Amperometric enzyme electrodes for the determination of volatile alcohols in the headspace above fruit and vegetable juices

摘要

We have investigated two amperometric biosensors for the determination of volatile alcohols in the headspace of fruit juices and vegetable juices. One type of sensor is based on the use of alcohol dehydrogenase (ADH) and the detection of NADH (at +300 mV vs. Ag/AgCl with phenothiazine as redox mediator), and the other on the use of alcohol oxidase (AOx) and the detection of hydrogen peroxide (at +600 mV vs. Ag/AgCl). Samples were analyzed with the AOx-based biosensor by measuring the alcohol concentration in the headspace above the liquid without prior sorption or pre-concentration. The sensor has a linear response in the range 0.1–20.0 mM of alcohol (referred to the concentration in the liquid sample). It has excellent stability in that the signal decreases by 4.5 % only over a 60 h operational period. However, a comparison of the AOx-based biosensor with HPLC and an enzyme test kit revealed an overestimation of ethanol levels in juices by the biosensor due to the simultaneous detection of methanol present in the samples. A flow-through version of the biosensor placed at the exit of a HPLC system proved this assumption. In order to improve the specificity for ethanol, the ADH-based sensor was studied. While showing no cross sensitivity to methanol, its stability was rather limited, this making it not suitable for practical applications. Headspace analysis offers advantages such as high selectivity (because it can be interfered by volatile substances only) and the lack of contamination of the sensor by species in the liquid juice.