion is time and temperature dependent, with higher temperaturs permitting faster conversion
• Requires the presence of water, as it is a hydrolization of the denaturing collagen proteins

Of course, it sounds like by braising for hours, you have met these basic criteria.

Here are some sources I found:

• why use a pressure cooker for collagen to gelatin conversion? Related question here, on why pressure cookers are effective at gelatin conversion
• Gizmodo article on duck confit, with really nice scientific introduction for someone not already familiar with the topic. No references.
• Science of Cooking suggests that acid actually aids in the process by breaking covalent links. Has references to more sources. Nicely balanced on the science without being impenetrable to the layperson.
• http://www.gelatin.co.za/gltn1.html indicates there are forms of gelatin from both acid and alkaline environments, so the pH of the cooking medium was probably not the major contributing factor. Extremely detailed including information on chemistry. Has extensive references to primary sources, but no description of the site itself.
• Gelatin Manufacturer's Industry Association article on manufacture, process, uses, chemistry and more. Extensive references.
• Hydrodynamic properties of gelatin: studies from intrinsic viscosity measurements Introduction has some good summary information on gelatin, and then full on science of the viscosity measurements. Lots of references.

At this point--I stopped, as I have learned more about gelatin than I need to know as a cook!