Post-Harvest Handling Decision Tool > Crop Groups > Dry-Cured Alliums
Dry-cured alliums include sweet, red, and yellow onions, as well as garlic and shallots. These crops all require a “curing” step after harvest to seal the leaf tissue that makes up the bulb against moisture loss and entrance by pathological organisms. In addition, dirt, loose skin, tops, and roots must be removed prior to sale.
Large, commercial operations use a variety of machines for removing scales from the bulbs, but these are not widely available for farms not operating in major onion-producing areas.
Diversified Upper Midwest market farms use a variety of innovative techniques for accomplishing these tasks using manual labor. Larger, diversified operations frequently invest in a roller topper to achieve the desired result with large table onions. Shallots and garlic are almost all cleaned by hand.
Curing
Onion and garlic bulbs are made up of leaf tissue, a part of the plant designed to transpire water obtained from the roots. As onions and garlic reach maturity, the outer leaves senesce, forming a dry layer that prevents this transpiration process. At harvest, growers take steps to fully dry the outer layer and seal the neck of the bulb and the leave pores against water loss and the invasion of disease-causing bacteria and fungi.
Most diversified Upper Midwest market farmers do not have specialized curing systems, instead putting crates or bins in an unused transplant-production greenhouse, or spreading the onions in a single layer on greenhouse benches. Portable barn fans are used to circulate air and promote drying.
Some farmers have developed specialized systems for curing onions by forcing air through the onions in a manner similar to that used in grain bins. Heating the air can reduce drying time and help to cope with wet periods that are not uncommon during the Upper Midwest onion harvest. At Common Harvest Farm in Osceola, Wisconsin, farmer Dan Guenthner uses a squirrel-cage fan to force air into a plenum under mesh benches in a pre-existing outbuilding. At Rock Spring Farm in Decorah, Iowa, the author, Chris Blanchard, uses a Sunderman Manufacturing greenhouse heater with a squirrel cage fan to force heated air into a plenum under stacked trays of onions and shallots.
Thorough curing greatly eases and speeds the removal of dirt, loose skin, tops, and roots. When removing dried alliums from refrigerated storage, condensation can cause the skins to soften and become difficult to clean.
Manual Cleaning
The most common method for removing the loose skin and roots of onions, shallots, and garlic is for workers to rub the bulbs between their hands. A number of inexpensive aids can facilitate the process and increase speed and effectiveness.
Because direct fresh-market sales, whether to stores, CSAs, or farmers markets, place a high value on earliness, growers frequently begin to process cured alliums before the curing process is fully complete. At this stage, the loose skins can be quite difficult to remove. A well-wrung kitchen towel can help add friction to slip the skins off at this stage, according to Hog's Back Farm’s David Van Eeckhout.
When skins are fully cured, nitrile surgical gloves and cotton gloves with vinyl dots can help to “pop” the skins off by applying pressure while moving the hands across the bulb.
Sharp bypass pruners can cut the tops off of hardneck garlic bulbs without crushing the stem.
Roller Topper
A roller topper consists of multiple pairs of longitudinal rollers that rip the necks and skins from fully-cured onions. The rollers have metal ridges that corkscrew down the length of the cylinder. The ridges on alternate rollers corkscrew in opposite directions, and alternate rollers rotate in opposite directions, resulting in a gripping and ripping action that moves the onions down the length of the sloped topping bed.
The rotation of the rollers bounces the onions around quite severely, so only very hard varieties should be cleaned in this way, and only just before sale. This piece of equipment is only suitable for use on round onions. Driftless Organics uses their roller topper to pre-clean yellow and red storage onions, then uses manual cleaning to achieve a finished product.
Market garden machines can be obtained on the used market and typically consist of two sets of four-foot long rollers. These are undoubtedly the most dangerous piece of packing shed equipment considered here, and their use should be carefully considered on farms with young or unskilled workers.
This information is part of a Post-harvest Handling Decision Tool developed by Chris Blanchard of Rock Spring Farm in Decorah, Iowa. The tool is a project of the Fruit and Vegetable Working Group affiliated with the Value Chain Partnerships program. This project was funded by the Iowa State University Extension Value Added Agriculture program and the Leopold Center for Sustainable Agriculture. Blanchard conducted case studies of three vegetable operations to gather information for this decision tool. Products referred to in this tool are not an endorsement by Iowa State University.