A bar of chocolate may seem simple, but producing its flavor, shine and satisfying snap requires remarkable control. Microbes help determine how the beans taste, carefully formed fat crystals create the right texture, and small changes in temperature can leave the surface covered in a pale film known as bloom.
Chocolate has been consumed as a drink for thousands of years, but the modern solid bar dates only to 1847. Even after nearly two centuries of commercial production, scientists are still investigating the chemical and physical changes behind it. Chocolate is "incredibly complex," says Caitlin Clark, a food scientist at Colorado State University.
Part of that complexity begins where cacao is grown. Different regions produce chocolate with distinctive fruity, floral or nutty notes. These differences have traditionally been attributed to plant genetics, soil and climate. More recently, however, researchers have discovered that fermentation may play a much larger role.
Each cacao pod contains 30 to 50 beans surrounded by white pulp. After harvesting, farmers place the beans in wooden boxes and allow them to ferment for about a week. Yeast first breaks down the pulp and consumes its sugars. Other microbes then arrive and produce acids, creating substances that influence the beans' final flavor.
By the end of fermentation, the beans can smell "very close to wet socks," according to plant geneticist David Gopaulchan. Fortunately, that smell fades as the beans dry.
Unlike the production of many fermented foods, cacao fermentation is only loosely controlled. Farmers may stir the beans occasionally, but different microbes are otherwise free to appear and disappear. The result can be highly unpredictable. For scientists, Gopaulchan says, the process has long resembled a "black box."
His team examined the genetic material of the microbes involved at each stage of fermentation. The researchers then trained a computer model to connect particular groups of microbes with particular flavors. Some acid-producing bacteria were associated with fruitier notes, while certain yeasts were linked to the familiar taste of cocoa. The team was later able to reproduce some of these flavors in the laboratory.
Fermentation is essential. Gopaulchan has made chocolate from beans that were not fermented, and the result was hardly tempting. "It's like eating leaves," he says. "It's very green."
Flavor is only part of chocolate's appeal. Texture depends largely on cocoa butter, the natural fat found in cacao beans. During production, cocoa butter is combined with refined cocoa, sugar and sometimes milk powder. The mixture then goes through tempering, a carefully controlled series of heating and cooling stages.
The amount of cocoa butter affects how easily the chocolate flows. More cocoa butter makes the mixture thinner, allowing it to reach the smallest details of a decorative mold. More sugar and milk particles make it thicker, helping chocolate chips and drop-shaped pieces remain upright as they cool.
Cocoa butter also gives a finished bar its shine, clean snap and ability to melt in the mouth. To create those qualities, its fat molecules must form the correct crystal structure. During tempering, the liquid fats arrange themselves into a solid network. Clark compares the process to people "linking arms, forming a chain."
Cocoa butter can form six different kinds of crystals, but only one produces the glossy chocolate that breaks sharply and melts close to body temperature. Because traditional tempering is slow and demanding, researchers are exploring possible shortcuts. Some are testing ultrasound waves that encourage the molecules to form the desired structure. Others are adding small amounts of particular fats to give crystals more places to begin growing.
Yet the same crystal structure that makes chocolate attractive can also create problems. One common explanation for white marks on chocolate is fat bloom. This pale coating is often mistaken for mold, but it forms when cocoa butter moves through the chocolate and recrystallizes on the surface.
Fat bloom usually appears during storage or transportation, especially when temperatures are warm or frequently changing. Some fat molecules remain liquid inside the chocolate. As the temperature rises and falls, they may travel toward the surface and form a duller, more stable type of crystal. Bloom can also slow the way chocolate melts, giving it a dry or chalky texture.
Food scientist Jiayang Jin investigated whether the shape of sugar crystals could affect this process. She added water to the chocolate mixture and then heated it until the moisture disappeared, smoothing the normally sharp surfaces of the sugar crystals. Chocolate made with these rounder particles developed bloom more slowly, possibly because the particles packed together more tightly and made it harder for liquid fat to reach the surface.
Careful tempering and stable storage temperatures can delay bloom, extend shelf life and reduce waste. Even so, scientists still do not fully understand why it forms.
The science of making chocolate may be precise, but enjoying it is more personal. During tastings, Clark recommends experiencing several textures at once, with some chocolate melted and some still solid. The more forms it takes in the mouth, she says, the more there is to notice.