## 4 Discussion  
During the solid-phase fermentation of willowherb leaves, various chemical reactions occur, during which complex compounds are broken down into simpler ones, some substances are transformed into others, and thus both qualitative and quantitative changes occur [9; 10; 11].  
Vitamin C is necessary for human health. Many diseases, such as infections, cancer, cardiovascular disease, stroke, diabetes, and sepsis, have been associated with poor vitamin C levels [12; 13]. Decreased vitamin C levels in disease are often explained by a combination of a sometimes massively increased turnover due to oxidative stress and inflammation and a decreased dietary intake of vitamin C associated with the disease [14; 15].  
From the presented data (Table 1) we can see that solid-phase fermentation significantly decreased the amounts of vitamin C in its oxidized form, dehydroascorbic acid, and L-ascorbic acid. Vitamin C highest content was found in unfermented willowherb leaves (678.35 mg 100 g\({}^{-1}\) DW). The vitamin C present in the leaves of willowherb is very sensitive to rubbing, softening, changes in temperature, oxygen, and the method of drying. Therefore, there were significantly higher amounts of vitamin C in the raw material of willowherb leaves than in fermented ones. As we can see from the obtained data (Table 1), the decrease in vitamin C content was most significantly influenced by the duration of fermentation: the longer the duration of the fermentation the less vitamin C remained in the leaves. For example, after 24 h aerobic SPF, vitamin C was 404.32 mg 100 g\({}^{-1}\) DW, after 48 h aerobic SPF decreased by 33.60% (to 268.47 mg 100g\({}^{-1}\) DW), and after 72 h aerobic SPF decreased by 40.31% (to 241.35 mg 100 g\({}^{-1}\) DW) compared to the control (678.35 mg 100 g\({}^{-1}\) DW).  
Chlorophylls fulfill various vital functions in plants. In the chloroplast, chlorophylls are the predominant light-absorbing pigment in photosynthesis [16]. The dominant pigment in pomelo leaves, chlorophyll, has been reported to possess many biological activities such as antioxidant, anti-inflammatory, and anticancer properties [17].  
The ratio of total chlorophyll \(a\) and chlorophyll \(b\) in the control (unfermented leaves of willowherb) was almost the same, but chlorophyll \(a\) increased significantly after fermentation. This difference was most pronounced after 72 h of anaerobic fermentation (chlorophyll _a_: 503.50 mg 100 g\({}^{-1}\) DW and chlorophyll _b_: 106.53 mg 100 g\({}^{-1}\) DW). This could be seen by the variation of intensity of the colors: in the control, the leaves were bright green, and during fermentation this color turned more and more brownish green until it turned almost brown after 72 h of fermentation. Moreover, during fermentation, chlorophyll compounds and other pigments undergo degradation due to the accumulation of lactic acid and the resultant decline in pH [18].  
Sugars, organic acids, and amino acids are the major energy sources. It is produced via photosynthesis and respiration in plants [19]. During solid-phase fermentation, one of the steps is the hydrolysis of primary polymeric substances to finer substances (e.g., disaccharides (sucrose) to monosaccharides (fructose and glucose)). From the presented data we can see that during aerobic fermentation both the total amounts of sugars and the amounts of fructose, glucose, and sucrose decreased significantly. However, during anaerobic fermentation, total sugars, fructose, and glucose increased, but sucrose decreased. This could be explained by the fact that under solid-phase fermentation the decomposition of disaccharide sucrose into monosaccharides occurs. In this case fructose and glucose occurs more intensively under anaerobic conditions than during aerobic fermentation.