A hardness test measures the force that makes a sample break, snap or collapse under compression at a controlled speed, and reports it as a number in newtons (N). For drug tablets and dietary supplement tablets, this value is often called tablet hardness, while the United States Pharmacopeia (USP) calls it tablet breaking force (United States Pharmacopeial Convention, 2023a). For pressed cosmetics, the peak force measured shows how well the product holds its shape and how it feels in use.
In solid products such as chewable tablets, dietary supplement tablets, pressed foundations and eye shadows, hardness is a parameter that determines both quality during production and the consumer’s experience of use. Chewable tablets that are too hard put consumers off after the first try, tablets that are too soft break easily during packing and transport, and pressed foundations that are too brittle may crack when dropped or knocked.
Hardness is not a value judged by human feel but a physical quantity measured with an instrument that applies controlled compression and records the peak force the sample withstands before it breaks or collapses. It is reported in newtons, although older instruments and some industry documents still use kilogram-force (kgf or kp), where 1 kilogram-force equals about 9.8 newtons. Setting a hardness specification for each product means finding the balance between holding shape during transport and the consumer’s experience of use.
VISBIO Laboratory, in collaboration with partner laboratories, provides hardness testing with a tablet hardness tester and a texture analyzer, instruments used in the pharmaceutical, food and cosmetics industries. The service covers compressed dietary supplement tablets, chewable tablets, pressed foundations, eye shadows and other pressed cosmetics.
Why hardness matters for both production and the consumer experience
In tablet production, hardness is a parameter that must balance robustness during transport against disintegration in the digestive tract. Tablets that are too hard withstand transport well but may disintegrate in the stomach more slowly than designed, which may affect the release of the active ingredient. Conversely, tablets that are too soft disintegrate quickly but crack or chip easily during packing and transport. Powder lost from cracked tablets makes tablet weights uneven, and the amount of active ingredient in a tablet may fall short of what the label states.
In pressed cosmetics, hardness determines both how well the product holds its shape and how it feels when swiped. If it is too hard, a foundation is difficult to pick up and loads unevenly onto the puff, so consumers find it hard to use. If it is too soft, the product breaks easily during transport or when dropped in normal use.
In chewable dietary supplement tablets and chewable confectionery with added active ingredients, hardness determines the feel when biting and chewing. Tablets that are too hard are difficult for older people and children to chew, so the product does not suit its target group, while tablets that are too soft may stick to the teeth and put consumers off buying again. Hardness specifications should therefore reflect the needs of end consumers, and products should be tested with real users before launch.
From a regulatory perspective, the Cosmetic Act B.E. 2558 (2015) and the Food Act B.E. 2522 (1979) do not set hardness values directly, but manufacturers of products that state the amount of active ingredient per serving on the label usually set a hardness range in the product specification, because controlled hardness helps keep tablet weight and active-ingredient content per tablet consistent. For herbal products in tablet form under the Herbal Products Act B.E. 2562 (2019), hardness is an in-process quality control value used together with the tablet dosage-form tests found in pharmacopoeias, such as disintegration, weight uniformity and friability, which measures the weight a tablet loses from tumbling and impact in a test apparatus (United States Pharmacopeial Convention, 2023b).
Principles of hardness testing and related techniques
The basic principle of hardness measurement is to apply compression to a sample at a constant speed or rate of force increase, record the resisting force against the travel of the probe, and read the peak force before the sample breaks. For compressed tablets, tablet breaking force is measured with two flat, parallel platens that compress the tablet across its diameter until it breaks. The peak force recorded (crushing strength) is reported in newtons (United States Pharmacopeial Convention, 2023a).
For pressed products other than tablets, such as pressed foundations and eye shadows, the test uses a texture analyzer with a cylindrical or flat-plate probe that presses into the center of the product at a set speed and distance. Reported results are the peak force at the initial yield point and the area under the force–distance curve, which represents resistance to collapse. Testing may also include a drop test, in which the product in its compact is released from a set height, to confirm that it withstands real conditions of use.
Technically, three factors must be controlled for results to be comparable. The first is controlling the temperature and humidity of the test room: hygroscopic products, such as sugar-containing chewable tablets and dietary supplement gummies, give very different hardness values when tested at 50% compared with 70% relative humidity. The second is choosing a compression speed that suits the product, for example in the range of 1–5 millimeters per second, and using the same speed every time results are compared, because different speeds give very different values. The third is testing enough samples: for compressed tablets, at least 10 tablets per batch are generally tested, and the mean is reported together with the standard deviation.
Scope of VISBIO Laboratory's hardness testing service
VISBIO Laboratory’s service, carried out in collaboration with partner laboratories, covers two groups of samples:
- Compressed tablets: compressed dietary supplement tablets, chewable tablets, herbal chewable tablets and film-coated tablets
- Pressed cosmetics: pressed foundations, eye shadows, pressed blushes and pressed powders
Results are reported as the peak force in newtons according to the test format: the breaking force for compressed tablets, and the force at the initial yield point with the area under the curve for pressed cosmetics, together with the mean and standard deviation. Clients can choose a single test for batch release or monitoring at several time points during a stability study.
Using hardness test results
Hardness test results can be used at several stages of a product’s life:
- Research and development: select the ingredient proportions and compression conditions that give a hardness matching the product concept, and compare the effect of changing a raw-material supplier, which may change the hardness of the finished product.
- Pre-launch: set the baseline for the specification and the acceptable hardness range for batch release.
- Commercial production: use as a batch-release criterion that helps detect deviations in the production process.
- Stability studies: combine with real-time and accelerated stability studies to confirm that hardness stays within limits throughout shelf life.
- Export: attach to the Certificate of Analysis (COA) for buyers who set an acceptable hardness range or change in hardness in their trade agreements.
- Product line extension: use the hardness of a formula already on the market as a reference point when scaling up or developing new formulas with a similar structure.
Frequently asked questions about hardness testing
What is a hardness test?
A hardness test measures the force that makes a sample break, snap or collapse under compression at a controlled speed, and reports it in newtons. It is used for compressed tablets, chewable tablets and pressed cosmetics.
What is the difference between tablet hardness and tablet breaking force?
Both terms mean the same value: the force that makes a tablet break when compressed across its diameter. The United States Pharmacopeia uses the term breaking force and reports it in newtons, while some older instruments still report it in kilogram-force.
Why are tablets that are too hard or too soft a problem?
Tablets that are too hard withstand transport well but may disintegrate more slowly than designed, and chewable tablets that are too hard are difficult for older people and children to chew. Tablets that are too soft crack easily during packing and transport, making tablet weight and active-ingredient content per tablet uneven.
How is the hardness of pressed cosmetics tested?
For pressed cosmetics such as pressed foundations and eye shadows, a cylindrical or flat-plate probe presses into the center of the product. The results are the peak force at the initial yield point and the area under the force–distance curve, which relate to how well the product holds its shape and how it feels in use.
Why must temperature, humidity and compression speed be controlled?
Hygroscopic products give very different hardness values at different room humidity, and different compression speeds give different measured values. Testing under the same conditions every time makes it possible to compare batches and time points.
How many tablets must be tested per batch?
For compressed tablets, at least 10 tablets per batch are generally tested, and the mean is reported together with the standard deviation.
References:
- United States Pharmacopeial Convention. (2023a). Tablet breaking force (USP <1217>). USP.
- United States Pharmacopeial Convention. (2023b). Tablet friability (USP <1216>). USP.
- Royal Thai Government Gazette. (1979, May 13). Food Act B.E. 2522 (1979). Royal Thai Government Gazette, 96(79 Kor), 1–24.
- Royal Thai Government Gazette. (2015, September 8). Cosmetic Act B.E. 2558 (2015). Royal Thai Government Gazette, 132(86 Kor), 1–22.
- Royal Thai Government Gazette. (2019, April 30). Herbal Products Act B.E. 2562 (2019). Royal Thai Government Gazette, 136(56 Kor), 116–161.