Feruloyl esterase

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Introduction

Plant biomass is an excellent renewable source of biopolymers. It provides the possibility for the development and production of sustainable raw materials and energy, and it fits perfectly with the development of a bio-based economy. Lignocellulosic biomasses from agriculture, crop, forestry waste, energy crops, and marine algae are all considered to be the expected raw materials for modern bioethanol and biochemical production. Compared with the chemical conversion in bioethanol production, the enzymatic hydrolysis of lignocellulosic biomass has many advantages, including the use of non-corrosive operating conditions; no substrate loss due to chemical modification; more environmentally friendly, etc.

However, plant cell walls have evolved to resist external factors, including mechanical, thermal, chemical, and biological stress. In order to depolymerize different types of lignocellulosic materials more effectively, a series of carbohydrate active and lignin-acting enzymes are required. Feruloyl esterases (FAEs, also known as ferulic/cinnamic acid esterases, EC 3.1.1.73) can remove ferulic acid residues and crosslinks from polysaccharides. They act as accessory enzymes, helping other enzymes to enter their sites of action during the biomass conversion process. In addition to its role in the production of bioethanol, FAE and its hydrolytic or transesterification products have applications in all aspects of biotechnology, especially as modified natural antioxidants or food flavor precursors. Therefore, academia and industry have considerable interest in discovering new FAEs with new characteristics.  

Industrial applications of FAEs

Due to its ability to remove hydroxycinnamic acid from plant cell walls, FAE plays an important role in various biotechnological processes. So far, FAE-related applications and patents have almost doubled compared to ten years ago. Here we emphasize and highlight the application of FAE in five major fields: (1) biomass processing, (2) pulp and paper, (3) ferulic acid and related fine chemical production, (4) feed, (5) seasoning and alcoholic beverages.

Figure 1. Schematic overview of industrial applications of FAEs (Dilokpimol, A.; et al.2016)

• Application in biomass processing

FAE is recognized as an accessory enzyme necessary for the hydrolysis of lignocellulosic biomass for bioethanol and other biorefineries. The activity of FAEs in combination with other hydrolases and oxidases on plant biomass not only increases the decomposition of plant materials, greatly improves the availability of fermentable carbohydrates, but also releases toxic esters and phenolic compounds.

• Applications in pulp and paper industry

Whiteness is an important characteristic of wood pulp, and closely related to the quality of the paper. The key step of bleaching is to remove residual lignin to make the pulp white. This process uses hazardous and expensive chemicals and has a bad impact on the environment. In the environmentally friendly biological bleaching process, FAEs can be used in combination with xylanases and lignin oxidases, which not only reduces energy consumption, but also significantly reduces the chemical oxygen demand (COD) of pulping wastewater.

• Application in the production of ferulic acid and related fine chemicals

Ferulic acid and other hydroxycinnamic acids are phenolic phytochemicals, which are widely used in the food and cosmetic industries.

Although the current commercial ferulic acid is mainly produced from rice oil, the focus of modern technology is the use of FAEs and other hydrolytic enzymes to produce ferulic acid. In addition to hydrolysis, FAEs can also synthesize ester-linked hydroxycinnamic acid through the ester exchange reaction between the organic group of an ester group (donor) and the organic group of an alcohol (acceptor).

Figure 2. Transferuloylation reaction (Dilokpimol, A.; et al.2016)

• Application in feed industry

Fiber digestibility is the basic criterion for animal feed. Improper digestion can hinder animal growth and cause immunological stress, which ultimately limits the profitability of farmers. Adding FAEs or a mixture of enzymes containing FAEs can improve the availability of main chain degrading enzymes, thereby improving fiber digestibility and bioavailability of phytonutrients.

• Application in seasonings and alcoholic beverage industry

FAE has been used to remove off-flavors/odors and enhance the aroma of various seasonings and alcoholic beverages. Flavor and odor are the key factors for the success of the premium fermented seasoning and alcoholic beverage industry. FAE can be used in the saccharification process as a FAE-producing koji (rice-fungal culture starter) and added together with xylanase and cellulase, thereby increasing the release of ferulic acid from the cell walls of rice and other grains, and finally transformed into aromatic derivatives in the process of fermentation and aging.