Are there advantages to an unlined copper jam pan?

As @caconym said, copper ions can bind to pectins, which are the gelling agents in jam.

The gelification behaviour of pectin depends on serveral factors, but for those in jams, the important ones are low water activity (which is due to the sugar content, and also the factor that prevents spoilage), and low pH. Addition of a small amount of copper ions could help making the gel firmer by crosslinking pectin chains. But as we are talking about very small amounts of copper ions, its influence compared to other factors is hard to determine.

Note that it's important that you keep unlined copper vessels very clean, with no traces of oxidation on the inside: while the metallic copper doesn't react very fast with the acids, and is thus relatively harmless for short contact times (like boiling jams), the oxididation products (e.g. verdigris, formed by contact with vinegar) will dissolve much more easily when using the vessels, which will lead to a higher copper intake (and might make your jam too solid ;) ).

But don't forget that copper is also a micronutrient, so you do need some copper in your food.

After periodically coming back to the question over a few months, I think I've found some answers. Remco's answer gives part of the justification (crosslinking pectin chains), but there are other complications.

The short answer is there may be minor chemical benefits to unlined copper for preserve-making, but one can likely achieve similar benefits by just using slightly more acidic recipes for jam. And the aversion to lined copper may be partly based in problems with old corroded tin linings in copper, which would discolor some jams. (Stainless linings shouldn't have these problems.)

The only culinary resource I've found on point (which also has a good scientific repuation) is Hervé This's Molecular Gastronomy, which addresses this question on pp. 65-67.

He begins by noting there is conflicting advice about preserving pans in traditional culinary sources. Many traditional sources going back to the 1800s advocate unlined copper pans. But Henri Babinski in Gastronomie pratique (1907) recommends against them for "preserves made with red fruits," claiming unplated copper pans transmit a "sharp taste." Hervé This also notes professors at the Ecole du Cordon Bleu in the early 1900s recommended against tin for preserves: "avoid using any iron or tin-plated vessel." Lots of sources recommend using pans that have good heat response for preserves (as copper obviously does).

So, there are really two questions here: (1) are there positive advantages to unlined copper, and (2) are there disadvantages to (some?) copper linings?

To answer the first question, there seem to be at least a few possible advantages:

• Hervé This focuses on the crosslinking pectin explanation. As he notes, pectin naturally contains chemical groups (carboxylic acid) that repel each other. Acidic conditions neutralize this effect somewhat and allow pectin to bond (hence one reason why some jam recipes add acid). Copper can provide positive ions that counteract the negative carboxylic ions in solution and thereby allow pectin to link a bit easier, hence "hardening preserves," as This claims.

• What surprises me about that answer is that it's so little studied. If it were a significant effect, there are plenty of food science journals. Industrial jam-making is a huge enterprise, and if this effect were worthwhile, it would likely have been quantified. (In addition to the article linked in comments, there are several articles dating back a couple decades on copper sequestration effects with pectin, so this is a somewhat well-known phenomenon chemically.) But Hervé This seems to be the only source out there that even mentions this effect. How significant is it?

• Other sources that claim positive benefits for unlined copper don't speak in those terms. In fact, the only other one I saw consistently mentioned is that unlined copper makes "sparkling" preserves. Oddly, yes, it's that specific adjective that I saw in several resources, both online and off. My English-language copy of Larousse Gastronomique under "Jams, Jellies, and Marmalades" claims:

Copper and aluminum [pans] should not be used as these metals react with the fruit and contaminate the preserve. (At one time, copper was favoured because the reaction between the metal and fruit produced a sparkling preserve, but this was before the risks of metal contamination were fully appreciated.)

• But what creates the "sparkling" effect? Why that specific word? This is my own theory, but I believe it is partly due to the likely effect copper ions also have on catalyzing the inversion of sugar in preserve-making. One of the goals in cooking preserves is to break down sucrose into its components glucose and fructose (i.e., "inverting sugar"). The catalysis of sugar inversion in unlined aluminum and copper is well-known to confectioners, which can sometimes be an advantage (to break down sugar into a stable syrup) or a disadvantage (when producing harder candies, it can soften the product too much). I've also seen vague references to copper jam pans perhaps helping against jam crystallization (where not enough sugar is inverted, so the sugar recrystallizes after setting). Increased fructose and glucose would also result in more "syrupy" preserves that would tend to be glossier, perhaps aiding this supposed "sparkle."

These advantages seem to be minor enough that no serious food science studies have tackled them. But what are the objections to lined copper?

• The likely culprit is again mentioned by Hervé This. Tin or silver linings themselves are not an issue. But tin and silver salts (i.e., formed through corrosion and left on pans that are not perfectly clean) will react with acidic preserve components: "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."

• However, This goes on to note: "Modern cleaning methods are superior to those in times past... and so the dictum must be amended: Red fruits should never be placed or cooked in unclean tin-plated copper pans."

A final point that is not addressed by This or any other source I could find is -- what about stainless steel linings, which are incredibly popular for copper pans these days? Since they are non-reactive, they would lack the minor advantages of copper's reactivity, but they would also avoid the disadvantages of bare pots (or tin or silver lined) which could introduce potentially poisonous salts if not cleaned properly, as well as creating discolored jam.

Why stainless-lined jam pans are basically unavailable is thus still a bit of a mystery to me. And what about the supposed benefits of unlined copper reacting to help the jam gel quickly (both through pectin crosslinking and perhaps through sugar inversion)? Well, both of these reactions can also be helped along naturally -- without copper -- in jam-making with sufficient acidity.

Thus, ironically, jam makers who buy expensive unlined copper pans and then avoid using very acidic recipes (to avoid excessive copper uptake) are actually working against the very chemical processes unlined copper might facilitate to produce better jam.

Just my two cents. The copper pan was traditionally used for its antimicrobial properties. The heating qualities were just a bonus. It was known that water would stay fresh longer in a copper container. Why it stayed fresh didn't matter. With additional knowledge we can fast-forward to copper plumbing. So, before water bath, steam, or pressure treatment were thought up, the finished product would have just been put in a jar. For a traditional french-style jam with minimal cooking and minimal sugar, the use of a copper pan might have had a significant impact on how long and how well the jam could be stored. (Another example of a traditional practice is that of placing a sprig of lavender blossom on top of the jam. Again, people didn't need to know why, even though now we know lavender oil discourages mold--nothing to do with the smell, taste, or beauty of the lavender. Somehow we lose sight of the fact that adding sugar to fruit has always been about food storage. The same applies to getting preserves to 'set' a skin across the top. This functioned as a sealant, just like sealing a jar with paraffin, or potting meat with a layer a fat. And now I'm thoroughly off topic. Sorry.) I don't have any scientific sources to reference and I can't prove anything about the impact of copper on health or microbes. But I do know what my grandma taught me, for better or worse. And since she lived healthy and independent for 96 years, I'm not going to worry about copper in my jam.