Convenient quantification of methanol in juices by methanol oxidase in combination with basic fuchsin

Combination of methanol oxidase (MOX) and basic fuchsin was confirmed effectively in determining methanol content in a model and real systems. The optimal reaction conditions for 20 ppm formaldehyde–0.1% basic fuchsin mixture were determined to be at 35 °C for 2 h in 0.25 N HCl with a maximal absorption wavelength of 560 nm, while those for MOX (0.8 unit MOX/mL)–methanol mixture were at 25 °C for 30 min. Gas chromatography (GC) also confirmed the method developed with an accuracy of >95%. Presence of food additives such as sulfite (100 ppm) interfered greatly (−92%) with the quantification of methanol, while fruit juice components, galacturonic acid, pectin, glucose, did not apparently interfere the quantification results of methanol. Ethanol (>100 ppm) presented competitive inhibition with methanol on MOX. In real samples, fresh fruit juices such as Sunkist, water melon, carrot, carambola, melon, tomato, and papaya were detected to contain 8, 31, 36, 17, 8, 42, and 38 ppm methanol, respectively, with an accuracy of 93–97%, as compared to that determined by a GC, suggesting the feasibility of MOX–basic fuchsin method developed for juice industry.