Onions, Dry

MATURITY & QUALITY

Maturity Indices

• Indicated when approximately 10 to 20 percent of tops have fallen over
• Conversion from active growth to dormancy accelerated by undercutting bulbs 1 to 2 inches
• Field-dry” maturity is indicated when bulb neck is completely dry to the touch and not slippery. Typically reached at 5-8% weight loss following harvest.

Quality Indices

• Mature neck and scales
• Firmness
• Diameter (Bulb size)
• Absence of decay, insect damage, sunscald, greening, sprouting, freezing injury, bruising, and other defects
• *Degree of pungency

TEMPERATURE & CONTROLLED ATMOSPHERE (CA)

Optimum Temperature

Curing. Field curing when temperatures are at least 24°C(75°F) or exposure for 12 hrs. to 30 to 45°C (86 to 113°F) for forced air-curing.

Storage. Mild onions: Typically 0.5 to 1 month at 0°C(32°F)
Pungent Onions: Typically up to 6 to 9 months at 0°C(32°F) depending on the cultivar

Optimum Relative Humidity

Curing. 75 to 80% for best scale color development

Storage. 65 to 70% with adequate air circualtion (1m3/min/ m3 of onion )

Rates of Respiration

Whole Onions. 3-4 ml/kg·hr @ 0-5°C (32-41°F); 27-29 ml/kg/hr @ 25-27°C (75-79°F). Storage between 5-25°C (41-75°F) favors sprouting and is not recommended for extended periods.

Diced Onions. 40-60 ml/kg·hr @ 0-5°C (32-41°F). 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

Whole Onions: < 0.1 µl/kg·hr at 0-5°C (32-41°F)

Diced Onions: NA

Responses to Ethylene

Ethylene may encourage sprouting and growth of decay-causing fungi.

Responses to Controlled Atmospheres (CA)

No commercial benefit has been identified for varieties with long storage potential. Onions are damaged by <1% O₂ and 10% CO₂. There is some commercial use of CA (3% O₂ and 5-7% CO₂) for sweet onion varieties (short storage potential) . Diced onions benefit from CA conditions of 1.5% O₂ and 10% CO₂.