## 3 Results and Discussion
### Comparison of Raman and MIR Spectroscopy
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & **Regression** & **Quality** & \multicolumn{3}{c}{**Raman System**} & \multicolumn{3}{c}{**MIR-ATR System**} \\  & **Model** & **Parameter** & **Glucose** & **Ethanol** & **Biomass** & **Glucose** & **Ethanol** & **Biomass** \\ \hline \multirow{6}{*}{A} & \multirow{2}{*}{2D MLR} & RMSEC (g L\({}^{-1}\)) & 1.83 & 0.68 & 0.86 & 1.06 & 0.61 & 0.66 \\  & & RMSEP (g L\({}^{-1}\)) & 1.86 & 0.71 & 0.86 & 1.18 & 0.77 & 0.73 \\ \cline{1-1} \cline{2-9}  & & RMSEC (g L\({}^{-1}\)) & 1.03 & 0.47 & 0.33 & 0.87 & 0.31 & 0.44 \\ \cline{1-1} \cline{2-9}  & & RMSEP (g L\({}^{-1}\)) & 1.08 & 0.50 & 0.36 & 0.97 & 0.38 & 0.47 \\ \cline{1-1} \cline{2-9}  & & PIS without PP & RMSEC (g L\({}^{-1}\)) & 0.90 & 0.41 & 0.30 & 1.00 & 0.45 & 0.47 \\ \cline{1-1} \cline{2-9}  & & RMSEP (g L\({}^{-1}\)) & 0.92 & 0.39 & 0.30 & 0.68 & 0.48 & 0.37 \\ \cline{1-1} \cline{2-9}  & & PLS with PP & RMSEC (g L\({}^{-1}\)) & 0.78 & 0.56 & 0.50 & 0.86 & 0.30 & 0.36 \\ \cline{1-1} \cline{2-9}  & & RMSEP (g L\({}^{-1}\)) & 0.76 & 0.56 & 0.48 & 0.76 & 0.28 & 0.33 \\ \hline \multirow{6}{*}{B} & \multirow{2}{*}{2D MLR} & RMSEC (\%) & 6.1 & 6.8 & 5.7 & 3.5 & 6.1 & 4.4 \\  & & RMSEP (\%) & 6.2 & 7.1 & 5.8 & 3.9 & 7.7 & 4.9 \\ \cline{1-1} \cline{2-9}  & & RMSEC (\%) & 3.4 & 4.7 & 2.2 & 2.9 & 3.1 & 2.9 \\ \cline{1-1} \cline{2-9}  & & RMSEP (\%) & 3.6 & 5.0 & 2.4 & 3.2 & 3.8 & 3.1 \\ \cline{1-1} \cline{2-9}  & & RMSEC (\%) & 3.0 & 4.1 & 2.0 & 3.3 & 4.5 & 3.2 \\ \cline{1-1} \cline{2-9}  & & RMSEP (\%) & 3.1 & 3.9 & 2.0 & 2.3 & 4.8 & 2.4 \\ \cline{1-1} \cline{2-9}  & & PLS with PP & RMSEC (\%) & 2.6 & 5.6 & 3.3 & 2.9 & 3.0 & 2.4 \\ \cline{1-1} \cline{2-9}  & & RMSEP (\%) & 2.5 & 5.6 & 3.2 & 2.5 & 2.8 & 2.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of calibration and validation results applying Raman and MIR-ATR spectroscopy for real-time monitoring of yeast fermentations. **(A)** Calibration (RMSEC) and prediction (RMSEP) errors in g L\({}^{-1}\) for glucose, ethanol, and biomass. **(B)** RMSEC and RMSEP in percent with respect to the maximum concentration of glucose (30 g L\({}^{-1}\)), ethanol (10 g L\({}^{-1}\)), and biomass (15 g L\({}^{-1}\)). and 3% for ethanol were obtained by [PERSON] et al. [20] applying 4D MLR models for the evaluation of Raman spectra. An MLR model was also used in [21] to calibrate the developed MIR-ATR sensor and a commercial FT-MIR spectrometer for monitoring of yeast fermentations. In that study, the glucose concentrations were predicted with a relative error of 6.2% in the case of the ATR sensor and 4.3% for the FT-MIR spectrometer setup. For the prediction of ethanol, the relative errors were 4.5% and 2.0%, respectively. For the evaluation of MIR-ATR spectra, [PERSON] et al. [22] developed a PLS method that was able to predict glucose and ethanol concentrations with relative errors of 0.9% and 2.2%, respectively. [PERSON] et al. [23] analyzed drawn samples from anaerobic yeast fermentations using an FT-MIR and an FT-Raman spectrometer. The initial glucose concentration of approximately 50 g L\({}^{-1}\) was converted into an ethanol concentration of 20 g L\({}^{-1}\) during the process. PLS models based on pre-processed MIR spectra generated the smallest error of prediction, which amounted to 1.8 g L\({}^{-1}\) for glucose (corresponding to 4% relative error with respect to the maximum concentration), 1.2 g L\({}^{-1}\) (6%) for ethanol, and 0.6 (20%) for the optical cell density. In the case of Raman spectroscopy, the weak intensity bands resulted in higher calibration errors in comparison to MIR spectroscopy. In that study, validation results (RMSEP values) for Raman were not given. This study combined real-time monitoring of yeast fermentations using Raman and MIR-ATR spectroscopy. For both spectroscopic setups, the time-dependent concentration courses in Figure 5, and the quality parameters in Table 2 were in good agreement with a slight advantage for MIR-ATR spectroscopy.