## 3 Results and Discussion
### Comparison of Raman and MIR Spectroscopy
The required cleaning of the crystal surface would cause considerable maintenance effort in a production plant. Gas bubbles can also have an influence on the evaluation of MIR-ATR and Raman spectra. Since the spectra of the calibration samples and the spectra for process monitoring were recorded at a constant aeration rate of 1.0 vvm, the effect of gas bubbles was largely compensated. Furthermore, neither the evaluation of MIR-ATR, nor Raman spectra was disturbed by the produced carbon dioxide. In the MIR spectrum, CO\({}_{2}\) exhibits two strong absorption bands at 2349 cm\({}^{-1}\) and 667 cm\({}^{-1}\)[4], which lie outside the evaluated spectral range from 850-1250 cm\({}^{-1}\). Raman peaks of CO\({}_{2}\) can be observed in the PLS range at 1282 cm\({}^{-1}\) and 1385 cm\({}^{-1}\)[40], but at very low intensities (see Figure 3) and without a spectral overlap with the regions for MLR evaluation. The measurement setup for process applications using Raman spectroscopy consists of an excitation laser, a fiber-optical Raman immersion probe, and a spectrometer. In contrast to MIR spectroscopy, quartz fibers with high light transmittance can be used in Raman spectroscopy, since the spectral range of the inelastically-scattered light lies in the visible or the near-infrared region depending on the wavelength of the excitation laser. Therefore, Raman spectroscopy combines the benefit of using quartz fiber optics with the high information content of MIR spectroscopy. The technological progress made in laser technology over the last few decades, as well as the development of efficient optical filters and highly-sensitive detectors led to an increased use of Raman spectroscopy during reaction monitoring [41]. Moreover, the penetration depth of the Raman laser beam into the sample (several millimeters) is higher by a factor of approximately 1000 in contrast to MIR-ATR setups (micrometers). Especially in the case of inhomogeneous samples, the higher optical path length could result in measuring representative sample volumes. Besides the fiber-optical Raman immersion probe, the measurement setup for process applications using Raman spectroscopy consists of a spectrometer and the excitation laser. The most common Raman lasers are the Nd:YAG laser (emission wavelength 1064 nm), GaAlAs laser diode (785 nm), Kr\({}^{+}\) laser (647 nm), HeNe laser (633 nm), frequency-doubled Nd:YAG laser (532 nm), and Ar\({}^{+}\) laser (488 nm or 514 nm) [42]. For recording the Raman spectra, either silicon charge-coupled device (Si CCD) array spectrometers or FT-NIR spectrometers with InGaAs or Ge detector are typically used. The type of spectrometer depends on the excitation wavelength of the laser and the wavelength range to be detected. Since the intensity of the Raman bands decreases with the fourth power of the laser wavelength, short excitation wavelengths lead to a high measuring sensitivity. Depending on the sample composition, short excitation wavelengths can cause a significant fluorescence background. As a consequence, only little fluorescence occurs when a Nd:YAG laser is used. However, the Raman signals are very weak when excited at 1064 nm. Therefore, an average laser wavelength of 785 nm represents a good compromise between measuring sensitivity and interfering fluorescence background for many applications. In Raman measurement systems, the 785-nm laser is often combined with a CCD spectrometer for the detection of light [17; 43; 44]. The penetration depth of the laser beam into turbid media is influenced by the light scattering of particles. Therefore, the measuring sensitivity of Raman spectroscopy can vary with particle diameter and concentration. In the case of yeast cells, cell densities (concentrations) below \(10^{8}\) cells per milliliter would cause nonlinearities in the calibration models. In this work, the Raman measuring sensitivity was approximately constant during the yeast fermentations, since cell densities were always above \(1.3\times 10^{8}\) cells per milliliter (biomass concentration 5 g L\({}^{-1}\)).