MUNDANE MYSTERIES: How Warm Is “Lukewarm”?

The term “lukewarm” is used commonly to mean “somewhere between hot & cold”. But, there are certain times that call for a more specific place on the thermometer, such as when you’re baking bread, bathing a baby, or having breakfast with Goldilocks. But, how warm is lukewarm really?

It depends, really. It was in the early 18th century that both Dutch physicist Daniel Fahrenheit & Swedish astronomer Andres Celsius each came up with their own respective temperature scales. At that time, “lukewarm” had already been in use for several centuries. And, according to the Oxford English Dictionary, the word derives from the Middle English word “lheuc”, which generally means “tepid”.

Basically, lukewarm wasn’t originally a scientific term (it still isn’t, either). Still, people do occasionally have a temperature range in mind when using it. Some consider lukewarm to be body temperature (98.6°F), while others characterize it to be closer to room temperature (around 70°F, give or take a few degrees). But, if you’re talking about bread or baths, lukewarm needs to be a bit warmer.

Pediatricians say that a baby’s bathwater should be right around 100°F; so, warm but not hot. Water to activate yeast for baking bread should be in that region, too, though it depends on what kind of yeast. Cake yeast (sometimes called “wet,” “fresh,” or “compressed” yeast) needs water between 90°F & 95°F, while dry yeast requires water between 110°F & 115°F. Instant yeast, on the other hand, usually needs even warmer water to activate, between 120°F & 130°F. And, it matters because, if your water’s too hot (140°F or higher) the fungi in the yeast won’t be able to grow at all & your dough won’t rise.

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