Tart yogurt means it has been traditionally fermented or aged. Traditionally fermented yogurt has far less lactose (naturally occurring milk sugar) and a significantly larger population of probiotics (live beneficial bacteria or culture). The beneficial bacteria eat the lactose and produce lactic acid, hence the tart taste.
A cup of whole milk contains 12g of lactose. White Mountain yogurt contains less than half that—5g—and, as a result, is easily digestible by our lactose-intolerant consumers. The live bacteria populate our digestive tract and help increase the nutritional value of all the food we eat through more efficient digestion. The bacteria also provide a significant source of B vitamins and trace minerals as a result of their life cycle.