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

skim or partially skimmed milk, turning these bacteria loose on high-fat cream will produce a much richer product.
    Unlike commercial sour cream, the homemade kind—provided you’ve made it with heavy cream—can be whipped. When whipping it, be careful not to overwhip, because it can turn suddenly to butter. Homemade sour cream, whipped or plain, makes a delicious topping for fruit tarts and for many chocolate desserts.
    Old-fashioned pasteurized cream will thicken in 24 hours and will have a delicious tang. Ultrapasteurized cream will take a little longer to thicken and will have a softer texture. Both can be kept in the refrigerator for a whole month; the sour cream will get thicker and more flavorful.
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    2  cups heavy cream
    5  teaspoons buttermilk
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    1.     Combine the cream and buttermilk in a screw-top glass jar. Shake the jar for 1 minute.
    2.     Let the jar stand at room temperature for 24 hours while the liquid thickens to the consistency of sour cream. If the room is especially cool, you may need to let it stand an additional 12 to 24 hours.
    3.     Refrigerate for at least 24 hours, or preferably longer for better flavor and consistency, before using.
    MAKES 2 CUPS
                        
HOW SOFT IS SOFT-SERVE?
                        
    I brought home a quart of Dairy Queen soft-serve ice cream and put it in my freezer. To my surprise, the next day it was just as hard as any ordinary ice cream. I thought the soft serves, like Dairy Queen and frozen yogurt, were special formulas that always had that lovely, voluptuous mouth feel that I like so much. What happened?
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    A (chocolate) Dairy Queen aficionado myself, I tried your experiment twice. Each time, as soon as I got the quart container into the car I measured its temperature by plunging a so-called instant-read thermometer into the middle and waiting for a couple of dozen “instants” until it reached its final reading. On the two occasions it measured 14 and 16°F ( 2 10 and 2 9°C). Then, after a couple of days in my freezer, somewhat diminished in quantity by after-dinner “scientific tests,” each quart measured 0°F ( 2 18°C). My soft-serve, like yours, had become just as hard as ice cream.
    Thus, Dairy Queen and the other soft-serves are nothing but ice cream at a warmer temperature. We love them not only because of their softer, smoother textures but because our palates are more sensitive to flavors at warmer temperatures.
    The American Dairy Queen Corp. lists the ingredients of its vanilla product as milk fat and nonfat milk, sugar, corn syrup, whey, mono- and diglycerides, artificial flavor, guar gum, polysorbate-80, carrageenan (the last three are thickeners), and Vitamin A palmitate. Milk fat, the main ingredient, is of course butterfat, and butter is harder at lower temperatures because more of its fat is crystallized. (See p. 83.) The mono- and diglycerides behave similarly to the butterfat’s whole fats ( triglycerides ), while the guar gum and carrageenan thickeners also tend to tighten up at lower temperatures. So it’s no wonder that the Dairy Queen is soft when you buy it but hardens in the freezer.
    At Dairy Queen, TCBY, Carvel, and many other franchised and independent stores, rivers of various soft ice cream products, including nonfat ice cream, low-fat ice milk (which perversely contains less milk than ice cream), frozen yogurt, and frozen custard, flow like lava from smug-looking machines that guzzle batches of packaged mixtures dumped into their maws. The machines mix and chill them, adjust their temperature and viscosity, and dispense them in a variety of flavors, even swirling two flavors together for irresolute customers who can’t decide between chocolate and vanilla. (Can there be any question?)
    Dairy Queen’s product is a reduced-fat ice cream containing 5 percent fat. TCBY sells both nonfat and 4-percent-fat (billed as “96 percent fat-free”)