Exopolysaccharide (EPS) gel strength determination is a critical characterization and quality control test for microbial polysaccharides widely used in food, pharmaceutical, and biomedical industries. This technical article explains the principle, sample types, testing items, standard operating parameters, and analytical methods for accurately measuring the gel strength of EPS produced by lactic acid bacteria and other microorganisms. Reliable gel strength data enable formulators, researchers, and QC laboratories to compare EPS batches, optimize fermentation processes, and predict textural performance in finished products.
Technical Overview
Exopolysaccharides (EPS) are high-molecular-weight carbohydrate polymers secreted by microorganisms, particularly lactic acid bacteria such as Lactobacillus, Lactococcus, Streptococcus thermophilus, and Bifidobacterium, as well as by fungi and microalgae. They are classified as homopolysaccharides (for example dextran, pullulan, curdlan, and bacterial cellulose) or heteropolysaccharides (for example kefiran, xanthan gum, and gellan gum). When dispersed in water under controlled conditions, many EPS molecules associate through hydrogen bonding, electrostatic interactions, or cation-mediated crosslinking to form a three-dimensional network that entraps large amounts of water, producing a viscoelastic gel.
Gel strength is defined as the force required to rupture or deform a gel of defined geometry and is typically expressed in grams-force (gf) or Newtons (N). It is the single most important texture-related functional parameter for gelling EPS, because it directly reflects the density and stability of the polymer network. Gel strength is influenced by EPS concentration, molecular weight distribution, monosaccharide composition, degree of substitution with acetyl or pyruvyl groups, solvent pH, ionic strength, thermal history, and measurement temperature. Quantitative determination is therefore performed under tightly standardized conditions to ensure reproducibility and comparability between batches and laboratories.
Testing Samples
Microbial EPS from Fermentation
This category includes crude and purified exopolysaccharides isolated from fermentation broth. Typical workflow samples include centrifuged and ethanol-precipitated crude EPS, dialyzed and deproteinated EPS fractions, and lyophilized EPS powders. These samples are rehydrated at defined concentrations to prepare model gels for strength evaluation, which is essential during strain screening and fermentation optimization studies.
Commercial Microbial Hydrocolloids
Industrial polysaccharides of microbial origin, including xanthan gum, gellan gum, curdlan, pullulan, dextran, and bacterial cellulose, are routinely tested for gel strength as part of incoming material inspection and supplier qualification. Ion-sensitive grades, such as low-acyl gellan gum, are tested with controlled calcium or potassium ion addition.
EPS-Containing Finished Products
Textured food products such as stirred or set yogurt, fermented milk, drinking yogurt, sauces, dressings, and desserts may be tested directly or with minimal preparation to quantify the textural contribution of in-situ EPS. Biomedical and cosmetic hydrogels, including wound dressings, moisturizing gels, and tissue engineering matrices, are also evaluated for gel firmness and structural integrity.
Testing Items
A comprehensive EPS gel characterization program typically includes the following measurable parameters:
- Gel breaking strength (rupture force), reported in gf or N
- Gel hardness and firmness at defined penetration depth
- Apparent rigidity or gel modulus derived from the initial force-deformation slope
- Elasticity, brittleness, and deformation at rupture point
- Texture Profile Analysis (TPA) parameters: hardness, springiness, cohesiveness, gumminess, chewiness, and resilience
- Dynamic rheological parameters: storage modulus (G'), loss modulus (G''), loss tangent (tan delta), and gelation temperature or gel point
- Water holding capacity and syneresis, as supporting indicators of gel network quality
Testing Methods
Gel Sample Preparation
Lyophilized EPS powder is accurately weighed and dispersed in deionized water or a defined buffer at a specified concentration, commonly 10 to 40 mg/mL (1 to 4 percent w/v) depending on the gelling capacity of the sample. The dispersion is hydrated by heating at 80 to 90 degrees Celsius for 30 minutes with continuous stirring to ensure complete dissolution. The hot solution is immediately transferred into cylindrical molds, typically 25 mm in diameter and 20 mm in height, taking care to eliminate air bubbles. Molds are cooled and conditioned at 4 degrees Celsius for 12 to 24 hours to allow full network maturation, then tempered at 25 degrees Celsius for 30 minutes before measurement. For cation-dependent EPS, a standardized calcium or potassium salt solution is incorporated at this stage.
Texture Analyzer Penetration Test
The primary method uses a texture analyzer or universal testing machine equipped with a cylindrical flat-faced probe, commonly a P/0.5R plunger of 12.7 mm diameter. Typical operating parameters are a pre-test speed of 2.0 mm/s, a test speed of 1.0 mm/s, a post-test speed of 10 mm/s, a trigger force of 5 g, and a penetration depth of 10 to 15 mm or until gel rupture. The peak force recorded during penetration is reported as gel strength, where 1 N corresponds to approximately 102 gf. A minimum of five to six replicate gel cylinders are tested, and results are reported as mean plus or minus standard deviation with a relative standard deviation target of below 10 percent.
Bloom Reference Method
For gelatin-like EPS systems, the Bloom method adapted from ISO 9665 may be applied. A 6.67 percent w/v solution is prepared, chilled at 10 degrees Celsius for 17 to 18 hours, and tested with a 12.7 mm plunger penetrating to a depth of 4 mm. This approach provides data directly comparable to commercial gel strength grades.
Rheological Verification
Small-amplitude oscillatory shear measurements on a rotational rheometer with 40 mm parallel plates and a 1 mm gap provide complementary structural information. Frequency sweeps from 0.1 to 10 Hz are performed within the linear viscoelastic region, typically at 0.5 to 1 percent strain, while temperature sweeps during cooling identify the gelation point where the storage modulus exceeds the loss modulus. Rheology confirms thermoreversibility, gel kinetics, and network stability that puncture testing alone cannot resolve.
Applications
Gel strength data for EPS support decision-making across multiple industries and research fields:
- Screening and selection of high-EPS-yielding lactic acid bacteria strains for improved dairy texture without added stabilizers
- Optimization of fermentation parameters such as carbon source, temperature, and incubation time based on functional gel performance
- Clean-label product development in yogurt, fermented milk, and plant-based analogues where EPS replaces synthetic hydrocolloids
- Quality control and batch release testing of commercial microbial hydrocolloids
- Design of pharmaceutical sustained-release matrices, encapsulation shells, and wound dressing hydrogels
- Development of biomedical scaffolds and cosmetic gel formulations requiring defined mechanical integrity
FAQ
Question: What concentration of EPS should be used for gel strength testing?
Answer: There is no single mandatory concentration. A concentration producing a measurable, self-supporting gel should be selected, commonly 10 to 20 mg/mL for strong gelling EPS and up to 40 mg/mL for weakly gelling heteropolysaccharides. Whatever concentration is chosen must be reported explicitly, because gel strength scales strongly with polymer concentration.
Question: How should gel strength results be expressed?
Answer: Results are reported as the peak rupture force in grams-force (gf) or Newtons (N), together with probe diameter, penetration depth, test speed, gel concentration, conditioning temperature, and conditioning time. Reporting these conditions is essential for inter-laboratory comparison.
Question: What is the difference between gel strength and viscosity?
Answer: Viscosity measures resistance to flow in a fluid or semi-fluid system, whereas gel strength measures the mechanical force needed to fracture an intact gel network. An EPS sample may show high viscosity without forming a true gel, so both parameters characterize different aspects of texture.
Question: Can fermentation broth be tested directly?
Answer: Direct testing of broth is possible for comparative purposes, but cell debris and proteins interfere with gel formation. For accurate functional characterization, EPS should be purified by ethanol precipitation, dialyzed, and lyophilized before standardized gel preparation.
Question: How can measurement variability be minimized?
Answer: Variability is controlled by strict standardization of dissolution temperature, mold geometry, cooling profile, conditioning time, and test temperature, combined with careful bubble removal, consistent probe calibration, and adequate replication across independent gel preparations.