Parma and Iberian hams, red from zinc
In a recent column in the New York Times, I mentioned the curious chemistry behind the redness of prosciutto di Parma, which unlike most salt-cured meats doesn't depend on the action of nitrite. Here are a few more details.
Though nitrites have been added to curing salts for centuries, some traditional hams, including the renowned prosciutto di Parma and many Spanish hams, are made with salt alone. Regulations for production of the prosciutto di Parma specify this; nitrite and nitrate are not allowed. The meat becomes dark rosy-red nevertheless. It's been proposed that salt impurities and/or bacterial growth provide nitrite, but meat scientists have shown that this is not the case. In the last couple of years, groups in Japan and Denmark have demonstrated that the pigment in these hams is not the usual combination of nitrite-derived nitric oxide with iron-containing myoglobin. Instead, it's the central cage of the myoglobin molecule, called protoporphyrin, with the usual iron replaced by zinc.
Zinc?
Zinc is an essential nutrient for animals. Dozens of enzymes use it to do their work. Pork meat contains two to three times more zinc than iron. So there's more than enough in the hams to displace the iron. Exactly why and how this displacement happens isn't yet known.
It turns out that there's an extensive medical literature on zinc protoporphyrin, because it can be found in human blood and is a sign of iron deficiency. When blood cells don't have enough iron for the protoporphyrin cages in hemoglobin, they put zinc in instead. The zinc version can't absorb oxygen, and so is useless in our blood. In long-cured hams, though, it makes meat color more stable to oxygen exposure. And it's less affected by heating than nitrite-cured myoglobin, though both are dulled by exposure to light.
The Danish scientists suggest that a better knowledge of the zinc story may make it possible to produce a broader range of cured meats without nitrite. At the moment, the main difficulty is time. Prosciutto di Parma is aged for 18 months, Iberian hams for as much as 30 months or more: and the development of the zinc pigment is very slow. In one experiment, the Danish group found that the pigment developed most rapidly between months 12 and 18. That's a long time to wait on hot dogs.
There are a couple of other issues as well. Nitrite prevents the growth of botulism bacteria, so some other antimicrobial additive would have to go into nitrite-free sausages. And nitrite also helps produce the characteristic flavor of cured meats, both by inhibiting fat oxidation (one of the most important sources of flavor development in long-cured hams), and by supplying nitrogen for the production of new aromatic compounds, especially during cooking. So nitrite-free salumi might have the right color, but not the right flavor--or in any case a different flavor.
Clearly we still have a lot to learn from some of our oldest, simplest foods.
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See the references here, and also:
Moller, J.K.S. et al. Mass spectrometric evidence for a zinc-porphyrin complex as the red pigment in dry-cured Iberian and Parma ham. Meat Science 75, 2007, 203-10.
http://dx.doi.org/10.1016/j.meatsci.2006.07.005
Timón, M.L. et al. A study of the aroma of fried bacon and fried pork loin. J. Sci. Food Agriculture 84, 2004, 825-31.
http://dx.doi.org/10.1002/jsfa.1740
Labbé, R.F. et al. Zinc protoporphyrin: a metabolite with a mission. Clinical Chem. 1999, 45, 2060-72.
