How spicy can mustard get? Depends on the soil

Serious wine drinkers often have their preferences: some prefer sweet hints of chocolate in a Malbec from Argentina, while others are drawn to a spicy and fruity Cabernet Sauvignon from Napa Valley. Wine connoisseurs firmly believe that the soil in which grapes are grown determines how it tastes.

Can microbes in the soil also contribute to taste?

In a recent study published in New Phytologist, former CIRES PhD student Corrine Walsh and Fellow Noah Fierer ran one of the first experiments to figure out if soil microorganisms like bacteria and fungi influence the flavor of a crop. Their target: the spiciness of mustard seeds.

"I thought that was an interesting question," Walsh said. "We know microbes and plants communicate via chemicals-could those chemicals impact plant flavor?"

Previous research has confirmed that soil properties influence plant characteristics like growth, seasonal cycles, disease resistance, and nutrient absorption. What remains a mystery is whether or not soil microbes influence a plant's flavor. Testing for this is difficult-past studies have surveyed plants grown in different locations and regions, making it tricky to isolate the role of microbes alone.

"It is often claimed that the types of microbes found in soil should influence crop flavor," Fierer said. "We set out to test this claim, but it wasn't easy-soil microbes are tough to study."

Walsh and Fierer decided to take a unique approach: They used a greenhouse study and grew mustard plants while inoculating the plants with a liquid inoculum of microbes from soils in aspen groves, fields of sagebrush, ponderosa pine forests, and agricultural pastures, all in Colorado. The potting soil, temperature, watering, and nutrients were held constant-only the microbes varied.

They chose mustard because it's easy to grow. Mustard is a part of the Brassicafamily which includes broccoli, cabbage, and horseradish-spicy and bitter vegetables. These spicy and bitter flavors, and the spicy flavor of mustard seeds, all come from glucosinolates, chemicals that help Brassicaplants defend against insects and pests.

"When you cook them they have that kind of sulfury smell," Walsh said. "The glucosinolates have a sulfur compound in them, and that's what you're smelling when you cook."

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