Proteins are formed from long chains of amino acids. By using enzymes and/or acids to perform a process called hydrolysis, these chains can be broken down into their constituent parts. The resulting substance, now called protein hydrolysate, hydrolysed proteins or simply digest, is then spray dried to form a powder which can be useful in pet food for a number of reasons.
The first and most common use is as a flavour enhancer since the hydrolysed proteins tend to taste and smell very attractive to dogs.
Second, while certain whole proteins might trigger allergic responses in some dogs, the much smaller molecules of protein hydrolysate go completely undetected by the immune system making them ideal for dogs that are highly prone to allergies.
And last, protein hydrolysate provides a useful source of relatively inexpensive but bio-appropriate amino acids.
Most often, protein hydrolysate is formed from 'slaughterhouse side streams', that is castoffs from the meat industry that, while not particularly appetising to you and me, serve perfectly well as a basis for hydrolysis. But some companies go much further using higher end meat ingredients or even whole animals for their protein hydrolysate.
Although protein hydrolysate is undoubtably useful, it has attracted criticism, especially from some natural feeding advocates who claim that the acidic/enzymatic reactions that are used to make it aren't really 'natural'. Another cause for concern for some lies in the fact that monosodium glutamate (MSG) can be formed during the hydrolytic breakdown of proteins. MSG is a very effective flavour enhancer which may cause dogs (and humans) to eat much more than is healthy and thus contribute to the growing pet obesity epidemic. When MSG is added to foods as part of a protein hydrolysate, it does not need to be declared on the label.
As with all meat ingredients, it's always better to make sure the animal species is specified. This means ingredients like 'chicken liver digest' or 'fish hydrolysate' are much better than simply 'digest' or 'meat hydrolysate'.
Find foods containing Digest / Hydrolysed Animal Proteins See the full Ingredient Glossary