Do onions need sunlight during curing?
Written by
Liu Xiaohui
Reviewed by
Prof. Charles Hartman, Ph.D.Sunlight exposure during curing causes extreme damage to onions. I have learned from experience that direct sunlight can cause irreparable sunscald to my bulbs. The extreme light intensity and heat destroy the protective compounds in the outer layers. Instead, I recommended using shaded areas allowing for indirect light and air movement.
Sunscald Development
- Direct sunlight heats bulb surfaces above 100°F causing cell death. This creates bleached soft spots that rot quickly. I prevent this using 70% shade cloth during field curing.
Flavor Compound Breakdown
- UV radiation degrades thiosulfinates responsible for onion flavor. Light-exposed bulbs develop bitter off-flavors. I cure in complete darkness preserving complex taste profiles.
Skin Barrier Damage
- Sunlight dries outer skins too fast causing cracks. These openings allow mold entry during storage. Indirect light allows gradual even drying forming intact barriers.
Indoor curing requires keen management of light. I have heavy blackout curtains in my curing room, and I utilize a ventilation fan. Place your lights with space in between. I use hygrometers to monitor humidity and prevent moisture buildup, especially in areas with limited light exposure.
Air circulation substitutes for sunlight's advantages. Fans moving 2-3 mph of air simulate wind drying without damaging the heat. I have oscillating fans 3 ft from bulbs. No stagnant air forms while preserving the darkness necessary for flavor retention.
Field Curing Technique
- Lay bulbs tops-over-bulbs under trees or shade structures. Turn twice daily for even drying. Cover with breathable row fabric if rain threatens.
Indoor Curing System
- Use mesh racks in dark well-ventilated spaces. Maintain 75-85°F with 60% humidity. Position fans for cross-ventilation without direct airflow on bulbs.
Variety differences matter when it comes to light sensitivity. Sweet onions, for example, such as the easy-to-grow Sweet onion, may endure some morning sun, but storage types prefer total darkness. I use the tools I have, knowing Copra onions will lose bulbs to storage quality with any light exposure in curing.
Read the full article: 7 Essential Signs for When to Harvest Onions
