The concentration of glucose, fructose, glycerol, ethanol, and acetic acid was determined using a Surveyor Plus Liquid Chromatograph (Thermo Fisher Scientific, Waltham, MA) equipped with a refraction index and a photodiode array detector (Surveyor RI Plus and Surveyor PDA Plus, respectively) on a 300 × 7.7 mm HyperREZTM XP Carbohydrate H+ (8 μm particle size) column and guard (Thermo Fisher Scientific). The column was maintained at 50 °C, and 1.5 mM H2SO4 was used as the mobile phase at a flow rate of 0.6 mL/min. Prior to injection in duplicate, the samples were filtered through 0.22 μm pore size nylon filters (Micron Analitica, Madrid, Spain) and diluted 10-fold in MilliQ water.
Yeast assimilable nitrogen in natural grape must was determined spectrophotometrically as the sum of the contributions of free ammonium and free amino groups. Ammonium was assayed using a specific R-Biopharm assay kit (Darmstadt, Germany). Free amino groups were determined with ο-phthaldialdehyde (Dukes and Butzke 1998 ).
Yeast assimilable nitrogen in natural grape must was determined spectrophotometrically as the sum of the contributions of free ammonium and free amino groups. Ammonium was assayed using a specific R-Biopharm assay kit (Darmstadt, Germany). Free amino groups were determined with ο-phthaldialdehyde (Dukes and Butzke 1998 ).