Quality & Specification
Foreign Matter in Pulses
What counts as foreign matter, why it is different from admixture, which cleaning stages remove which contaminants, and why stone tolerance is usually the tightest number in a contract.
· 7 min read
Foreign matter is everything in a lot that is not the contracted commodity. In a pulse consignment arriving from smallholder production, that covers a wide range: pod fragments, stalk, leaf and straw from threshing; soil, mud balls and small stones picked up from drying on the ground; string and sacking fibre from handling; dust; and seeds of other plants, both crop and weed.
Foreign matter is not admixture
The two terms are frequently confused and they are not interchangeable. Foreign matter is material that is not the commodity at all. Admixture is the commodity, but the wrong type — a red pigeon pea in a white pigeon pea lot, or a light speckled bean in a red kidney lot. They are separated in a specification because they are removed by different means and because they matter to a buyer for different reasons.
- Foreign matter
- Not the commodity: stone, soil, straw, pod, string, dust, other species of seed. Removed by cleaning.
- Admixture
- The commodity in the wrong type, colour or variety. Removed by segregation at buying and storage, and by optical sorting.
Why stone gets the tightest tolerance
Among all the categories of foreign matter, stone is the one buyers care about most, and for a practical reason: it damages equipment. A dal mill runs the grain between abrasive rollers, and a stone entering that process damages the surface and can stop the line. A canner or packer has a different concern — a stone that reaches a consumer is a food safety incident and a product recall.
Stone is also the hardest contaminant to remove by screening, because a stone the same size as a pigeon pea passes through the same aperture. Removing it requires a density-based separation, which is exactly what a destoner is designed for: a fluidised inclined deck that separates material of the same size but higher specific weight.
Which stage removes what
| Contaminant | Removed by | Why |
|---|---|---|
| Straw, pod, stalk, leaf | Pre-cleaning and aspiration | Larger or much lighter than the grain |
| Dust and chaff | Aspiration | Very low weight, lifted by the air stream |
| Large stones and clods | Pre-cleaning screens | Dimensionally larger than the grain |
| Small stones and soil pellets | Destoning | Same size as the grain but denser |
| Sand and fine grit | Screening (lower deck) | Smaller than the grain |
| Other seeds, similar size | Colour sorting | Same size and density, different colour |
| Ferrous fragments | Magnets and metal detection | Magnetic, or detectable in-line |
Where foreign matter comes from
Most of it is introduced before the crop reaches a cleaning line at all. Threshing by hand or with simple equipment leaves pod and stalk in the grain. Drying on bare ground or on a road surface introduces soil, stone and grit. Transport in reused sacks introduces fibre and residue from whatever the sack carried previously. Storage on an unswept floor introduces everything on that floor.
The practical implication is that foreign matter levels vary substantially between suppliers within the same origin and the same season, depending on handling practice at aggregation. It is one of the clearest signals of how well a supply chain is being run, which is why intake sampling matters as much as line capability.
The full cleaning sequence, stage by stage. How pulses are cleaned for export