=Commercial Buttermilk= or =Cultured Milk= is simply carefully soured milk. It can be made at home from fresh milk either whole or skimmed or partly skimmed. Partially skimmed milk containing from 1% to 2% butter-fat is plenty rich enough and even better for most purposes than whole milk. The essential qualities of good buttermilk depend upon the proper ripening of the cream or milk, the development of a pure “breed” of healthful bacteria in a clean field free from weeds. Such a plantation or “culture” may be grown in milk as well as in cream. Its function is to turn the sugar of milk into lactic acid under the development of pleasing flavors and whether the butter-fat is removed by the separator or by churning makes little difference. In natural buttermilk there is always a little butter-fat—at least ½%—left, mostly in the form of fine granules, too small to be retained in the butter. If the same amount of butter-fat is left in skim milk and that is ripened and churned, the product will be identically the same as natural buttermilk from ripened cream. =Ripening.=—For best result the milk should be pasteurized, not necessarily as thoroughly as for starters, but sufficiently so as to destroy all obnoxious bacteria and give those introduced through a pure culture starter a chance to grow. Buttermilk may, however, also be made from good, clean, unpasteurized milk of good flavor. Whether pasteurized or not the milk is set to ripen with from 5 to 10% starter at a temperature of from 65 to 75°. The preparation of starters is described under “Bacteria” and the ripening of the milk for “buttermilk” is essentially the same process (see also under _Ripening_ of cream for butter). When ripened to the desired acidity,—say .5% to .6% by the acid test,—stop further fermentation by thorough cooling. =Breaking up the Curd.=—After cooling, the ripened milk may be broken up fine and if vigorously shaken or “churned” it will remain smooth and creamy. Otherwise it may separate into curd and whey. If churned long enough for the butter to form, it becomes absolutely identical with real buttermilk. But, for all practical purposes, a vigorous shaking for a few minutes is enough. =Thick Milk.=—“Thick Milk” as eaten in Scandinavia is made in the same way as commercial buttermilk, except that the milk—rich whole milk—is set to ripen in the bowl in which it is to be served. Instead of being churned or stirred, it is left thick, to be served as a pudding, like Junket made from sweet milk. The rich layer of cream that forms on top is excellent. “Thick Milk” is eaten plain with the oatmeal for breakfast, or as a dessert with grated stale bread and sugar spread over it. The uses of buttermilk in making pancakes and for many other culinary purposes are mentioned in the chapter on “Milk Cookery.” [Illustration: Dr. Elie Metchnikoff, author of “The Prolongation of Life” ]