• Pastes, gels, ointments and creams are closely related to suspensions, liquids and emulsions except that they are products with higher viscosities

• The scale up involves many of the same factors that are considered in lower viscosity products but high viscosity renders certain aspects of the scale up of semi-solid products more critical


• The natural turbulence created by mixers used to make liquid suspensions or emulsions is not adequate to produce a homogenous product or cream

• The mixing equipment must be capable of effectively and continuously moving the semi-solid mass from the outside walls of the mixing kettle to the centre and from the bottom to the top of the kettle


Distribution of ingredients

Efficient heat transfer during heating and cooling

High Shear homo mixer

Power required for Mixing

• Power required for mixing depends on the viscosity of the product

• Motors used to drive the mixing system must be appropriate to handle the product at its most viscous stage

• They need to have variable speed mixing

Low speed à Low viscosity

Moderate speed à High viscosity

High speed mixing à causes à Air entrapment

High speed mixing à Prevented by à Use of vessels that operate under controlled vacuum

Temperature of process

• Mixing of oil and water phases during emulsification, homogenization, addition of ingredients, product transfer are carried out at carefully predetermined temperatures

• Temperature is critical to the quality of final product

Temperature of process‐ Emulsions

• In the formation of cream, the aqueous phase and oil phase must be heated to a temperature above the solidification point of the oil phase and then emulsified

If temperature of both phases are not maintained correctly:

è Improperly dispersed wax leading to a poor quality product

è Wide ranges in product vicosity

Temperature of process ‐ Suspensions

• Improper temperature control can have adverse effects on the particle size of poorly soluble active ingredients

If insoluble ingredients added at high temperatures


Solubility is artificially increased creating a metastable product


On cooling – crystal growth or recrystallisation from a saturate solution


Change of particle size distribution, gritty product, altered stability and biologic activity


• Many cream formulations and gel products are shear sensitive

Shear sensitive liquids change viscosity when under stress or pressure, such as when they are hit by the impeller inside a pump:

v  Some liquids become less viscous with increased force (called shear thinning or pseudoplastic)

v  Some become more viscous with increased force (called shear thickening or dilatant)

Shear thinning or pseudoplastic à Ketchup‐ becomes runnier when shaken

Shear thickening or dilatant à Corn starch suspended in water ‐ when stirred slowly it looks milky, when stirred vigorously it feels like a very viscous liquid.

• Handling such products during transfer from the manufacturing kettle to holding tanks to filling lines require consideration of shear properties

• Changes in measured viscosity are seen when viscous products are pumped through long transfer lines or during filtration

Critical processing steps

• Emulsification of 2 phases

• Dispersion of any suspended active ingredients

• Hence equipment selection is important

High Shear homo mixer


Colloid mill

Transfer pumps

• Transfer pumps (must be able to move viscous material without applying excessive shear and without incorporating air)

• While choosing the size and type of pump,

a. Product viscosity

b. pumping rate

c. Product compatibility with the pump surface

d. Pumping pressure

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