| Spray drying is a common method used in the dehydration of foods. However, volatile flavor and aroma components are lost from individual drops during drying. The goals of this project are to identify the mass-transfer mechanisms occurring within drops while drying. Above the boiling temperature, a complicated process known as morphological development occurs. In this process, bubbles grow and often burst, releasing volatile components.; Precise experiments were performed in which single drops of coffee extract and sucrose solutions were dried. A mock-flavor molecule, sulfur hexafluoride, was added to the solutions before drying. Up to four different types of measurements were taken simultaneously for each drop throughout the entire drying process: the instantaneous drying rate, the instantaneous loss rate of SF6, the drop temperature, and the physical appearance of the drop, as recorded by a videocamera.; Several important discoveries resulted from the experimental work. First, loss of SF6 into bubbles was found to be a liquid-phase diffusion-controlled process. Second, the overall rate of water loss during morphological development was found to be somewhat independent of the type of morphological development. Water loss during this regime is determined by gas-phase heat transfer, which is driven by the difference in temperature between the drying gas and the boiling temperature of the drop. This discovery led to a simple drying model that predicts the drying rate and temperature history of a drop during morphological development. Third, a new mass-transfer mechanism, called "ruptureless inflation", was discovered. In this mechanism, water is selectively removed during morphological development because the bubbles inside drops do not burst.; A numerical model was also developed to solve the convective-diffusion equations for SF6 and water in a drop of sucrose solution. The model drop included a single, centrally located bubble that changed size according to the drop temperature and water activity. A ternary model was used to treat the diffusion of SF6. The model predictions for instantaneous losses of water and SF6 from the drop were in good agreement with the experimental data. |
