Capsules are made of gelatin, sugar, and water and contain about 10% to 15%
moisture. Gelatin can absorb up to ten times its weight in water. So if gelatin
capsules are placed in areas of high humidity, they will become malformed or
misshapened as they absorb moisture. On the other hand, if capsules are placed
in low humidity, they become dry and brittle and may crack. To protect capsules
from the extremes of humidity, they should be dispensed in plastic or glass
vials and stored in a cool, drug place. It appears that a storage relative humidity
of 30% to 45% is best. Cotton can be placed in the top of the vial to keep the
capsules from rattling.
If powders that are being mixed before encapsulation are very light and fluffy
and "difficult to manage," add a few drops of alcohol, water, or mineral oil.
As an alternative, mix these powders in a plastic bag. If the powders seem to
have a "static charge," use about 1% sodium lauryl sulfate.
Magnesium stearate (less than 1%) can be added to powders to increase their
"flowability" which makes filling capsules easier. However, magnesium stearate
is a hydrophobic compound and may interfere with the dissolution of the powders.