Fundamentals of spray freezing of instant coffee

The freezing of single 2 mm diameter droplets of coffee solution in cold moving air down to −15 °C has been studied for 10–50 wt.% coffee solutions in a novel temperature monitoring apparatus and in a similar device installed within a nuclear magnetic resonance (NMR) spectrometer. Heat transfer measurements and modelling confirmed that droplet internal temperatures were almost uniform (low Biot number regime), which was consistent with the microstructures generated. Fundamental parameters which could be extracted from the data included nucleation temperature, degree of recalescence, freezing temperature, and extent of freezing and freeze concentration. The mode of nucleation (spontaneous or forced) was observed to depend on concentration, cooling rate and air temperature.