Analysis of volatile alcohols in apple juices by an electrochemical biosensor measuring in the headspace above the liquid

• Published in: Sensors and actuators. B, Chemical Chemical, 2011-11, Vol.158 (1), p.313-318
• Main Authors: Hämmerle, Martin, Hilgert, Karin, Horn, Marcus A., Moos, Ralf
• Format: Article
• Language: English
• Subjects: absorption; alcohol dehydrogenase; Alcohol oxidase; Amperometric biosensor; Apple juice; biosensors; detection limit; electrochemistry; electrodes; Ethanol; Gas-phase sensor; gases; headspace analysis; high performance liquid chromatography; hydrogen peroxide; Malus; Methanol; Pichia pastoris; platinum; polytetrafluoroethylene; qualitative analysis; refractive index
• ISSN: 0925-4005; 1873-3077
• DOI: 10.1016/j.snb.2011.06.026

An electrochemical biosensor was optimised for the analysis of volatile alcohols directly from the gas phase without prior absorption or pre-concentration. The sensor is based on the alcohol oxidase ( Pichia pastoris) catalyzed conversion of ethanol and the amperometric detection of the generated hydrogen peroxide. Key part of the three-electrode set-up was a gas-diffusion working electrode (potential: +600 mV vs. Ag/AgCl) that consisted of a porous Teflon membrane coated with a thin platinum layer. Headspace samples were analysed for alcohols and used to derive alcohol concentrations in the liquid phase. The biosensor had a sensitivity of 3.43 μA/mM for ethanol, a response time of 69 s, a linear dynamic range of 0.10–30 mM, a theoretical detection limit (3 < S/N) of 9.9 μM, and a stability of 86% during continuous operation (18 h @ 1 mM ethanol). Using one sensor on three consecutive days, the mean coefficient of variation was 1.3% (three measurements each day @ 10 mM ethanol). Alcohol contents of three apple juices determined with the biosensor were in the range 0.30 g/l–0.67 g/l (equivalent to 6.51 mM–14.5 mM). However, ethanol contents determined by high pressure liquid chromatography coupled to refractive index detection (HPLC-RI) and by a commercial enzyme test kit based on alcohol dehydrogenase ranged from 0.12 g/l to 0.38 g/l (equivalent to 2.60 mM–8.25 mM). Both indicate that the biosensor detected alcohols other than ethanol in the apple juices. HPLC-RI coupled to the biosensor in a flow-through configuration demonstrated that the biosensor detected methanol concomitant to ethanol. Thus, the biosensor could perform a qualitative analysis of the total content of volatile alcohols in apple juices by analysing the gas phase above the sample. This offers the additional advantage that possible, non-volatile interfering substances in the liquid sample cannot impair the measurement.