Inhibitors can be divided into furan derivatives, such as furfural and 5-hydroxymethylfurfural (HMF), phenolic compounds, weak acids (levulinic and formic acid) and ions of heavy metals (nickel, aluminium, chromium, etc.) [58]. Furfural and HMF are two furan derivatives formed by the hydrolysis of sugars, hexoses and pentoses, respectively. These furans are present at relatively high concentrations in the hydrolysate and can act as potential microorganism inhibitors. Furans are probably the most important group of inhibitors, since the fermentability of hydrolysates obtained by acidification is inversely related to the concentration of these compounds. In addition, phenolic compounds cause loss of integrity in biological membranes, affecting their ability to serve as selective barriers and enzyme matrices [59]. Acetic acid, the main aliphatic compound present, is released from acetyl-hemicellulose groups. In turn, formic acid is a product of HMF degradation.