Enzymatic hydrolysis of lignocellulosic biomass: converting food waste in valuable products

Highlights

• Second generation ethanol production does not compete with food production.

• The high-cost of enzymes is a challenge to be overcome for biomass hydrolysis.

• New technologies are being studied to find more effective enzymes for saccharification.

• Fermentation of pentoses in high yields has yet to become viable.

Second generation ethanol production has great potential to be a sustainable reality, especially in Brazil due to the large quantity of sugarcane bagasse encountered as industrial waste. Moreover, this process does not require that food crops are converted into biofuels. Biomass hydrolysis continues to be a bottleneck of the overall process due to the high costs of enzyme production. Therefore, efforts have been taken to make the process more cost-effective with regards to the discover of more effective enzymes. New sources of enzymes are continuously encountered and several strategies of enzyme prospection and production have been studied. The fermentation step is also an area of interest for biochemical alteration since pentoses are not effectively fermented to ethanol via traditional methods.

Introduction

Lignocellulosic biomass consists of forestry, agricultural, agro industrial and food wastes that are abundant, renewable and inexpensive energy sources. These lignocellulose wastes accumulate in large quantities and can cause environmental problems. Since the chemical composition of these materials consists mainly of polymer sugars (cellulose and hemicellulose) and lignin, these chemical components can be recycled and used for the production of a number of value added products, such as ethanol, food additives, organic acids, enzymes, and others.

The production of biofuels and alternative chemical products from agricultural residues is considered one of the most promising strategies to replace non-renewable fossil fuels. Most biofuels are produced from first generation substrates including sugarcane, corn, sugar beet, etc. that directly compete with food production. For this reason, more attention has been given to the development of biofuels from agricultural residues such as corn stover, wheat straw, rice straw and sugarcane bagasse. First generation biofuels are produced from simple vegetal components including sucrose and starch, while second generation biofuel production requires the conversion of lignocellulosic biomass into simple sugars. This sustainable method requires complexes enzyme mixtures due to the different compositions of lignocellulose from different agricultural residues.

This process of second generation ethanol production has great potential because cellulose is the most abundant renewable resource in the world. Moreover, this process contributes to improve energy security and to decrease air pollution by reducing CO₂ accumulation in the atmosphere [1].

Brazil is the largest producer of sugarcane in the world and the 2013/2014 sugarcane harvest was 653.32 million tons [2]. Sugarcane is used in the food industry for production of brown, raw and refined sugars, syrup and ‘cachaça’. As a general rule, in Brazil one ton of raw sugarcane generates 260 kg of bagasse [1]. About 50% of this residue is used in distilleries as a source of energy and the remainder is stockpiled [2]. Due to the large quantity of this biomass as an industrial waste, it presents potential for application of the biorefinery concept which permits the production of fuels and chemicals that offer economic, environmental, and social advantages (Figure 1).