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A reader left a voicemail asking: "Where can I get some good sweet onions that are not bitter ... like ones like you get at McDonald's."

The short answer to finding onions short on bitterness is to buysweet onion varieties with names like Vidalia and Walla Walla.

As far as finding McDonald's onions at the local grocery storewell, you don't become a global restaurant powerhouse by broadcasting trade secrets.However, there are plenty of online posts on how to replicate McDonald's onions at home by hydratingminced onions.

I tried a method that callsfor 1 tablespoon of minced onions mixed with an cup of water mixed in a bowl. Microwavethe soaked onion bits for 30 seconds and let sit for 15 minutes.

It worked. The onions were sweet and plumped up but no larger than the minced onions on McDonald's burgers.

Hydrating dried minced onions creates tiny pieces that add onion flavor with minimal bitterness.(Photo: Daniel Higgins/USA TODAY NETWORK-Wisconsin)

Still, there's no need to reconstitute dried onions to satisfy your sweet onion tooth. Sweet onions are available nearly year-round in Wisconsin.

University of Wisconsinhorticulture professor Irwin Goldman wrote in an email response to my onion inquiries that most sweet onions are grown in the southern United States, Mexico, and both Central and South America. A smaller amount are grown in the Pacific Northwest.

When it comes to onions, Goldman has many layers of knowledge. Since joining the UW faculty in 1992 he's headed up the Goldman Lab that focuses on research, breeding and genetics of table beets, carrotsand onions.

Goldman explains the science of why onions make us cry and the varying bitterness as follows:

Before being cut, compartments in the onion's cells isolate a specific enzymefrom a sulfur-based substrate a substrate is a substance acted upon by an enzyme.

When the onion is cut,the cells are ruptured,allowing the enzyme and substrate to combine and produce propanethial sulfoxidethatacts a little like sulfuric acid on the nerve cell membrane of the eye and causes tearing.

The substrate concentration levels vary based on the onion variety and where and how the onion is grown. Higher substrateconcentrations and a more activeenzyme can lead to larger amounts of propanethial sulfoxide.

Variety, growing conditions and how long onions have been stored all impact their flavor.(Photo: Daniel Higgins/USA TODAY NETWORK-Wisconsin)

Onions grown in soil with lower levels of sulfur produce substrates with lower sulfur concentrations and therefore result in a milder flavor. Soil with lower levels of sulfur is more widely found in states like Georgia, Floridaand Texas. This is why sweeter, milder onions typically come from the southern states, whereas stronger flavored onions come from northern regions.

Stored onion bulbs generally increase in pungency up to about 90 days after harvest,and some continue to increase up to 120 days. There are a few that get milder with storage, but most onions simply lose water and further concentrate the substrate, which in turn makes the onion a bit more pungent with time. Also, the onion bulb goes dormantafter harvest, but its dormancy is broken after a few months. Onions that are a few months old may be producing green sprouts because their dormancy has been broken and the sulfur compounds in the substrate are being mobilized into the new leaves.

The greatest pungency of the onion is found in the tissues at the base of the bulb. Cutting through that part of a bulb releases the most pungency and would make you tear up faster than if you kept the basal portion intact and cut other parts of the onion.(Photo: Daniel Higgins/USA TODAY NETWORK-Wisconsin)

The greatest pungency of the onion is found in the tissues at the base of the bulb, near where the stem is located. If you were holding an onion in the palm of your hand with the roots at the bottom, the base would be the centimeter or so of tissue closest to your palm. Disrupting this part of a bulb releases the most pungency and would make you tear up faster than if you kept the basal portion or the onion bulb intact and cut other parts.

My thanks to professor Irwin Goldman for answering our onion questions. If you have a food question, send it my way via email or leave a voicemail message. I can't promise every answer will come from an expert of Goldman's stature, but I will get your questions answered.

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Contact Daniel at (920) 996-7214or dphiggin@gannett.com. Follow himon Twitter and Instagram at @HigginsEats.

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Answering your sweet onion question and the science of why onions make you cry - Green Bay Press Gazette