If flour is the star of bread making, salt is the director, invisible in the dough but controlling its action and timing. Salt adds flavor. It slows fermentation. It tightens gluten and makes the dough less gloppy. And one of its lesser known jobs is protecting the flour’s carotenoids.
The most famous carotenoid, carotene, gives carrots their bright orange color. Carotenoids also contribute color and flavor to flour and bread. (In bleached flour, they have been destroyed.) Some people claim you can watch the color of dough change as you knead, from an orangey hue to a whiter color. What’s happening to the carotenoids? They’re oxidizing. Call in the salt! How can it slow down this oxidation?
First of all, the carotenoids in flour are lutein, lutein monoester, and lutein diester . Lutein is a long hydrocarbon with a carbon ring and a hydroxyl group (an oxygen atom bonded to a hydrogen atom, also called an OH‑group) on each end. The OH‑group is the reactive part of the molecule.
To make a lutein monoester (below) or diester, one or two fatty acids react with the OH‑groups. Each fatty acid also has an OH‑group, and it pulls the hydrogen off lutein’s OH‑group.The remaining part of the lutein molecule joins the fatty acid, while the lost atoms (O, H, and H) form a water molecule. Lutein monoester still contains one reactive OH‑group.
When you start kneading your dough, you work air into it, and air contains oxygen. This oxygen steals hydrogen from the lutein's OH‑group, as well as stealing a hydrogen from the neighboring carbon. (The two stolen hydrogens and the thieving oxygen form water.) Since the lutein's oxygen now has an extra electron (the electron it formerly shared with the stolen hydrogen atom) as does the robbed carbon atom, the oxygen and carbon turn to each other and form a stable double bond. (Previously they had one bond between them, and they use the extra electrons to form another.)
In wet dough, salt dissolves into ions: positive sodium and negative chloride. These ions move around, and, if they encounter an opposite charge, they are attracted to it.
Lutein's reactive OH‑group has a slightly negative charge on the oxygen atom and a corresponding positive charge on the hydrogen atom. (The bigger oxygen hogs the shared electrons, which are more attracted to the big oxygen nucleus than to the weeny hydrogen nucleus.) The hydrogen’s slight charge normally attracts incoming oxygen atoms, resulting in oxidation. But if a negative chloride ion is nearby, it shields the hydrogen and prevents oxidation.
 Lepage, M. and R.P.A. Sims, “Carotenoids of Wheat Flour: Their Identification and Composition,” Cereal Chemistry 45 (November, 1968) 600-604. Read it online: http://www.aaccnet.org/cerealchemistry/backissues/1968/chem45_600.pdf
Update (September 15th, 2013): Realizing I’d made mistakes, I further researched this topic. Here is what I’ve discovered.
It is accepted in the literature that the “bleaching” or oxidation of lutein in wheat dough is part of a two-step process. In the first step, the enzyme lipoxydase oxidizes linoleic acid, an unsaturated fatty acid in the dough; the result is an unsaturated fatty acid hydroperoxide. The hydroperoxide then oxidizes lutein in step two. The path to discovery of this mechanism is as follows: first, a study linked carotenoid oxidation to oxidation of unsaturated fats (1); then, studies related loss of yellow lutein pigment to lipoxygenase activity (2); then, studies found linoleic acid to be necessary as the substrate (3, 4, 5); and finally, a study found a correlation between bleaching and lipoxygenase activity (6). More recent studies of lipoxygenase activity and carotenoid loss confirmed the results (7, 8).
When carotenoid oxidization happens (at the carotenoid’s chain, as described in the comments), the carotenoid’s chain of double bonds is either cleaved or added to (9). A fairly recent (2007) paper, however, claims there is not enough information available on carotenoid oxidation (10). It lists many studies but notes that the conditions used were different than those found in food. (The data presented in the paper is from a different system than that of wheat dough.)
The original question of this post, how salt slows the oxidation of carotenoids in dough, is still unanswered. I only found one reference in the literature that included salt, which stated that adding salt to dough did not affect the amount of linoleic acid oxidized (11). As suggested in the comments, one would suspect that salt affects the yeast, but throughout the literature, the enzymes involved are said to be from the wheat (7, 12) with no mention of yeast at all. In fact, studies that looked at carotenoid oxidation during dough kneading and rising found that barely any oxidation occurred during rising; the reason given was that the yeast are now competing for the oxygen (12). This seems to imply that salt slowing down yeast functions would make more oxygen available for oxidation reactions.
Perhaps the effect of salt slowing carotenoid oxidation is something that has been observed by bakers (as a color change) but not recorded in the scientific literature. I don’t want to hazard a guess as to where the salt is interfering. If you have any ideas, please post them!
(1) Sumner, J. and R. Sumner. “The coupled oxidation of carotene and fat by carotene oxidase.” J Biol Chem 134 (1940) 531-533.
(2) Irvine, G. and C. Winkler. “Factors affecting the color of macaroni. II. Kinetic studies of pigment destruction during mixing.” Cereal Chem 27 (1950) 205-218. Irvine, G. and J. Anderson. “Variation in principal quality factors of durum wheats with a quality prediction test for wheat or semolina.” Cereal Chem 30 (1953) 334. Please note I have not been able to read these references. They were cited by McDonald (6).
(3) Guss, P., T. Richardson, and M. Stahmann. “Oxidation of various lipid substrates with unfractionated soybean and wheat lipoxidase.” J Am Oil Chem Soc 45 (1968) 272-276.
(4) L. Dahle. “Factors affecting oxidative stability of carotenoid pigments of durum milled products.” J Ag Food Chem 13 (1965) 12-15.
(5) Matsuo, R., J. Bradley, and G. Irvine. “Studies on pigment destruction during spaghetti processing.” Cereal Chem 47 (1970) 1-5.
(6) McDonald, C. “Lipoxygenase and lutein bleaching activity of durum wheat semolina.” Cereal Chem 56 (1979) 84-89.
(7) Leenhardt, F., B. Lyan, E. Rock, A. Boussard, J. Potus, E. Chanliaud,and C. Remesy. “Genetic variability of carotenoid concentration, and lipoxygenase and peroxidase activities among cultivated wheat species and bread wheat variesties.” European Journal of Agronomy 25 (2006) 170-176.
(8) Leenhardt, F., B. Lyan, E. Rock, A. Boussard, J. Potus, E. Chanliaud, and C. Remesy. “Wheat lipoxygenase activity induces greater loss of carotenoids than vitamin E during breadmaking.” J Ag Food Chem 54 (2006) 1710-1715.
(9) Krinsky, N. and E. Johnson. “Carotenoid actions and their relation to health and disease.” Molecular Aspects of Medicine 26 (1005) 459-516. See pages 476-479.
(10) Rodriguez, E. and D. Rodriguez-Amaya. “Formation of apocarotenals and epoxycarotenoids from beta-carotene by chemical reactions and by autoxidation in model systems and processed foods.” Food Chemistry 101 (2007) 563-572.
(11) Mann, D. and W. Morrison. “Effects of ingredients on the oxidation of linoleic acid by lipoxygenase in bread doughs.” J Sci Food Ag 26 (1975) 493-505.
(12) Hidalgo, A., A. Brandolini, and C. Pompei. “Carotenoids evolution during pasta, bread and water biscuit preparation from wheat flours.” Food Chemistry 121 (2010) 746-751.