## 3 Discussion
Some of the identified aroma compounds were uniquely associated to a specific substrate (Table 2), whereas the profile of volatile metabolites of some fermented substrates, e.g., onion pulp, red grape marc, Sauvignon blanc grape marc, and kiwifruit skins showed far fewer compounds compared to substrates such as carrot, apple and orange pomaces (Figure 1). As all substrates were used in an unsupplemented state (the only treatment was autoclaving to sterilise the substrates), this leaves the potential to investigate supplementation or fortification of the substrates with nutrients, e.g., nitrogen/phosphorus/potassium and/or sugars. For example, as red grape marc is pre-fermented by yeast and kiwifruit skins are naturally very low in sugar, both substrates may perform better if supplemented with additional sugar. On the other hand, Sauvignon blanc grape marc is high in fermentable sugars; however, it also has a high lignin content, which can be slow to break down. A two-step fermentation using a lignin-degrading fungus such as _P. cinnaburinus_ followed by a second fungus could be an option to help increase the available nutrients and produce more of a desired compound (30). Nonetheless, potentially some unique aroma molecules were found in the headspace of these poorly fermented substrates such as traces of (E)-cinnamaldehyde (cinnamon aroma) in red grape marc fermented by _A. oryzae_ and vanillin in olive cake fermented by _P. camemberti_, which are worth further investigation (Table 2). Among the filamentous fungi used in this study to ferment different fruit and vegetable by-products, _P. cinnaburinus_ and _A. niger_ showed the best performance regarding the production of volatile compounds (Figure 1). These two fungi also produced the largest number of unique volatile compounds independent of substrate (Figure 2). Comparatively, _A. oryzae_ presented the lowest number of unique volatiles, being generally outperformed by the other three fungi regarding their volatilome complexity (Figures 1 and 2). However, only _A. oryzae_ was capable of producing cinnamon aroma from red grape marc, albeit in limited amounts. Similarly, _P. camemberti_ was the only fungus capable of producing trace amounts of vanillin from olive cake. To the best of our knowledge, this is the first-time vanillin has been detected in _P. camemberti_ fermentation, suggesting this fungus is capable of producing this aroma compound from a precursor available in olive cake (Table 1, Figure 2). Many filamentous fungi are capable of biosynthesis vanillin from different phenolic acids, including _P. cinnaburinus_(31; 32; 33). However, our strain of _P. cinnaburinus_ did not produce this aroma compound in any fermentation. Olive cake is a substrate rich in lignin, polyphenols, and phenolic acids (34; 35). Therefore, _P. camemberti_ may have converted some phenolic acids commonly present in olive cake into vanillin, such as ferulic acid and or vanillic acid (35). Nine different substrates were investigated, of which apple, carrot, and orange pomaces appeared to perform the best, producing the most volatiles. Therefore, any further screening experiments should focus on these substrates where available. Further experiments could also include investigating different fungal strains for their potential to produce high-value 1-octen-3-ol. Many edible mushrooms--especially _Agaricus bisporus_--produce high quantities of 1-octen-3-ol and should be investigated for their potential to ferment agro-industrial by-products such as onion pulp. As onions present high levels of linoleic acid, which is a precursor in the production of 1-octen-3-ol, this could be a very promising combination of substrate and fermenting fungi [36]. Solid-state fermentation is still in its infancy compared with industrial liquid-state fermentations. While there is potential to produce more complex secondary metabolites by using SSF, there are still problems with the large surface area required, air circulation, assuring consistent fermentations, as well as a heightened risk of contamination [14].