Issue Date: August 25, 2008
It’s one of the most wrenching deflation-elation-deflation cycles for chocoholics: They first suspect they might be living in a chocolate-free house, then frantically search the kitchen for an overlooked piece of their desire and actually find one, only to discover that the coveted viand is covered with a grayish-white film, or what chocolate industry insiders call “FAT BLOOM.” Worse than the aesthetic turnoff is the violence the bloom can do to the chocolate’s texture, which can make a piece of chocolate unappealing even to an addict.
“This is a multi-billion-dollar problem,” laments nutritionist and food structure scientist Dérick Rousseau of Ryerson University, in Toronto.
In what stands as an act of great compassion as well as the best possible way to put an atomic force microscope (AFM) to use, Rousseau partnered with food technologist Sopark Sonwai of Silpakorn University, in Thailand, and secured more than 750 bars of milk chocolate specially formulated for the researchers by Cadbury Adams Canada. They then examined, by way of their AFM and several other analytical tools, how a fat bloom comes to violate the surfaces of chocolate. They report their results in the journal Crystal Growth (DOI: 10.1021/cg070503h).
“Chocolate is highly dynamic,” Rousseau says. “As soon as it leaves the production line, it starts to evolve.” Fat phases ooze around and sugar and protein crystals grow slowly, for example. “I like to compare this to plate tectonics,” Rousseau says.
The researchers’ goal is to uncover what makes different proportions of cocoa butter, milk fat, and cocoa butter equivalents (CBEs)—vegetable fats such as palm kernel oil masquerading as cocoa butter—more or less prone to forming fat blooms as chocolate sits on a shelf. Chocolate makers turn to CBEs because, compared with cocoa butter, these ingredients can be more uniform, cheaper, and more reliably procured. But they also affect the fat-bloom issue.
The fat-bloom conundrum amounts to a materials science problem in which the challenge is to identify storage conditions, such as time and fluctuating temperatures, and details of chocolate formulation that are related to the formation of “light diffusing triglyceride fat crystals >5 µm in length,” which is Rousseau’s and Sonwai’s way of saying “fat bloom.”
The scientists’ primary finding is that the stage for fat bloom is set when pores, cracks, and other flaws form in newly poured chocolate’s matrix, which they describe as “sugar crystals, milk solids, and cocoa solids interspersed within a continuous fat phase consisting of crystalline and liquid fat.”
During storage, lower melting fat components—in the chocolate matrix or the oil-rich filling that it may envelope—percolate to the surface, forming volcano-like cones several micrometers across at the base. From their microtopographic AFM data, the researchers conclude that the cones “act as nurseries for surface crystal growth.” That’s a drag on chocolate’s mission, which is to bestow upon its consumer a fully pleasurable combination of taste and, in food-industry parlance, mouthfeel.
One potentially useful pointer for chocolatiers seeking to offer products with longer shelf lives is that formulations with up to 30% milk fat along with the legal limit (5%) of cocoa butter equivalents are bloom-resistant compared to formulations with less of these fats.
In their paper, the researchers noted, with a euphemism usually reserved for biologists, that “all samples were sacrificed following analysis.” When asked for clarification on this dubious claim, Rousseau first quipped that he and Sonwai “put the samples on a pole and lit a fire underneath.” Then he got serious, saying “we simply did not reuse the samples after they had been analyzed once.” Then he even told the truth: “Actually, I have had chocolate samples ‘mysteriously’ disappear from the lab.”
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