Most browning in dehydrated onion is non-enzymatic. Natural sugars and amino acids in the onion react when they are heated, and the reaction speeds up with higher temperature and moisture. Poor raw onion and scorching in the dryer add to the problem.
It is prevented by good raw material, controlled drying, low final moisture and cool, dry storage in sealed packs. No single step solves it. Each stage can add colour that cannot be removed later.
Where browning starts
- Raw onion: bruised, sprouted, rotting or long-stored onion gives a darker product.
- Delays: sliced onion left waiting before drying can discolour.
- Dryer temperature: too much heat, especially near the end of drying when the onion is almost dry, scorches pieces.
- Uneven drying: thick slices or overloaded belts leave some pieces wet while others overheat.
- Residual moisture: onion packed above spec keeps browning slowly in storage.
- Storage heat: warm warehouses and long voyages through hot climates speed up colour change.
How good plants control it
Mahuva plants typically dry sliced onion on continuous belt dryers with hot air, in several zones. Control of temperature in each zone, belt speed and bed depth is the main defence against scorching. Ask how the plant sets and records these.
- Raw onion is checked on arrival and poor lots are rejected or sorted.
- Onion is peeled, sliced and washed without long delays before drying.
- Slice thickness is kept even so pieces dry at the same rate.
- Drying temperatures are lowered towards the end of the process.
- Dried flakes are sorted, by hand or by colour sorter, to remove dark pieces.
- Product is packed promptly in sealed liners and stored cool and dry.
Browning after shipment
If onion arrives darker than the pre-shipment sample, look at moisture and storage first. Product just over the moisture limit, held in a hot port or warehouse, can darken over weeks. That is one reason to check moisture on the pre-shipment sample and store goods well once they land.
Steam sterilisation can also darken onion slightly, as covered in whether dehydrated onion is steam sterilised. If you need sterilised goods, approve a sterilised sample, not an untreated one.
What to put in your spec
- A colour grade with a sealed reference sample.
- A limit on dark, brown or scorched pieces.
- A maximum moisture with a named method.
- Packing in sealed polyethylene liners inside cartons or bags.
- Storage advice for your own warehouse, based on the supplier's shelf life statement.
How we help
When we audit a plant we look at raw onion intake, slicing delays and how dryer zones are controlled, because that is where colour is won or lost. We also compare pre-shipment lots with your reference sample. For the wider checks, see auditing a dehydration plant in Mahuva, or start with a free sourcing review.