Recent Advances in Methods for Measuring Plant Fiber-Related Indices and Application-Oriented Selection
Recent Advances in Methods for Measuring Plant Fiber-Related Indices and Application-Oriented Selection
Plant cell-wall fiber components (including cellulose, hemicellulose, and lignin) constitute key structural foundations that determine tissue mechanical strength, digestibility, processing quality, and stress resistance. In plant physiology and quality research, “fiber” is not a single concept. Crude fiber is a traditional operational index that primarily reflects recalcitrant residues remaining after strong acid and strong alkali treatments. Holocellulose and α-cellulose are closer to chemical fractionation descriptors of the polysaccharide framework of the cell wall. Lignin reflects the deposition level of aromatic polymers. The detergent fiber system (NDF, ADF, ADL) emphasizes a reproducible washing-based fractionation framework that facilitates linkage to feed digestibility and cell-wall structural parameters. Dietary fiber (total/soluble/insoluble) focuses on polysaccharides that are not degradable by human digestive enzymes; methodologically, it emphasizes enzymatic removal of starch and protein followed by gravimetric mass balance. Selecting an appropriate index system aligned with the research objective, and standardizing pretreatment, separation conditions, and quality control, are prerequisites for generating comparable and interpretable data.
Keywords: plant fiber; hemicellulose; cellulose; α-cellulose; holocellulose; lignin; detergent system; NDF; ADF; ADL; dietary fiber; enzymatic–gravimetric method; gravimetry; spectrophotometry
I. Chemical Fractionation with Colorimetric/Gravimetric Determination: Quantification of Structural Polysaccharides and Lignin
1.1 Hemicellulose: Hydrochloric Acid Hydrolysis Method
(1) Principle
Hemicellulose is generally more susceptible to dilute-acid hydrolysis than cellulose. In the hydrochloric acid hydrolysis method, acidolysis conditions are controlled to promote preferential depolymerization of hemicellulose into measurable fractions. Hemicellulose content is then obtained using defined separation and conversion procedures, enabling comparison of cell-wall polysaccharide composition and assessment of structural changes.
(2) Key Procedural Considerations
① Consistent sample pretreatment: Drying procedure, particle size after milling, and optional defatting/depigmentation influence hydrolysis efficiency and comparability.
② Controlled hydrolysis conditions: Keep HCl concentration, temperature, duration, and solid-to-liquid ratio strictly consistent to avoid under-hydrolysis or parallel degradation.
③ Standardized separation and neutralization: Fix clarification, separation, and neutralization steps after hydrolysis to reduce systematic bias from residual acid.
④ Blanks and recovery assessment: Include reagent blanks and process blanks; when needed, verify reliability using spike-recovery tests and/or repeatability assessments.
(3) Advantages, Limitations, and Applicability
① Advantages: Mechanistically aligned with hemicellulose acid lability; suitable for trend analysis of compositional change.
② Limitations: Highly sensitive to hydrolysis conditions; lignification level and wall compactness alter hydrolysis kinetics.
③ Use scenarios: Comparative profiling of cell-wall polysaccharides; evaluation of hemicellulose dynamics during maturation or stress.
1.2 Cellulose: Anthrone–Sulfuric Acid Colorimetric Assay
(1) Principle
Under acidic conditions, cellulose is hydrolyzed to glucose and/or related dehydration products. In a concentrated sulfuric acid system, anthrone reacts with carbohydrate dehydration products to form a colored complex. Absorbance is measured spectrophotometrically and quantified with a standard curve, supporting batch-wise comparison of cellulose content.
(2) Key Procedural Considerations
① Interference reduction and pre-separation: For pigment-rich samples, apply appropriate depigmentation/cleanup. If higher specificity is required, remove soluble sugars prior to hydrolysis.
② Unified hydrolysis and color-development conditions: Standardize sulfuric acid addition order, reaction temperature, and the color-development time window to minimize within-batch drift.
③ Standard curve and linear range verification: Build a calibration curve using glucose standards; dilute samples to ensure readings fall within the valid linear range.
④ Background correction: Include reagent blanks and sample blanks; use matrix-matched blanks when background absorbance is substantial.
(3) Advantages, Limitations, and Applicability
① Advantages: Relatively high throughput; suitable for relative quantification across multiple treatments.
② Limitations: Sensitive to acid strength and temperature; inadequate pretreatment can allow other carbohydrate sources to contribute background signal.
③ Use scenarios: Cellulose screening; comparison of cellulose changes among tissues or treatments.
1.3 α-Cellulose: Alkali Insolubility Method
(1) Principle
α-Cellulose is defined as the cellulose fraction insoluble in an alkali solution of specified concentration. The alkali method removes part of the alkali-soluble polysaccharides by controlled alkaline extraction, retains the insoluble residue, and determines α-cellulose content gravimetrically. This index is used to evaluate the proportion of the insoluble cellulose framework.
(2) Key Procedural Considerations
① Fixed alkali concentration and treatment conditions: NaOH concentration, temperature, duration, and agitation intensity define the solubility boundary.
② Residue recovery and adequate washing: Loss of residue causes underestimation; insufficient washing leaves salts that inflate mass and cause overestimation.
③ Standardized drying to constant mass: Keep drying temperature, drying time, and constant-weight criteria consistent to reduce moisture-related error.
④ Precision control: Include replicates and process blanks to monitor within-batch precision.
(3) Advantages, Limitations, and Applicability
① Advantages: Emphasizes the insoluble cellulose backbone and supports structural comparisons.
② Limitations: Operationally defined and condition-dependent; separation efficiency is more sensitive in highly lignified samples.
③ Use scenarios: Maturity and lignification studies; evaluation of cellulose structural stability.
1.4 Holocellulose: Sodium Chlorite Method
(1) Principle
Holocellulose typically refers to the polysaccharide framework retained after delignification. In the sodium chlorite method, selective delignification is performed under acidic conditions, polysaccharide residues are retained, and holocellulose is quantified gravimetrically. This provides an estimate of overall cell-wall polysaccharide abundance.
(2) Key Procedural Considerations
① Controlled delignification: Keep sodium chlorite dosage, acidity, temperature, and reaction time strictly consistent.
② Safety and side-reaction management: The strong oxidizing system requires controlled reagent addition and temperature to prevent vigorous reactions and sample loss.
③ Residue washing and constant-weight determination: Wash thoroughly to remove residual salts and acids; dry to constant mass before calculation.
④ Applicability verification: For lignin-rich materials, verify delignification sufficiency using reference or control samples.
(3) Advantages, Limitations, and Applicability
① Advantages: Provides total polysaccharide framework information and supports joint interpretation with lignin indices.
② Limitations: Sensitive to reaction conditions; both insufficient and overly aggressive delignification can introduce systematic bias.
③ Use scenarios: Comparison of overall cell-wall polysaccharide levels; analysis of lignification effects on polysaccharide proportion.
1.5 Crude Fiber: Gravimetric Method
(1) Principle
Crude fiber is a traditional gravimetric index. After sequential acid boiling and alkali boiling to remove soluble components, the remaining residue is dried and corrected by ashing to obtain crude fiber content. It provides a rapid descriptor of recalcitrant residues.
(2) Key Procedural Considerations
① Fixed acid/alkali treatment conditions: Standardize acid/alkali concentration, boiling duration, solid-to-liquid ratio, and filtration mode.
② Control of filtration and transfer losses: Filter media, washing intensity, and transfer handling determine recovery and repeatability.
③ Ash correction: Standardize ashing temperature and duration to subtract inorganic contributions.
④ Interpretive boundary: Suitable for trend comparison, but it should not be used as a direct substitute for NDF/ADF or dietary fiber when structural interpretation is required.
(3) Advantages, Limitations, and Applicability
① Advantages: Mature and widely used for routine quality evaluation and high-throughput screening.
② Limitations: Biochemical meaning is relatively coarse; compositional detail is strongly compressed.
③ Use scenarios: Preliminary quality screening; rough characterization of recalcitrant residue levels.
1.6 Lignin: 72% Sulfuric Acid Hydrolysis Coupled with Titration
(1) Principle
High-concentration sulfuric acid hydrolyzes polysaccharides and leaves lignin-related residues. Lignin content is then determined using titration or an associated quantification scheme. This method supports evaluation of lignification degree and its relationship to digestibility and mechanical strength.
(2) Key Procedural Considerations
① Strictly consistent acidolysis conditions: Fix sulfuric acid concentration, temperature, duration, and sample pretreatment.
② Adequate separation and washing: Wash residues thoroughly to remove acid and solubles, minimizing titration background interference.
③ Standardized titration system: Standardize titrants, endpoint determination, and blank subtraction.
④ Safety control: Concentrated sulfuric acid is highly corrosive and strongly exothermic upon mixing; strict protective measures are required.
(3) Advantages, Limitations, and Applicability
① Advantages: Enables quantitative comparison of lignin levels and supports linkage to structural traits.
② Limitations: Highly sensitive to condition consistency; operationally demanding.
③ Use scenarios: Assessment of lignification degree; studies of maturity- or stress-induced lignin deposition.
II. Detergent Fiber System: Construction and Application of Cell-Wall Fractionation Indices
2.1 Neutral Detergent Fiber (NDF): Detergent Method
(1) Principle
Neutral detergent extraction dissolves cell contents and part of the soluble fraction, retaining cell-wall-related residues. The residue mass is determined gravimetrically to obtain NDF, representing overall cell-wall abundance.
(2) Key Procedural Considerations
① Standardize particle size, solid-to-liquid ratio, treatment temperature, and duration to reduce kinetic variability.
② Use consistent filter media and washing steps to control residue carryover and recovery.
③ Apply ash correction when needed, and include replicates and reference samples to monitor between-batch comparability.
(3) Advantages, Limitations, and Applicability
① Advantages: Good reproducibility; suitable for assessing overall cell-wall level and for digestibility linkage studies.
② Limitations: Sensitive to particle size and filtration handling.
③ Use scenarios: Feed-quality evaluation; comparative assessment of cell-wall deposition.
2.2 Acid Detergent Fiber (ADF): Detergent Method
(1) Principle
Acid detergent extraction further removes portions of hemicellulose and related components, leaving residues dominated by cellulose and lignin. Gravimetric determination yields ADF, reflecting the recalcitrant cell-wall framework.
(2) Key Procedural Considerations
① Keep acid detergent formulation, temperature, duration, and agitation intensity consistent.
② Standardize filtration and residue recovery to avoid systematic bias from fine-particle carryover.
③ Interpret ADF together with NDF; when using differences to infer components, increase replication to control error amplification.
(3) Advantages, Limitations, and Applicability
① Advantages: More sensitive to recalcitrant structural fractions.
② Limitations: Requires strict consistency in washing and filtration operations.
③ Use scenarios: Maturity and lignification studies; feed digestibility prediction frameworks.
2.3 Acid Detergent Lignin (ADL): Acid Detergent Followed by Sulfuric Acid Hydrolysis
(1) Principle
Starting from the ADF residue, sulfuric acid hydrolysis is applied to remove cellulose and other polysaccharides, retaining lignin-related residues. Gravimetric determination yields ADL, representing lignin fractionation within the detergent system.
(2) Key Procedural Considerations
① Ensure stable acquisition and thorough washing of ADF residues to minimize effects of residual acid/salts on constant-weight determination.
② Keep sulfuric acid treatment conditions strictly consistent; wash to remove acid and dry to constant mass before calculation.
③ Interpret ADL jointly with NDF/ADF; it is not recommended to compare ADL across studies outside the detergent framework.
(3) Advantages, Limitations, and Applicability
① Advantages: Forms a closed interpretive system with NDF/ADF, facilitating structured interpretation and feed evaluation.
② Limitations: High-acid handling demands; sensitive to residue recovery and constant-weight control.
③ Use scenarios: Feed digestibility studies; graded characterization of lignification differences.
III. Dietary Fiber: Structural Nutritional Evaluation by the Enzymatic–Gravimetric Method
3.1 Total/Soluble/Insoluble Dietary Fiber: Enzymatic–Gravimetric Method
(1) Principle
The enzymatic–gravimetric method removes starch and protein using α-amylase, protease, and amyloglucosidase, retaining fiber fractions not degradable by human digestive enzymes. Total dietary fiber (TDF) is obtained via filtration, precipitation-based separation, and gravimetric mass balance. Additional fractionation steps distinguish soluble dietary fiber (SDF) and insoluble dietary fiber (IDF), supporting nutritional quality and functionality evaluation.
(2) Key Procedural Considerations
① Fix enzyme activity, temperature, pH, and reaction time; monitor enzyme-batch differences using controls or check samples.
② The completeness of starch/protein removal determines accuracy; fix ethanol precipitation concentration (v/v) and separation conditions to stabilize fractionation boundaries.
③ Apply ash and residual protein corrections when necessary; include replicates and stable reference samples to monitor between-batch comparability.
(3) Advantages, Limitations, and Applicability
① Advantages: Consistent with the nutritional definition of dietary fiber; enables discrimination of SDF vs. IDF.
② Limitations: Time-consuming; requires strict control of enzyme activity and separation consistency.
③ Use scenarios: Nutritional evaluation of foods and feeds; assessment of processing effects on dietary fiber.
IV. Overall Comparison of Indices and Methods and Application-Oriented Selection
Index | Recommended method | Sensitivity | Key control points and common interferences | Use scenarios |
Hemicellulose | HCl hydrolysis | Medium | Consistent hydrolysis/neutralization/separation; lignification affects kinetics | Cell-wall composition comparison; hemicellulose trend assessment |
Cellulose | Anthrone–sulfuric acid colorimetry | Medium | Sensitive to acid and temperature; incomplete pretreatment increases carbohydrate background | Cellulose screening and batch comparison |
α-Cellulose | Alkali insolubility (NaOH) | Medium | Solubility boundary depends on concentration/time; residue recovery and constant weight | Insoluble cellulose framework proportion evaluation |
Holocellulose | Sodium chlorite method | Medium | Balance between delignification completeness and polysaccharide loss; oxidant control | Total polysaccharide framework assessment; joint analysis with lignin |
Crude fiber | Gravimetric method | Medium | Acid/alkali treatment and ash correction; filtration/transfer losses | Routine evaluation; rapid screening |
Lignin | 72% H₂SO₄ hydrolysis + titration | Medium | High-acid consistency; titration endpoint and blank subtraction | Lignification assessment; linkage to strength/digestibility |
NDF | Detergent method | Medium | Washing/filtration recovery; particle size and ash effects | Overall cell-wall level; feed evaluation models |
ADF | Detergent method | Medium | Acid detergent consistency; filtration and particle carryover | Recalcitrant framework; maturity/lignification studies |
ADL | Acid detergent + sulfuric acid hydrolysis | Medium | ADF residue quality; sulfuric acid consistency; constant weight | Lignin fractionation within detergent framework |
TDF/SDF/IDF | Enzymatic–gravimetric | Medium | Enzyme activity and removal efficiency; ethanol precipitation boundary; ash/protein correction | Nutritional evaluation; processing impact studies |
V. Aladdin-Related Products
Name | CAS No. | Applicable indices | Typical use/notes |
Sodium hydroxide | Neutralization after acidolysis; α-cellulose (alkali method); crude fiber | Neutralization; alkali fractionation; alkali boiling | |
Anthrone | Cellulose (anthrone–H₂SO₄ colorimetry) | Color reagent | |
D-Glucose | Cellulose (anthrone–H₂SO₄) | Standard for calibration curve | |
Sodium chlorite | Holocellulose (chlorite method) | Selective delignification oxidant | |
Acetic acid (glacial) | Holocellulose (chlorite method) | Provides acidity; system adjustment | |
Alpha-amylase | TDF/SDF/IDF | Starch removal (enzymatic digestion) | |
Protease | TDF/SDF/IDF | Protein removal (enzymatic digestion) | |
Amyloglucosidase | TDF/SDF/IDF | Deep hydrolysis of starch/dextrins |
VI. Method Combinations and Experimental Design Recommendations
6.1 Cell-Wall Composition and Maturity Studies
(1) Recommended combination: holocellulose (chlorite method) + lignin (sulfuric-acid hydrolysis system) + cellulose (colorimetry or fractionation-based index).
(2) Interpretive framework: Use polysaccharide framework abundance and lignin deposition as dual axes to support structural interpretation of mechanical strength and maturity.
6.2 Feed Quality and Digestibility Association Evaluation
(1) Recommended combination: NDF + ADF + ADL (detergent fiber system).
(2) Key requirements: Strictly standardize particle size, detergent systems, filter media, and constant-weight criteria. When inferring components via differences, increase replication to control error propagation.
6.3 Nutritional and Functional Fiber Evaluation in Foods
(1) Recommended combination: TDF/SDF/IDF (enzymatic–gravimetric) supplemented by NDF/ADF/ADL when structural interpretation is needed.
(2) Key requirements: Monitor enzyme-batch variability and completeness of starch/protein removal; fix ethanol precipitation and separation conditions to stabilize fractionation boundaries.
Plant fiber-related indices provide a multilayered characterization system spanning cell-wall framework polysaccharides, lignin deposition, detergent-based fractionation, and the nutritional definition of dietary fiber. Chemical hydrolysis/delignification coupled with colorimetric or gravimetric determination is suitable for quantitative comparison of structural components. The detergent fiber system offers consistency and a clear interpretive framework for feed evaluation and structural-parameter modeling. The enzymatic–gravimetric method is more consistent with dietary fiber definitions and functional evaluation. In practice, index combinations should be built around the scientific question, and result comparability and interpretive robustness should be strengthened through standardized pretreatment, separation conditions, constant-weight determination, blanks, and QC strategies, thereby supporting more accurate structure–function association analyses and application decisions.
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[1] Research Progress in Detection Technologies for Soluble Sugars in Plants