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Powder rheology (properties of powder flow)
Powder density
The specific weight of α-lactose monohydrate is 1.53 kg / l (at 20ºC). Pure β-lactose is somewhat heavier at1.59 kg / l at 20ºC. The density of lactose powder is much lower than the values cited above. The reason for this is the incorporation of air between the individual lactose particles in the powder. Powder density is not as well defined a characteristic as the true density (= specific weight). Powder density depends heavily on the granulometry of the powder. Coarse powders have a higher powder density than fine powders. Powders tend to compact spontaneously over time, which will cause the apparent powder density to increase. It is common practice, therefore, to always give two values for the density of a powder:
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the bulk density, which is the powder density of fresh, loosely discharged powder
the tapped density, which is the density after compacting the powder by tapping it in a well-defined way |
The table below gives some typical values for Friesland Foods Domo lactose products.
Typical values of Bulk Density, Tapped Density, Hausner ratio and Carr’s Index
Lactose product
Lactochem DOMO |
Bulk
Density |
Tapped
Density |
Hausner
ratio |
Carr's
Index in % |
Coarse crystals
Crystals
Extra fine crystals
Powder
Fine powder
Extra fine powder
Super fine powder |
0.75
0.74
0.73
0.64
0.61
0.45
0.47
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0.88
0.86
0.86
0.89
0.84
0.74
0.74 |
1.2
1.2
1.2
1.3
> 1.3
> 1.3
> 1.3 |
15
15
15
>25
>25
>25
>25 |
The ratio of the bulk density and the tapped density is called the Hausner ratio. This ratio is strongly related to the flow properties of lactose powder. A slightly different way of calculating gives the Carr’s Index, which also is strongly indicative of flow properties.
Powder flow
The laws governing powder flow have been extensively studied and described in the technical literature. We will take a more qualitative view of the subject.
The ability of a lactose product to flow freely is determined by factors such as mean particle size, particle size distribution and particle shape. The Hausner ratio is also important. A limited number of general rules of thumb apply and are useful for lactose as well.
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Powders consisting of regularly shaped particles flow better than those consisting ofirregularly shaped particles.
The more the particles in a powder resemble spheres, the better the powder flows.
Coarse powders in general have better flow properties than fine powders.
Powders with narrow particle size distributions flow better than powders with wide particle size distributions.
Powders with a low Hausner ratio (or a low Carr’s Index) flow better than powders with a high Hausner ratio (or a high Carr’s Index). |
In spray-dried lactose, most particles have shapes closely resembling spheres. The flow properties are, therefore, excellent. Crystalline α-lactose monohydrate, consisting of regularly shaped, undamaged crystals, also has good flow properties.
In milled lactose powders, damaged crystals and irregularly shaped crystal fragments are predominant. Consequently, flow properties are not as good as those of unmilled lactose products. Micronised lactose (e.g. Friesland Food Domo’s Lactochem Microfine ) does not flow at all.
The table below gives some typical flowability values of some standard Friesland Food Domo lactose products (Hosokawa Powder Tester)
Hosokawa Powder Tester results of some Friesland Foods Domo lactose products
| Product |
Hosogawa
flow index |
Flowability-
qualification |
Coarse crystals
Crystals
Fine crystals
Extra fine crystals
Coarse powder
Powder
Fine powder
Microfine |
79
75
75
74
43
38
26
11,5
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good
good
good
good
not good
bad
bad
very bad |
The higher the Hosokawa Flow Index, the better the powder flows. For further reading on flow properties in relation to functionality of lactose as an excipientin various applications, see section ’’Functionality of lactose’.’
Solubility
Lactose is freely soluble in water. However, the solubility of lactose is much lower than that of other common sugars. Solubility increases with increasing temperature.
β-lactose dissolves more readily than α-lactose, as is apparent from their very different initial rates of solubility. Final solubility is the same for α- and β-lactose because of the mutarotation equilibrium that is eventually reached in solution.
The particle size of the lactose influences its dissolving velocity. Coarse lactose crystals dissolve much slower than tiny lactose particles. Dissolving velocity, and hence particle size, does not alter final solubility.
Final solubility of lactose dependent on temperature.
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