A standardised extract has two specifications living in one powder. The chemical specification — marker percentage, identity, safety limits — gets all the attention. The physical specification — particle size, bulk density, flow, moisture — gets almost none, until something goes wrong on the line. Yet it is the physical properties that decide whether your extract meters evenly into a capsule, compresses into a stable tablet, disperses cleanly in a drink, or segregates out of a blend on the way to the filler. This article is about the numbers that govern that behaviour, and why they belong on every purchase specification.
Particle size and mesh
Particle size is the single most influential physical property of a powder. It is reported either as a mesh specification (e.g. "passes 80 mesh" — meaning the powder passes through a sieve with 80 openings per linear inch) or as a particle-size distribution (D10/D50/D90 in microns, from laser diffraction). A finer powder dissolves and disperses faster and blends more intimately; a coarser, more granular powder flows better and dusts less. Neither is universally better — the right size depends entirely on what you are making.
Why a single mesh number isn't enough
"Passes 80 mesh" only tells you the upper bound — nothing about the spread below it. Two powders that both pass 80 mesh can have completely different distributions: one mostly coarse particles just under the cutoff, another dominated by fines. The distribution (D10–D90) is what predicts flow, segregation and dissolution. When particle behaviour matters, ask for the distribution, not just the mesh cutoff.
Bulk and tapped density
Bulk density is the mass of powder per unit volume as it naturally settles (g/mL); tapped density is the same after standardised tapping consolidates it. These two numbers drive practical decisions you cannot avoid: whether a dose fits in a given capsule shell, how big a tablet will be, and how much a drum actually weighs for the volume it occupies. A low-density, fluffy spray-dried powder may simply not fit the target dose into a size-0 capsule; a dense vacuum-dried grade might.
Carr's Index and the Hausner Ratio — flow from density
The relationship between bulk and tapped density is itself a flow indicator. Carr's Compressibility Index and the Hausner Ratio are derived from the two figures: a powder that consolidates a lot under tapping (high compressibility, Hausner ratio well above ~1.25) is cohesive and flows poorly; one that barely changes flows freely. These cheap, standard calculations predict hopper and feeder behaviour before you ever run the powder.
Flowability
Flow is the property your equipment feels most directly. Poor flow shows up as bridging (the powder arches over a hopper outlet and stops feeding), rat-holing (it channels down the centre and leaves the sides stuck), erratic fill weights, and tablet weight variation. Flow is characterised by Carr's Index / Hausner Ratio, angle of repose, and — for serious work — flow-function testing on a shear cell. Cohesive, fine, low-density botanical extracts are often the poorest-flowing ingredient in a blend, which is exactly why flow aids exist.
- Angle of repose — the angle of a freely poured cone; lower angles (≲30°) indicate good flow, high angles (≳40°) indicate cohesive, poorly-flowing powder.
- Flow aids — a small percentage of an anti-caking / glidant such as silicon dioxide markedly improves flow; it should be a low, declared figure, not a major fraction.
- Moisture interaction — a hygroscopic powder that has picked up water flows worse and cakes; flow and moisture must be read together.
Blend uniformity and segregation
The moment your extract joins other ingredients, a new risk appears: segregation. If the extract's particle size or density differs markedly from the rest of the blend, the components can separate during handling, transfer and filling — so the powder at the start of a run carries a different dose than the powder at the end. This is the physical root of many content-uniformity failures, and the marker percentage on the COA is blameless: each particle is potent, but they didn't stay mixed.
The physical spec at a glance
These are the physical parameters worth putting on a purchase specification, what they govern, and how they are measured.
| Parameter | What it governs | How it's measured |
|---|---|---|
| Particle size / mesh | Dissolution, flow, segregation, dustiness | Sieve (mesh) or laser diffraction (D10/D50/D90) |
| Bulk & tapped density | Capsule fit, tablet size, packing, flow | Graduated cylinder, tapped-density tester |
| Flow (Carr's / Hausner / repose) | Hopper & feeder behaviour, fill-weight consistency | Density ratio, angle of repose, shear cell |
| Loss on drying (LOD) | Caking, microbial & hydrolysis risk, true potency basis | Moisture balance / oven |
| Water activity (Aw) | Microbial stability, free-water spoilage risk | Water-activity meter |
| Colour / odour | Process consistency, thermal-abuse signal | Visual / organoleptic vs. reference |
Specifying physical properties without over-specifying
You do not need every parameter on every extract — you need the ones your process is sensitive to. Specify deliberately.
- 1Lead with your dose form — capsule, tablet, beverage, gummy, stick-pack — so the supplier can recommend a sensible particle size and density.
- 2Set a mesh or particle-size target, and ask for the distribution if flow or dissolution is critical.
- 3Specify bulk density when capsule fit or tablet size is tight; confirm the dose actually fits before scaling up.
- 4Ask for flow data (Carr's Index / Hausner Ratio / angle of repose) for any high-speed automated line.
- 5Pin LOD (and Aw where available) to protect flow, stability and a true potency basis.
- 6Request a representative sample and run it on your equipment before committing to bulk — physical behaviour is best confirmed, not assumed.
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