Almost every dry pet food on the shelf is made the same way. A dough of meat meal, starch and vitamins is forced through a screw at pressure, heated past 140°C, and cut into shapes as it expands out of the die. The process is called extrusion, and it is a marvel of food engineering: cheap, shelf-stable, and capable of turning very low-grade inputs into something a dog will eat.
It also has a cost, and the cost is measurable.
Heat and the amino acids
Protein is a chain of amino acids, and the shape of that chain is what makes it digestible. Above roughly 120°C, in the presence of the starches extrusion requires, some of those amino acids bind irreversibly to sugars — the Maillard reaction, the same chemistry that browns a steak. In a steak it is flavour. In a food that has to deliver complete nutrition, it is loss: lysine, the amino acid most vulnerable to the reaction, can drop by a fifth or more, and the damage is invisible on the label, because a crude protein assay counts the nitrogen whether or not the animal can use it.
This is why extruded foods are so often sprayed with a vitamin and amino-acid premix after cooking. The nutrition on the bag is real. It is just not all coming from the ingredients.
Why 68°C
68°C, held for long enough, is the temperature at which the pathogens that matter — salmonella, listeria, E. coli — are reliably destroyed. It is also low enough that proteins denature gently rather than crosslinking, and low enough that the water stays in the food.
That last part matters more than it sounds. A gently cooked bowl is about 74% moisture; kibble is about 8%. Dogs and cats evolved taking most of their water with their food, and cats in particular are poor voluntary drinkers. Feeding a dry diet for a lifetime means asking an animal to make up a water deficit every day, from a bowl, by choice.
The honest caveat
Extruded food is not poison, and a well-formulated kibble will keep a dog alive and healthy for years. Our argument is narrower: that when you can afford to cook properly, cooking properly is better — and that a food which needs a premix sprayed on afterwards is telling you something about what happened inside the machine.