Myoglobin

Myoglobin is the pigment of muscle, and the colour of all meat, fresh and cured, is at bottom the colour of this one molecule in whatever state it happens to hold. It is a small protein that stores oxygen on an iron atom cradled in a flat ring called a haem, and it is far more concentrated in muscles built for endurance, which is why beef and the leg of a hard-working pig run dark while veal and the loin run pale.

In fresh meat the iron sits in a balance the curesmith learns to read by eye. A fresh-cut surface shows the deep purple of deoxymyoglobin, holding no oxygen. Left to the air it blooms to the bright cherry-red of oxymyoglobin, the colour a butcher's counter is arranged to display. Pushed too far by time, light or warmth, the iron oxidises to brown metmyoglobin, the tired look of meat past its best. These three interconvert constantly, and the freshness a shopper judges is really this equilibrium tilting one way or the other.

Curing lifts the pigment out of that restlessness and fixes it. Nitric oxide, formed from the nitrite of the cure, binds the haem iron to make nitrosomyoglobin, the raw pink of bacon and fresh salame; heat then sets the protein around that bond into nitrosylhaemochrome, the stable pink of cooked ham that neither browns on the plate nor fades in the light as an uncured roast would. This one reaction is the whole reason cured meats look as they do. There is even a route with no cure at all: across the long ageing of a Parma ham the meat's own enzymes swap the iron for zinc, building the separate, remarkably stable red of zinc protoporphyrin.

For the curesmith, then, myoglobin is not a detail but the whole subject of colour. Every colour fault is this pigment reporting a problem: the grey core of an under-cured ham where no nitric oxide reached, the green ring of a contaminated sausage, the slice that fades on the board. Read the colour and you are reading the state of the pigment.