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Would an onion by any other name taste as sweet as a Vidalia?

  • Posted on: April 11, 2017
Would an onion by any other name taste as sweet as a Vidalia?

At the University of Georgia Crop and Environmental Laboratory in Athens, lab manager Daniel Jackson and his staff are getting to the root of why onions taste the way they do. The data coming out of the lab is important to Georgia’s multimillion-dollar Vidalia® onion industry and will impact not only the farmers who produce this unique Georgia crop, but also fertilizer manufacturers and onion breeders. To listen to Jackson explain it, biting into an onion does not just produce a burst of flavor that is sweet, hot or pungent; it is the trigger for a series of chemical reactions releasing pyruvic acid, lachrymatory factor and methyl-thiosulfinates, as well as various sugars. These reactions make an onion taste like an onion.

In the past, researchers focused on the sugar content. It was assumed the more sugar the onion contained, the sweeter it tasted. It turns out the story is more complex with some compounds masking the sugars. For example, lachrymatory factor is responsible for the upfront heat and pungency that can turn a hardened onion eater teary-eyed, and methyl-thiosulfinates are responsible for oniony aftertaste and onion breath. High levels of these and pyruvic acid mean a hot, pungent onion while low levels mean a sweet, mild onion.

Extracting these compounds is an exacting process. Jackson and his staff core 10 onions from a single 10-pound bag to make one sample. That sample is then placed in a hydraulic piston where the juice is analyzed for each of the three flavor compounds. The lab processes about 40 of these samples per day, or 2,000 pounds of onions per week during peak season. Testing requires careful timing. For example, lachrymatory factor peaks at 30 seconds, methyl-thiosulfinates at two minutes, and pyruvic acid at 20 minutes. At these points the chemical reactions are halted and the samples are placed into a spectrophotometer or gas chromatograph to get analytical readings.

Determining levels of these compounds in onions is one thing; determining what levels are acceptable is something else. That involved a two-year study with the U.S. Department of Agriculture and the UGA Food Science Department, and meant training participants to be on a taste-test panel to score onions on sweetness, burning sensation and aftertaste. The participants did not know the analytical readings the onions had received but the scientists did, and this enabled them to determine a consumer acceptability threshold for the offending compounds.

Knowing what is in an onion and knowing what consumers like are not useful unless you can do something to modify one to match the other. Fortunately, there is something to be done. The lachrymatory factor, methyl-thiosulfinates and pyruvic acid are linked to the amount of sulfur in the soil: the lower the sulfur, the lower the levels of these compounds.

When some onions grown in the Vidalia region tested for higher levels of these compounds, the researchers were faced with something of a mystery. Soils in Georgia’s Vidalia onion-growing region were known to be naturally low in sulfur, and soil samples taken from Vidalia onion fields were showing low sulfur levels. However, most soil samples were taken from the top six inches of soil. Jackson and researchers took deeper soil samples and found that sulfur from fertilizers had leached through the sandier topsoil and had built up in the clay level farther down. Onion roots were extracting sulfur the soil tests were showing wasn’t there. Sulfur is an essential plant nutrient so you can’t remove it altogether, but this knowledge may mean new fertilizers with lower percentages of sulfur as well as new growing recommendations. Some farmers have already changed their fertilizing regimens.

It may also mean more onion varieties will be acceptable to grow as Vidalia onions. Many people think a Vidalia onion is one specific variety, but actually a group of varieties may be grown as Vidalias. Jackson told how 45 varieties were analyzed and tested by the taste panel in 2015 with only 12 passing the consumer acceptability test. The same varieties were tested in 2016 after being grown with reduced sulfur levels and 34 passed. Having more varieties in the Vidalia onion arsenal could mean more disease resistance, cold tolerance or earlier or later-maturing varieties, Jackson explained. It could also mean even sweeter Vidalia onions in the future.

Jackson’s research is a cooperative effort among researchers, farmers, the Georgia Department of Agriculture, the Vidalia Onion Committee and UGA Extension staff. “This is applied research, and the benefit for the industry and the consumer is a consistently good product,” Jackson said. “Everyone wants that.”

By Arty Schronce