Onions are one of the most widely used food items in the US. They are an important flavoring component in countless dishes and are even eaten raw on sandwiches and salads. Onions are also rich in antioxidants and flavonoids which may bring a number of healthful benefits including anti-cholesterol, anti-inflammatory, and anti-cancer. For onions to properly serve their crucial culinary and nutrition niche, they must be stored, cooled and transported carefully in ways that take the quality and freshness needs of the onion into consideration.

General Facts About Onions

The phrase onions encompasses a wide array of different bulb vegetables in the Allium genus, including such exotic members as the Egyptian onion (A. ×proliferum), The Japanese bunching onion (A. fistulosum), and the Canada onion (A. canadense) to name just a few. However, most varieties of common onions are in the A. cepa species, including red onions, yellow onions, white onions, shallots, and potato onions, each of which has a different and distinct flavor.

Onions pose a unique challenge to growers because unlike many other fruits and vegetables which can go straight from harvest into cooling and storage, onions need to be dried first. In some regions onions are dried in the field; however, areas with high humidity and rainfall during onion harvesting season must instead dry the onions by other means such as in bins and barns, often with carefully regulated humidity and temperature.

General Information About Cooling and Storing Onions

Like other fruits and vegetables onions have specific cooling needs which include factors such as temperature, humidity, handling, and storage. As discussed above onions also require drying. Depending on the resources available some farmers have had success repurposing equipment and facilities used for drying tobacco, peanuts, and other crops into use for drying onions.

Drying – Drying is crucial to reduce the onion’s susceptibility to decay organisms, extend their shelf life, and vulnerability to bruising. Even dried onions are of course still subject to eventual decay and physical damage, but drying helps provide them with extra time and resistance. Drying times will vary from two or three days if heated air is used, to anywhere upwards of one or more weeks if only ambient air is used.

Humidity – During drying onions should be exposed to a relative humidity of about 65%. Higher humidity levels will lengthen drying times or even prevent full drying. This will also increase the onion’s risk of neck rot and other problems. Once the onions are dried and are in cold storage a slightly higher humidity of about 70% to 75% is generally acceptable.

Temperature – During drying heated air temperature should be 100°F. Higher temperatures should be avoided since anything over 110°F can damage the onion. Once the onion is dried it should be stored in a cooler or cool environment with a temperature between 32°F and 36°F. It is important to prevent the temperature from reaching 31°F or lower because his will subject the onion to freeze damage.

Storage – Temperature and humidity greatly impact how long an onion can viably be stored and at what rate the onion loses mass. Onions stored in temperatures 50°F or higher may be subject to sprouting.

Exposure to Other Produce – Onions should also be kept separate from other fruits and vegetables, both to protect the onion and the other food. That is because onions will readily leach moisture from other produce, drying out the other food and exposing the onion to too much moisture for proper storage. Onions also have a very pungent odor and many other types of produce may absorb the smell.

Cooling Method for Onions

Onions should be cooled using the room cooling method. This involves placing them in a refrigerated room or large walk-in cooler. It is important to continue monitoring temperature and humidity levels while the onions are in the cooler.

Long-Term Storage

Onions can also successfully be placed in controlled atmosphere storage for even longer-term storage. This involves carefully regulating not only temperature and humidity levels, but also the onion’s exposure to gases such as oxygen, nitrogen, and carbon dioxide. Onions in controlled atmosphere storage may last up to 8 months with minimal quality loss.

Semco offers quality cooling equipment that can be customized and designed to fit the needs of onions. We can also tailor our systems to the particular capacity demands and other requirements of our customers. Please contact us for more information.