Brown onion, bulb onion, common onion, yellow onion
Onions come in various shapes and colours, the most common being round to oval-shaped and slightly smaller than a tennis ball. Onions are a bulb. Various colours are available, including white, brown, red and yellow, which have corresponding differences in flavour and use.
Onion quality is determined by neck and scale maturity, bulb firmness and size, an absence of decay, insect damage, sunscald, greening, sprouting, freezing injury, bruising and other defects, and the degree of pungency. Onions may be field cured when temperatures are at least 24°C or forced air-cured by exposing them to 45°C for 12–30 hours.
Postharvest storage temperature
Mild onions should be stored up to 1 month at 0°C. Pungent onions can be stored up to 9 months at 0°C depending on the cultivar. Onions are susceptible to freezing injury and symptoms include soft water-soaked scales that rapidly decay from subsequent microbial growth.
Controlled atmosphere storage
Onions are damaged by <1% O2 and 10% CO2. there is some commercial use of controlled atmospheres (3% O2 and 5–7% CO2) for sweet onion varieties (short storage potential). Diced onions benefit from controlled atmosphere conditions of 1.5% O2 and 10% CO2.
Ethylene may encourage sprouting and growth of decay-causing fungi.
During curing onions should be exposed to 75–80% relative humidity for best scale colour development. During storage onions should be exposed to 65–70% relative humidity with adequate air circulation.
Disease & infection
Onions are susceptible to a range of pathogen-causing rots. Rots can be minimised by harvesting only at full maturity, proper drying and curing, minimising bruising and scraping damage, and maintaining proper storage conditions to prevent condensation from forming on the bulbs.
Chop onions early in the preparation phase, then cover and set aside for a few minutes to enhance the flavour.