Hydrolysis is governed by the law:

(C6HwOs )n + nH2O ^ nC 6H nO6 (3)

and can be mainly of two types: acid (using diluted or concentrated acids) or enzymatic. A lignocellulose biomass is more complicated to hydrolyze than pure cellulose because it contains components that are not glucose-based, such as hemicellulose and lignin.

A lignocellulose biomass undergoing acid hydrolysis mainly produces xylose, while the lignin and cellulose fractions remain unchanged. This is because xylan is more susceptible to hydrolysis in moderately acid conditions because of its amorphous structure, while cellulose demands more severe conditions because of its crystalline nature.

If hydrolysis is implemented using 1% diluted sulfuric acid, the hemicellulose is depolymerized at a lower temperature than the cellulose. This process is usually conducted in two consecutive stages.

One of the most important characteristics of this type of hydrolysis is the rate of the reactions involved, which facilitate the continuity of the process. To speed up the diffusion of the acid, the raw material is mechanically reduced to pieces a few millimeters in size. Hydrolysis with concentrated acids (10-30%), on the other hand, rapidly and completely converts cellulose into glucose and hemicellulose into xylose, with some degree of degradation. The acids most often used are sulfuric and hydrochloric acid, and hydrogen fluoride.

This type of acid hydrolysis has the great advantage of recovering the sugars very efficiently (approximately 90% of hemicellulose and cellulose are depolymerized into monomeric sugars). From an economic standpoint, this process enables a reduction in production costs by comparison with the diluted acid solution, especially if the acids are retrieved and reconcentrated. The acids and sugars in solution are separated by ion exchange so the acid is reconcentrated by passing it through a series of multiple-effect evaporators. The remaining solid fractions, which are rich in lignin, are collected and can be made into pellets for use as fuel.

So, in short, we can divide concentrated acid hydrolysis into two stages: in the first stage, the concentrated acid (70%) destroys the crystalline structure of the cellulose, breaking up the hydrogen links between the cellulose chains; in the second stage, hydrolysis induces a hydrolytic reaction in the single isolated cellulose chains.

The enzymatic hydrolysis of natural lignocellulose materials is a very slow process, because it is hindered by several structural parameters of the substrate, such as its of cellulose and hemicellulose content, and the surface area and crystallinity of the cellulose. Pretreatments are consequently needed to make the biomass more susceptible to attack by hydrolysis. For the same reason, a cocktail of enzymes has to be used that is capable of breaking the links in the polymeric chains. This cocktail is usually a mixture of various hydrolytic enzymes, including cellulase, xylanase, hemicellulase and mannoxidase. Enzymatic cellulose degradation is a complex process because it takes place in limit conditions between the solid and liquid phases, where the enzymes are the mobile components. Generally speaking, degradation is characterized by a rapid initial phase followed by a slower second phase that can continue until all the substrate has been used up. The reason for this behavior is usually assumed to be because the accessible fraction of cellulose is quick to hydrolyze, followed by the slow activation of the absorbed enzyme molecules.

Chopping up the biomass increases the surface area accessible to the enzymes and reduces the polymerization and crystallinity of the cellulose, thus enabling a smaller quantity of enzymes to be used and the production costs to be contained.

Both bacteria and fungi can produce the cellulase for the hydrolysis of lignocellulose materials. The bacteria may be aerobic or anaerobic, mesophylic or thermophylic. The bacteria most often used are Clostridium, Cellulomonas, Bacillus, Thermomonospora, Ruminococcus, Bacteriodes, Erwinia, Acetovibrio, Microbispora and Streptomyces. The enzymes are usually classified according to their reaction site, so they may be intracellular (or cell-associated) or extracellular. The main function of extracellular enzymes is to convert the substrate into an external medium by taking effect on the cell mass constituents. Conversely, intracellular enzymes need the substrate to spread through the cellular mass before it can be converted.

The most widely accepted mechanism for the enzymatic hydrolysis of cellulose involves the synergic action of the enzymes endoglucanase (or endo-1,4-P-glucanase, EG), exoglucanase (or cellobiohydrolase, CBH), and P-glucosidase. Both EG and CBH are extracellular enzymes, while P-glucosidase is intracellular. EG randomly disrupts the cellulose chains, consequently inducing their strong degradation. It takes effect by hydrolyzing the P-1,4- glucoside bonds, creating new ends in the chains. Exoglucanase breaks up the ends of the chains, thus enabling the release of soluble cellobiose or glucose. BGL hydrolyses the cellobiose into glucose, thus eliminating the inhibitory cellobiose; then BGL completes the process by catalyzing the hydrolysis of cellobiose into glucose. Most cellulase and hemicellulase producers are microorganisms such as the filamentous fungi, e. g. Trichoderma sp., which can be used in their natural form or genetically modified (Trichoderma viride, Trichoderma reesei, Trichoderma longibrachiatum). CBH I and CBH II are the main enzymes of Trichoderma reesei, while EG I and EG II are the dominant endoglucanases.

Enzymatic activity is influenced by various parameters, such as temperature (a 20-30°C increase in temperature leads to a 3- to 5-fold increment in the end products). The crucial issue of temperature lies in the risk of an unwanted denaturation when the temperature is too high (Balat et al., 2008). Enzymatic hydrolysis, with or without the addition of catalysts, has generally proved capable of a high yield of both glucose (>90%) and xylose (>80%).