The citrus family is hard to keep up with. Its few well-defined species happily hybridize with each other, so they've produced many not-so-well-defined species and varieties, only a few of which we see outside of Asia. And citrus chemistry is highly variable as well, which is why oranges, lemons, limes, and grapefruits share a common character but are so distinctive. A handful of publications over the last year suggests how many varieties we have yet to taste, what useful substances they contain, and how they can affect other ingredients in the kitchen.
Eye-friendly pigments in oranges: From Spain, a study of the fate of carotenoid substances in fresh, pasteurized, and electrically treated orange juices. Despite its color, orange juice doesn't contain much of the beta-carotene or chemical relatives that our bodies can turn into vitamin A. But my eye was caught by the numbers for lutein and zeaxanthin, two carotenoids that accumulate in the retina and apparently protect the eye from damage that can lead to macular degeneration. By my calculation, a cup of orange juice contains about a third to a half the protective carotenoids found in an egg yolk, one of our richest sources. And of course, freshly squeezed juice contains the most; processing and time both take a toll.
Vitamin C map of the orange: From Brazil, a survey of vitamin C concentrations throughout the orange fruit, taken by tracing across cut fruit surfaces with a platinum electrode. The highest concentrations are nearest the skin and at the bottom end of the fruit, furthest from the stem.
Oil-breaking effects of citrus aromatics: From South Korea, a study of the effect of citrus peel aromatics on vegetable oils. This study seems to have been motivated by two Japanese reports that the main aromatic in raspberries "melts human fat" and is good for weight loss! That sounds pretty dubious, but Hyang-Sook Choi found something interesting by looking at the chemical changes in olive oil caused by the addition of various citrus peel aromatics. The citrus aromatics break apart the molecules of the olive oil, and release free oleic acid. Free oleic acid is a defect in olive oils. It can have an irritating effect in the mouth, and it destabilizes emulsions like mayonnaise. So an oil flavored with lemon may have a lovely aroma, but it can be less pleasant in the mouth or in a sauce.
Anti-browning activity of citrus aromatics: From Japan, a study demonstrating that many citrus aromatics inhibit the activity of browning enzymes, the catalysts in fruits and vegetables that cause a brown discoloration when the tissues are damaged and exposed to the air. The broad chemical family of aldehydes is especially effective, and this includes the main flavor compounds in anise and cumin. Most cooks know that the acidity of lemon juice is good for delaying browning, but citrus peel oils and spice essential oils may also be useful. The brown spots in aging human skin are created by our own browning enzymes, and the authors of this paper suggest cosmetic as well as culinary uses for citrus oils.
Unfamiliar citrus varieties: Both the Korean and the Japanese studies mention species that not only are new to me, but that aren't even listed in my plant bible, Stephen Facciola's 1998 Cornucopia II: A Source Book of Edible Plants. Among these are the Korean hallabong, a three-way hybrid, and the Japanese mochiyuzu, kabosu, naoshichi, kimikan, keraji, and kiyookadaidai (respectively Citrus inflata, sphaerocarpa, taguma-sudachi, flaviculpus, keraji, and, for kiyookadaidai, unspecified species).
Citrus, we hardly know ye!
Allegrone, G. et al. Comparison of volatile concentrations in hand-squeezed juices of four different lemon varieties. J. Agric. Food Chem. 2006, 54: 1844-1848.
Cortes, C. et al. Carotenoid profile modification during refrigerated storage in untreated and pasteurized orange juice . . . . J. Agric. Food Chem. 2006, 54: 6247-54.
Paixao, T.R.L.C. et al. Use of an electrochemically etched platinum microelectrode for ascorbic acid mapping in oranges. J. Agric. Food Chem. 2006, 54: 3072-77.
Choi, H.-S. Lipolytic effects of citrus peel oils and their components. J. Agric. Food Chem. 2006, 54: 3254-58.
Matsuura, R. et al. Tyrosinase inhibitory activity of citrus essential oils. J. Agric. Food Chem. 2006, 54: 2309-13.