Following evaluation parameters are carried out for emulsion:
Ageing and temperature:
An emulsion is said to be stable if it meets the following:
Freeze thaw cycle:
In this test, emulsion is subjected to store at too high temperature and too low temperature in alternating cycles. This exposure is repeated many times and emulsion stability is evaluated after
Centrifugation of emulsion increases the gravity and increase in gravity leads to increase in creaming (instability of emulsion) which can be used to evaluate the shelf life of emulsion rapidly. Centrifugation at 3750rpm in a centrifuge for 5 hours is equivalent to effect of gravity for one year.
It is considered that excess agitation of emulsion cause breaking of emulsion or prevent the formation of stable emulsion. Example: Production of butter from milk.
The phase separation can be determined by visual inspection but rate and extent of phase
separation of emulsion can be determined by only by measuring the volume of separated phase which involves the withdrawl of small amount of emulsion from top and small amount from bottom of emulsion and compare the composition of both by analysis of oil content, water content and any other additives.
Viscosity is an important parameter to assess the shelf life of the emulsion. Any change in
viscosity can cause instability of emulsion. Viscosity of emulsion is determined by cone plate type device because most of the emulsions are non-newtonian. Decrease in viscosity indicates the poor shelf life of emulsion because decrease in viscosity causes increase in coalescence due to increase in globule size.
Zeta potential is determined by moving boundary method and micro electrophoresis method. Emulsions having high zeta potential (negative or positive) are electrically stabilized while emulsions having low zeta potential tend to flocculate, coagulate leading to poor stability. Generally at high zeta potential, force of repulsion exceeds the force of attraction causing stable system.