Rock Sugar Crystallization: The Effect of Mineral Impurities

Rock sugar is a common Asian ingredient produced by recrystallizing refined sugar to form large sugar crystal agglomerates. The yield and quality of the rock sugar varies highly based on the refined sugar used. To investigate this, rock sugar recrystallization was conducted using six different commercial sugar samples. The purity and mineral profiles of the commercial sugars were analyzed by high-performance liquid chromatography (HPLC) and inductively coupled plasma—optical emission spectroscopy (ICP-OES). Pure sucrose was then doped with sodium, potassium, calcium and magnesium chloride respectively and recrystallization was performed using the doped sucrose. The results showed that rock sugar crystals made from low purity sugar samples are small (< 1 cm), while purer sugars (> 84%) produce larger crystals (2–5 cm). The yield of rock sugar is strongly correlated to the amount of trace minerals. In general, crystal size and yield decrease with increasing mineral levels. However, at low level of potassium (0.1%), crystallization yield increases. In general, commercial sugar samples with ~ 0.1% potassium and low level of calcium or magnesium will produce large rock sugar crystals with decent yields.