Petr Vesely (1,2), Antonia Volgyi (1), Lance T. Lusk (1), Gabriela Basarova (2), Alfonso Navarro (1), John Seabrooks (1), and David Ryder (1). 1. Miller Brewing Company, 3939 W. Highland Boulevard, Milwaukee, Wisconsin, 53208, U.S.A. 2. Institute of Chemical Technology, Technicka 5, Prague 6, 166 28, Czech Republic.

Abstract
Yeast extracellular esterase is mostly destroyed during pasteurization. Aseptically packaged, unpasteurized beer, which has become an effective alternative for providing microbiologically stable, packaged beer, does not inactivate esterase activity. The effect of esterase activity on the volatile compound composition and flavor stability of aseptically packaged, unpasteurized beer during storage was studied. Analysis of beer volatile compounds using an SPME/GC/MSD method (solid-phase microextraction, separated by gas chromatography, and detected by a mass selective detector) showed that octyl acetate, its level significantly decreased in beer during storage at 30°C, was a good substrate for yeast esterase. Esterase also acted on ethyl hexanoate, which resulted in slightly increased levels of hexanoic acid. However, since the flavor changes resulting from oxidative and other degradation reactions that occur in beer during aging have a more profound impact than those brought about by yeast esterases, a sensory panel was not able to detect a flavor difference between this unpasteurized beer and pasteurized beer stored at 30°C for up to 12 weeks.
Keywords: esterase, flavor stability, SPME/GC/MSD

Síntesis
La esterasa extracelular de la levadura es destrozada casi completamente durante la pasteurización. La cerveza sin pasteurizar, envasada asépticamente (una alternativa efectiva para mercadear cerveza envasada microbiológicamente estable), mantiene su actividad de esterasa completa. Se estudió el efecto de la actividad de la esterasa sobre los compuestos volátiles y la estabilidad sensorial de cerveza durante el almacenamiento de cerveza no pasteurizada envasada asépticamente. El análisis de compuestos volátiles de cerveza mediante SPME/GC/MSD (microextracción de fase sólida separado por cromatografía de gases y detectado mediante un detector selectivo de masa) mostró que el acetatato octílico (con su nivel reducido en cerveza almacenada a 30°C), era un buen sustrato para esterasa proveniente de la levadura. La esterasa también actúa sobre hexanoato de etilo, dando niveles levemente aumentados de ácido hexanóico. Sin embargo, un panel sensorial no pudo distinguir una diferencia entre cerveza pasteurizada y no pasteurizada almacenadas a 30°C por hasta 12 semanas, debido a que los cambios sensoriales ocasionados por reacciones oxidativas y degradativas durante el almacenaje tienen un mayor impacto sensorial que aquellos causados por la esterasa proveniente de la levadura.
Palabras claves: esterasa, estabilidad sensorial, SPME/GC/MSD