## 3 Results and Discussion
### Maturity Index

Table 3: Maturity index values, fruit colors and maturity levels for hot peppers. fruits appearing mostly green (green), ripening fruits with a color change from green to orange (orange) and fully ripe fruits colored red to dark red (red). Immuture peppers were associated with a maturity index between 0.5 and 2. The rather strong variation can again be explained in terms of the low carotenoid and higher chlorophyll content, which gave rise to poor Raman spectra yielding low and hence somewhat noise-afflicted intensity values. The index values of ripening peppers, immature and harvestable, also varied, since at this stage, an uneven carotenoid distribution exists along the fruit and thus inhomogeneous color gradients were perceived. The spectrum and thus the index depended on the investigated spot on the fruit surface. Averaging over several test spots did not reduce the variations to a larger extent, as was shown above. However, a minimum value of about 2.3 could be recognized. Above this value, the analyzed fruits started to display a reddish gloss. The strong variation within the group of ripening fruits was hence due to the changing colors. Some fruits had a red tip but were green otherwise. However, other fruits exhibited a discoloration in the middle. The fully ripe state (red) was associated with a value of 4 or larger, while the index variation decreased significantly. The variation within this group could be traced back mainly to the aging process of pepper, as some fruits were already overripe. The lower carotenoid concentrations in tomatoes as compared to hot peppers can be traced back to the higher water content. The Raman intensities observed for both fruit species were therefore different. As a consequence, the tomato maturity index assumed a range different from the one obtained for hot peppers. Tomato fruits seemed immature up to a value of 1.5. Due to the strong uneven discoloration, a similar variability occurred for the ripening tomatoes as for the peppers. Unlike for peppers, a clear distinction between the individual categories could not be recognized, cf. Figure 6. In summary, the maturity index is specific for one fruit species. Values need to be determined specifically. Within this study, a total of seven different visual color classes were discerned for hot peppers. As a consequence of the spectroscopic performance, they were reduced to the four categories indicated in Table 3. For tomatoes, no similar detailed classification was achieved in contrast to previous studies [26; 27]. Three categories as depicted in Figure 6 were attainable at this level. Again, the ripening fruit stage was the least well defined in terms of the maturity index. The last 14 data points stemmed from overripe tomatoes, which, through the different rotting processes, led to dispersed values of the maturity index. Figure 6: Maturity index for each fruit sample calculated according to equation 1 for hot peppers (**left**) and tomatoes (**right**) at different maturity stages. Color coding corresponds to visually perceived green (green), orange to orange/red (orange) and red fruit (red), cf. Table 3.  
While the PCA model distinguished well between ripe and unripe hot peppers but lacked the intermediate stages, one to two ripening categories could be defined using the maturity index. Applying more sophisticated multivariate methods might overcome the disadvantage. The signal variability due to the biological variation in the sample might deteriorate the accuracy of such a chemometric model as well. Hence, the univariate maturity index might be a simple alternative. Raman spectral recording, peak picking, intensity extraction and average ratio computation are easily automated as well as single value comparison to the calibrated index range.