2.3. Analytical determinations

Fourier-Transform-Middle-Infrared-Spectroscopy (FT-MIR) was used to assess total extract, density, pH, glycerol and SO₂ in initial grape juice and wines. The method was used according to Baumgartner et al., 2001, Patz et al., 1999 and the Standard Operating Procedure SOP-WG1-84 of the HGU’s Department of Beverage Research.

Measurements of non-volatile organic acids, ethanol and residual sugars were performed by HPLC (High Performance Liquid Chromatography) according to Schneider, Gerbi, and Redoglia (1987) with modifications of Semmler, Sponholz, and Rauhut (2017). For this purpose, the '1100 Series' system by Agilent Technologies (Santa Clara, USA) equipped with an Allure Organic Acids column (Restek GmbH, Germany) (250 mm × 4.6 mm I.D. × 5 μm grain size × 60 Å pore size) preceded by a 4 mm × 3.0 mm I.D precolumn (Security Guard C18™, Phenomenex, Germany) was used. Detection was performed using a refractive index detector (RID) and a multi wavelength detector (MWD). The samples were analysed in scan mode.

The quantification of esters, higher alcohols and fatty acids were performed with a modified method of Rapp, Yavas, and Hastrich (1994) using a ‘GC 5890 Series II’ gas chromatograph (Hewlett-Packard, Palo Alto, USA). For the sample preparation, 2 g sodium chloride (Carl Roth, Karlsruhe, Germany) were weighed into a 15 mL sample vessel and 10 mL wine was added. 10 µL of the internal standard 2,6-dimethyl-5-hepten-2-ol (DMH) (stock concentration 1219 µg L⁻¹) (Carl Roth, Karlsruhe, Germany) was added for the quantification, 10 µL of the internal standard cumol (Honeywell, Morris Plains, USA) (stock concentration 170 µg L⁻¹) as control and 160 µL of 1,1,2-trichlorotrifluoroethane (Merck, Darmstadt, Germany) was added as extractant. The mixture was agitated for 20 min and centrifuged for 8 min (3000 rpm; 1700 g). The extract was removed with a glass pipette and transferred to a sample vessel for analysis. The cold feed system 'KAS 3′ (Gerstel, Mülheim an der Ruhr, Germany) was used for sample feeding. 2 µL of sample was injected in splitless mode (start temperature = 40 °C, heating rate: 3 °C min⁻¹ to 125 °C, holding time: 4 min and 6 °C min⁻¹ to 200 °C, holding time: 14.2 min). It was equipped with a Varian VF-5MS Agilent column (Santa Clara, USA) with the dimensions of 60 m × 320 µm × 1 µm. Helium (Linde Gas, Bingen, Germany) was used as carrier gas at a flow rate of 1 mL min⁻¹. The detection was performed by mass spectrometry ('5972 MSD', Hewlett-Packard) in scan mode covering a mass-to-charge ratio from m/z 35 to 250. Voltage of electron – impact was set at 70 mV. The analysis of the sulphur compounds was carried out using the method according to Rauhut, Beisert, Berres, Gawron-Scibek, and Kürbel (2005) with the modification according to Beisert and Rauhut (2017). Free monoterpenes and C13 – norisoprenoids were quantified using a HS-SPME-GC-MS according to Câmara, Alves, and Marques (2006) adapted by Brandt, Scheidweiler, Patz, Rauhut, Zorn, and Stoll (2018)