Milk is loaded with milk sugar, lactose. Lactose is the cause of milk related problems for many. Lactobacteria consume the lactose during fermentation and produce lactic acid. Commercial yogurt typically has about 50% of the milk Sugar converted to lactic acid. A person making yogurt at home can lengthen the fermentation time to where the resulting yogurt is essentially lactose free. There are other benefits to fermemting milk, as well.