Most bulb onions offered for sale in retail grocers are dried for long-term storage and an extended in-store shelf life. Onions are also further dried in commercial dyers for use in spices and flavorings.
Bulb onions reach maturity when their tops start to yellow and fall over. When 70 to 80% of the tops have fallen they should be harvested. ‘Topping’ the onions is optional. The bulbs should be undercut by 1-2 inches when harvested to enhance conversion to dormancy. If the weather conditions are suitable the bulbs can be left to dry in the field for 1-14 days, a process called ‘windrowing’.
Windrowing is successful only if the temperature remains between 75 and 90°F (24 and 32°C) and the humidity isn’t too high. In the absence of good windrowing weather, onion drying can be done indoors with forced air at 75-90F (24-32°C) and 60-70% relative humidity. A dry onion has a tight neck, brown outer skin, and a rustling sound when handled, and water content is reduced by 3-8%. Once dried, the onions can be stored for months at 32°F (°C) and 60-70% relative humidity. Be careful that the temperature doesn’t reach 41F (5°C) or the onions will sprout and be ruined.
To achieve the necessary dryness level for milling onions into chips and powders, the onions are harvested and immediately cured. They are then washed and sliced, and loaded onto a conveyer belt that carries them through three to four onion drying steps. During the first onion drying step the water content of the slices is reduced from about 83 to 25% using temperatures between 82 and 104°F (28 and 40°C). The second stage reduces the water content to about 10%, but with cooler air.
The third stage reduces water content to about 6% and requires strict control of temperature and air flows to ensure a quality product. If the ambient humidity is below 10%, further onion drying typically isn’t necessary; otherwise a special dryer (Bry-Air) is utilized to bring the water content down to about 4%. This is accomplished by passing ambient air through silica gel beds to remove moisture, treating the onions with the desiccated air, and then removing the moisture from the silica gel beds using heated air. The dried onions are then sent to a milling facility that first removes the skins and then chops or grinds the onions into the desired size.