European butters are churned from milk that has first been “soured” with bacterial starter cultures. This not only gives the butter a bit of a tangy-taste, it lets the butter keep better without the addition of salt being necessary.
Butterfat cream (the cream in which the butterfat in the milk has been extracted from the rest of the milk.) is put into holding tanks for a day to settle during which time live cultures are added which will consume the milk proteins to make lactic acid. The bacterial cultures can include “Streptococcus diacetilactis” and “Streptococcus lactis.”
This has two benefits: the lactic acid helps Butterfat to separate more easily in the upcoming churning process, and the butter has a longer storage life because the milk proteins, if left in the milk, would cause the butter to go rancid more quickly. Beyond that, the culturing also gives the butter a good taste. The “Streptococcus diacetilactis” bacteria also produces “diacetyl”, which helps improve storage life.
After the cultures have had a day to work, the cream is then churned for about half an hour to make the butter.