Swell volume is the volume, in millilitres, that one gram of psyllium husk occupies after soaking in water under set conditions. It is reported as ml per gram and is the most direct test of how much active fibre the husk carries.
The value to ask for depends on your grade and use. Set a minimum figure based on tested samples and your customer's needs, and always name the test method.
Why swell volume matters
The mucilage in the husk absorbs water and forms a gel. That gel is what gives psyllium its effect in fibre supplements and its binding role in food. If swell volume is low, the product does less per gram. A medicine may then underperform, and a recipe may not hold together.
Swell volume also helps catch problems that a purity figure can hide. Husk blended with lower grades, old stock, or husk damaged by rain at harvest often swells less than expected.
How it is tested
A weighed sample is placed in a graduated cylinder with water, shaken at set intervals and left to stand for a fixed time. The settled volume of swollen material is read off. Pharmacopoeia monographs describe their own methods, and mills often use a simpler in-house version.
Small differences in method change the result: the amount of water, the soaking time, the temperature and how often the cylinder is shaken. That is why a figure without a method is weak evidence.
What value to ask for
There is no single correct number. As a general pattern, higher purity grades give higher swell volumes, and lower grades or powders with more fines give lower ones. Use these steps to set your figure.
- If you sell under a USP or EP monograph, use the limit in the current edition as your floor. See whether psyllium needs to meet USP or EP standards.
- If your customer has a spec, copy their minimum and method.
- Otherwise, test samples from two or three mills and set a minimum that good lots meet with some margin.
- Name the method, the soaking time and the lab in the contract.
- Ask for swell volume on every lot certificate, not just on the first sample.
Linking swell volume to other tests
Swell volume works best alongside purity, light extraneous matter and ash. A lot that claims high purity but swells poorly needs a closer look. The relationship between grade and properties is covered in what the psyllium purity grades mean.
Keep a retained sample of every shipment. Swell volume can be retested if your customer complains.
How Creston & Co handles this
We agree the swell volume method with you and the mill before the order. We have approval and pre-shipment samples tested at an NABL-accredited lab and compare results lot by lot. Ask for a free sourcing review.