Effect of steam treatments on the availability of various families of secondary metabolites extracted from green sweet sorghum

Both free carbohydrates and hemicelluloses from green sweet sorghum [Sorghum bicolor (L.) Moench] were extracted using a steam process to produce combined first and second generation ethanol. Once the carbohydrates removed, Soxhlet extraction was used to extract the lipophilic secondary metabolites from the plant fibers and the results were compared to those obtained with the biomass extracted without prior steam treatments. This allowed assessing if the carbohydrate removal process had an impact on the secondary metabolites that could be recovered from the biomass. Soxhlet extracts where characterized using several techniques such as gas chromatography-mass spectrometry, liquid chromatography-triple quadrupole mass spectrometry and liquid chromatography-UV detection followed by high-resolution mass spectrometry. A total of 13 compounds were detected and quantified in the non-polar soxhlet fraction and were mainly fatty acids (6), phenyl glycosides (5) and sterols (2) at concentrations ranging between 0.015 g kg−1 and 1.5 g kg−1 (dry biomass). Comparison of the extractive content before and after steam process showed that most of the compounds were found in a lesser extent (up to 71%) although still recoverable after the steam process while other compounds (namely phytosterols) were as concentrated as they were prior to steam treatments. These results show that in the case of sweet sorghum, the preliminary extraction of the carbohydrate content with a steam process may not completely hinder the possibility to extract high value secondary metabolites, which would in this case come second in a biorefinery approach.