Temperature control facilitates chocolate quality.

The migration of oil from high-oil-content centers into chocolate coatings can change the quality of the chocolate. Such changes in quality may include a loss of texture in the chocolate and filling, and the onset of fat bloom on the chocolate's surface.

Historically, this oil migration has been thought to be primarily diffusion-driven. But recent testing doesn't indicate that the migration is being driven solely by differences in chemical potential. Some believe that capillary forces are an important mechanism. The relative importance of capillary forces and diffusion may depend on storage temperatures, since the underlying structure of the solid phase changes dramatically from 20 C to 30 C.

Scientists at the University of California at Davis examined the effects of temperature on fat migration in a model chocolate-peanut butter system. Their work reinforces the importance of controlling temperature to maintain the quality of filled chocolates.

In experiments, the investigators used a model system that consisted of a 1.25-cm peanut butter filling layer over a 1.25-cm layer of chocolate. Researchers stored three samples at 20 C, 25 C and 30 C. They used magnetic resonance imaging to measure liquid oil signal as a function of position over six weeks.

The pattern of oil migration differed significantly as a function of temperature in the three sets of samples. At 20 C, oil migration in the chocolate phase was slight. The dominant migration involved peanut oil separating at the top of the filling layer. At 25 C, oil migration into the chocolate phase exhibited a qualitative profile primarily involving Fickian-based diffusion--the basic relationship used to describe the nonconvective mass flux of a solute in a porous medium. A gradual gradient of liquid oil formed from the peanut butter into the chocolate.

At 30 C, the gradient in the liquid oil from the peanut butter into the chocolate changed rapidly. The liquid oil fraction in the entire chocolate region increased. This overall increase and the lack of concentration gradient are not consistent with a Fickian-based diffusion mechanism.

Higher temperatures do not give better control over oil migration. When two oil-containing layers are next to each other, as in peanut butter cups, the peanut goes into the chocolate faster at 30 C than at 20 C. And generally, consumers agree that the quality is lower with hard peanut butter filling and softer chocolate.

Further information. Kathryn L. McCarthy, Department of Food Science and Technology, University of California, One Shields Ave., Davis, CA 95616; phone: [phone]; fax: [phone]; email: [e-mail].