Lactic butter is made from whole milk that has been fermented with lactic acid. It has a characteristic low moisture content and tangy flavor that some people find preferable to sweet cream butter. Depending on where in the world a person is, this product may or may not be easy to find; it's readily available in Europe. It can also be made at home, by people who can maintain sterile conditions for butter making.
Butter from cow's milk breaks down into one of two types: lactic and sweet cream butter. Traditionally, lactic butter was made by allowing milk to sit for several days, which would encourage the formation of beneficial lactic acid, fermenting the milk into a product almost like yogurt, which could then be churned into butter. Sweet cream butter is made from fresh milk that is skimmed to separate the cream and the milk. Only the cream is churned for sweet cream, while the milk is used elsewhere.
Once either type of butter has been churned, it is typically washed before being packaged, and it may be salted as well to keep it stable in storage. Traditionally, salted butter was heavily salted and fresh butter was a rare treat; most modern butter is only lightly salted to evoke the flavor of old-fashioned butter. Just like sweet cream butter, the lactic type is available in salted or unsalted varieties, and it is extremely suitable for baking; some people find that it actually yields better baked goods because of the lower moisture. It also has a higher smoking point than sweet cream butter, which makes it a preferred cooking butter in some regions of the world.
Some producers call their products cultured butter, in a reference to the cultures that the whole milk is inoculated with to mimic the conditions of sitting for several days to ferment. Others may refer to it as “ripened butter,” also in a reference to the ripening of these cultures.
Sweet cream and lactic butters also taste different. The lactic variety has a full, rich creamy flavor with a hint of a tang, while sweet cream butter is more sweet and flat in taste. Cultured butter is also perfectly safe to eat; the fermentation process is halted through pasteurization, and it is carefully packaged to keep it shelf stable, just as with sweet cream butter. In regions where this butter is not readily available, people might want to try checking with small local dairies, which sometimes produce small batches, or with Internet retailers who can ship it.