Feruloyl esterase is a class of carbohydrate esterases that hydrolyze ester bonds between ferulic acid and polysaccharides, oligosaccharides, or alcohols. In feed and food fermentation systems, feruloyl esterase often acts synergistically with xylanase, cellulase, pectinase, β-glucanase, and other enzymes to promote plant cell wall depolymerization, release of bound phenolic acids, improvement of substrate availability, and enhancement of fermentation quality. It is an important functional enzyme in the deep conversion of plant-derived raw materials.

Keywords: feruloyl esterase; feed fermentation; food fermentation; ferulic acid; plant cell wall; hemicellulose; phenolic acid release; multienzyme synergy

1 Basic Concepts of Feruloyl Esterase

1.1 Enzymatic Definition

(1) Ester bond hydrolysis

Feruloyl esterase mainly catalyzes the hydrolysis of ester bonds between ferulic acid and arabinoxylan, oligosaccharides, pectin, or alcohols, releasing bound ferulic acid as free ferulic acid. This reaction reduces the degree of phenolic acid crosslinking in plant cell walls and improves the accessibility of hemicellulose and cellulose structures.

(2) Carbohydrate esterase characteristics

Feruloyl esterase is generally classified as a carbohydrate esterase. Its substrates are not single small molecules, but complex phenolic acid ester structures in plant cell walls. Therefore, its actual catalytic efficiency is affected by substrate polymerization degree, steric hindrance, branch distribution, cell wall compactness, and pretreatment method.

(3) Function in fermentation systems

In feed and food fermentation, feruloyl esterase not only participates in substrate degradation but also affects reducing sugar release, microbial utilization efficiency, antioxidant compound release, flavor precursor formation, and the composition of fermentation end products.

1.2 Forms of Ferulic Acid in Plant Cell Walls

(1) Bound ferulic acid

In cereal bran, corn bran, wheat straw, rice husk, rice bran, and various plant by-products, ferulic acid is often linked to arabinoxylan side chains through ester bonds. Bound ferulic acid can further form dimers or participate in polysaccharide crosslinking, thereby increasing cell wall rigidity.

(2) Arabinoxylan crosslinking

Ferulic acid residues can connect hemicellulose chains and may also form crosslinks between different polysaccharide chains through diferulic acid structures. Such crosslinking restricts the access of xylanase, cellulase, and other cell wall-degrading enzymes to the interior of substrates, reducing the degradation efficiency of plant materials.

(3) Significance of enzymatic release

After feruloyl esterase cleaves ester bonds, the cell wall structure becomes more susceptible to further hydrolysis by other enzymes. Free ferulic acid can also act as an antioxidant compound, flavor precursor, or microbial metabolic substrate in subsequent fermentation processes.

2 Catalytic Mechanism of Feruloyl Esterase

2.1 Substrate Recognition

(1) Recognition of phenolic acid ester structures

Feruloyl esterase recognizes ester bonds between ferulic acid and glycosyl residues. Its active site generally needs to accommodate the aromatic ring, methoxy group, hydroxyl group, and ester bond region simultaneously. Therefore, different enzymes vary in affinity toward methyl ferulate, ethyl ferulate, feruloylated arabinose, or complex arabinoxylan substrates.

(2) Effect of steric hindrance

In plant cell walls, ferulic acid is often embedded in complex polysaccharide networks, making it difficult for enzyme molecules to access ester bonds. The denser and more highly crosslinked the substrate, the more limited the release efficiency of feruloyl esterase alone.

(3) Role of substrate pretreatment

Grinding, steam explosion, extrusion, acid or alkali pretreatment, and prehydrolysis with multienzyme systems can improve substrate accessibility, allowing feruloyl esterase to enter the cell wall structure more effectively.

2.2 Catalytic Process

(1) Nucleophilic attack

Feruloyl esterase usually cleaves ester bonds through a serine hydrolase-like catalytic mechanism. The nucleophilic residue in the active center attacks the carbonyl carbon of the ester bond, forming an acyl-enzyme intermediate.

(2) Intermediate hydrolysis

Water further participates in the reaction, hydrolyzing the acyl intermediate to release free ferulic acid and regenerate the active enzyme. This process allows the enzyme to enter the next catalytic cycle.

(3) Product release

The reaction products include free ferulic acid and the corresponding glycosyl or alcohol residues. If the substrate is arabinoxylan, ferulic acid release reduces the crosslinking degree of polysaccharide chains, thereby improving the efficiency of subsequent xylanase, arabinofuranosidase, and cellulase action.

2.3 Enzymatic Synergy

(1) Synergy with xylanase

Xylanase cleaves the xylan backbone and reduces substrate polymerization degree, while feruloyl esterase cleaves ferulic acid ester bonds and removes side-chain crosslinks. Their combined use can significantly improve arabinoxylan depolymerization efficiency.

(2) Synergy with arabinofuranosidase

Arabinofuranosidase removes arabinose residues from xylan side chains. If ferulic acid is attached to the arabinose side chain, feruloyl esterase and arabinofuranosidase can act synergistically to promote complete side-chain degradation.

(3) Synergy with cellulase and pectinase

After hemicellulose and phenolic acid crosslinking are reduced, cellulose microfibrils become more exposed, facilitating further cellulase action. In fruit and vegetable by-products or soybean residue materials, pectinase can also synergistically disrupt cell wall structures, improving phenolic acid release and substrate softening.

3 Application Value in Feed Fermentation

3.1 Plant Cell Wall Degradation

(1) Loosening of fibrous structures

The fiber structure in feed raw materials is often composed of cellulose, hemicellulose, lignin, and phenolic acid crosslinks. Feruloyl esterase releases bound ferulic acid, reduces cell wall compactness, and improves the degradation efficiency of multienzyme systems toward substrates.

(2) Utilization of non-starch polysaccharides

Cereal bran, corn bran, and oilseed meal materials contain relatively high levels of non-starch polysaccharides. Feruloyl esterase can promote arabinoxylan depolymerization, converting part of the complex polysaccharides into oligosaccharides or fermentable sugars and increasing the level of microbial-available carbon sources.

(3) Improvement of fermentation efficiency

After the substrate structure is opened, lactic acid bacteria, Bacillus, yeasts, or molds can more fully utilize released sugars and small-molecule nutrients, improving fermentation acidification rate, microbial proliferation, and final product stability.

3.2 Improvement of Nutrient Digestibility

(1) Energy release

Enhanced cell wall degradation makes starch, proteins, and lipids originally encapsulated in feed more readily released, increasing opportunities for contact with animal digestive enzymes. This process helps improve the available energy value and nutrient release efficiency of raw materials.

(2) Reduction of antinutritional factors

Non-starch polysaccharides such as arabinoxylan can increase intestinal digesta viscosity and affect nutrient absorption. Feruloyl esterase and xylanase can synergistically degrade related structures and reduce some antinutritional effects.

(3) Support for intestinal microbiota

Oligosaccharides and phenolic acids released by enzymatic hydrolysis can participate in intestinal microbial metabolism. Moderately released ferulic acid and its metabolites may contribute to antioxidant activity and barrier regulation, but the specific effect depends on dose, animal species, and intestinal microbiota status.

3.3 Improvement of Fermented Feed Quality

(1) Organic acid accumulation

When substrate availability is improved, fermenting microorganisms such as lactic acid bacteria can produce lactic acid, acetic acid, and other organic acids more efficiently, reducing pH and inhibiting the growth of spoilage microorganisms.

(2) Flavor and palatability

Plant cell wall degradation and phenolic acid release can alter the aroma, acidic fragrance, and taste of fermented feed. Moderate fermentation helps improve palatability, whereas excessive degradation or excessive phenolic acid accumulation may produce irritating flavors.

(3) Storage stability

Good fermentation acidification and substrate conversion can reduce the risk of mold, contaminating microorganisms, and spoilage bacteria. Feruloyl esterase is not a direct preservative, but it can indirectly improve feed stability by promoting the fermentation process.

4 Application Value in Food Fermentation

4.1 Cereal Fermentation

(1) Utilization of bran and whole grains

Whole wheat, oats, corn, rye, rice bran, and other materials are rich in arabinoxylan and bound phenolic acids. Feruloyl esterase can release bound ferulic acid and increase phenolic acid availability and functional component release in cereal fermentation systems.

(2) Dough and fermentation structure

In flour products and cereal fermentation, changes in hemicellulose structure affect water retention, viscoelasticity, and microbial fermentation status. Moderate enzymatic hydrolysis can improve raw material dispersibility and fermentation substrate accessibility, whereas excessive degradation may impair structural support.

(3) Release of functional components

Free ferulic acid has strong antioxidant activity and can also participate in subsequent microbial conversion. Releasing bound phenolic acids during cereal fermentation helps increase detectable levels of free phenolic acids in fermented products.

4.2 Brewing and Flavor Formation

(1) Ferulic acid release

Wheat, malt, corn, and certain spice materials contain bound ferulic acid. Feruloyl esterase can increase free ferulic acid levels and provide precursors for subsequent flavor conversion.

(2) Formation of 4-vinyl guaiacol

Some yeasts or microorganisms possess ferulic acid decarboxylation capacity and can convert ferulic acid into volatile phenolic compounds such as 4-vinyl guaiacol. This reaction has specific aroma significance in certain beers, fermented beverages, and flavor foods.

(3) Flavor control

Excessive ferulic acid release or excessive downstream conversion may lead to phenolic, medicinal, or smoky odors. Therefore, when feruloyl esterase is used in food fermentation, enzyme dosage, fermentation strains, and reaction time should be controlled.

4.3 Fermentation of Fruits, Vegetables, and Plant By-Products

(1) Cell wall softening

Fruit and vegetable peels, pomace, pulp, soybean residue, and plant processing by-products contain pectin, hemicellulose, and bound phenolic acid structures. Feruloyl esterase can act synergistically with pectinase and xylanase to facilitate cell wall dissociation.

(2) Polyphenol release

After bound phenolic acids are released, the level of free polyphenols and antioxidant capacity in the fermentation system may increase. This effect can be used in the development of plant-based fermented beverages, fruit and vegetable fermented slurries, and functional food ingredients.

(3) By-product valorization

Bran, rice bran, corn bran, fruit pomace, and soybean residue are often underutilized because of high fiber content and coarse texture. Feruloyl esterase-assisted fermentation can improve structure, release bioactive components, and increase the utilization value of raw materials.

5 Factors Affecting the Application Effect of Feruloyl Esterase

5.1 Enzyme Source

(1) Fungal sources

Feruloyl esterases from fungi such as Aspergillus and Penicillium are widely used and often show good adaptability to plant cell wall substrates. Fungal enzyme systems are frequently secreted together with xylanase, cellulase, and other enzymes, making them suitable for complex substrate fermentation.

(2) Bacterial sources

Some lactic acid bacteria, Bacillus, and intestinal-associated bacteria can produce feruloyl esterase. Bacterial enzymes have potential application value in feed fermentation, intestinal release, and food fermentation.

(3) Recombinant enzymes

Recombinant expression can improve feruloyl esterase yield and purity, facilitating mechanistic studies and formulation optimization. In industrial applications, expression cost, stability, and compatibility with food or feed regulations should also be considered.

5.2 Reaction Conditions

(1) pH range

The pH of feed and food fermentation systems often changes with microbial metabolism. Feruloyl esterases with good activity under neutral or weakly acidic conditions are more suitable for lactic fermentation and acidifying systems.

(2) Temperature conditions

The optimal temperature differs greatly among enzymes from different sources. The temperature of solid-state feed fermentation and liquid food fermentation differs; therefore, enzyme preparations should be selected according to process temperature.

(3) Moisture and substrate state

In solid-state fermentation, water activity, raw material particle size, and mixing uniformity can limit enzyme diffusion. In liquid fermentation, substrate suspension state, viscosity, and particle sedimentation also affect reaction efficiency.

5.3 Enzyme Compatibility

(1) Xylanase ratio

Feruloyl esterase often needs to be used together with xylanase. After the xylan backbone is cleaved, ferulic acid ester bonds are more easily exposed; after ferulic acid ester bonds are cleaved, xylanase can more readily enter the substrate structure.

(2) Assistance from cellulase and pectinase

For complex plant raw materials, targeting hemicellulose alone is not always sufficient. Cellulase, pectinase, and β-glucanase can jointly improve cell wall depolymerization efficiency.

(3) Compatibility with microbial strains

Whether fermentation strains can utilize released sugars, tolerate phenolic acids, and further convert ferulic acid significantly affects final outcomes. Enzyme preparations and strains should be optimized as an integrated system.

6 Detection and Evaluation Methods

6.1 Enzyme Activity Assays

(1) Small-molecule substrate method

Methyl ferulate and ethyl ferulate can be used as model substrates to detect feruloyl esterase hydrolytic activity. After the reaction, ferulic acid release can be detected by HPLC, UV, or colorimetric methods.

(2) Natural substrate method

Wheat bran, corn bran, arabinoxylan, or destarched cereal flour can be used as natural substrates and are closer to practical fermentation applications. This method more realistically reflects the enzyme’s ability to act on complex plant cell wall substrates.

(3) Enzyme activity units

Feruloyl esterase activity is commonly expressed as the amount of ferulic acid released per unit time. If different substrates, pH values, or temperatures are used, enzyme activity values should not be directly compared.

6.2 Detection of Ferulic Acid Release

(1) HPLC detection

HPLC can quantify free ferulic acid and distinguish related phenolic acids such as p-coumaric acid, caffeic acid, and sinapic acid. This method is suitable for phenolic acid release analysis in food, feed, and fermentation broth.

(2) Total phenolics and antioxidant capacity

Ferulic acid release is often accompanied by changes in total phenolic content and antioxidant capacity. Folin-Ciocalteu, DPPH, ABTS, and FRAP methods can be used as auxiliary evaluations, but they cannot replace ferulic acid-specific detection.

(3) Distinction between bound and free forms

To evaluate enzymatic release effects, free ferulic acid and bound ferulic acid should be distinguished. Total ferulic acid usually requires alkaline hydrolysis before detection, whereas free ferulic acid can be detected by direct extraction.

6.3 Evaluation of Fermentation Quality

(1) Feed fermentation indicators

Common indicators include pH, organic acids, reducing sugars, crude fiber, neutral detergent fiber, acid detergent fiber, degree of protein degradation, and microbial counts. The effect of feruloyl esterase should be comprehensively evaluated together with these indicators.

(2) Food fermentation indicators

In food systems, free phenolic acids, flavor compounds, texture, viscosity, acidity, sensory characteristics, and antioxidant capacity should be considered. For beverages or cereal fermentation, precipitation, turbidity, and mouthfeel should also be evaluated.

(3) Safety indicators

Both fermented feed and food require attention to contaminating microorganisms, molds, mycotoxins, biogenic amines, and over-acidification risks. Feruloyl esterase itself cannot replace fermentation safety control and must be managed together with strains, temperature, and hygienic conditions.

7 Application Characteristics in Different Raw Materials

7.1 Cereal Bran Raw Materials

(1) Wheat bran

Wheat bran is rich in arabinoxylan and bound ferulic acid and is a typical substrate in feruloyl esterase application research. Enzymatic treatment can increase free ferulic acid content and improve hemicellulose degradation efficiency.

(2) Rice bran

Rice bran contains dietary fiber, lipids, and various bound phenolic acid structures. Using feruloyl esterase during fermentation helps release phenolic acids, but lipid oxidation and rice bran rancidity should also be considered.

(3) Corn bran

Corn bran has high hemicellulose content and substantial ferulic acid crosslinking. The synergy between feruloyl esterase and xylanase can improve polysaccharide depolymerization and phenolic acid release, making it an important direction in feed and by-product valorization.

7.2 Straw and Roughage

(1) Wheat straw

Wheat straw has a compact cell wall structure, and feruloyl esterase alone has limited effect. Combined use with alkali treatment, steam explosion, or multienzyme systems can improve degradation efficiency.

(2) Corn straw

Corn straw is rich in hemicellulose and phenolic acid crosslinks and is suitable for improving fermentation utilization through combined use of feruloyl esterase with cellulase and xylanase.

(3) Silage systems

Adding related enzyme systems to silage can improve cell wall degradation and substrate supply for lactic acid bacteria, but enzyme activity must adapt to the gradually acidifying environment. If the enzyme loses activity rapidly at low pH, the practical effect may be limited.

7.3 Fruit and Vegetable Processing By-Products

(1) Peels and pomace

Fruit peels and pomace contain pectin, hemicellulose, and bound phenolic acids. Combining feruloyl esterase with pectinase can improve cell wall dissociation and phenolic acid release.

(2) Soybean residue and plant protein by-products

Soybean residue and related by-products contain dietary fiber and residual proteins. Using multienzyme systems during fermentation can improve texture and nutrient release. The main contribution of feruloyl esterase lies in phenolic acid release and reduction of cell wall crosslinking.

(3) Development of functional ingredients

Enzymatic fermentation can release phenolic acids and improve the functional properties of plant by-products, making them more suitable for fermented beverages, dietary fiber ingredients, or functional feed additives.

8 Common Problems in Application

8.1 Insufficient Ferulic Acid Release

(1) Poor substrate accessibility

When the cell wall structure is too compact, enzymes have difficulty accessing ferulic acid ester bonds. In this case, grinding particle size, heat treatment, alkaline pretreatment, or multienzyme combinations should be optimized.

(2) Insufficient enzyme compatibility

If xylanase or arabinofuranosidase is absent, feruloyl esterase may not fully exert its effect. The ratio of multienzyme components should be adjusted according to raw material structure.

(3) Mismatch of reaction conditions

Unsuitable pH, temperature, and moisture conditions reduce enzyme activity. In solid-state fermentation, moisture uniformity and local temperature elevation should be given particular attention.

8.2 Abnormal Fermentation Flavor

(1) Excessive phenolic acid release

High levels of free phenolic acids may cause astringency, bitterness, or phenolic flavors. In food systems, enzyme dosage and reaction time should be controlled.

(2) Excessive microbial conversion

Some strains can convert ferulic acid into volatile phenolic compounds. If the yield is too high, medicinal, smoky, or irritating odors may be produced.

(3) Oxidative side reactions

Phenolic acids and polyphenols may cause color darkening or flavor changes under oxidative conditions. Oxygen exposure and metal ion-catalyzed oxidation should be controlled during fermentation.

8.3 Inconsistent Detection Results

(1) Confusion between free form and total amount

Direct detection measures free ferulic acid, whereas detection after alkaline hydrolysis measures total ferulic acid. Failure to distinguish between the two can easily lead to misjudgment of enzymatic release efficiency.

(2) Differences in sample extraction efficiency

Different solvents, pH values, and extraction times affect ferulic acid recovery. Extraction conditions should be kept consistent when comparing different treatment groups.

(3) Continued metabolism after fermentation

Released ferulic acid may be further converted by microorganisms, causing free ferulic acid levels to decrease. In this case, conversion products and microbial metabolic capacity should be detected simultaneously.

9 Reagent and Detection Material Selection Related to Feruloyl Esterase

9.1 Substrates, Standards, and Reagents Related to Enzyme Activity Detection

9.2 Enzyme Activity and Sugar Release Detection Products Related to Feruloyl Esterase Fermentation Applications

The core value of feruloyl esterase in feed and food fermentation lies in cleaving phenolic acid ester bonds in plant cell walls, thereby promoting hemicellulose depolymerization, release of bound ferulic acid, and improvement of fermentation substrate availability. In practical applications, enzyme dosage should be controlled according to raw material structure, target products, strain metabolic capacity, and flavor requirements. Comprehensive evaluation should integrate ferulic acid release, fiber degradation, sugar release, antioxidant capacity, and fermentation quality indicators.

For more related articles, please see below:

[1] Ferulic Acid: Structural Features, Preparation Routes, and Key Points for Research Applications