Back before there was baking powder to use as a quick leavener in baking, there was clabber. This was something that every farm wife or person with access to fresh milk could make, no additional ingredients necessary!
To make clabbered milk, the fresh, raw milk was simply left out at room temperature. Bacteria in the milk would start converting the lactose (sugars) into an acid, causing the milk to thicken and sour while also keeping it from actually spoiling. According to the Joy of Cooking, clabber is milk that has "soured to the stage of a firm curd, but not to a separation of the whey."
In baking, clabber would react with the baking soda to give baked goods a quick rise in the oven. It was also sometimes curdled further to make cottage cheese or simply fed to livestock as part of their feed.
We're speaking in past tense here because clabber isn't something that can be found or even made very easily anymore. It waned in popularity as baking powder (which doesn't require an acid to activate) became more commercially available and as regulations began to require that milk be pasteurized. The pasteurization process kills the bacteria necessary for this kind of souring.
Has anyone ever made, tasted, or baked with real clabber?
(Image: LIFE Magazine via Google Images)