Kinetic study of crystal colouration during crystallization of sucrose

Abstract

Although, existence of the coloured syrup layer surrounding the crystal surface is <i>fait accompli, </i>there is no definite explanation for the colour inside the crystal. In the present investigation, therefore, whether the colour is adsorbed or occluded or both, three types of artificial colouring matters; caramel, iron-phenol complex and reducing sugar-amino acid browning reaction products are used to examine the effect of each on crystal colouration. By static and dynamic analysis, it is concluded that different colourants have different affinities for the crystals and cause different colouration under the same condition; <i>viz. </i>caramel is the slowest, RS-AA browning product is intermediate and the iron-phenol complex is the fastest. At a slow rate of the crystallization (<i>k</i>=0.86-6.05×10⁵ mg/m²/h), colouration is caused mainly by adsorption. However, when the rate increases, this may also be accompanied by another mechanism such as occlusion, although adsorption is still in effect. The quantity of colouration decreases with rise in crystallization temperature, the extent of which decreases with increase in super-saturation. By reducing the amount of these colourants of higher affinity which are present in the ·mother liquor and under varying crystallization conditions, different effective measures may be adopted to overcome the colouration. Thermodynamic

data of the crystal colouration are also evaluated and data indicate a loss in the entropy change in the process.