Brix - Measurement

Measurement

As specific gravity was the basis for the Balling, Brix and Plato tables dissolved sugar content was originally estimated by measurement of specific gravity using a hydrometer or pycnometer. In modern times hydrometers are still widely used but where greater accuracy is required an electronic oscillating U-tube meter will be employed. Whichever means are used, the analyst enters the tables with specific gravity and takes out (using interpolation if necessary) the sugar content in percent by weight. If he uses the Plato tables (maintained by the American Society of Brewing Chemists) he reports in °P. If using the Brix table (the current version of which is maintained by NIST and can be found on their website) he reports in °Bx. If using the ICUMSA tables, he would report in mass fraction (m.f.). It is not, typically, actually necessary to consult tables as the tabulated °Bx or °P value can be printed directly on the hydrometer scale next to the tabulated value of specific gravity or stored in the memory of the electronic U-tube meter or calculated from polynomial fits to the tabulated data. Both ICUMSA and ASBC have published suitable polynomials, in fact the ICUMSA tables are calculated from the polynomials. The opposite is true with the ASBC polynomial. Also note that the tables in use today are not those published by Brix or Plato. Those workers measured true specific gravity reference to water at 4 °C using, respectively, 17.5 and 20 °C, as the temperature at which density of the sucrose solutions was measured. Both NBS and ASBC converted to apparent specific gravity at 20°C/20°C. The ICUMSA tables are based on more recent measurements on sucrose, fructose, glucose and invert sugar and tabulate true density and weight in air at 20 °C against mass fraction.

Dissolution of sucrose and other sugars in water changes not only its specific gravity but its optical properties in particular its refractive index and the extent to which it rotates the plane of linearly polarized light. The refractive index, nD, for sucrose solutions of various strengths by weight has been measured and tables of nD vs. °Bx published. As with the hydrometer, it is possible to use these tables to calibrate a refractometer so that it reads directly in °Bx. Calibration is usually based on the ICUMSA tables, but the user of an electronic refractometer should verify this.

Sugars also have known infrared absorption spectra and this has made it possible to develop instruments for measuring sugar concentration using NIR (Near Infra Red) and FT-IR (Fourier Transform Infrared Spectrometry) techniques. In the former case, in line instruments are available which allow constant monitoring of sugar content in sugar refineries, beverage plants, wineries, etc. As with any of the other instruments NIR and FT-IR instruments can be calibrated against pure sucrose solutions and thus report in °Bx but there are other possibilities with these technologies as they have the potential to distinguish between sugars and interfering substances.