(Inside Science) -- A streaming cocoa curtain gently drapes rows of baked goods — marzipan, cookies, caramel-filled nougat — to enrobe the confectionery in a rich, glossy layer of chocolate. Chocolate manufacturers begin producing masses of these treats months in advance for the holiday season.
Over time, however, chocolate-coated desserts can acquire a dull white film. The layer, known as cocoa (or fat) bloom, is formed when fats in the chocolate separate and deposit on the surface. Even though it is harmless, this fat bloom can ruin the appeal of dessert — and can spell significant losses for the chocolate industry.
One way to prevent these cocoa blooms is by seeding molten chocolate with microscopic particles known as “precrystallized” chocolate. When heated and cooled to precise temperatures, or "tempered," the fat molecules in molten chocolate arrange themselves into the regularly repeating crystal patterns present in the seed particles. Forming these chocolate crystals is the key to high-quality chocolate. Tempered chocolate sets with a glossy finish and snaps sharply when broken.
Fat molecules in poorly tempered chocolate clump in microscopic, disorganized heaps—making the final product crumbly, duller, and more susceptible to forming fat blooms when stored. Seeding molten chocolate with a few pre-formed crystals helps the molten product set in a similar crystal structure, ensuring the final quality of the chocolate product.
But in some instances, even well-tempered chocolate is prone to forming a fat bloom as a result of the way confectionery is manufactured, according to a study led by Wolfgang Danzl, a food engineering researcher at the Fraunhofer Institute for Process Engineering and Packaging in Germany.
To test why this might happen, Danzl and his co-workers analyzed marzipan, nougat, cookies, caramel and a variety of other commercially produced chocolate-coated treats. All were produced by passing the products under a stream of precrystallized chocolate. Using three different tests, the researchers found that although the streaming cocoa was well-tempered, the finished products’ surfaces were more likely to carry the bloom-causing fats through the filling or baked goods being coated.
The fats slip in during the enrobing process, Danzl explained.
“As liquid chocolate flows like a curtain over the products, some of it sticks. Some flows off and gets tempered again within the machine, and washes off some of the [fats used in the chocolate fillings] with it.”
This backflow of filling fats is more severe in smaller machines, and thus likely to create more significant issues for small- and mid-sized manufacturers, said Danzl.
“We work a lot with chocolate companies and this is something that we do think can happen,” said Claudia Delbaere of Cacaolab, an experimental chocolate production facility at Ghent University in Belgium. “But this is the first [study] to my knowledge; there is no scientific publication about it yet.”
Unlike cocoa butter, the filling fats in some chocolate treats include a variety of oils from nuts, coconut oil or palm kernel oil, which can muddle tempered chocolate’s organized crystal lattices, making it softer and more crumbly. Using two different methods, the researchers measured softening of the chocolate and the proportion of fats from product filling in the crystallized chocolate. They found that the more filling fat there was within the chocolate, the softer it became and more likely to grow a cocoa bloom. The extent of this “contamination” was surprising, said Danzl. “In some cases, we found up to 10 percent of filling fats in the chocolate,” he said. “Normally, it is about half to 2 percent in most products.”
Filling fats flowing back during production is only one reason for fat bloom. Others include improper storage temperatures, prolonged storage periods, or simply mixing less compatible fats, such as coconut oil or palm kernel oil, into chocolate. The thickness of a chocolate shell or geometry of a piece of confectionery can also influence bloom formation.
“Because these products are so highly individual, it is difficult to model or predict [fat bloom formation],” said Danzl. “Now we know of one more factor that might influence certain products.”
Although the enrobing process itself is difficult to modify, chocolate makers may be able to tweak their recipes to reduce the amounts of incompatible fats, according to Delbaere, who was not involved with the study. She added that proving that filling fats can flow back into tempered chocolate in this way is “an important step for chocolate manufacturers.”
Jyoti Madhusoodanan is a science writer based in San Jose, Calif. She tweets at @smjyoti