Curious Cook in the New York Times: Organic produce and yak cheese

In today's Curious Cook column I write about nutritional claims made for organic fruits and vegetables and for yak cheese made in Nepal.

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Benbrook, C. et al. New Evidence Confirms the Nutritional Superiority of Plant-based Organic Foods. The Organic Center, March 2008.
http://www.organic-center.org/science.nutri.php?action=view&report_id=126

Or-Rashid, M.M. et al. Fatty acid composition of yak (Bos grunniens) cheese including conjugated linoleic acid and trans-18:1 fatty acids. J. Agricultural and Food Chemistry 2008, 56: 1654-60. http://dx.doi.org/10.1021/jf0725225

Curious Cook in the New York Times: Wild and crazy apples

In the November 21st issue of the New York Times, I write about rare and wild apples, and apples of the future.

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New developments in tomato flavor, part 2: How about some lemon basil in that tomato?

The second of two recent studies of tomato flavor involves genetic engineering, and offers a scent of tomorrow's tomatoes. 

A group of plant scientists in Israel and at Rutgers and the University of Michigan reported their success in transferring a gene from the basil plant into tomato plants. This particular gene diverts molecules in the pathway toward becoming the red pigment lycopene, and sends them instead onto the pathway that generates aroma molecules. The engineered tomato plants produced fruits that were paler than usual, but also had a stronger aroma and smelled distinctly of perfume, rose, geranium, and lemongrass. More than half of a panel of taste testers preferred the engineered tomato to its unengineered parent. 

This experiment may be a harbinger of things to come, a new era of plant modification in which flavor combinations once created by cooks will be re-created--or precreated--by breeders in the plants themselves. 

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Davidovich-Rikanati, R. et al., Enrichment of tomato flavor by diversion of the early plastidial terpenoid pathway. Nature Biotechnology 2007, online publication 24 June.
doi:10.1038/nbt1312

New developments in tomato flavor, part 1: Save the seeds

Two interesting studies of tomato flavor have appeared in the last month. One originated in the kitchen and may immediately change the way you taste and use tomatoes. The other involves genetic engineering, and offers a scent of tomorrow's tomatoes. Here's the first; check back in a few days for the second.

In classic French cooking, it's only the fleshy walls of the tomato fruit that get used in any kind of prominent way. The skin is peeled off and the seeds and their jelly are scooped out, perhaps to be used in a stock.

I've grown a number of different tomato varieties in my garden over the years, and in the course of comparing them in detail, found that I really liked the jelly more than the flesh. It has a wonderful slippery consistency, and it has more flavor. I thought that it was especially acidic and helped balance the sweetness of the flesh.

A few years ago, Heston Blumenthal at The Fat Duck near London tasted the seedy jelly of a tomato and was struck by what seemed to him a surprisingly intense umami taste, that savory, mouth-filling sensation created by MSG (monosodium glutamate, the sodium salt of glutamic acid) and several compounds called nucleotides. Heston maintains both formal and informal collaborations with several food scientists, and he asked Donald Mottram of the University of Reading whether there is more glutamic acid and nucleotides in the jelly than in the flesh. No one had asked the question before. So Professor Mottram's group did the analysis. The report has just come out, with Chef Blumenthal as a co-author.

Heston was right. The Reading group analyzed 14 different tomato varieties grown in a half dozen countries, and found that all of them had significantly higher glutamate contents in the jelly than in the flesh. The average ratio was nearly 4 to 1, and in some varieties was more than 6 to 1. The same general trend was found for several nucleotides, and for other free amino acids, which may contribute to the fullness of flavor. Though the salt content and pH weren't significantly different between jelly and flesh, the tasting panels consistently rated the jelly higher in perceived saltiness and acidity.

So: tomato jelly is packed with flavor. Taste it and use it! Several years ago at El Bulli in Spain, well before the Reading analysis, Ferran Adrià served clusters of tomato seeds and their jelly intact, as the central elements of a dish, to be admired for their glistening translucence and savored on their own. Why not?

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Oruna-Concha, M.-J. et al. Differences in Glutamic Acid and 5'-Ribonucleotide Contents between Flesh and Pulp of Tomatoes and the Relationship with Umami Taste. J. Agric. Food Chem. 2007, 55, 5776-80.
doi: 10.1021/jf070791p

Olive oil pungency and bitterness

Here are a couple of interesting details to add to the story of olive oil's back-of-the-throat, peppery pungency.

In 2003, a research group at Unilever reported on the sensory effects of several of the phenolic compounds found in olive oil, including the cough-inducing substance later named oleocanthal. Oleuropein, the phenolic compound that makes the fruit of the olive bitter, is water-soluble rather than fat-soluble, so it doesn't get transferred into the oil when the fruit is pressed. But a number of oleuropein relatives do end up in the oil, and the Unilever researchers tasted them in pure form. They found these compounds to contribute mainly bitterness and drying astringency, along with some numbing, cooling, sour, salty, and tingling sensations. Relatives of pungent oleocanthal also taste somewhat astringent and bitter. So an oil rich in phenolics can have a very complex taste and mouthfeel indeed.

Other studies have found that heating olive oil reduces the levels of most phenolic compounds. And pepperiness gradually fades as an oil ages. The balance of flavors in an excellent olive oil is thus temporary and so especially worth savoring when you find it. The deterioration of olive oil is slowed by keeping it cool and protected from light.

The balance of phenolics and of so of an oil's taste depends on the variety of olive, the way the oil is pressed and handled, and on the growing conditions for olive trees--especially the water supply. The olive tree is native to the arid Mediterranean and can get by with very little water. According to a number of recent reports, including one from the University of California at Davis, water stress produces very bitter and astringent olives, while copious irrigation produces olives and oils with very little bitterness, astringency, or fruity flavors. So the olive is another example of the many fruits and vegetables whose quality suffers when the plants are pampered.
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Andrewes, P. et al. Sensory properties of virgin olive oil polyphenols: identification of deacetoxy-ligstroside aglycon as a key contributor to pungency. J. Agric. Food Chem. 2003, 51, 1415-20.

Carrasco-Pancorbo, A. et al. Evaluation of the influence of thermal oxidation on the phenolic composition and on the antioxidant activity of extra-virgin olive oils. J. Agric. Food Chem. 2007, 55, 4771-80. http://dx.doi.org/10.1021/jf070186m

Berenguer, M.J. et al. Tree irrigation levels for optimum chemical and sensory properties of olive oil. HortScience 2006, 41, 427-32.

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

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.

Kuerle fragrant pears

Last October I read a report about a kind of Asian pear that I'd never heard of--and then this last weekend I found it in the 99 Ranch Market in nearby Milpitas. It's different, delicious, and worth trying.

All European pears are varieties of the species Pyrus communis. Asian pears are varieties of one or two different species that have variously been called Pyrus pyrifolia, Pyrus ussuriensis, and now usually Pyrus serotina. Most of the Asian varieties grown in North America were developed in Japan, have crisp, juicy, off-white flesh, a round, apple-like shape, often a russet skin, and a distinctive aroma that to me seems both flowery and slightly alcoholic. Alcohol (ethanol) is in fact one of their characteristic chemical components. The flavor is refreshing and heady at the same time.

The Kuerle variety of Asian pear comes from China, mainly from the northwest near Xinjiang. According to scientists from Shihezi University in that city and colleagues at the China Agricultural University in Beijing, the Kuerle pear was developed more than a thousand years ago, and has been valued for its "super white" flesh, elongated shape, jade green skin, and special fragrance. Substantial quantities are now exported. The fruits are harvested when they've matured enough to contain about 12% sugars and organic acids in their cell fluids. By refractometer, my samples were indeed around 13%. They are small as pears go, around 3 inches /7.5 cm long, light green with a waxy feel to the skin, familiarly crisp and juicy like a Shinsui or 20th Century.

But the delicate aroma is different. My first bite reminded me immediately of bubble gum! Vintage 1960; I haven't had bubble gum in a while. Not nearly as heady and volatile as the usual Asian pear varieties, with a light, candy-like fruitiness that probably wouldn't work too easily with other ingredients, but that's charming on its own. The food scientists analyzed the Kuerle aroma and found several components that it has in common with European pears, including the characteristic European "pear ester" (an ethyl decadienoate, or a combination of an alcohol molecule with a ten-carbon fragment of a fatty acid). But to me the Kuerle was nothing like a Bartlett or Comice or Bosc. It's a distinctive addition to the flavors of autumn, if you can find it.

Chen, J.L. et al., Changes in the volatile compounds and chemical and physical properties of Kuerle fragrant pear during storage. J. Agric. Food Chem. 2006, 54, 8842-47.

Hold that watermelon

With the exception of fruits in the process of ripening, most produce deteriorates once it has been harvested, and benefits from being cooled down to refrigerator temperature, where its metabolism is slowed down. Thanks to Penelope Perkins-Veazie and Julie K. Collins at the USDA lab in Lane, Oklahoma, we now know that watermelon is a big exception. The red color of watermelons comes from lycopene, the valuable antioxidant relative of carotene that also colors red tomatoes. The USDA scientists found that watermelons held for two weeks at room temperature continue to produce lycopene and so deepen in color, and end up with from 10 to 40% more pigment than freshly harvested melons. Conversely, refrigerated melons lose lycopene and tend to develop areas where the cells are damaged and leaky. (The watermelon plant came originally from hot, arid regions of Africa, and its fruits just don't do well in the cold.) So it's actually a good thing to let watermelons sit out after harvest, and chill them only just before eating.

I haven't been able to find out how the watermelon stores its lycopene, but it appears to be in a form that's more available to the human body than the lycopene in raw tomatoes. Other studies show that fresh-frozen watermelon juice is a good source, roughly equivalent to canned tomato juice.

Perkins-Veazie, P. and J.K. Collins, J. Agric. Food Chem. 2006, 54 (16) 5868

Carotene pigments in mango and carrot

Mangoes and carrots are beautiful to look at because they contain rich deposits of carotene pigments. The carotenes come in many different variations, and range in color from yellow to deep orange. Beta carotene in particular is a valuable nutrient because it's an antioxidant, and because our bodies can convert it into vitamin A, which has important roles in our eyes and in other tissues. So we can count on orange-colored fruits and vegetables to be especially good for us. But often we don't get as much of their goodness as we might think.
Carotenes are much more soluble in fats and oils than in water. The cells of plants are mostly water, so the cells have to package the carotenes in special structures. One common structure is a solid crystalline mass. This is what carrot cells contain, and as a result, raw carrots give up a relatively small proportion of their carotenes. When we eat a raw carrot, the crystals only partly dissolve in the water-based mass of carrot, and only some of the carotene molecules are free for our intestinal cells to absorb. Cooked carrots are much more nutritious. The heating process disrupts the cells and carotene crystals, mixes the carotene molecules with other fatty materials in the carrot tissue and the rest of our meal, and makes them more readily available for absorption.
Mangoes are a different story. Reinhold Carle and colleagues at Hohenheim University recently studied mango cells and found that they store their carotene pigments not in solid crystals, but in microscopic oil droplets, where they are predissolved and so presumably much more available for our bodies to absorb them, even when we eat the fruit raw.
In a 2003 paper, Carle and colleagues reported that dried mangos are a concentrated source of beta carotene, even though the drying process does destroy some of the pigment. Sun-dried fruit suffer the greatest losses.

Vasquez-Caicedo, A.L. et al., J. Agric. Food Chem. 2006, 54 (16) 5769.