A few years ago, a small baker from the West Coast had a problem. A day or so after baking chocolate chip cookies, the chocolate chips would develop an unpleasant white haze. Confused, she reached out to Richard Hartel, a professor of food science at the University of Wisconsin. Hartel studies fo ods like chocolate and ice cream, and he gets questions like this all the time. So what was going on with those chocolate chips?  What is chocolate bloom?   Chocolate may look uniform and solid to the naked eye, but if you looked at it under a microscope you would see that it’s actually a mixture of cocoa particles, sugar crystals, and (in milk chocolate) milk powder, all held together by cocoa butter.  Sometimes, some of those ingredients move around—and that’s what makes the chocolate turn white or “bloom.” There are two main kinds of chocolate bloom: sugar bloom and fat bloom. Sugar bloom: the fridge effect When you take a cold chocolate bar out of the fridge and leave it unwrapped on the counter, water from the surrounding warmer air can condense on the cold chocolate’s surface (just like condensation on a cold window pane). That moisture dissolves some of the sugar on the surface of the chocolate. When the water evaporates, the sugar is left behind in the form of tiny crystals, creating a white, powdery coating. This is called sugar bloom. The chocolate is still good to eat, just not very pretty. To avoid it, Hartel says “chocolate should be wrapped well for storage in the refrigerator or freezer, then allowed to warm to room temperature before unwrapping.” This prevents condensation from forming on the chocolate. In order to achieve chocolate’s glossy look and melt-on-your-tongue texture, bakers must carefully heat and cool the chocolate in a process known as tempering. Image: DepositPhotos Fat bloom: when chocolate gets old or warm Chocolate can also turn white even when it’s not stored in the fridge. “Think of leaving a chocolate bar in the car on a hot summer day,” says Hartel. “After it cools, it often develops a white, hazy coating.” (Of course, you might not have noticed because you ate the chocolate too fast. That’s totally normal.) This white, hazy coating is called fat bloom, and it happens when cocoa butter inside the chocolate slowly changes its shape. Cocoa butter is made of fat molecules that can arrange themselves in six different crystal shapes (which chemists call ‘polymorphs’). Chocolate makers want one special form—called Form V—because it gives chocolate that special glossy look and melt-on-your-tongue texture. They create this form by carefully heating and cooling the chocolate in a process called tempering. But over time, especially in warm places (like a sunny windowsill or a hot car), Form V can change into a more stable form called Form VI. These larger crystals scatter light instead of reflecting it, making the chocolate look dull and white. Hartel explains that fat bloom “can form on chocolate at almost any temperature, although it is generally slowed as temperature goes down.” That’s why chocolate brands recommend storing chocolate in a cool environment, ideally around 57 to 61 degrees Fahrenheit (the equivalent of 14 to 16 degrees Celsius). Some chocolates are more susceptible to fat bloom than others. In 2008, scientists from Canada and Sweden looked at chocolate under a microscope. They discovered that chocolate with a microscopically rough surface was more likely to form fat bloom. All those tiny cracks create more places for fat crystals to grow. Filled chocolates—like ones with peanut butter centers—are even more likely to bloom. The liquid fat from the filling can move into the chocolate shell, speeding up the process and making the chocolate soft and messy. Related 'Ask Us Anything' Stories The best chocolate chip cookie recipe, according to science Can one big meal really make you gain weight? Is microwave cooking nuking all the nutrients? What are ultra-processed foods and are they bad for me? Does eating spicy food help you lose weight? Science has a clear answer. So what about that baker’s cookies? Hartel studied the problem and found that the cookie dough didn’t have enough fat. During baking, fat from the dough normally moves into the chocolate chips, changing how the cocoa butter cools and helping prevent bloom.  “Think about the texture of chips that have been baked in the cookie, it’s soft and gooey, not at all hard like the original chip that was mixed in the dough,” says Hartel. “That’s the result of fat migration, whether it’s butter or shortening.” Without enough fat, the chips cooled the “wrong” way and bloomed. Once the baker added more fat to her cookie dough recipe, the problem disappeared. “It was so successful she sent me an enormous basket of goodies for Christmas that year,” Hartel recalls. “Another case of being paid in food or candy as a consultant.” The bottom line Chocolate bloom is caused by sugar or fat molecules moving around, and usually it’s just a cosmetic issue.  If it’s only on the surface, the chocolate will taste fine—“there is no effect other than the visual turn-off,” says Hartel.  But if the chocolate is very old and the bloom has spread deep inside, the chocolate will be cracked, dry and crusty, possibly with a waxy aftertaste, says Hartel.  He confesses to having a chocolate Santa that’s about 30 years old. “Not for eating,” he says. In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? 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