Gel strength determination, also known as jelly strength or Bloom value testing, is the fundamental quality control measurement for gelatin and other gelling agents, quantifying the rigidity of a standardized gel in grams-force. This article explains the technical principles behind gel strength determination, the applicable standards such as ISO 9665 and GB 6783, sample preparation and testing procedures, and its applications across the food, pharmaceutical, and industrial sectors.
Gel strength is the single most important specification parameter for gelatin and many other hydrocolloids, directly determining functional performance in gelling, film-forming, and texture applications. Determination of gel strength (jelly strength, Bloom value) is routinely performed by accredited third-party laboratories and in-house quality control departments to verify product grade, support lot release, and qualify suppliers.
Technical Overview
Gel strength determination measures the force, expressed in grams, required for a standard cylindrical plunger to penetrate 4 mm into the surface of a gel prepared and conditioned under strictly standardized conditions. The result, known as the Bloom value or jelly strength, reflects the molecular weight distribution and gelling capability of the protein or polysaccharide sample. The test was originally performed on a mechanical Bloom gelometer and is now almost universally carried out on calibrated texture analyzers equipped with a dedicated Bloom probe and load cell.
- Standard test concentration for gelatin: 6.67% w/v (7.50 g of sample in 105 mL of water)
- Plunger: polished cylindrical probe, 12.7 mm in diameter, with a flat or slightly rounded face
- Penetration depth: 4 mm below the gel surface
- Conditioning: maturation in a controlled water bath, typically at 10 °C for 16–18 hours
- Result: peak force in grams (Bloom value), reported as the mean of replicate determinations
The principal standards governing the method include ISO 9665 (adhesives — animal glues — sampling and testing), GB 6783 (food additive gelatin), JIS K 6503 for animal glue testing, and the gelatin monographs of major pharmacopoeias such as USP and Ph. Eur. Gelatin grades are conventionally classified by Bloom value: low Bloom grades (below approximately 150 g), medium grades (approximately 150–220 g), and high Bloom grades (above approximately 220 g), with commercial products typically ranging from about 80 to 300 g.
Testing Samples
Edible Gelatin
Food-grade gelatin derived from porcine skin, bovine hide, or bone is the most frequently tested sample type. Gel strength determination confirms the declared Bloom grade and supports conformance with national food additive standards such as GB 6783.
Pharmaceutical Gelatin
Gelatin used for hard capsules, softgel shells, tablet binding, and microencapsulation requires tightly controlled Bloom values, typically in the medium-to-high range, to guarantee shell mechanical strength, dissolution behavior, and drug release performance. Testing is performed against pharmacopoeial requirements and internal specifications.
Industrial Gelatin and Animal Glues
Technical gelatin, bone glue, and hide glue used in adhesives, paper sizing, matches, abrasives, and photographic emulsions are tested for jelly strength under ISO 9665 or JIS K 6503 to classify quality and predict bonding and film-forming performance.
Other Hydrocolloids and Functional Gums
Agar, carrageenan, konjac glucomannan, pectin, and mixed gelling systems can also be evaluated. For these materials, the test concentration (commonly 1–2% gels), probe geometry, and conditioning regime differ from the gelatin Bloom method, and results are often expressed in g/cm² based on the effective probe area, in accordance with the relevant product standard.
Testing Items
Typical testing items offered for gel strength determination include:
- Bloom (jelly) strength of gelatin at the standard 6.67% w/v concentration
- Gel breaking strength and penetration strength of hydrocolloid gels at specified concentrations
- Comparative gel strength at alternative concentrations or conditioning temperatures for formulation studies
- Supporting characterization such as viscosity, gelation temperature, melting temperature, pH, moisture, and ash content, which are commonly reported together with gel strength for full specification release
Testing Methods
Sample Preparation
A representative portion of the sample, typically 7.50 g of gelatin, is weighed into a standard 150 mL Bloom bottle and 105 mL of distilled water is added. The granules are allowed to hydrate (bloom) at room temperature for a defined period, generally 1–4 hours, until fully swollen. The bottle is then heated in a water bath at approximately 60–65 °C for 10–20 minutes, with gentle swirling, until the gelatin is completely dissolved. Overheating above approximately 70 °C or prolonged heating must be avoided, as thermal degradation lowers the measured gel strength.
Conditioning
The dissolved solution is topped up to the standard fill level, capped, and allowed to stand at room temperature for about 15 minutes before being placed in a controlled water bath at 10 °C, maintained to within ±0.1 °C, for 16–18 hours (commonly 17 ± 1 hours). Consistent maturation time and temperature are essential because gel networks continue to develop during aging.
Determination Procedure
The conditioned bottle is centered on the platform of a calibrated texture analyzer, ensuring that the gel surface is free of condensation and that the bottle is not disturbed. The 12.7 mm cylindrical plunger is driven into the gel surface at a constant crosshead speed as specified by the referenced method, typically 0.5 or 1.0 mm/s, to a depth of 4 mm. The peak force recorded during penetration, expressed in grams, is the Bloom value. Determinations are performed in duplicate or triplicate, and the mean is reported, usually rounded to the nearest gram. Replicate results should normally agree within approximately 2–3%.
Quality Control and Influencing Factors
Key parameters that must be controlled to obtain accurate and comparable results include:
- Accuracy of the sample mass and final solution concentration, including allowance for moisture content of the sample
- Precision of the conditioning bath temperature, with deviations as small as 1 °C measurably shifting the result
- Hydration time, dissolution temperature, and heating duration during preparation
- Conformity of the Bloom bottle geometry and the dimensional accuracy, surface finish, and vertical alignment of the plunger
- Regular calibration of the load cell and verification of crosshead speed with certified reference weights and equipment
- Use of defoamed, bubble-free gel surfaces and undisturbed, unmarked gels at the moment of testing
Applications
Gel strength data are used throughout the supply chain of gelatin and hydrocolloid products:
- Food industry: grade classification of edible gelatin; formulation and texture control of gummy confectionery, marshmallows, jelly desserts, aspic, and meat products; incoming material inspection
- Pharmaceutical industry: qualification of gelatin for hard capsules and softgel shells; control of shell mechanical strength and disintegration; support of regulatory submissions
- Industrial applications: specification of animal glues for adhesives, paper and textile sizing, abrasive binders, and photographic emulsions
- Cosmetics and consumer products: performance evaluation of gel-based face masks, styling gels, and encapsulated actives
- Research and development: comparison of extraction processes, enzymatic or hydrolyzed gelatins, bloom and set-time studies, and development of mixed hydrocolloid systems
- Quality assurance and trade: lot release testing, supplier qualification, certificate of analysis verification, and dispute resolution by accredited third-party laboratories
FAQ
What is the Bloom value?
The Bloom value is the force in grams required for a 12.7 mm diameter plunger to penetrate 4 mm into the surface of a 6.67% gelatin gel that has been matured at 10 °C for 16–18 hours. It is the internationally recognized index of gelatin gel strength and grade.
Which standards are used for gel strength determination?
The most commonly applied standards are ISO 9665 for animal glues, GB 6783 for food additive gelatin, JIS K 6503 for animal glue testing, and the gelatin monographs of major pharmacopoeias. The basic test conditions of these methods are harmonized around the classical Bloom procedure.
How much sample is required and how long does testing take?
Approximately 20–50 g of sample is generally sufficient to prepare replicate determinations. Because the gel must be conditioned for 16–18 hours, results are typically available within 2–3 working days after sample acceptance.
Why is temperature control so critical?
Gel network formation is strongly temperature dependent. Both the conditioning temperature and its uniformity across the aging period directly affect the measured force; therefore, the water bath is maintained at 10 °C with a tolerance of ±0.1 °C to ensure reproducibility between laboratories.
What can cause abnormally low or variable gel strength results?
Frequent causes include thermal degradation during dissolution, incomplete hydration, incorrect weighing or final volume, disturbed gel surfaces, inconsistent aging time, incorrect plunger dimensions or alignment, foam on the gel surface, and degradation of the sample during storage due to moisture or heat exposure.
How is gel strength different from viscosity?
Gel strength measures the rigidity of the formed gel network and correlates with the higher molecular weight fraction of the polymer, whereas viscosity measures the flow resistance of the solution at a given temperature and concentration. Both parameters are usually specified together, as they describe complementary functional properties of gelatin.