# Upgrading of Mixed Food Industry Side-Streams by Solid-State Fermentation with _P. ostreatus_
## 3 Results and Discussion
### Composition of the SSF Product and Autolyzate  
The product of SSF of the mixed AISS (substrate S7) by _P. ostreatus_ was analyzed for protein, total fat, ash, essential elements (Ca, Mg, Fe, Cu), and volatiles content (Table 4). The protein and fat levels were relatively high, higher than those that were reported in other works for fermented barley or dry citrus pulps [35; 36; 37], which makes the enriched material a potential nutritious food and feed supplement. The protein content was also higher when compared to that of the same AISS substrates that were fermented by SSF with _kefit_ (23.6%), and slightly lower than those that were fermented by _S. cerevisiae_ and _K. marxianus_ (38.4% and 33.7%, respectively) [10]. The fat content (14.4%) was slightly higher than those of the _S. cerevisiae_ and _kefit_ fermented AISSs (12.9% and 12.3%, respectively), but lower than that of _K. marxianus_ (25.5%) [10]. Regarding the minerals content, the _P. ostreatus_ SSF product had similar amounts of Ca and Cu content with those of _S. cerevisiae_, _kefir_, and _K. marxianus_[10], but much higher Fe content when compared with these products (0.9-2.3 mg/Kg) [10]. Regarding Mg, the substrate that was fermented by _S. cerevisiae_ contained the highest content of all compared products (771.1 mg/Kg) [10]. In general, it can be said that these essential minerals were found in significant amounts in the fermented AISS products. The composition of headspace volatile compounds that were identified by SPME GC/MS (totally 30 different compounds) in the same AISS as well as in their mixtures fermented by _S. cerevisiae_, _K. marxianus_, and kefir, has been reported in a previous study [10]. The volatile compounds that contribute to the aroma of fermented products are produced during fermentation or derive from the raw materials. Some of the major volatile by-products that are produced by fermentation are acetaldehyde, ethyl acetate, propan-1-ol, isobutanol, and amyl alcohols (Table 4). These compounds were all found (by GC/FID analysis) in the SSF product of _P. ostreatus_, especially acetaldehyde, which was found at relatively high levels. Acetaldehyde has a pungent, ethereial, fruity, or musty odor, depending on its concentration. Also, it may accumulate during mushroom growth at high temperatures due to anaerobic respiration and can have an inhibitory effect on mushroom growth \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Characteristic** & **Content** \\ \hline  & Moisture & 76.0\(\pm\)2.3 \\  & Dry matter & 24.0\(\pm\)2.1 \\  & Protein (\(\%\omega\)/\(\omega\)) & 30.3\(\pm\)1.9 \\  & Ash (\(\%\omega\)/\(\omega\)) & 0.93 \(\pm\) 0.1 \\  & Fat (\(\%\omega\)/\(w\) dry matter) & 14.4\(\pm\)1.1 \\  & Ca (mg/Kg) & 91.1\(\pm\)8.3 \\  & Mg (mg/Kg) & 324.5\(\pm\)24.1 \\ _SSF product_ & Fe (mg/Kg) & 17.8\(\pm\)2.1 \\  & Cu (mg/Kg) & 21.8\(\pm\)2.1 \\ \cline{2-3}  & Major volatile compounds (mg/Kg) (GC/FID) & \\  & _Acetaldehyde_ & 277.7\(\pm\)26.3 \\  & _Ethyl acetate_ & 142.4\(\pm\)18.7 \\  & _Propan-1-ol_ & 21.5\(\pm\)3.2 \\  & _Isobutyl alcohol_ & 31.9\(\pm\)4.3 \\  & _Amyl alcohols_ & 7.5\(\pm\)1.3 \\ \hline _SSF product after autolysis_ & RNA (mg/100 g dry extract) & 721.8\(\pm\)31.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Composition of the product of S7 SSF by _P. ostreatus_ and its autolyzate. (_Spawn-burning_ syndrome) [38]. Therefore, inefficient oxygen supply may be the reason for the high acetaldehyde levels and the inability of the mushroom to grow fruiting bodies, and, in the future, further research will be needed focusing on the optimization of growth temperature and oxygen supply for scale-up of edible mushrooms production on these AISS. Similar but fewer compounds were (positively) that were identified by SPME GC/MS analysis of the headspace of the _P. ostreatus_ AISS fermentation product (Table 4). The principal compounds that are responsible for the aroma of raw and thermally processed mushrooms are aliphatic alcohols and ketones with eight carbon atoms [39].