Onions, Green Bunch

MATURITY & QUALITY

Maturity Indices

Maturity of green onions is determined primarily by size which is largely determined by seeding density. Green or “bunching” onions are selected varieties of white onion (Allium cepa) planted at high density or from the non-bulbing onion group (Allium fistulosum) generally called Japanese-bunching. Harvest maturity is generally accepted as mean diameter of 0.6 to 1.3 cm (1/4 to 1/2 inch) in diameter at the base plate of the immature bulb.

Quality Indices

Quality green onions have a thin, white shank or neck at least 5 to 7.5 cm (2-3 inches) in length. Green onions should be well-formed (at most slightly curved or angular), uniform in shape, thin-necked, turgid, bright in color, well cleaned, and free from excessive roots, decay, insect-injury, mechanical damage, broken or crushed leaves, or dehydrated clipped-ends.

U.S. Grade No. 1, No. 2 (Standards established June 1947)

TEMPERATURE & CONTROLLED ATMOSPHERE (CA)

Optimum Storage

0°C (32°F); >98% R.H.

Green onions held at 32°F and 98 to 100 % relative humidity will remain fresh and flavorful for up to 4 weeks. Green onions are highly perishable and normally marketed over a short period. Lowering and removing the heat of respiration as well as preventing water loss is critical. Package-icing and perforated polyethylene film liners are used to maintain quality. Typically, storage life of green onions at 10°C (50°F) is 7 to 10 days. Higher temperatures greatly accelerate yellowing and decay of the leaves. Green onions benefit from light misting.

Rates of Respiration

To calculate heat production, multiply ml CO₂/kg·hr by 440 to get BTU/ton/day or by 122 to get kcal/metric ton /day.

Rates of Ethylene Production

<0.1 µl/kg·hr at 20°C (68°F)

Responses to Ethylene

Green onions are not sensitive to external ethylene.

Responses to Controlled Atmosphere (CA)

Information varies widely on the optimal conditions and extent of benefit of CA for green onions. In general, a controlled atmosphere of 2 % oxygen with 5% carbon dioxide at 0°C (32°F) should allow 6 to 8 weeks storage. Visually, green onions tolerate 1% O₂ and 10% CO₂ but off-flavors have been associated with extended storage above 5°C (41°F).