You were running late for class, so in an effort to be efficient, you decided to combine your milk and orange juice in one glass so that you could drink both together. Not only did it taste gross, but the milk actually curdled. What caused the milk to curdle? (hint: orange juice is an acidic product) The orange juice changed the primary structure of the milk proteins. The milk proteins acted as an emulsifier between the orange juice and the lipids in the milk. The milk proteins were denatured due to the acidity of the orange juice. Gelation occurred between the milk protein and the pulp in the orange juice.