They are designed to melt at body temperature.
1. Theobroma Oil (Cocoa butter)
It is a mixture of glyceryl esters of different
unsaturated fatty acids.
Cocoa Butter is a triglyceride, yellowish-white,
solid, brittle fat, that smells and tastes like chocolate. Its melting point is
between 30-35°C, its iodine value is "between" 34-38 and its acid
value is not higher than 4, because cocoa butter can melt and rancid. So it
must be stored in a cool dry place protected from light.
Overheating changes its physical characteristics and
it tends to adhere to the mold when solidified. It may exist in four
crystalline states.
α Form: This form is
obtained by suddenly cooling the melted mass to 0°C. Its melting point is
24°C.
β Form: This form is
obtained when cocoa butter is melted at 35 to 36°C and slowly cooled. It melts
at 18 to 23°C.
β' Form: It reverts to 3
forms and melts at 34 to 35°C.
γ Form: It is obtained by
pouring a cool (20°C) cocoa butter into a container before it is solidified and
cooled at deep-freeze temperature. It melts at 18°C.
All four forms are unstable and are converted to
stable forms over several days. Thus, extreme care should be exercised while
melting and cooling cocoa butter. In general, the minimal use of heat during
the melting process is recommended.
To overcome the drawbacks of cocoa butter, emulsified
theobroma oil, hydrogenated palm kernel and soyabean oils have been
suggested.
Advantages
• A melting range of 30 - 36°C (solid at room
temperature but melts in the body).
• Readily melted on warming, rapid setting on
cooling.
• Miscible with many ingredients.
• Non-irritating.
Disadvantages
• Polymorphism: When melted and cooled it solidifies
in different crystalline forms, depending on the temperature of melting, rate
of cooling and the size of the mass.
• If melted at not more than 36°C and slowly cooled
it forms stable beta crystals with normal melting point.
• If overheated and then cooled it produces unstable
gamma crystals which melt at about 15°C or alpha crystals melting at 20°C.
• Cocoa butter must be slowly melted over a warm
water bath to avoid the formation of an unstable crystalline form.
• Adherence to the mould.
• Softening point too low for hot climates.
• Melting point reduced by soluble ingredients.
• Rancidity on storage.
• Poor water-absorbing ability: Improved by the
addition of emulsifying agents.
• Leakage from the body.
2. Emulsified Theobroma Oil
When large quantities of aqueous solutions are
required to be incorporated then emulsified theobroma oil as a base can be
used. There are many agents which are used to form emulsified theobroma oil,
for example: 2-3% cetyl alcohol, 4% glyceryl monostearate, 10% lanette wax, 4%
beeswax, and spermaceti up to 12% can be utilised for emulsified theobroma oil
suppositories.
3. Hydrogenated Oils
They are used as a substitute for theobroma oil, many
hydrogenated oils are used as a substitute, for example, coconut oil, palm
kernel oil, hydrogenated edible oil, and a mixture of oleic acid and stearic
acid. They are known as synthetic fat bases.
Advantages
The synthetic fat bases have advantages over theobroma
oil are as follows:
• Their solidifying points are unaffected by
overheating.
• Because of the lower content of unsaturated fatty
acids they have good oxidation resistance.
• The difference between melting and setting points is
small. Hence, they set quickly, and the risk of sedimentation of suspended
ingredients is low.
• Lubrication of mould is not necessary because they
contract significantly on cooling.
• They are marketed in a series of grades with
different melting point ranges, which can be chosen to suit particular products
and climatic conditions.
• They produce colourless, odourless and elegant
suppositories.
• They contain a proportion of w/o emulsifying agents,
and therefore, their water-absorbing capacities are good.
Disadvantages
• Brittle if cooled rapidly, avoid refrigeration
during preparation.
• The melted fats are less viscous and more fluid than
theobroma oil because of that there is a greater risk of drug particles to
sediment during preparation.