Dehydrated onion specs commonly set a maximum moisture somewhere around 5 to 6 percent, and some buyers ask for lower. These are typical trade figures for illustration, not a universal standard.
The right limit depends on the form, how long you will store it, how it is packed and what your own customers require. Agree the number and the test method in writing before you order.
Why moisture matters so much
Dried onion is hygroscopic. It pulls water from the air, and it does so faster as the particles get smaller. A product that leaves the dryer well within spec can pick up moisture during milling, packing, transit or storage.
- Caking: powder and granules form lumps and then hard blocks.
- Browning: higher moisture speeds up colour change during storage, especially in warm conditions.
- Microbial growth: very dry product does not support growth, but damp pockets can.
- Short weight in reverse: you pay for water instead of onion.
- Shelf life: products with higher moisture usually keep their colour and flow for less time.
Form changes the risk
Flakes and kibbled pieces have less surface area and tolerate small moisture changes better. Powder is the most sensitive, so buyers often set a tighter limit for powder than for flakes. If you buy several forms, set a limit for each one rather than one figure for all.
How moisture is measured
Labs usually measure moisture by loss on drying in an oven, under set temperature and time conditions. Rapid moisture analysers are common in plants for routine checks. Different methods can give slightly different results, so name the method in the contract and use the same one at the plant and at your lab.
Some buyers also set a limit on water activity, which shows how available the water is for microbes and reactions. It is a useful extra measure for powders. Ask your lab whether it suits your product.
Keeping moisture down after drying
- Pack soon after drying and milling, without leaving product open to humid air.
- Use cartons with sealed polyethylene liners, as covered in how dehydrated onion should be packed.
- Store packed goods off the floor in a dry warehouse.
- Load containers in dry weather where possible, and consider container desiccants for long, humid voyages.
- Test moisture on the pre-shipment sample, not only on the dryer output.
Writing the limit
Put the maximum moisture, the method and the sampling point in your spec, alongside the other physical limits. The general guide to moisture, purity and foreign matter tolerances explains how to set tolerances that a good plant can actually meet, and what to do when a lot is just over.
Where we come in
We check how each plant measures moisture and how quickly it packs after drying. Pre-shipment samples are tested at an NABL-accredited lab using the method in your contract, and we watch packing and loading conditions on the day. Ask for a free sourcing review.