Ever wondered what makes an onion sweet? Well, it’s basically how much water is in it. Onions contain roughly 90% water. Sweetness in onions is another way of saying that they lack pungency and are relatively mild. Surprisingly, sugars have nothing to do with it. Some pungent onions actually have more sugar in them than sweet onions, but the sugar is masked by a high amount of sulfur compounds. 

what makes onions sweet


The primary factor that determines onion sweetness is genetic, relating to the pyruvic acid level. The pyruvic acid content of storage onions ranges from 10-13%, while it is below 5% in sweet onions.

Other factors that can affect pungency levels are growing conditions. Triggers such as high temperatures, drought, and insects will increase the pungency, thereby decreasing the sweetness.

Sulphur Content

All onions need sulphur for proper growth, but sweet ones contain far less sulphur than pungent onions. The low amount of sulphur compounds in sweet onions allows the sugar flavor to come
through. Sulphur helps the plants take up nitrogen, and fight off pests and disease. Most of the onion’s sulphur uptake occurs late in the season during bulb growth.

For healthy onion production, apply sulphur three times during the growth phase using ammonium sulfate (21-0-0) at the rate of one cup per 20 feet of row.

white onion

The Sweet Harvest

The height of sweetness occurs when onions are harvested; over time, pungency increases. When preparing onions for meals, cut them as close to cooking time as possible to get the best flavor.

Want to add some sweet onion varieties to your garden? We have yellow, red, and white choices for all growing areas right here at the farm.