There are two types of gelatin: acidic - gelatin A and alkaline - gelatin B, which differ in some physicochemical properties. Usually, mixtures of gelatins A and B are used in the manufacture of pharmaceutical capsules, since this allows one to obtain masses for the manufacture of capsule shells with the most optimal rheological characteristics (in particular, in terms of strength, viscosity, pH, iron content, etc.). Gelatin is easily and quickly absorbed by the human body, even with severe disorders of the gastrointestinal tract, is non-toxic and has no adverse reactions. A characteristic property of gelatin (from lat. Gelare - to harden) is the ability of its solutions to gel with cooling, forming a solid gel. The manufacture of gelatin capsules is based on this property of gelatin. To obtain a stable capsule shell, the gelatin base can contain various auxiliary substances that are allowed for use: plasticizers, film formers, stabilizers, preservatives, corrective substances, dyes and pigments. In order to improve the structural and mechanical properties and ensure appropriate elasticity, increase strength and reduce the fragility of the shells, plasticizers are introduced into the gelatin mass. The most popular plasticizers are glycerin, sorbitol, PEO-400, polyethylene glycol, polypropylene, polyethylene sorbitol (3-15%) with hydroxyethylene (4-40%), hexanthrope, etc. To make hard capsules, the gelatin mass should contain a small amount of plasticizers (0 , 3-1.0%), for soft - their number increases to 20-45%.
In some cases, gelatin capsules become more stable with partial or complete replacement of glycerol with sorbitol, PEO-400, or other plasticizers in the shell.
Among the disadvantages of gelatin capsules can be noted their high sensitivity to moisture, which requires compliance with certain storage conditions of the capsules. Therefore, sometimes special coatings are applied to gelatin capsules - film-forming agents that reliably protect the shell from moisture, while at the same time not preventing their rapid destruction in the stomach.

Such film-forming agents include sugar para-aminobenzoates and amino derivatives of cellulose that improve the resistance of gelatin capsules to moisture. Another way to overcome the moisture sensitivity of gelatin capsules is to use zein or other moisture-resistant film-forming substances instead of gelatin. To encapsulate complex vitamin formulations, Japanese researchers have proposed a method for producing “double” capsules: water-soluble vitamins are coated with a film of wax-like substances, and then with a hydrophilic film of gelatin. Gelatin mass is a favorable environment for the reproduction of microorganisms. To ensure the antimicrobial stability of the shells, preservatives are introduced into the mass: a mixture of salicylic acid (up to 0.12%) with potassium (sodium) metabisulfite (up to 0.2%), benzoic acid and sodium benzoate (0.05-0.1%) , nipagin (0.1-0.5%) ”In order to give capsules an attractive presentation or protect active substances from photochemical reactions, corrective excipients are introduced into the gelatin mass. Sometimes flavoring agents (essential oils, essences, ethyl vanillin 0.1%) are added to the gelatin base, giving the capsule a pleasant smell. The addition of sweet substances (sugar syrup, sucrose, glucose, etc.) improves the taste of the capsules and reduces the unpleasant feeling when swallowed. Currently, all capsules (with rare exceptions) are painted to give them a more aesthetic appearance. For this purpose, dyes are introduced into the gelatin mass. Different stains are also convenient for the production of various types of preparations in the form of capsules, since they make it possible to distinguish products by color. As dyes, substances of natural origin (carminic acid, chlorophyll, beta-carotene, etc.) can be used, the low toxicity of which allows them to be used without restrictions in most countries of the world, inorganic pigments (yellow, red and black iron oxide, titanium dioxide), as well as organic dyes approved for medical use. The number of dyes in one capsule, as a rule, does not exceed 50 μg. Capsules intended to be filled with photosensitive substances must be opaque. For this purpose, opacifiers are used - substances that make it possible to obtain opaque capsules, due to the ability to form a stable finely dispersed suspension in the gelatin mass.

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