# Upgrading of Mixed Food Industry Side-Streams by Solid-State Fermentation with _P. ostreatus_
## 3 Results and Discussion  
### Composition of the AISS  
The sugar, protein, fiber, and mineral content of the sixAISS samples, as well as the presence of minor compounds, such as vitamins and trace elements, makes them suitable for microbial conversion. Previous analysis of these exact substrates showed that BSG, MSR, and cheese whey had a considerable protein content of about 31, 24, and 14 %_w/w_ (on dry matter), respectively [9]. In brief, molasses, orange, and potato pulps had lower amounts of protein (av. 7.3, 2.9, and 2.5 %_w/w_ on dry matter, respectively) [9]. Molasses contained the highest amount of fermentable sugar (42.5 %_w/w_), while orange pulp had the highest amount of phenolics content (15.8 mg/g dry matter in aqueous extract, and 5% mg/g DM in organic extract) [9]. On the other hand, despite the total phenolic content, cheese whey presented the highest antioxidant capacity (4.35 mg/mL IC\({}_{50}\)) [9]. In this study, the analysis of Ca, Mg, Fe, and Cu (Table 2) showed that molasses had the highest minerals content (Ca and Mg) and orange pulp had the lowest. MSR also contained significant amounts of Mg, as reported in previous studies [20]. BSG also had a considerably high content of these minerals. Why was found to be rich in Mg (2 g/kg), however, when compared with the concentrations that were reported in other publications, there seem to be discrepancies. The results that were obtained in this study are similar to those reported by [PERSON] and [PERSON] [21] with respect to Fe and Cu, but significantly higher than other references [22; 23; 24] regarding Mg. These differences are expected due to the different raw materials (milk) and different production process. BSG were also found to be rich in Mg. Also, the amount of Cu contained was much higher than that required daily by the human body [25], while the content of Fe was significantly lower [26]. It has been reported that BSG contain significant amounts of minerals and trace elements and are therefore suitable substrates for the production enzymes by SSF [27]. In the case of the other two metals, there are differences in the levels, which may be due to the way barley that is cultivated, the different harvest time, and the beer production process [27]. Table 2 shows that the orange pulp has a high Ca content, the second largest among the six AISS, and at the same time, the lowest Mg content. Still, the percentage of Cu that is contained in orange pulp is low (1 Kg of pulp would be required to cover the average daily diet requirements). Wide variations in the orange composition have also been reported in the literature, which are affected by several factors, including availability in soil, soil pH, uptake ability of the plant, use of fertilizers, irrigation water, etc. [28]. Finally, potato pulp seems to be another good source of Mg since its percentage is quite high. On the other hand, Fe was not detected, which is not in line with the common knowledge that potatoes are rich sources of iron [29; 30]. However, there are also discrepancies in the literature, which may also be attributed to the different soil composition, plant physiology and agricultural practices.