**7. Other pretreatment processes**

According to the foregoing, this chapter has covered the most relevant pretreatment techni‐ ques for sugarcane and woody biomass as well as the new trends in this field. Below, we present other pretreatments, such as alkaline, ammonia fiber expansion and biological, which are also of relevance. Other important methods such as organosolv, ammonia percolation, and oxidative reactions using hydrogen peroxide or ozone will be dealt with elsewhere.

drolysis of cellulose and hemicellulose was obtained after AFEX pretreatment of bermuda‐ grass (approximately 5% lignin) and bagasse (15% lignin) [157]. Although hardwood pretreatment, like poplar, requires harsher AFEX conditions to obtain equivalent sugar yields upon enzymatic hydrolysis, poplar (*Populus nigra x Populus maximowiczii* hybrid) AFEX-pretreated at 180 °C, 2:1 ammonia to biomass loading, 30 minutes residence time by using various combinations of enzymes (commercial cellulases and xylanases) achieved

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This process presents some disadvantages, such as the use of ammonia solvent itself, that should be recycled and handled with caution to make the process environmentally feasible, and also from an economic point of view the ammonia consumption needs to be minimized [47]. However, there are some advantages in this pretreatment, like the feasibly solvent re‐ cover and the hydrolysate from AFEX is compatible with fermentation microorganisms

Biological pretreatment employs various types of rot fungi, being the white-rot fungi the most effective for biological pretreatment of lignocellulosic biomass. The aim of biological pretreatment processes are the lignin degradation by microorganisms, through the action of lignin degrading enzymes such as peroxidases and laccases [2]. The most investigated fun‐

The biological pretreatment of sugarcane straw was evaluated by screening eight microor‐ ganisms, including bacteria and fungi, for an incubation time of 30 days. The fungus *Asper‐ gillus terreus* was found as the most effective strain, resulting in 92% reduction in the lignin content [162]. The pretreatment of sugarcane straw was also evaluated using the fungus *Car‐ iporiopsis subvermispora* with the objective to reduce cooking times and chemicals load for the organosolv pulping. The pretreatment was effective regarding the decomposition of lignin, however high cellulose losses were pointed as negative side effects [163]. Another study evaluated the pretreatment of sugarcane bagasse with the white-rot fungus *Pleurotus sajorca‐ ju* PS 2001 using a 45 days incubation time, in order to modify its lignin content. However, in this case, the aim of the study was to provide a more digestible substrate for the produc‐

The pretreatment of the Japanese red pine *Pinus densiflora* was studied using three white-rot fungi. The fungus *Stereum hirsutum* was able to selectively degrade lignin resulting in a less recalcitrant biomass after eight weeks of pretreatment. As consequence, the sugar yields ob‐ tained after the hydrolysis of the pretreated red pine with commercial enzymes was 21%

The main advantages of such processes are the low capital cost, low energy, no requirement for chemicals, fewer hydrolysis and fermentation inhibitors produced during pretreatment and mild environmental conditions [166]. However, the biological processes require a very long residence time, when compared to other pretreatment techniques and result in very low reaction rates. Additionally, most microorganisms consume part of the substrate as a

high glucan and xylan conversion (93 and 65%, respectively) [159].

gus for lignin degradation is *Phanerochete chrysosporium* [161].

tion of cellulases by the fungus *Penicillium echinulatum* [164].

higher when compared to non pretreated control samples [165].

without the need for conditioning [160].

**7.3. Biological pretreatment**
