Dear Anthony

I have used milk effectively in the field to interfere with non persistent virus vectoring. I think the protein in the milk sticks to the viral coat and then the virus cannot be taken up and recognized. 

as you know for virus infection to proceed the complete virus must pass into the cell and lose its protein coat and attach itself to replicating RNA or DNA. 

I do not if this might help but I know of no information that proteases are involved in that stage and its step of involvement. 

good Luck