When mixed with milk, protein powder becomes a very gooey substance. I have tried many different types of protein powders and some are more gooey than others.
What makes the suspension so gooey? Is it an additive of the powder or the powder itself? Maybe an interaction between the powder and the milk?
It’s the protein itself that makes the gooey mess, not some additive.
Proteins are big long chains of amino acids, and include chemical groups with both positive and negative charges. When you add dryed protein to a liquid, the charge groups on one protein chain are attracted by the opposite charges on one ore more other protein chains. This produces randomly sized clumps of entangled proteins; goo.
(This can be avoided if you treat your protein well, and take some care in dissolving, but we’re talking bulk lyophilized powders here, not crystals of A. niger sulfhydryl oxidase).
The amount of gooeyness you get depends on the source of the protein (collagen, meat, soybeans etc.) and how long the individual protein chains are. Some protein powders are partially digested into short chains, which dissolve more readily; or you can go with totally digested mixtures of amino acids.
I have noticed casein protein is far more gooey than whey protein. I don’t know the reason though.