Whether it comes in a slab or block, the butter must contain a minimum of 82% dairy fats and 2% dry matter (lactose, lactic acid, proteins and vitamins). For a long time the butter was salted as a way of preserving it. Today it is more a question of taste and culinary chemistry.
Why is butter in Brittany salty?
The answer perhaps lies in the fact that the area was for many centuries exempt from paying the “gabelle”, a heavy tax on the salt trade - or because the area is close to Guérande which is famous for its salt!