Turkey Day Chemistry in the Kitchen

A Calculated Cranberry Sorbet

On the spectrum of pure ice to soft sorbet, texture can range from crumbly and crunchy to creamy and smooth. In each case, the same process is at work. Sugar is interfering with the formation of ice crystals and, in doing so, is lowering the mixture's freezing point. The smaller the crystals are, the creamier the ice and the more scoopable it will be straight from the freezer. The larger they are, the more the frozen mixture will resemble sharp shards and remain impenetrable unless partially thawed.

The trick to making a perfect ice, whether a flaky granita or a smooth sorbet, is calculating the perfect balance of water to sugar for freezing while keeping the tartness just right for taste, and stirring or pureeing the mixture to break up the crystals as it chills. This can usually be assessed by calculating the total percentage by weight of the sugar in the mixture. From 15 percent to 20 percent, the ice that would freeze solid if left undisturbed will make a granita if stirred frequently during freezing. Above 35 percent, it will remain syrupy. At 30 percent, it will be solid yet smooth. Between the two, something in between.

So then you just need to figure out the fructose and glucose in the fruit; the amount of sucrose-or table sugar-you need to add for your desired texture; and the amount of acid-or lemon juice-necessary for a nice acid balance. It's simple . . . if you have a refractometer or saccharometer to measure the sugar in your fruit, and can do the math.

In his second book, The Curious Cook, McGee kindly does these calculations for us, listing multiple fruits and three consistencies of ice and how much fruit, water, sugar and lemon juice is needed for each. Cranberry sorbet presents a special situation, though, if you want to use the fresh fruit. Pureed uncooked cranberries separate into unpleasant dry bits and liquid-not good for a sweet frozen dessert. Cooked, though, as the cell walls break down, two significant events occur. The fruit becomes soft enough to be pureed into a smooth mixture, and pectin is released to thicken it. But that pectin throws off the straight sorbet calculation, because will keep a less sweet sorbet creamy at a lower temperature. You also need to add back the water that evaporated off during cooking.
Here's the recipe:

Combine 1 cup of fresh whole cranberries, I cup of water, 6 tablespoons of sugar and 2 long strips of orange zest (taken with a vegetable peeler from a navel orange), thinly sliced, in a small saucepan. Bring to a boil, reduce the heat, and simmer until the cranberries have burst and are soft, about 5 minutes. Cool, add cold water to make 2 cups of liquid (about a 1/2 cup), puree, and transfer the mixture to a metal cake or pie plate and freeze, stirring occasionally with a fork, until solid, for about 6 hours. Puree in a food processor before serving.