It has to do with how long the cheese is aged. Generally speaking, the older the cheese, the less lactose it has. As the chief sugar in milk, lactose is the main food source for the various species of Lactobacillus used in making most kinds of cheese. These bacteria, which are also responsible for sourdough bread, yogurt, kimchee, and dozens of other fermented foods, turn lactose into lactic acid, which is easily digested by humans, even those who are lactose intolerant. The longer a cheese ages, the more of its lactose is consumed by the bacteria. “In theory,” University of Wisconsin-Madison food science professor Scott Rankin says, “most of the lactose is gone after three months of aging.”
Processed cheese has the most lactose. Christine Gerbstadt, a dietitian and a spokeswoman for the American Dietetic Association, points to Velveeta, which has 9.3 percent lactose—as much as whole milk. (Milk is 9 to 14 percent lactose, with skim on the high side and whole on the low side.) Not only is Velveeta unaged, but it also contains added lactose-laden milk solids. Fresh and/or unripened cheese, including Mexican queso fresco, farmer’s cheese, some mozzarella, paneer, cottage cheese, and cream cheese, contains the second-greatest quantities of lactose.
A good rule of thumb: The harder the cheese, the older it is, and the lower the lactose. So the eight-month-old, rock-hard Parmesan will contain less lactose than a softer (younger) Parmesan. Aged cheeses, including Roquefort and some goat cheeses, typically contain around 2 percent lactose.
But there’s no hard-and-fast rule for lactose-intolerant folks to follow—everybody can handle different concentrations. In fact, all people are lactose intolerant to some extent: Eat a whole bunch of pure lactose and you’ll wind up unhappy, no matter who you are. If you have trouble with dairy but still love cheese, Rankin suggests working your way up. Start with small quantities of aged cheeses: a little Parmigiano-Reggiano on your pasta, or some sharp cheddar in an omelet. It is possible to develop a better tolerance for lactose through exposure, so that bowl of Velveeta-and-salsa dip at your Super Bowl party might not be out of reach forever.