While lukewarm can generally be used to mean “somewhere between hot and cold,” there are certain instances that call for a more specific spot on the thermometer. Say, for example, you’re baking bread, bathing a baby, or breakfasting with Goldilocks. So how warm is lukewarm? The short answer is: It depends.
When Dutch physicist Daniel Fahrenheit and Swedish astronomer Andres Celsius came up with their respective temperature scales in the early 18th century, the word lukewarm had already been on the scene for several hundred years. According to the Oxford English Dictionary, it derives from the Middle English word lheuc, (also spelled leuk or lewke), which basically means tepid. “Als a lewke bath nouther hate ne calde,” an anonymous author wrote in a 14th-century poem called Prick of Conscience. In modern English, that’s “Also a lukewarm bath neither hot nor cold.”
In short, lukewarm wasn’t originally a scientific term, and it still isn’t. That said, people do occasionally have a temperature range in mind when they use it. As The Spruce Eats reports, some consider lukewarm to be body temperature (98.6°F), while others define it as closer to room temperature: around 70°F, give or take a few degrees. If you’re talking about bread or baths, however, lukewarm is a bit hotter.
The Mayo Clinic and Nationwide Children’s Hospital both advise that a baby’s bathwater should be right around 100°F—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. According to Red Star Yeast, cake yeast (sometimes known as “wet,” “fresh,” or “compressed” yeast) requires water between 90°F and 95°F, while dry yeast calls for water between 110°F and 115°F. Bob’s Red Mill explains that instant yeast usually needs even warmer water to activate: between 120°F and 130°F. If your water is too hot—140°F or higher—the fungi won’t be able to grow at all.
As for Goldilocks, there’s still no word on what her perfect porridge temperature is—maybe spring for bagels instead.
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