The most common foreign matter in dehydrated onion is onion skin, root and stalk pieces, black or burnt particles, sand and small stones. Less often, buyers find hair, insects, plastic, string or metal.
Plants remove these at several steps: peeling and washing before drying, then sieving, aspiration, magnets, colour sorting, hand picking and metal detection after drying. No single step catches everything, so the whole line matters.
Common types and where they come from
- Onion skin and peel: left after poor peeling, or blown back into the product.
- Root and stalk pieces: from incomplete trimming of the bulb.
- Black, brown or burnt particles: from over-dried pieces, dryer deposits or mouldy onion.
- Sand, soil and stones: from the field and from raw onion stored on the ground.
- Hair and fibres: from workers and from jute or PP bags used in handling.
- Insects and insect fragments: from open dryers, storage or the field.
- Plastic, string and paper: from packaging and from the plant itself.
- Metal: from worn slicers, sieves, screws and mill parts.
How plants remove it before drying
Good removal starts with raw onion. Bulbs are topped, tailed and peeled, often with a mix of machines and hand work. They are washed, sometimes several times, to take off soil and sand. Clean, sorted raw onion makes every later step easier.
Slicers and conveyors should be maintained and checked, since broken blades are a common source of metal.
How plants remove it after drying
- Sieving: removes fines, dust and oversized pieces, and sets the particle size of each form.
- Aspiration: air currents take out light skins and dust.
- Magnets: catch iron and steel fragments.
- Colour sorting: optical sorters reject dark, burnt or discoloured pieces. See our guide to machine-cleaned versus sortex-cleaned.
- Hand picking: workers remove visible defects on inspection belts or tables.
- Metal detection: final check on packed or packing product. Ask for test records with standard test pieces.
Writing it into your specification
"Free from foreign matter" is not enough on its own. Separate the types and set a limit for each. For illustration, buyers often set nil for stones, glass, metal, plastic and insects, and a small limit for onion skin and dark particles. The right limits depend on the form, the grade and your use.
Agree how you will test. A visual count per 100 g, a sieve analysis and a sealed reference sample avoid arguments later. Our article on Grade A and Grade B onion explains how defect limits separate grades.
What to do when you find it
Photograph the item with the lot label and a ruler. Keep the item and a retained sample. Report it quickly to the supplier with the lot number. A good plant will investigate the source and tell you what they changed.
Where we come in
We inspect the cleaning, sorting and detection steps during production, and check finished lots against your foreign matter limits before loading. To tighten foreign matter control on your next order, start with a free sourcing review.