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Roasting Green Chile with the Power of the Sun

Updated: Dec 5, 2022

Greener approach opens possibilities for food roasting


Every August and September the unmistakable pungent aroma of roasting green chile permeates the air across New Mexico. This delectable staple of regional cuisine is green in color, but roasting the pepper to deepen the flavor and make the inedible skin easier to remove is hardly environmentally friendly. In New Mexico alone, burning propane to roast the peppers leads to a seasonal emission of approximately 7,800 metric tons of carbon dioxide—the equivalent of driving 1,700 cars for a year.

Sandia National Labs engineer Ken Armijo, who grew up on a chile farm in Sabinal, thought there was a “greener” way. The results of his experiments with concentrated sunlight were shared at the American Society of Mechanical Engineers conference in July. “The principle behind this research was to see if high-temperature food roasting (not just peppers) could be done with solar and produce comparable results to traditional propane roasting. The answer is yes,” Armijo said.

“Combining state-of-the-art facilities and research with the culture, food and people of New

Mexico is just so special,” he said. “What other national lab in the world would have done

this?” For decades, Sandia has developed technologies that convert renewable resources like wind and sunlight into electricity and useful heat without producing greenhouse gases. Armijo’s demonstration using solar power to roast green chile could inspire new applications of solar technologies and new avenues of research.

With the assistance of several engineers, technologists and interns, Armijo got a traditional steeldrum tumbling chile roaster to the top of the 200-foot tower at Sandia’s National Solar Thermal Test Facility. Armijo’s father, who grows organic, heirloom chile from seed passed down through generations, donated several burlap sacks of green chile and his experience assessing properly roasted chile.

“This has a lot of potential for

roasting chile more quickly, with better

quality. And it’s a greener process.”

Using 38 to 42 of the 212 heliostats— mirror-like devices used to focus sunlight, Armijo was able to achieve a temperature above 900 degrees F uniformly across the roasting drum. This is comparable to a propane chile roaster. He used concentrated solar to roast three batches of green chile: two that had been washed immediately prior to roasting and one that was dry-roasted. The washed chiles took slightly longer to roast, but the amount of charring was

more uniform, and the flavor profile was preferred by green chile connoisseurs, Armijo said.

Afterward, the team returned the chile roaster to the ground and roasted three more batches using propane. Propane was slightly faster, taking four minutes to roast washed chiles, compared to six minutes for the fastest solar roast. With further experimentation, and using more heliostats, Armijo thinks they can roast chile even faster than propane, but he didn’t want to scorch the chile during his first experiments. “With solar roasting we were actually able to achieve a more uniform distribution of heat,” he said. “With propane, you just get heat right where the burners are, but all the chile piled on top isn’t really getting heated as efficiently. We saw with our infrared cameras that with solar, it’s more uniform; the heat is reaching all the chile in the front of the roaster. In practice, this has a lot of potential for roasting chile more quickly, with better quality. And it’s a greener process.”


For each of the traditional roasts, Armijo recorded the amount of propane used for 22 pounds of green chile and found that switching to solar power would reduce greenhouse gas emissions by 2.68 pounds. If the whole state of New Mexico switched to solar roasting, the net result would be the equivalent of planting 130,000 tree seedlings and letting them grow for 10 years.

It’s great to roast green chile sustainably; however, if consumers don’t like the taste, it will never gain acceptance. That’s where the second part of Armijo’s study comes in. He presented 14 green chile connoisseurs with both solar-roasted and propane roasted

chile, and surveyed them. He found that on average, the respondents favored the solar-roasted chiles by 18 percent for flavor, 12 percent for smell and 2 percent for ease of peeling

the skin. However, the respondents preferred the texture of the propane-roasted by 4 percent. “Overall, the participants preferred the solar-roasted to the propane-roasted,” Armijo said. “That was shocking to me. They preferred the taste because it didn’t have as burnt a taste. They said it tastes cleaner of green chile.”

Armijo acknowledged that it’s not feasible to build a tower and field of heliostats just for roasting foods like green chile, coffee or grains. However, he and his colleagues are exploring a much smaller, more modular solar system for roasting small batches like the propane-burning steel-drum roasters use. “In the future, I hope chile roasters will pull up to farmers’ markets, grocery stores and festivals with a trailer with a modular mirrored roaster,” he said. “They just pour the chile in, point the system at the sun and let it roast. That would be awesome.”

Concentrated sunlight could also roast food such as soybeans at 840 degrees F for animal feed and human food; grains for beer at 200-400 F; almonds and cashews at around 300 and 266 F. French roast coffee is roasted until the beans reach 464 F, and coffee beans 350-400 F for light roast. Traditionally, fossil fuels like propane or natural gas are used. New Mexico has practically perfect weather for solar roasting, with an average of 300 days of sunshine each year. The almond production region of California gets 260 days of sunshine, particularly in the summer and fall. Two companies in California are working on pilot plants to use concentrated solar power for lower-temperature processes, such as pasteurizing almonds.

Coffee farmers in Colombia, Ecuador and Peru grow and roast at high altitudes, which is also perfect for using solar. When it comes to the future of green, solar-roasted foods, the sky’s

the limit.



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