Preparation of sugar solution. The concentrated sugar solution was prepared by mixing aqueous solutions of D-Fructose (31.5% w/w) and D-Glucose 10.5% (w/w). The mixture was heated to 65 °C with constant stirring until a homogeneous solution was obtained. To this solution 10 ppm (by weight) of sodium azide was added that prevents growth of bacteria. The final solution was stored in a refrigerator, but was brought to room temperature before further use.