Free What Einstein Kept Under His Hat: Secrets of Science in the Kitchen by Robert L. Wolke

is lighter (less dense) than water, it will float to the top, and the fat-rich portion—the cream—can be poured off.
    But dairies separate the globules of fat much more quickly and efficiently from the rest of the milk by using centrifuges or so-called cream separators—machines that spin the whole milk around at thousands of revolutions per minute as if it were laundry in the spin cycle of a washing machine gone berserk. The heavier (more dense), watery skim is forced outward more strongly than the fat and migrates toward the outer portions of the bowl-shaped container, while the less dense fat globules linger nearer the center. A stack of conical vanes collects products of various densities, that is, products with various percentages of fat.
    The U.S. Department of Agriculture (USDA) regulates the labeling of creams of various fat contents. Heavy cream, sometimes called heavy whipping cream, is literally the crème de la crème , because it contains the highest percentage of butterfat: from 36 to 40 percent. Lighter whipping creams may contain from 30 to 36 percent butterfat, but anything less than 30 percent won’t whip. Light cream, sometimes called coffee cream, contains 18 to 30 percent butterfat.
    Half-and-half is supposedly half milk and half cream, but that’s not to be taken literally; its butterfat content depends on how heavy or light the “cream” half is. Half-and-half can run from 10.5 to 18 percent butterfat.
    Because the label wordings can still vary within the USDA regulations, just select your cream in the market by looking at the fat content printed on the container.

    A small, hand-cranked cream separator. The cream comes out of one spout (at left) and the milk comes out of the other.
(Courtesy Hoegger Goat Supply.)
                            
Pillowcase Sweets
                        
    T his is an adaptation of a handwritten recipe by American painter Mary Cassatt (1844–1926). She made these soft caramels for company when she and the impressionist painter Edgar Degas entertained in Paris. Cassatt’s recipe books have not survived, and this is the only recipe attributed to her. These homemade cocoa-covered caramels, each shaped like a tiny pillow, were placed on guests’ bed pillows during turn-down service at the grand opening of the Renaissance Hotel in Pittsburgh. (Cassatt was born in Allegheny City, Pennsylvania, now part of Pittsburgh.)
    Follow the recipe exactly. If the kitchen is too warm, the candies tend to soften and spread. In a cooler room, they keep their shape. If you like fudge, you’ll love these pillows.
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    6        ounces bittersweet chocolate
    1 1 / 2     cups confectioners’ sugar
    6        tablespoons ( 3 / 4 stick) unsalted butter
    1        cup honey
    1        cup heavy cream
    1 / 4      cup unsweetened cocoa powder for dusting
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    1.     Lightly oil a marble or granite surface or a cookie sheet. Do not use a Teflon or similarly coated cookie sheet. Set aside until needed.
    2.     On the large holes of a handheld grater, grate the chocolate into a medium bowl or pie tin.
    3.     Place the grated chocolate and all the remaining ingredients except the cocoa in a heavy, medium saucepan and mix them together with a wooden spoon.
    4.     Stirring occasionally, bring to a boil over medium-high heat and continue cooking until the mixture reaches 238°F on a candy thermometer. This will take about 10 minutes.
    5.     Carefully pour the hot mixture onto the oiled surface—do not spread it—and allow it to cool.
    6.     Using a bench scraper, gather the candy into 4 balls and dust them lightly with some of the cocoa powder. Roll them into logs about 1 inch in diameter. Dust again with the cocoa powder, place on a cookie sheet, and cover with plastic wrap. Refrigerate for several hours until firm, or up to overnight.
    7.     Uncover the logs and slice them into