Determination of Original Gravity and Alcohol Content of Dark Starkbiers with the Refractometer

Peter Koestler and W. Hagen. Poster presented at the MBAA 111th Anniversary Convention, Minneapolis, Minnesota, September, 1998. Full paper previously published in BRAILIWELT, Issue 46/47, 137, 1997 - presented as a poster with permission from BRAILIWELT.

Abstract
The advantage of the determination of original gravity of beer with the refractometer procedure lies generally in its cheap, fast and simple performance in actual practice. Formerly used nomograms (as the ones of Schild or Gerum and Wissner) are more and more replaced by regression formulas, as described in the Brewing Technology Analysis Methods by MEBAK (Central European Brewing Technology Analysis Commission). The main reasons are the possible integration in electronic data processing systems and the higher accuracy in comparison to the distillation procedure as the official reference method. Also one avoids false readings that can occur through the handling of the nomograms. These are based on formulas developed in the 1950s by using a refractometer made by the Zeiss Company, Jena, Germany. The divergences in the readings caused by the use of a flow-through refractometer by Zeiss, Oberkochen, Germany, were so significant that they led to an adjustment by new formulas for extract, alcohol and original gravity determination. But these were still not satisfactory for dark Starkbiers, which showed up to 0.98% divergence in original gravity and 0.49% (by volume) in alcohol content. The results for real extract correlated well with the reference method. It could be shown that the influence of the original gravity and the beer color were statistically highly significant. The higher the o.g. and the color the higher was the divergence. This linear relation enabled the use of the multiple linear regression calculation with the analysis data of 34 dark Starkbiers, which led to the suggestion of new regression formulas for the determination of alcohol content and original gravity of dark Starkbiers.
Keywords: original gravity, alcohol content, refractometer procedure, reference method, dark starkbiers  

Sintesis
Las ventajas de la determinacion de gravedad original de la cerveza con el procedimiento de refractometro esta en que es barato, rapido y simple de realizar en la practica. Nomogramas anteriormente utilizados (como los de Schild o Germurn y Wissner) estan cada vez mas siendo reemplazados por formulas de regresion, como estan descritas en los Metodos de Analisis Cerveceros de MEBAK (Comision Central Europea de Analisis Tecnico). Las principales razones son la posible integracion en sistemas de procesamiento, de datos electronicos y el nivel mas alto de exactitud en comparacion al procedimiento de destilacion como el metodo oficial de referencia. Adicionalmente uno evita lecturas erroneas que pueden ocurfir por el manejo de los nomogramas. Estos estan basados en formulas desaffolladas en los afios cincuenta usando, un refractometro hecho por la compaiiia Zeiss en Jena, Alemania. Las divergencias en las lecturas causadas por el uso de un refractometro de flujo por medio de Zeiss, Oberkochen, Alemania, eran tan significativas que Ilevaron a un ajuste de nuevas formulas para la determinacion de extracto, alcohol y gravedad original. Pero estos ajustes aun no eran satisfactorios para Starkbiers oscuros, que daban divergencias de hasta 0.98% en gravedad original, y 0.49% (por volumen) en contenido de alcohol. Estos resultados para extracto real correlacionaban bien con el metodo de referencia. Se puede demonstrar que la influencia de la gravedad original y del color de la cerveza son estadisticamente altamente significativos. Mientras mas alta la gravedad original y el color mas alta era la divergencia. Esta relacion lineal nos permitio el uso de calculo de multiples regresiones lineares con el analisis de datos de 34 Starkbiers oscuros, los cuales nos Ilevaron a la sugerencia de nuevas formulas de regresion para la determinacion de contenido de alcohol y gravedad original de Starkbiers oscuros.