Dehydrated Onion

Dehydrators do not buy the onions that supermarkets buy. The economics of drying are dominated by how much water has to be evaporated, so the industry uses cultivars bred for high solids content and grown specifically for the purpose — typically white, strongly pungent onions rather than the mild sweet types prized for fresh eating. More solids per bulb means more product per tonne delivered and less energy spent boiling off water.

Bulbs are topped and tailed, peeled, inspected, and sliced, then dried on moving belts through successive stages, each drier than the last — the bulk of the water leaves early and quickly, while the last of it is removed slowly and gently, because that final stage is where heat damage and browning happen. Dried slices are then broken and screened into size grades, with powder produced by milling. Flakes, granules, and powder are the same dried material differentiated by particle size, so a single line yields the whole range from one raw stream.

Two problems shape the process. Onion tissue browns as it dries, from reactions between its own sugars and amino acids, and the drier and hotter the material gets the faster that runs — so the endgame of drying is a balance between removing water and keeping the product pale. Onion flavour is also volatile and is generated by enzyme action when tissue is cut; some of it inevitably leaves with the steam. How much survives is a real difference between plants and a genuine quality variable in the trade.

Buyers specify particle size and its uniformity, colour and freedom from scorched or browned pieces, pungency or flavour strength, residual moisture, rehydration behaviour, and microbiological condition. Colour is watched closely because it is the visible record of how well the drying was controlled, and because dark specks in a pale powder are conspicuous in a finished food.

The dominant storage property is that the product is strongly hygroscopic — it pulls moisture from the air, and it does so eagerly. Water taken back up causes caking, restarts browning, and undoes the stability the drying paid for, so dehydrated onion is packed in moisture-barrier containers and kept sealed and dry. It also carries a penetrating aroma that transfers readily to other goods, which makes segregated storage a practical requirement rather than a nicety.

• Food manufacturing — the dominant use, in soups, sauces, seasonings, snacks, and prepared foods
• Food service and retail — packed as a dried culinary ingredient