## 3 Discussion  
Thousands of tonnes of phenylethyl alcohol are consumed across the food, beverage, cosmetics, and fragrance industries every year. Most of this is synthetically produced from benzene, styrene, or toluene due to the extremely low yield of natural product from its traditional source--rose petals [15; 16]. The yield of rose essential oil from rose petals is 0.03-0.04% and comprises up to 60% phenylethyl alcohol [15; 17]. Our yield was significantly higher than this; moreover, spent brewer's grain is a cheap substrate (\(\sim\)USD 300/tonne), while rose petals cost upwards of USD 3000/tonne. With further optimisation, the fermentative production of phenylethyl alcohol from grain could be a competitive, natural source of this molecule. 1-Octen-3-ol was originally discovered in a Japanese mushroom, _Armillaria__matsuake_[16; 18; 19]. Currently, it is synthesised through a reaction between magnesium amyl \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Compound** & **Value (USD/kg) a** & \multicolumn{2}{c}{**Annual Consumption (kg) b**} & **Substrate** & **Microorganism** & **Yield (g/kg) c** \\ \hline Methyl benzoate & USD 335 * & 590 & Orange & _Aspergillus niger_ & 0.173 \(\pm\) 0.0003 \\ Phenylacetaldehyde & USD 450 * & 106 & SBG & _Pycnorporus_ & 1.493 \(\pm\) 0.384 \\
1-Octen-3-ol & USD 4800 * & 250 & Onion & _Aspergillus oryzae_ & 1.297 \(\pm\) 0.107 \\ Phenylethyl & USD 500 * & 1240 & SBG & _Pycnorporus_ & 0.970 \(\pm\) 0.242 \\  & & & & _cinnabarinus_ & \\ \hline \hline \end{tabular}  
* Value per kilogram obtained through personal communication with [PERSON] at Excellentia International.
* Prices quoted in US dollars correct as of September 2017. b Annual consumption of compound as a flavour additive only ([PERSON], 2005). Excludes other uses, e.g., fragrance and cosmetics industry. c Yield represents average dry weight of compound produced per kilogram of fermented substrate (wet weight) \(\pm\) 2 standard deviations (n = 9).
* Prices quoted in US dollars correct as of September 2017. Substrates: Orange = orange pomace, SBG = spent brewer’s grain, Onion = onion pulp.  
\end{table} 
Table 3: Yield of commercially relevant flavour and aroma compounds produced from fermented substrates. Figure 2: Based upon the volatile metabolite level ratios and statistical significances (\(p\) value \(<\) 0.05), we visualise similarities and differences in metabolite profile among fungal species independent of the substrate fermented. The fold-change values are found in Table 1.  
bromide and acrolein (16). However, there is demand for natural 1-octen-3-ol as a mushroom flavouring in food products. As a high-value chemical (USD 4800/kg) with over 250 kg used annually in the food and beverage sector alone, our fermentation process could be a good option for the natural production of this metabolite (16). It is well known that many fungal species produce 1-octen-3-ol as part of their volatilome, including _Penicillin_, _Aspergillus_, and many edible mushroom species (20; 21; 22; 23; 24). [PERSON] (22) investigated natural 1-octen-3-ol extracted from several edible mushroom varieties finding high optical purity and yields of over 6 mg/100g in various types of edible mushrooms. These could be good candidates to include in further trials using fruit and vegetable by-products to increase and optimise fungal production of this compound. Phenylacetaldehyde is another volatile compound identified and quantified in fermented spent brewer's grain that has potential as a natural flavour and fragrance ingredient. It is currently produced chemically from benzaldehyde through a Darzen glycidic ester synthesis (16). However, it can also be produced through an expensive biotechnological route from phenylalanine and therefore carry a natural label.henylacetaldehyde is found as a component of many essential oils and contributes to the flavour of foods such as chocolate, cheeses, coffee, fruits, wine and breads (25; 26; 27; 28; 29).