Destoning Standards: How We Measure Clean
When a supplier invoice says destoned, it tells you one thing: stones were targeted. It says nothing about which foreign matter categories were addressed, to what residual level, or by what method. A spec that says only destoned leaves every dispute open. This post explains what destoned means in practice at KVM & Co., Mysore, what thresholds to write, and the one omission that costs buyers on nearly every contested lot.
What "destoned" actually means in trade practice
In APMC yard usage, destoning is shorthand for the gravity destoner pass — targeted removal of mineral matter. At Bandipalya we run a full cleaning sequence: primary sieving, magnetic separation, gravity destoning, and hand sorting. Most invoices cover only the gravity step. That gap is where disputes start.
Foreign matter in a pulse lot falls into four categories, each requiring a different removal method and carrying its own threshold under FSSAI and BIS standards for whole pulses.
- Mineral matter
- Stones, gravel, sand, soil clumps. The gravity destoner's primary target — it separates on density differential. At Bandipalya, post-monsoon arrivals from heavy clay soils routinely carry hardened clods inside the bag.
- Vegetable matter
- Crop husks, broken pods, leaves, weed seeds, other crop seeds. Lower density than stones — the gravity destoner passes most of it as product-side material. Sieving and hand sorting are the removal methods here.
- Animal matter
- Insect fragments, frass, droppings. Caught at the hand-grading table; not amenable to mechanical removal at trace concentrations. FSSAI prohibits visible animal matter in retail-grade pulses.
- Metallic matter
- Iron filings, wire bits, nails. Removed by the magnetic separator upstream of the destoner. Export consignments require a documented magnet-pull step.
Typical thresholds by grade
The following thresholds reflect common trade practice and BIS IS grading specifications for notified pulses. Exact figures vary by commodity; confirm against the relevant IS standard for your pulse. AGMARK grade parameters align closely.
| Grade | Total foreign matter | Mineral matter | Animal matter | Metallic matter |
|---|---|---|---|---|
| Industrial / Standard | ≤2.0% | ≤0.5% | Not specified | Not specified |
| Premium / Retail | ≤1.0% | ≤0.1% | None visible | Not specified |
| Export / FSSAI-compliant | ≤0.5% | ≤0.05% | None visible | Magnet-cleared, nil |
How mechanical and manual destoning differ
Mechanical destoning
A gravity destoner runs on a vibrating inclined deck with upward airflow. Denser material — stones, gravel, soil clods — migrates high-side and discharges separately from the lighter pulse seed. Magnetic separators upstream pull ferrous matter; primary sieving removes oversized debris and dust. A single pass recovers roughly 95–98% of stones by weight. It cannot reliably remove vegetable matter of similar density to the seed, or animal matter at trace concentrations.
Manual destoning
Workers at the hand-grading table pick out what the machines missed — a flat stone that matched seed density, a dried pod, a dark weed seed. Throughput is 80–120 kg per worker per shift; accuracy for visually distinct debris is high. For export and premium lots, this step is where the 0.05% mineral matter target becomes achievable.
KVM's destoning workflow at Bandipalya
At KVM & Co., Mysore, every cleaning lot follows a fixed sequence:
- Arrival and primary sieving — oversized debris and fine dust removed through graded screens.
- Magnetic separator — ferrous metallic matter pulled before the lot reaches the destoner.
- Gravity destoner — stones and heavy mineral matter discharge high-side; clean pulse discharges low-side.
- Hand-grading table — workers remove what the machines missed under good light.
- Final inspection and packing — a 200 g sample is sorted to confirm the contracted foreign matter threshold is met before bags are sealed.
How to write a cleanliness clause
A cleanliness clause should name three things: the threshold, the inspection method, and the rejection trigger.
- Threshold by category: total foreign matter as % by weight, mineral matter separately. Example: total ≤1.0%, mineral ≤0.1%, no visible animal matter, metallic nil (magnet-cleared).
- Inspection method: 200 g minimum sample, visual sort + 1 mm mesh sieve + magnet pull, balance to 0.01 g resolution.
- Rejection criteria: sample count that triggers rejection. Example: 2 of 5 samples fail = full lot rejected or returned for re-cleaning.
The most common mistake we see at Bandipalya is a purchase order that says destoned with no numeric threshold. When a dispute arises — a stone found after delivery, a failed third-party inspection — there is nothing to measure against. The supplier can show the gravity destoner was run; the buyer cannot show the output failed. One line fixes it: mineral matter ≤0.1% by weight, 200 g sample. Without it, a dispute that should take minutes takes weeks.
For the regulatory basis, FSSAI Food Safety and Standards Regulations specify foreign matter limits for packaged whole pulses. BIS IS standards for individual commodities — IS 1196 for tur dal, IS 1185 for moong, and so on — carry grade-wise foreign matter limits you can reference directly in a procurement specification.