In sugar factories, the purity analysis of liquid sugar is a fundamental element of process monitoring and quality control. Continuous supervision of impurities such as non-sugar components and residual by-products enables the early detection of deviations during clarification, crystallization, and refining. Consistent purity measurement not only supports compliance with industry quality standards but also contributes to optimized processing efficiency and uniform product quality for further applications.
Sugar purity is generally defined as the ratio of sucrose content to the total dissolved solids in a sugar solution, expressed as a percentage. In industrial practice and laboratory analysis, this value is obtained by combining polarimetric and refractometric measurements in accordance with ICUMSA® recommendations (GS5-1, GS5-2, GS7-31).
There are two different methods:
1. Schmitz table:
One commonly applied approach is based on standardized reference data known as the Schmitz table method. In this method, the optical rotation of the sugar solution is measured to determine the sucrose-related value, while the refractive index is used to quantify the dry substance content in Brix. The values in that table from ICUMSA data are approximated by the formular
%Pol = (Factor1 – Factor2 · Brix) · Pol
and the purity is defined as
Purity = %Pol / Brix · 100.0
2. Direct method
An alternative approach is the direct method, which is particularly suitable for prepared sugar solutions. In this procedure, the original solution is analyzed to determine its dry substance content in Brix. A second, diluted standard solution is prepared, typically by diluting a defined amount of sugar solution with distilled water, and measured to obtain the optical rotation value. The purity is subsequently calculated using the formula
Purity = °Z / °Brix × 100.
This method provides a straightforward and reliable means of purity determination while also allowing continuous calculation based on live measurement values. Both methods are recognized in the sugar industry and form the analytical basis for accurate purity control in production and laboratory environments.
The Purity Analyzer from SCHMIDT + HAENSCH offers an integrated and automated solution for sucrose purity measurement. The system combines polarimetric and refractometric measurement principles within a modular platform that enables simultaneous determination of sucrose content, dry substance concentration, and calculated purity. The polarimetric component of the system can be either a Saccharomat® quartz-wedge polarimeter or a Polartronic circular polarimeter, while the refractometric measurement is performed using the ATR P refractometer. All measured values are processed through a unified electronic control unit and displayed via a single user interface, ensuring ease of operation and clear data visualization. The system performs automatic purity calculations according to both the Schmitz table and direct methods in full compliance with ICUMSA® standards.
The system continuously verifies that the polarimetric values and refractometric values remain within the defined validity ranges to ensure reliable purity calculations:
1.0 < °Z < 110.0
1.0 < Brix < 28.5
0.06 < °Z / Brix < 4.5
By eliminating manual calculations and minimizing the risk of operator errors, the Purity Analyzer delivers high analytical precision, reproducibility, and speed.
