Development and validation of a HPLC-DAD method for simultaneous determination of main potential ABE fermentation inhibitors identified in agro-food waste hydrolysates

Lignocellulosic agro-food wastes are regarded as interesting carbohydrate sources for acetone-butanol-ethanol (ABE) fermentation. However, the physicochemical and enzymatic pretreatments applied to release their sugars generate inhibitory compounds that hinder the fermentation. The release of inhibitory compounds in the hydrolysates of four agro-food industrial wastes [apple pomace (AP), potato peel (PP), brewers' spent grain (BSG) and coffee silverskin (CS)] obtained after various chemical pretreatments (acid, alkali, organic solvents and surfactant pretreatments) was analyzed. Sixty-seven potential inhibitors were identified by gas chromatography-mass spectrometry (GC-MS) and were classified into non-aromatic compounds (aliphatic acids, nitrogen-containing compounds, furans and fatty acids) and aromatic compounds (phenolics and non-phenolics). Then, a high performance liquid chromatography method with diode array detection (HPLC-DAD) was developed and validated for the quantification of the main potential inhibitors identified in the hydrolysates (i.e. gallic, 3,4-dihydroxybenzoic, 2,5-dihydroxibenzoic, 4-hydroxybenzoic, 3-hydroxybenzoic, vanillic, caffeic, syringic, p-coumaric, and ferulic acids, vanillin, syringaldehyde and caffeine). The proposed HPLC-DAD method was simple, fast and robust and allowed the direct injection of samples without previous treatment, enabling the simultaneous quantification of the abovementioned compounds for the first time. The method was successfully applied to the analysis of AP, PP, BSG and CS hydrolysates.