Inhibitors

Inhibitors based on their chemical functional groups can be grouped as aldehydes, ketones, phenols, and organic acids.

  • Aldehyde inhibitors: compounds with one or more functional aldehyde groups, regardless of the base structure of a furan ring, a benzene ring or a phenol-related structure, including
      o furfural and HMF
           Cell walls and membranes of yeast cells grown under furfural and HMF-challenged conditions appear damaged when compared to those of controls grown in the absence of any inhibitor, resulting in the delay of cell growth and reduction of ethanol productivity
           Furfural and HMF inhibit cell growth and ethanol production rates at lower concentrations. Individual strains have been isolated that retain their ability to produce ethanol in the presence of 10 to 79 mM of either furfural or HMF, including strains of these species: Saccharomyces cerevisiae, Pichia stipitis, Candida shehatae, Corynebacterium glutamicum, Zymomonas mobilis, and Escherichia coli.
      o 4-hydroxybenzaldehyde, vanillin, syringaldehyde, isovanillin,ortho-vanillin, Cinnamaldehyde
         more inhibitory than those derived from sugar dehydration

• Ketone inhibitors:4-hydroxyacetopheone, acetovanillone and acetocsyringone
         exert a greater inhibitory effect on bacteria such as Thermoanaerobacter mathranii than on yeasts, in terms of reduced growth and ethanol yield

• Organic acid inhibitors: sharing a common carboxylic acid functional group, all contain a carboxyl functional group such as
       o acetic acid,formic acid, levulinic acid, caproic acid,furoic acid, 4-hydroxybenzoic acid, 3-hydroxybenzoic acid, 2-hydroxybenzoic acid, 2,5-dihydroxybenzoic acid, protocatechic acid, vanillic acid, gallic acid, syringic acid, 4-hydroxycinnamic acid, homovanillic acid, guaiaclyglycolic acid, and sinapic acid. These inhibitors are thought to be exert their inhibitory actions via their carboxyl functional groups.
             more toxic to isolates of bacteria than yeasts at low concentration
             The toxicity has been correlated with their degree of hydrophobicity, suggesting the involvement of a hydrophobic target such as the cell membrane

• phenol-based inhibitors: phenol, benzene-1,2-diol (catechol) etc.
      o cause increased membrane fluidity and affect membrane permeability, which may enhance synergistic inhibition when combined.
      o Phenols such as cathecol, hydroquinone, and coniferyl alcohol almost completely inhibit E. coli , but are relatively less toxic to yeast. However, eugenol and isoeugenol are inhibitory to yeasts at low concentrations.
     o The three main phenol structure building blocks in lignin show the order of inhibitory effects, ranked from strong to weak, as (1) hydroxyphenol, (2) guaiacyl, and (3) syringyl

In general, aldehydes and phenols are more toxic than organic acids.

In addition, some inhibitions will be amplified through process. A typical example is that fermentative microorganisms will be inhibited by concentrated non-volatiles such as lignin derivatives and extractives after concentrating although low initial inhibitor concentration in hydrolyzate.