Why are unplated copper pans recommended for cooking fruit preserves?
LET’S EXAMINE A FEW MORE DICTUMS. L.-E. Audot, author of La cuisinière de la campagne et de la ville (1847), says that in order to make fruit preserves “it is indispensable to use an unplated copper pan (earthenware or terracotta ones being liable to burn [the preserves] or impart a bad taste).” Sixty years later, geologist Henri Babinski, in his Gastronomie pratique (1907), advised, “For preserves made from red fruits, it is preferable to use an enameled pan, which does not transmit any sharp taste, as often happens with unplated copper pans.” During the same period, professors at the École du Cordon Bleu recommended that cooks “avoid using any iron or tin-plated utensil.”
What is one to make of these conflicting opinions? Should copper or enameled cast iron be used? If copper, tin-plated or unplated? Although copper preserving pans may retain a certain luster that encourages culinary nostalgia and adds to the aesthetic quality of the kitchens in which they are displayed, they are also a bother because they have to be thoroughly cleaned (which cannot be done with ammonia, by the way, because this would give the preserves a disagreeable taste) before being used. Why not use stainless steel pans or enameled containers instead? Does copper give better results because of its superior thermal conductivity? Or does it possess other unsuspected properties that make it preferable to these alternatives?
The Role of Copper in Preserves
Nothing beats an experiment. Let’s begin by putting red currants or raspberries in an unplated copper pan. To be rigorous about it, let’s first measure the pH of the pan’s contents (pH is a measure of acidity running from 0, for very strong acids, to 14, for very strong bases). The acidity of such fruits sometimes is surprisingly high. Indeed, a pH of about 3—which is to say about as much as certain vinegars—is not unheard of. Next, tilt the pan and you will see that the copper has been stripped away by the fruit and its juice. In other words, the copper ions covering the metal have dissolved.
Do these ions have an effect on the preserves? Let’s conduct another experiment, dividing a previously cooked batch of preserves in a chemically inert container (glass, for example) and then adding a copper salt to one of the two halves. When the two portions have cooled, one observes that the one containing copper ions is firmer than the other. Why? Because the solidity of the preserves depends on the presence of pectin molecules, extracted from the fruits, which form a network that traps the water, sugar, and fruits. Adding lemon juice generally promotes gellification because pectin molecules contain carboxylic acid –COOH groups that, depending on the degree of acidity, may or may not combine. If the environment is insufficiently acidic, the carboxylic acid groups are ionized in –COO– form so that the electrical charges they carry have a mutually repulsive effect; in an acidic environment, by contrast, these groups are neutralized and the pectin molecules no longer repel one another.
What is the role of copper in all of this? In preserves copper is found in the form of ions and possesses two positive electric charges that interact with the two negatively charged groups, causing the pectins to bond with one another. In other words, copper reinforces the pectin gels, hardening preserves, as experience shows.
And Tin?
Given that copper is a suitable material, why should the tin that covers the inner surface of old preserving pans be harmful? Could it be that the old dictums are nothing more than the worthless residue of empirical advances in culinary practice? As it turns out, putting red fruits such as raspberries or currants in tin-plated containers produces no unwelcome consequences. Because tin does not act on red fruits, one might suppose that copper is the culprit, but fruits placed in copper are not altered either.
It is nonetheless generally the case that metals act through their salts. Try sprinkling a pinch of various metallic salts—silver, aluminum, copper, tin, iron, and so on—over red fruits. The tin salts immediately cause a disagreeable purplish color to appear because the pigments in the red fruits have combined with the metallic atoms; and because the electrons responsible for this bond are differentially distributed in the molecules of the pigments, these pigments absorb light to different degrees. Thus silver salts cause raspberries to whiten a bit, whereas copper ions give them a fine red-orange color. Tin ions trigger the purple tinge that has given rise to the prejudice against tin-plated pans.
Modern cleaning methods are superior to those in times past, however, and so the dictum must be amended: Red fruits should never be placed or cooked in unclean tin-plated copper pans.