EPS Polysaccharide Detection by the Anthrone-Sulfuric Acid Method: Principles, Procedures, and Appli

Exopolysaccharide (EPS) quantification by the anthrone-sulfuric acid method is a widely adopted colorimetric technique for determining total polysaccharide content in microbial fermentation broths, probiotic products, and functional food samples. This article provides a comprehensive technical overview of the detection principle, sample preparation workflow, operational procedure, and practical applications of the anthrone-sulfuric acid assay for EPS analysis.

Technical Overview

Exopolysaccharides (EPS) are high-molecular-weight carbohydrate polymers secreted by microorganisms such as lactic acid bacteria, fungi, yeasts, and microalgae. Accurate quantification of EPS is essential in probiotics research, fermented food development, pharmaceutical screening, and biofilm studies. The anthrone-sulfuric acid method is one of the most established and reliable colorimetric assays for total polysaccharide determination.

The principle of the method is based on the strong dehydration capability of concentrated sulfuric acid. When a polysaccharide sample is mixed with anthrone reagent in concentrated sulfuric acid, the glycosidic bonds are hydrolyzed and the resulting monosaccharides are dehydrated to furfural or hydroxymethylfurfural derivatives. These intermediates subsequently condense with anthrone (9,10-dihydro-9-oxoanthracene) to form a blue-green colored complex. The absorbance of this complex is measured spectrophotometrically at 620 nm, and the polysaccharide concentration is calculated against a standard curve, typically prepared with glucose as the reference standard.

The method offers several advantages, including high sensitivity, simple operation, low equipment requirements, and broad applicability to both neutral and acidic polysaccharides. However, because the assay measures total carbohydrate content, free sugars and other interfering reducing substances must be removed during sample pretreatment to ensure accurate results.

Testing Samples

Microbial Fermentation Broths

Fermentation liquids from lactic acid bacteria (such as Lactobacillus, Streptococcus thermophilus, and Bifidobacterium), Bacillus species, and other EPS-producing strains are the most common sample type. The EPS is typically harvested from the cell-free supernatant after centrifugation.

Fungal and Mushroom Polysaccharide Extracts

Extracellular polysaccharides produced by submerged cultivation of medicinal fungi, including Ganoderma lucidum, Cordyceps militaris, and Pleurotus ostreatus, can be quantified using this method after ethanol precipitation and purification.

Microalgal and Cyanobacterial Cultures

EPS released by microalgae and cyanobacteria in photobioreactor cultures is frequently monitored by the anthrone method in algal biotechnology and wastewater treatment research.

Functional Foods and Dairy Products

Yogurt, fermented milk, probiotic powders, capsules, and functional beverages containing microbial EPS or plant polysaccharides can be analyzed after appropriate extraction and deproteinization.

Environmental and Biofilm Samples

EPS extracted from activated sludge, granular sludge, and biofilms in environmental engineering studies is routinely quantified to evaluate microbial aggregation and flocculation performance.

Testing Items

The anthrone-sulfuric acid assay primarily addresses the following analytical parameters:

• Total polysaccharide content in crude or purified EPS samples, expressed as glucose equivalents (mg/g or mg/mL)
• EPS yield in fermentation processes, reported as EPS concentration per unit volume of broth or per unit biomass
• Polysaccharide purity evaluation in purified fractions after deproteinization and dialysis
• Standard curve linearity verification, with correlation coefficients (R²) typically required to exceed 0.999
• Method validation parameters, including precision (relative standard deviation), recovery rate, limit of detection, and limit of quantification

Testing Methods

Reagents and Equipment

The essential reagents include analytically pure anthrone, concentrated sulfuric acid (95–98%), and a glucose standard. Anthrone reagent is freshly prepared by dissolving anthrone (commonly 0.1–0.2 g) in 100 mL of concentrated sulfuric acid, typically prepared as a 0.1% (w/v) solution. Required equipment includes a UV-visible spectrophotometer, water bath or boiling bath, ice bath, centrifuge, precision balance, and volumetric glassware.

Sample Pretreatment

For fermentation broths, the culture is centrifuged at high speed (for example, 8,000–12,000 × g for 10–15 minutes) to remove cells. The supernatant is then mixed with three volumes of cold absolute ethanol and held at 4 °C overnight to precipitate the EPS. The precipitate is collected by centrifugation, redissolved in deionized water, and deproteinized using the Sevag method (chloroform:n-butanol, 4:1 v/v) or trichloroacetic acid (TCA) treatment. Dialysis against distilled water is performed to remove small molecules, free sugars, and salts. For food and biological matrices, additional homogenization, degreasing, and hot-water extraction steps may be required.

Standard Curve Preparation

A glucose stock solution (for example, 0.1 mg/mL) is prepared and diluted into a series of working standards covering an appropriate range, such as 0 to 0.1 mg/mL. Each standard is mixed with freshly prepared anthrone reagent, and the calibration curve is constructed by plotting absorbance at 620 nm against glucose concentration.

Color Development and Measurement

The typical operational procedure is as follows:

• Pipette 1.0 mL of appropriately diluted sample or standard solution into a dry test tube cooled in an ice bath
• Add 4.0 mL of freshly prepared anthrone-sulfuric acid reagent slowly along the tube wall while maintaining cooling, as the dilution of sulfuric acid is strongly exothermic
• Mix thoroughly and heat the sealed tubes in a boiling water bath for 10 minutes
• Remove the tubes and cool them rapidly in an ice-water bath to room temperature
• Measure the absorbance of the blue-green complex at 620 nm against a reagent blank
• Calculate the polysaccharide concentration from the standard curve regression equation and apply dilution factors to obtain the final EPS content

Quality Control Considerations

Anthrone reagent must be prepared freshly or stored protected from light for a limited period, as degraded reagent increases blank absorbance. Sulfuric acid concentration, heating time, and cooling conditions must be strictly controlled because color intensity is time- and temperature-dependent. Replicate measurements (at least triplicates) are recommended, and results should be rejected if the sample absorbance falls outside the linear range of the calibration curve. Contamination with free reducing sugars, proteins, nucleic acids, or phenolic compounds can cause positive bias, so thorough purification is critical.

Applications

The anthrone-sulfuric acid method for EPS detection is extensively applied across multiple disciplines:

• Probiotics and strain screening: comparing EPS production capacity among lactic acid bacterial isolates to identify high-yield strains with technological potential
• Fermented dairy science: evaluating EPS content in yogurt and cultured milk, where EPS improves texture, viscosity, water-holding capacity, and syneresis control
• Functional food and nutraceutical development: quantifying bioactive polysaccharides added to beverages, health supplements, and edible films
• Pharmaceutical research: supporting purification monitoring and batch consistency testing of polysaccharide-based therapeutics and immunomodulatory agents
• Environmental engineering: profiling EPS in activated sludge and aerobic granular sludge to understand flocculation, dewaterability, and membrane fouling mechanisms
• Algal biotechnology and biorefinery: tracking EPS accumulation in microalgal cultures used for biopolymer and biofuel production
• Academic research: providing total carbohydrate data for structural characterization studies combining gel permeation chromatography, FT-IR, and monosaccharide composition analysis

FAQ

Why is glucose used as the standard for EPS quantification?

Glucose is used because it is the most common constituent monosaccharide of EPS and yields a stable, reproducible color reaction with anthrone. Results are conventionally expressed as glucose equivalents. For heteropolysaccharides rich in other sugars, such as fructose or galactose, the use of a corresponding monosaccharide standard can improve accuracy.

What is the detection wavelength for the anthrone method?

The blue-green anthrone-carbohydrate complex exhibits maximum absorbance at approximately 620 nm, sometimes cited in the range of 580–630 nm. Consistent use of 620 nm is recommended for comparability.

How does the anthrone method differ from the phenol-sulfuric acid method?

Both are total carbohydrate colorimetric assays. The anthrone method uses concentrated sulfuric acid alone and requires boiling-water heating, while the phenol-sulfuric acid method combines phenol with sulfuric acid and develops color at room temperature. The phenol method typically measures at 490 nm, whereas the anthrone method measures at 620 nm. Choice between them depends on sample matrix, sensitivity requirements, and laboratory convention.

What are the main sources of interference?

Free monosaccharides and disaccharides remaining in the sample react directly and cause overestimation, so ethanol precipitation and dialysis are essential. Proteins, nucleic acids, pigments, and phenolic compounds may also interfere. Deoxyribose from DNA can contribute to absorbance, which is why deproteinization and nucleic acid removal are recommended for purified EPS fractions.

What is the typical sensitivity and working range of the method?

The anthrone-sulfuric acid method reliably quantifies carbohydrate in the range of approximately 10 to 100 micrograms per assay tube, with detection limits typically in the low microgram range. Samples must be diluted to fall within the linear portion of the standard curve for accurate quantification.

Are there safety precautions to observe?

Yes. Concentrated sulfuric acid is highly corrosive and its dilution is strongly exothermic. Anthrone reagent must be added to cooled tubes in a fume hood or with appropriate personal protective equipment, including acid-resistant gloves, goggles, and a lab coat. Heating sealed tubes should be performed carefully to prevent pressure buildup and splash hazards.