In-Situ X-Ray Studies of Cocoa Butter Droplets Undergoing Simulated Spray Freezing

Spray freezing offers a novel manufacturing route to fine powders with controlled crystalline structures. Here we simulate this process by freezing (using a cold dry air flow) suspended 2-mm diameter droplets of cocoa butter such that X-ray diffraction observation of the droplet's evolving crystalline structure is possible in situ. Initially the Form I polymorph is observed in the droplets: this transforms to Form II over a few minutes and then to Form III over a few hours, even at 0 degrees C. If the droplet is then warmed to 24 degrees C, further transformation to Form IV and then Form V occur over approximately 2 h. These phase transformations are similar to those which would be expected in a bulk cocoa butter sample, but occur significantly faster in the droplets. Small crystal sizes in the frozen droplet, resulting from the droplet's low Biot number (and thus even temperature distribution), is postulated as being the cause for the unexpectedly rapid evolution in the crystal habit.