By Harold McGee
In France today, crème fraîche means cream with 30% fat that has been pasteurized at moderate temperatures, not UHT pasteurized or sterilized. (Fraîche means “cool” or “fresh.”) It may, however, be either liquid (liquide, fleurette) or thick (épaisse). The liquid version is unfermented and has an official refrigerated shelf life of 15 days. The thick version is fermented with the typical cream culture for 15 to 20 hours, and has a shelf life of 30 days. As with all fermented milks, the thickening is an indication that the product has reached a certain acidity (0.8%, pH 4.6) and so a distinct tartness. Commercial American crème fraîche is made essentially as the French fermented version is, though some manufacturers add a small amount of rennet for a thicker consistency. A distinctly buttery flavor is found in products made with Jersey and Guernsey milks (rich in citrate) and with diacetyl-producing strains of bacteria.