Cultured buttermilk is probably the easiest and most fool proof fermented milk product to make. (Note that cultured is different than "old fashioned buttermilk.") All you need is active cultured buttermilk for the starter, and fresh milk for it to act on. The formation of buttermilk is based on the fermentation by the starter bacteria which turns milk sugar (lactose) into lactic acid. As lactic acid is formed, the pH of the milk drops and it gets tart. Milk proteins, most notably casein, are no longer as soluble under acid conditions and they precipitate out, causing what we recognize as clabbering. Thus the two marked characteristics of buttermilk, its tartness and its thickened nature, are both explained by the presence or the action of lactic acid. Additional by-products of fermentation give subtle variations in buttermilk flavor. The acidity of buttermilk also explains its long refrigerator shelf life. Acid is a natural preservative because it inhibits the growth of pathogenic bacteria. Thus buttermilk keeps easily for weeks in your refrigerator. If you keep it longer, it may develop mold on the inner walls of the jar. This mold belongs to the same group of fungi which grow on cheese and is not dangerous. Remove it and the buttermilk can still be used for baking. However, because the desired bacteria may have died in older samples, buttermilk older than three to four weeks may not work as an inoculum to make buttermilk.