There are two types of gelatin: acid gelatin A and alkaline gelatin B, which differ in some physical and chemical properties. Usually in the manufacture of pharmaceutical capsules use mixtures of gelatin A and B, as it allows 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 digested by the human body even with severe gastrointestinal disorders, non-toxic and has no adverse reactions. A characteristic property of gelatin (from Latin gelare – to solidify) is the ability of its solutions to gel when cooled, forming a solid gel. Production of gelatin capsules is based on this property of gelatin. To obtain a stable capsule shell, the composition of the gelatin base may include various auxiliary substances approved for use: plasticizers, film formers, stabilizers, preservatives, correcting agents, dyes and pigments. In order to improve structural and mechanical properties and to ensure appropriate elasticity, increase strength and reduce brittleness of shells, plasticizers are introduced into the composition of gelatin mass. The most popular plasticizers are glycerin, sorbitol, PEO-400, polyethylene glycol, polypropylene, polyethylene sorbitol (3-15%) with oxyethylene (4-40%), hexanthropol and others. For the manufacture of hard capsules gelatin mass should contain a small amount of plasticizers (0.3-1.0 %), for soft – their amount increases to 20-45 %.
In some cases, gelatin capsules become more stable with partial or complete replacement of glycerol in the shell with sorbitol, PEO-400 or other plasticizers.
Among the disadvantages of gelatin capsules can be noted their high sensitivity to moisture, which requires compliance with certain conditions of storage of capsules. Therefore, sometimes gelatin capsules are coated with special coatings – film formers, which reliably protect the shells from moisture, while not preventing their rapid destruction in the stomach.

Such film formers include para-aminobenzoate sugars and cellulose amino derivatives, which improve the moisture resistance of gelatin capsules. Another way to overcome the moisture sensitivity of gelatin capsules is to use zein or other moisture-resistant film-forming agents instead of gelatin. For encapsulation of complex compositions of vitamins Japanese researchers proposed a method of obtaining “double” capsules: water-soluble vitamins are covered with a film of wax-like substances, and then a hydrophilic film of gelatin. The gelatin mass is a favorable environment for the reproduction of microorganisms. To ensure antimicrobial resistance of shells in the composition of the mass introduced preservatives: 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%)” To give capsules an attractive commercial appearance or protect the active substances from photochemical reactions, the composition of the gelatin mass introduced corrective auxiliary substances. Sometimes aromatizing substances (essential oils, essences, ethyl-vanillin 0.1%) are added to the gelatin base to give capsules a pleasant smell. The addition of sweet substances (sugar syrup, sucrose, glucose, etc.) improves the taste of capsules and reduces the unpleasant sensation when swallowing. Currently, all capsules (with rare exceptions) are colored to give them a more aesthetic appearance. For this purpose, coloring agents are introduced into the gelatin mass. Different coloring is also convenient for the release of different names of drugs in the form of capsules, as it allows you to distinguish products by color. As dyes can be used substances of natural origin (carminic acid, chlorophyll, beta-carotene, etc.), the low toxicity of which allows their use without restrictions in most countries, inorganic pigments (yellow, red and black iron oxide, titanium dioxide), as well as organic dyes approved for medical use. The amount of dyes in one capsule, as a rule, does not exceed 50 mcg. Capsules intended to be filled with photosensitive substances should be opaque. For this purpose, obscuring agents are used – substances that make it possible to obtain opaque capsules due to their ability to form a stable fine suspension in the gelatin mass.