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Why Slow-Cook Onions in Oil?

So many recipes calling for softening onions in oil over low heat, but is this really the best method?

Invariably, the first step in any soup or sauce recipe is “Heat oil over low heat and cook onions until soft but not browned.” But why not accelerate the process by turning up the heat? And is “pre-cooking” in oil even necessary?


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.


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.”


Onions contain different types of sulfur molecules. Low heat, and chopping, release the enzyme alliinase, which zeroes in on some of these molecules, breaking them in half and producing pungent compounds that, over time, transform into sweeter-tasting disulfides and trisulfides. 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 to 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 against the reaction with water, so that fewer of these objectionable molecules can form.

The bottom line: Take the time to slow-cook onions in oil. The added complexity is worth it.

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