It doesn't have to be sour, I've done it with regular milk too! I just needed another good use for my sour milk other than yogurt. But the milk draws out the strong taste of beef liver and makes it more like the mild taste of chicken liver! Only way I'll eat it 🤣 

 Although actually now that I think about it, I did see recipes for soaking liver in buttermilk so maybe it is better that way. I didn't look into it!