The making of yogurt and some medical tests have the process of lactose fermentation in common. With the help of bacteria, lactose fermentation -- the breaking down of the sugar lactose into an acid -- is used to make fermented dairy foods and to test for food poisoning. Lactose fermentation also occurs in your body if you are lactose-intolerant.
Examples of foods made using lactose fermentation include yogurt, cheese and fermented milk beverages such as kefir. The fermentation process helps limit the spread of unwanted bacteria and other pathogens because it increases the acidity of the food. It also enhances flavors. When making yogurt, for example, the Streptococcus thermophilus and Lactobacillus bulgaricus bacteria feed on the lactose in the milk, producing lactic acid, which changes the structure of the proteins in the milk and causes it to curdle. Just keep in mind that pasteurization destroys the beneficial bacteria, called probiotics, in these fermented foods. Look for products that contain live active cultures if you want to increase your probiotic intake.
In Your Body
Normally, lactose -- the sugar in milk -- is digested in the small intestine by an enzyme called lactase. People who are lactose-intolerant don't have enough of this enzyme, so the lactose travels undigested to the large intestine, where the bacteria break it down into simple sugars that can be absorbed, in the process creating the gas that commonly occurs after a lactose-intolerant person eats dairy.
Lactose-fermented products, such as yogurt and kefir, have less lactose because of the fermentation process, making them easier for those with lactose intolerance to tolerate. Yogurt can have more than 50 percent less lactose than milk, and kefir has about 30 percent less lactose, according to a classic study published in the *Journal of Dairy Science*.
Some tests to determine what type of food poisoning someone has involve taking advantage of the principles of lactose fermentation. The sample is placed in certain types of agar -- a growing medium for bacteria -- and the type of bacteria can be determined based on the color the agar turns over time. Only certain types of bacteria can use lactose as a food, and the acidity created by the process changes the color of the agar.
Giving It a Try
You can make your own yogurt without purchasing any special equipment as long as you have a thermometer. Heat milk to a temperature of 185 degrees, let it cool until it is about 110 degrees and then stir in about 3 tablespoons of plain yogurt with live active cultures. Pour this mixture into a sterilized quart jar and cover it. Until your yogurt is formed, or for about four to six hours, keep the jar at a temperature of 110 degrees. A March 2005 article in *Mother Earth News* notes that you can do this by putting your yogurt in a thermos, wrapping the jar with a blanket, putting the jar in a cooler with a few hot water bottles, or you can put it in a gas oven with a pilot light. Once the curd is formed, refrigerate the yogurt until you're ready to enjoy it.