Interesting!
When they have got together a great quantity of milk, which is as sweet as cow’s as long as it is fresh, they pour it into a big skin or bottle, and they set to churning it with a stick […] and when they have beaten it sharply it begins to boil up like new wine and to sour or ferment, and they continue to churn it until they have extracted the butter. Then they taste it, and when it is mildly pungent, they drink it. It is pungent on the tongue like rapé wine when drunk, and when a man has finished drinking, it leaves a taste of milk of almonds on the tongue, and it makes the inner man most joyful and also intoxicates weak heads, and greatly provokes urine.”
"In its natural form, mare’s milk has almost 40% more lactose than cows milk and is not commonly imbibed without fermentation as it is a very strong laxative."
It was also churned by just leaving it in a saddle bag and letting it bounce around on the ride!