expired long life milk

We bought a 6 pack crystal Valley long life milk at shoprite last month and the milk was sour... the expiry date is 30 april 2016. We have 3 boxes left and cant use it to cook with or bake or even drink...this is unacceptable in this day where food prices are high.. either the lab guys are not Doing their job right or the ***s need to be fed Correctly. . I used to work in a milk factory and I know quality is of great importance.