By protein, I just meant what type of meat did you use.
I just started using meat binders a couple of years ago and I am a proponent of them.
They essentially help the sausage retain moisture and add a little collagen to hold the meat together so it doesn't crumble or come off with the casing when you remove the casings.
I have found that for my preference I like to use amount 3/4 of what they say to use though. I have had a few batches that turned out a little rubbery (too much collagen) when using the recommended amount.
Austin Walton made a really good video that discusses the various meat additives (including meat binders) that can be used when making sausage and is worth the time to watch if you are a sausage maker:
https://www.youtube.com/watch?v=V-3vZygdVvM&list