Salting tomatoes in the greenhouse

Tomato lovers know that a sprinkling of salt enhances the flavor of even the best field-ripened specimen. Some recent news that bodes well for improved flavor in greenhouse tomatoes: you can enhance tomato flavor by salting the plant as the fruit grows! At the Institute of Vegetable Science in Freising, German scientists grew hydroponic tomatoes in a solution that was 0.1% sodium chloride, about one-thirtieth the salinity of seawater. The plants produced fruits with significantly higher levels of flavorful organic acids and sugars, and as much as a third more vitamin C and beta-carotene (the precursor to vitamin A) and the antioxidant red pigment lycopene. The researchers don’t say whether the tomatoes were saltier than usual. They were smaller, so salting the growing medium may be the hydroponic equivalent of dry-farming, which restricts the availability of water to the plant and the dilution of flavor and nutrients.

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Krauss, S. et al. The Influence of Different Electrical Conductivity Values in a Simplified Recirculating Soilless System on Inner and Outer Fruit Quality Characteristics of Tomato. J. Agric. Food Chem., 2006, 441 -448. http://dx.doi.org/10.1021/jf051930a

Progress in sparkling wines: bubbles and flow

A progress report from Gérard Liger-Belair, the world authority on wine bubbles, and his quest to define the ideal glass for enjoying the effervescence of champagne. Bubbling really stirs up a glass of wine, and if you're going to etch a glass to generate bubbles, you'll need to adjust the pattern to the glass shape.

When a sparkling wine is poured into a glass, the bubbling delivers aroma and pleasantly irritating carbon dioxide to our nose. At the same time it depletes aroma, gas, and its own activity. If a glass of sparkling wine bubbles vigorously, it loses the advantages of effervescence quickly; if it bubbles too slowly, it has no charm. Liger-Belair has shown that steady, regular, "pleasing" bubbling is caused by plant dust: microscopic cellulose fibers from the dish towel or released into the air from such things as clothes and paper. Intentionally scratching or etching the bottom of the glass creates pits that induce more predictable bubble formation, but the bubbling is faster, coarser, and more chaotic.

For this new study, Liger-Belair and colleagues added tiny reflective plastic beads to bottles of champagne, poured the wine into glasses with bubble-forming pits etched just above the central stem, illuminated the glasses from the side with a laser beam, and used time-lapse photography to follow the movements of the beads.

They found that bubbles rising from the bottom of the glass pull the surrounding liquid along with them, setting up regular lines of flow from the bottom of the glass to the top and then back down. In a tall flute, the lines of flow run through the full volume of the liquid, and deliver bubbles directly to the edge of the glass to form the desirable collerette or bubble collar. But in a broad and shallow coupe, the flow lines ran only in the center of the glass, leaving a surrounding "dead zone" of little or no effervescence that prevented the delivery of bubbles to the edge.

So glasses with different shapes will have to be etched with different patterns of pits to deliver the same desirable effervescent effects. The large surface area of wine exposed in a coupe also means a more rapid loss of gas, so the etching in a coupe may need to be sparser than in a narrow flute.

Next on the agenda for the Liger-Belair lab: "quantitative measurements of the release of volatile organic compounds and carbon dioxide from glasses showing various engravement and shape conditions, our final goal being to scientifically identify the best glass for the tasting of champagne and sparkling wines in terms of gas discharge and flavor release."

In the meantime, fans of sparkling wine have more to look for in the glass.

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Liger-Belair, G. et al. Visualization of mixing flow phenomena in champagne
glasses under various glass-shape and engravement conditions. J. Agric. Food Chem. 2007, vol. 55.
Published on the web: http://dx.doi.org/10.1021/jf062973+

In the dark: olive oil, milk, butter, and beer

In my last post I mentioned that olive oil is best stored in the dark. The same is true for milk and butter and beer. It's turning out that all these foods are sensitive to light for similar reasons.

When milk is exposed to light, especially sunlight or to the fluorescent lights in a market, it develops an unpleasant, sulfurous "sunlight" or "lightstruck" flavor. It's been known for a long time that the vitamin riboflavin is involved in this reaction, and a recent report by David Min and colleagues at Ohio State summarizes the current understanding of what happens. It turns out that the off flavor signals significant nutritional losses. When riboflavin absorbs certain frequencies of light, it catalyzes the conversion of ordinary oxygen to an especially reactive "singlet" form. Singlet oxygen in turn attacks the milk fat, producing fragments with grassy aromas, and it attacks the amino acid methionine, producing a compound with an overcooked-vegetable aroma (dimethyl disulfide). It also attacks both the riboflavin that made it, and vitamin D, which we need to absorb the calcium in milk efficiently.

Exposure to light also damages the flavor of beer, which accumulates a characteristic "skunky" sulfur compound known as MBT (3-methyl-2-butene-1-thiol). Earlier studies had shown that MBT is produced when flavor compounds from hops, the hop acids, react with sulfur-containing compounds. But the hop acids themselves don't absorb the wavelengths of light that cause skunkiness. It appeared that that the energy for the reaction was supplied indirectly, and probably by the same molecule that damages milk-- riboflavin! Richard Pozdrik and colleagues in Melbourne, Australia have strengthened the case against riboflavin by showing that light absorption by riboflavin in beer correlates well with the development of skunkiness.

According to a new study of butter done in Norway and Denmark, riboflavin isn't the only "photosensitizer" in dairy products. J.P. Wold and colleagues found that traces of chlorophyll and related substances in butter also absorb light energy and transfer it to other butter components, thus causing oxidation reactions and unpleasant flavor changes. This makes sense, because absorbing and transferring light energy is exactly what chlorophyll is designed to do in the leaf of a living plant. And it's the lovely green chlorophyll and related molecules that are the major photosensitizers in olive oil.


So it's a good idea to buy and keep all these foods in opaque or at least dark containers. If they're in clear glass or plastic, or the butter is wrapped in light wax paper, then keep them in the dark as much as possible.

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D.G. Bradley et al. Effects, quenching mechanisms, and kinetics of water-soluble compounds in riboflavin photosensitized oxidation of milk. J. Agric. Food Chemistry 2006, 54, 6016-20.

R. Pozdrik et al. Spectrophotometric method for exploring MBT formation in lager. J. Agric. Food Chemistry 2006, 54, 6123-29.

J.P. Wold et al. Active photosensitizers in butter detected by fluorescence spectroscopy and multivariate curve resolution. J. Agric. Food Chemistry 2006, 54, 10197-10204.

Keeping olive oil cool

Refrigeration usually slows the deterioration of our foods by slowing the chemical reactions that cause it. But extra-virgin olive oil turns out to go rancid at about the same rate in the cold as it does at room temperature. Chemists at the University of Udine in Italy found that the benefits of slowed reactions at low temperature are counterbalanced by the gradual crystallization of the more saturated oil molecules. This process leaves the remaining liquid oil with a higher proportion of vulnerable unsaturated fats, and with a smaller proportion of antioxidant substances, which tend to get trapped in the crystals. So oxidation continues at about the same pace despite the low temperature.

There's no need, then, to crowd the fridge with bottles of special olive oil. Just keep them in a cool, dark place. And this is especially true for unopened bottles. Another study from the Universities of Milan and Castilla-La Mancha reports that freshly bottled Italian and Spanish oils high in antioxidants retained much of their antioxidant capacity after as much as 240 days storage at 40 degrees Centigrade, or 104 degrees F. One oil made from the Picual olive even remained fully within the specifications for extra-virgin olive oil.

One other useful fact to be gleaned from the first report: the viscosity of olive oil nearly triples as it cools from room to refrigerator temperature. As many cooks know, you can make a plain vinaigrette very thick and creamy simply by serving it and the salad ice-cold.


Calligaris, S. et al. Influence of crystallization on the oxidative stability of extra virgin olive oil. J. Agric. Food Chem. 2006, 54, 529-35.

Lavelli, V. et al. Effect of storage on secoiridoid and tocopherol contents and antioxidant activity of monovarietal extra virgin olive oils. J. Agric. Food Chem. 2006, 54, 3002-07.

Citrus news: varieties and powers

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.

Lemon varieties and flavors: From Italy, a study of the aromatic substances in the hand-squeezed juices of four Sicilian lemon varieties grown near Siracusa. It found not just variations in flavors, but a huge twenty-fold range in the total quantities of aromatics, with Verdello Siracusano having the most and Femminello Siracusano the least. Where would our standard Eureka or Lisbon place? We may never have tasted a truly intense lemon.

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.

Curious Cook in the New York Times: Absinthe and champagne

In today's dining section I write about a recent analysis of the fact and fiction surrounding absinthe and its distinguishing ingredient, wormwood. And about the latest from the bubble laboratory of Gérard Liger-Belair, deep in champagne country. And what happens when you mix absinthe and champagne.

Anyone who enjoys sparkling wine will love Professor Liger-Belair's informative and poetic book Uncorked: The Science of Champagne, published by Princeton University Press in 2004.


Lachenmeier, D.W. et al. Absinthe: a review. Critical Reviews in Food Science and Nutrition, 2006, 46: 365-77.

Liger-Belair, G. et al. Modeling the kinetics of bubble nucleation in champagne and carbonated beverages. J. Physical Chem. B 2006, 110: 21145-51.

Liger-Belair, G. Et al. Champagne experiences various rhythmical bubbling regines in a flute. J. Agric. Food Chem. 2006, 54: 6989-6994.