Why Does Pineapple Spoil Milk?

Recently, when developing a recipe for baked oatmeal with pineapple, the Eating Well Test Kitchen editors noticed something unusual: Despite the fact that they started with sweet, fresh fruit, the finished product had a bitter taste. The recipe did contain two dairy products (low-fat yogurt and reduced-fat milk), and it seemed as though they had spoiled during the cooking process. Yet when the editors made the recipe using nondairy versions of the yogurt and milk, it turned out perfectly—try the recipe for Vegan Pineapple & Coconut Baked Oatmeal, pictured below to see for yourself. (Check out these 10 Bad Cooking Habits You Should Break.)

To figure out why this happened, we turned to food scientist Christopher Loss, Ph.D., the Louis Pasteur lecturer at Cornell University, who explained that proteins—and some chemistry concepts—hold the key to the answer. "Pineapple contains bromelain, which is an enzyme, or a type of protein that has chemical activity, and it can break down animal proteins," says Loss. In fact, bromelain is used in powdered meat-tenderizing products such as McCormick seasonings. Some people take bromelain as a dietary supplement for nasal and sinus inflammation (research shows it may help these conditions), and it's also being studied for its effects on asthma. (Check out our Buyer's Guide to Nondairy Milks.)

When pineapple mingles with dairy products such as milk and yogurt, its bromelain breaks down their casein, which is a major protein in milk. "The casein is chemically chewed up by bromelain, which will result in peptides, or smaller protein pieces, that can be perceived as bitter," he says. Loss also notes: "There are probably a variety of other enzymes in pineapple that would also play a role, but bromelain might be the biggest contributor."

If you're hoping to avoid this reaction, you have a few options. You can try gently heating the pineapple on the stove or in the microwave, allowing it to cool and then using it in your recipe. You can also use canned pineapple, which has been heat-processed, but aim to choose a brand that's been packed in water and has no added sugar. "The heat will denature the bromelain enzyme, rendering it inactive," says Loss. Or, if you've found a gorgeous, fresh pineapple for your smoothie and can't bear to mess with it, have fun experimenting with nondairy options like oat milk and coconut yogurt for your recipe. After all, there's nothing wrong with leaning a bit more toward a plant-based diet.