2.5. NIR Measurements

For the NIR measurements, a Büchi NIRFlex N-500 spectrometer (Flawil, Switzerland) equipped with a solids cell attachment was used. The spectra were measured in the range from 10,000 to 4000 cm⁻¹ in diffuse reflection mode with a wavenumber accuracy of 2 cm⁻¹ and a relative reproducibility of 0.2 cm⁻¹. Each sample was measured 9 times with 64 scans with a spectral resolution of 8 cm⁻¹ that was automatically interpolated by the software “NIR Ware 1.4.3010” (Büchi, Flawil, Switzerland) to a data point interval of 4 cm⁻¹. Technical characteristics of the instrument are highlighted in Table 1. An external Teflon reference was measured after each new sample. The cuvettes used (Hellma GmbH & Co. Kg., Müllheim, Germany) had a technical path length of 2 mm and were made from Quartz SUPRASIL 300 providing transmission of >80% in a spectral range of 50,000 to 4000 cm⁻¹ (as indicated by the manufacturer). The samples were unfrozen and measured as soon as they reached room temperature. During measurement, elderberries were compressed using a pressing stamp and a spinner add-on was used for better consistency of measured spectra. (Figure 2).

Elderberries compressed for near-infrared (NIR) measurements.

Technical characteristics of Büchi NIRFlex N-500 spectrometer.