## 3 Results and Discussion
### Browning Index  
The browning index (BI) is an easily determined indicator for the Maillard reaction between the proteins and the reducing sugars [30]. The BI was calculated (Equations (1) and (2)) using the measured L*a*b* color values (Figure S1) from the unheated and heated fiber samples to detect color active compounds formed during the Maillard reaction. The BI acts as an indirect indicator of the progression of the Maillard reaction. If the focus lays on the glycation of proteins via the first stage of the Maillard reaction, a light and little pigmented sample with a low BI is favorable. If possible, the reaction should be terminated after the Schiff reaction in order to prevent color formation [18]. As expected, the browning index increased within the first 6 h for all blends (Figure 4). Between 6 and 24 h, a slight decrease for all ratios was observed and all samples showed an increase in BI after 48 h heating. However, the BIs of each blend showed mainly significant differences between the unheated and heated fibers (6 up to 24 h) and after 48 h (Figure 4). After 48 h, blends B, C, and D showed similar BIs of approximately 10 without significant difference, whereas the browning index of blend A was significantly higher with 14.7 \(\pm\) 0.7. One reason for the enhanced browning of blend A could be its very small mean fiber diameter (Figure 1)  
Figure 3: Protein content (mean \(\pm\) standard deviation) for electrospun fibers from blend A (80:10:5), B (80:10:10), C (80:10:15), and D (80:10:20) as a function of heating time (the different letters indicate significant differences (\(p<0.05\)). that might have accelerated the Maillard reaction due to the increased surface. Another potential explanation is the composition of blend A. Due to the lower protein content of blend A, the carbohydrate content was proportionally higher. Some recent studies showed that the carbohydrate content is the limiting factor for Maillard reaction because, in terms of molecular weight, potato protein has more reactive lysine groups than maltodextrin has carbonyl groups [28,38]. Visual observation of the fiber mats (Figure 5) showed no visible difference between the individual blends of the unheated fibers. All were equally white and voluminous, although the mean values of the browning indices of the unheated fibers showed a significant difference. As shown in Figure 5, after 48 h heating at 65 \({}^{\circ}\)C and 75% RH, the fibers shrunk together and developed a very light yellowish/greyish color. The structures of blend A, which had the lowest protein content of 5% \(w/w\), hardened. With increasing protein contents, the heated fibers appeared increasingly porous. After grinding the mats to a powder, a slight brownish, greyish coloration became visible with increasing heating times, which is characteristic of a light degree of nonenzymatic browning [30]. The browning of the heated fibers can be explained by the formation of yellow compounds in the intermediate and brown compounds (melanoidins) in the last stage of the Maillard reaction [40]. Temperature, humidity, and time have a significant effect on browning, which increases with rising temperature and time [30]. The increase in color during heating can be explained by the reaction of carbonyl groups with free amino groups to form a Schiff base. Some intermediates can then polymerize in the further stages of the Maillard reaction and form brown pigments [9,41]. In a study with whey protein and maltodextrin, it was shown  
Figure 4: Browning index (mean \(\pm\) standard deviation) versus the heating time for the different ratios (A (80:10:5), B (80:10:10), C (80:10:15), and D (80:10:20)) of the fibers (the different letters indicate significant differences (\(p\) ¡ 0.05) between the heating times of each blend (lowercase) and between blends at the same heating time (upper case)). that a BI between 10 and 17 corresponded with slight yellow coloration [30]. At a BI of over 25, the products showed a distinct browning [30].