Raw materials for hard gelatin capsule shell manufacturing

Hard gelatin capsule shell is composed largely of gelatin. Other than gelatin, it may contain materials such as plasticizer, colourants, opacifying agents, and preservatives which either enable capsule formation or improve their performance. Hard gelatin capsules also contain 12–16% water, but the water content can vary, depending on the storage conditions.

a. Gelatin

Gelatin is by far the most common and most well-known material used to produce hard capsule shells. It is a generic term for a mixture of purified protein fractions obtained from irreversible hydrolytic extraction of collagen obtained from the skin, white connective tissue, and bones of animals.

Depending on the source of the collagen and the method of extraction, two types of gelatin can be produced – type A gelatin and type B gelatin. Type A gelatin is made from pork skin via acid hydrolysis and has an isoelectric point between 7.0 and 9.0. Type B gelatin is prepared by alkaline hydrolysis of bovine bones and has an isoelectric point between 4.8 and 5.0. Because of this difference in isoelectric points, both gelatins show solubility differences at different pH values.

Traditionally capsules may be manufactured by using both types of gelatin, but combinations of pork skin and bone gelatin are often used to optimize shell characteristics because bone gelatin contributes firmness, whereas pork skin gelatin contributes plasticity and clarity.

Gelatin derived from Gelatin grade is further specified by bloom strength. This is measured in a Bloom gelometer which determines the weight in grams that is required to depress a standard plunger in a 6.67% w/w gel under standard conditions.

Gelatin is stable in air when dry but is subject to microbial decomposition when it becomes moist.

b. Plasticizer

Plasticizers are added to gelatin to reduce the rigidity of the polymer and make it more pliable. Common examples of plasticizers are glycerine and polyhydric alcohol. Water is also a good plasticizer and is naturally present in the gelatin.

c. Colourants

Most frequently, hard gelatin capsules are coloured to enhance the aesthetic properties and also to act as a means of identifying the product. Colorants used must meet the regulatory requirements of those countries where the product will be sold. Examples of commonly used capsule colourants include synthetic dyes such as azo dyes and xanthene dyes. Iron oxide pigments are also used.

d. Opacifying agents

Opacifiers (e.g., titanium dioxide) may be included to make clear gelatin opaque. Opaque capsules may be employed to provide protection against light or to conceal the contents.

e. Preservatives

Preservatives (often parabens esters) were formerly added to hard capsules as an in-process aid in order to prevent microbiological contamination during manufacture. Manufacturers operating their plants to Good Manufacturing Practice?(GMP) guidelines no longer use them. In the finished capsules, the moisture levels, 12–16% w/ v, are such that the water activity will not support bacterial growth because the moisture is too strongly bound to the gelatin molecule.