Tablet coating in dietary supplement manufacturing – film coating, enteric coatings, and controlled release

Two tablets may look almost identical – both smooth, colored, and uniform. Yet in terms of their quality profile, they can be worlds apart. A coating that appears from the outside to be a purely aesthetic element in fact determines whether the active substance will reach its site of action intact, when it will begin to be released, and at what rate.

A coating is not decoration. It is a functional element of the formulation that controls when, where, and how the active substance is released from the tablet. One mechanism is responsible for basic mechanical protection, another for resistance to the acidic environment of the stomach, and yet another for prolonged release distributed over many hours.

For a client using the services of a contract manufacturer of dietary supplements (CDMO), the choice of coating type and its application parameters is a decision with direct quality and regulatory consequences. An incorrectly selected or unevenly applied coating means the risk of failing to meet pharmacopoeial criteria and the rejection of the entire batch. Below, we explain how the individual types of coatings work and what determines their correct application.

What tablet coating is and why it matters in production

Tablet coating involves applying a thin polymer layer to the surface of the finished tablet core. The process is carried out in a drum coater or a fluidized-bed coater, where a polymer suspension is sprayed onto tumbling tablets in a stream of heated air (Major and Rathore, 2020).

Coating serves several distinct purposes:

• mechanical protection – protection against abrasion and chipping during transport and packaging;
• moisture barrier – slowing the penetration of moisture into the tablet;
• taste and odor masking – concealing bitter or unpleasant notes of active substances;
• easier swallowing – a smooth, slippery surface;
• visual identification – a colorant or imprint that makes it possible to distinguish the product;
• modification of the release profile – in the case of functional coatings (enteric and controlled-release coatings).

The key quantitative indicator of the process is coating weight gain, i.e., the ratio of the mass of the applied polymer to the mass of the core. For non-enteric film coating, it typically falls within the range of 2-5% of the tablet weight, while for enteric coatings it ranges from 5 to 15% (van den Ban et al., 2017). Both excess and insufficient coating constitute a batch rejection criterion – a layer that is too thin will not provide the intended function, while one that is too thick may delay release beyond the specification.

Good to know: types of tablet coatings – quick overview

Film coating (non-enteric) – materials: HPMC, HPC, PEG; purpose: protection and aesthetics; pH-independent release profile; dissolution in water or at pH 6.8 within 30 minutes.Enteric coating – materials: HPMC-AS, Eudragit L100/S100; purpose: protection against gastric acid; pH-dependent profile (disintegration above pH 5.5-7.0); dissolution: no disintegration at pH 1.2, disintegration at pH 6.8 within 60 minutes.Controlled release – materials: ethylcellulose, Eudragit RS/RL; purpose: prolonged or pulsatile action; profile: diffusion or osmotic kinetics.

Film coating (non-enteric coating) – mechanism and materials

Aqueous film coating involves spraying a suspension containing a polymer, a plasticizer, and a pigment onto tablets in a heated drum. The water evaporates, and the polymer forms a continuous, uniform coating covering the core (Major and Rathore, 2020). This type of coating does not change the release profile of the active substance – it is independent of the pH of the gastrointestinal environment.

Materials used for film coating:

• HPMC (hydroxypropyl methylcellulose) – the industry standard; moisture-resistant, does not alter the release profile, used in ready-made systems such as Opadry;
• HPC (hydroxypropyl cellulose) – produces a coating with lower hardness, used for light coating;
• plasticizers (PEG 400/6000, triacetin) – increase coating flexibility; without a plasticizer, the polymer layer cracks during drying and under mechanical stress;
• pigments (e.g., iron oxides and hydroxides) – provide opacity and color, serving an identification function.

Process parameters are crucial to coating quality. The inlet air temperature, suspension spray rate, and drum rotation speed must be balanced with one another. Too low an inlet temperature causes excessive wetting of the tablets (overwetting) and sticking, whereas too high a temperature leads to drying of polymer particles while still in the air, before they reach the tablet surface (spray drying). Both conditions result in coating defects – roughness, uneven thickness, or lack of continuity. The boundary between a correct and a defective process (transition boundary) is critical for layer homogeneity and must be defined for each formulation (van den Ban et al., 2017).

Enteric coatings – protection against gastric acid and targeted release in the intestine

An enteric coating is a pH-sensitive coating: it remains stable at the low pH of the stomach (pH 1.2-2.0) and dissolves only at the higher pH of the small intestine (pH above 5.5). As a result, it protects the active substance from the acidic environment and releases it in the target section of the gastrointestinal tract (Mohan et al., 2015).

The materials and their pH solubility thresholds make it possible to precisely select the release site:

• Eudragit L100-55 – pH threshold 5.5; release in the proximal part of the small intestine (duodenum). Below pH 5.5, virtually no release occurs; above this value, the coating dissolves rapidly (Qiao et al., 2013);
• Eudragit L100 – pH threshold 6.0; release in the middle part of the small intestine;
• Eudragit S100 – pH threshold 7.0; release in the distal part of the small intestine and in the large intestine;
• HPMC-AS (HPMC acetate succinate) – a pH-dependent ionomer whose solubility threshold depends on the ratio of acetyl to succinoyl groups (Mohan et al., 2015).

The thickness of an enteric coating typically falls within the range of 30-100 µm, which ensures effective gastric protection (Qiao et al., 2013). In supplement practice, enteric coatings are used primarily for:

• digestive enzymes (e.g., bromelain, lipase, amylase) – deactivated in the acidic environment of the stomach; the coating ensures delivery to the intestine in active form;
• probiotics with acid-sensitive strains (e.g., Lactobacillus acidophilus) – protection during gastric transit;
• ingredients that irritate the gastric mucosa – reducing the risk of irritation by shifting release to the intestine.

Quality verification is carried out in accordance with the European Pharmacopoeia, method 2.9.3. An enteric tablet should not disintegrate for 2 hours in 0.1 mol/L HCl (simulation of the gastric environment), and should then disintegrate within 60 minutes in a phosphate buffer at pH 6.8. Failure to meet any of these criteria means the batch is rejected (Ph.Eur. 11.0, method 2.9.3).

Good to know: why the dissolution test is a critical coating quality control point

The disintegration and release test (dissolution) is not a formality, but the only in vitro verification of whether the coating actually performs in accordance with the specification. A tablet with a non-enteric coating that looks correct but has uneven layer thickness may fail the dissolution criterion – and the batch must be rejected. In turn, an enteric coating with too low a coating weight (coating weight gain below the minimum) may begin to disintegrate already in the stomach, exposing both the active substance to degradation and the gastric mucosa to irritation. Ph.Eur. 2.9.3 and USP <711> precisely define the test conditions (pH, temperature, time, and apparatus) – these are parameters that the CDMO must validate for each formulation.

Modified-release coatings – extended release and pulsatile release

Unlike film coating and enteric coatings, modified-release coatings are designed to control the diffusion kinetics of the active substance, rather than merely protect it during transit. Their purpose is to distribute release over time or to release it at a planned moment.

Materials and mechanisms:

• ethylcellulose (EC) – a non-ionic, water-insoluble cellulose polymer; it forms a diffusion membrane through which the active substance passes at a rate proportional to the concentration gradient. The ratio of ethylcellulose to plasticizer regulates the diffusion rate; application: extended release (12-24-hour action);
• Eudragit RS/RL – acrylic trimethylammonium copolymers, pH-independent; the RS grade has low water permeability, while RL has higher permeability. An RS/RL blend allows the release rate to be precisely regulated independently of pH (Fan et al., 2001);
• pulsatile systems – combining an enteric coating (Eudragit L) with an ethylcellulose layer provides a pH-dependent delay combined with diffusion through the membrane; after the appropriate pH is reached, a larger portion of the substance is released at once in a pulse (Fan et al., 2001).

In supplementation, modified-release coatings are used where action distributed over the entire day is desirable without the need for multiple dosing (e.g., magnesium, vitamin C), as well as in formulations with chronopharmacological characteristics, in which the timing of substance release is important.

Plan tablet coating with Eubioco

The choice of coating type depends on the characteristics of the active substance (stability in an acidic environment, hygroscopicity, odor), the expected site and timing of release, and the requirements of the target market. Eubioco, as a contract manufacturer operating to GMP and ISO 22000:2018 standards, performs tablet coating on an industrial scale, selecting the coating type and process parameters according to the specification of each formulation. Contact us to discuss your project: sprzedaz@eubioco.eu

Bibliography

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