Moisture, purity and mesh explained

Three numbers appear in most agricultural specifications: moisture, purity and mesh. Each is simple, and each is often written ambiguously. Here is what they mean and how to specify them.

Moisture

Moisture is the water content of the product, usually expressed as a percentage by weight, and often measured as loss on drying (LOD). It matters for four reasons:

• Shelf life. More water means faster deterioration.
• Microbial risk. Mould and bacteria grow where moisture is high.
• Caking and clumping. Hygroscopic products such as psyllium husk swell or cake if they take up moisture.
• Contaminants. In green coffee, high moisture is associated with ochratoxin A.

Specify a maximum, for example "moisture 7% maximum", and name the method. Packing matters as much as the number at the factory: moisture-barrier liners protect a good lot in transit.

Purity

Purity says how much of the product is the product. The meaning changes with the commodity:

• In psyllium husk, purity is the husk content: 85%, 95% or 98 to 99% or more, and it is the main price differentiator.
• In makhana, purity often means freedom from foreign matter and broken or immature kernels.
• In tamarind block, the equivalent is residual seed content, so premium means about 5% seed or less.

Always state what the percentage refers to and how it is measured.

Mesh

Mesh describes particle size for powders. It counts the openings per inch in the screen the powder passes through, so a higher mesh number means a finer powder. Approximate openings for common sieve sizes are:

A mesh figure on its own is a convention, not a guarantee. Say how much of the powder must pass the screen, for example "95% through 80 mesh".

Write it as a range, with a method

A specification should give a maximum or a range, the unit, and the test method. "Moisture 7% max by LOD" is clear. "Dry" is not.

See how these numbers are used in moringa leaf powder quality parameters and psyllium husk quality parameters.