Reducing sugars facilitated carbonyl condensation in detoxification of carbonyl aldehyde model compounds for bioethanol fermentation

Abstract

We investigated the inhibitory effects of ortho-phthalaldehyde (OPA) as a carbonyl aldehyde model compound on the fermentation and growth of Saccharomyces cerevisiae and its alkaline detoxification. OPA was a potent inhibitor on the fermentation and growth of S. cerevisiae compared to vanillin, furfural and hydroxymethyl furfural (HMF) at the same concentration level. The inhibition of OPA on the fermentation and growth of S. cerevisiae was dose dependent. Ethanol production and growth of S. cerevisiae were both completely inhibited in the presence of 1.0 mM OPA. The inhibition of the fermentation and growth of the yeast decreased with the decrease in the OPA concentration between 0.02 mM–1.0 mM. OPA at 0.02 mM showed no inhibition on both the fermentation and growth of yeast, the ethanol final yield was even increased by 4.6% compared to the control. The inhibition of OPA at a low concentration could be overcome by increasing the inoculation size of the yeast. Most interestingly, we found OPA inhibition could be detoxified under alkaline conditions (pH∼10) at 60 °C for 2 h in the presence of a reducing sugar (ketone or aldose), but not a non-reducing sugar. Mass spectral analysis of the OPA reaction products in negative ion mode revealed a high intensity mass at 313.09 m/z ([M − H]⁻). This molecule (314) was predicted to be the aldol reaction product of the reducing sugar and OPA under alkaline conditions. One of the CHO groups on OPA was converted into a hydroxyl group by nucleophilic addition of the enolate ion of the reducing sugar. Loss of one CHO group of OPA could be the key factor for the removal of OPA inhibition.