What makes cranberries gel?
One word: pectin.
Pectin is a complex carbohydrate (a polysaccharide) found mainly in the cell walls of fruits and certain other parts of plants. Apples, citrus rinds and cranberries are particularly high in pectin.
When an acid is present, pectin reacts with sugar to form a gel, a homogeneous dispersion of water molecules throughout a molecular network within the solid carbohydrate. The resulting substance has jellylike properties. It behaves somewhat like a liquid, yet holds its shape when cooled, like a solid.
As in most chemical reactions, pectin and sugar must be present in a certain proportion for the conversion to occur properly. The right proportions, by weight, are one part sugar and one part water to two parts cranberries. For example, 6 ounces (about 3
4 cup) each of sugar and water to a 12-ounce bag of cranberries. The berries themselves contain the necessary acid. Insufficient sugar, in particular, can lead to a sauce that won’t gel, and you may wind up with cranberry soup.
Too much water — more than is necessary to dissolve the sugar and hydrate the pectin — also will keep the sauce from setting. An easy way to fix soupy sauce is to cook it longer to evaporate excess water. Just remember that it won’t “set up” until it has cooled so the melted gel solidifies. (Strictly speaking, the words “melt” and “solid” don’t apply to gels.) But be careful: If you cook the sauce too long, you’ll wind up with red rubber.
Hint: The directions on a bag of fresh cranberries can’t be beat.
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Wolke is a professor emeritus of chemistry at the University of Pittsburgh and a former Food section columnist.