*2.6.3. Acetoin concentration determination*

Acetoin was determined by the modified Voges-Proskauer (VP) reaction of Westerfield [30]. An aliquot of the sample solution was pipetted into a 25 cm3 calibrated flask. A total of 2.5 cm3 of 1-naphthol solution and 1.0 cm3 of creatine solution were added. After adjusting the mixture to volume and shaking vigorously, the solution was kept at 30°C. The color intensity of the complex was determined by measuring the absorbance after 40 min at 530 nm using a UV–Visible Spectrometer of 2020 GBC Cintra model [31].
