The first step in any soup or sauce recipe is usually, “Heat oil over low heat and cook onions until soft but not browned.” We wondered if this was necessary; why not accelerate the process by turning up the heat? And was “pre-cooking” in oil even necessary? We wanted to get to the bottom of this ubiquitous recipe step, so we devised an experiment to see just how important this whole “slow-cooking” thing was, anyway.
We made three batches of tomato sauce and tasted them side by side. In the first, we slow-cooked the onions for 10 minutes in oil over low heat before adding the tomatoes; in the second, we cooked the onions in oil over high heat for 8 minutes; in the last batch, we dumped the onions and tomatoes into the oil all at once.
What were the results? The wisdom of the ages proved correct: The sauce with gently cooked onions was strongly preferred by the majority of tasters, who praised its “rich,” “round, sweet flavor.” The sauce with onions cooked over high heat was deemed “sharp” and “flat,” while the sauce made with raw onions was even more “thin-tasting.”
The reason behind our findings is that onions contain different types of sulfur molecules. Low heat and chopping releases the enzyme alliinase, which zeroes in on some of these molecules, breaking them in half and producing pungent compounds that transform into sweeter-tasting disulfides and trisulfides over time. The longer the exposure to low heat, the more such molecules are produced—and the greater complexity the onions can add to a sauce. High heat, on the other hand, deactivates the enzymes, so that fewer of these flavor molecules are produced.
Slowly cooking the raw onions in oil is also important for better flavor. Cooking onions in water (or watery substances like tomatoes) triggers the release of smelly and unpleasant-tasting sulfur compounds (boiled onions, anyone?). But when oil coats the onions during cooking, it protects them against the water, so that fewer of these objectionable molecules can form.
THE BOTTOM LINE: Take the time to slow-cook your onions in oil. The added complexity is worth it.
MAKE IT NOW: Our recipe for Carbonnade à la Flamande is free through April 2, 2013.