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

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.

Grass-Fed Beef vs. Farmed Salmon

My post last August about the weak claim of grass-fed beef to healthful quantities of omega-3 fatty acids has drawn a skeptical response from Robert Buxbaum, who found a contrary view in Michael Pollan's important new book, The Omnivore's Dilemma. I'd like to dissect the discussion a bit, because I think it's a cautionary example of how easily speculations about food and nutrition can stray from the facts, become accepted as fact themselves, and end up being unintentionally misleading.

For context: I've long been a believer in the importance of local, small-scale, sustainable food production, greatly admired Michael Pollan's address in the moving opening ceremonies at Terra Madre last month, and have been a fan ever since reading his 1991 book Second Nature.

Bux wrote:
I have some comments and a question about your August 25 report on
omega-3 fatty acids in grass-fed beef. Are you comparing farm raised
salmon or wild salmon when comparing grass-fed beef to salmon? As I
posted on [Michael] Ruhlman's blog: Pollan, in "The Omnivore's Dilemma,"
mentioned that our impression of salmon as superior to beef as a
source of omega 3 fatty acids was based on measurements taken at the
time in which we first became conscious of omega 3s. That was when
most available beef was corn fed and most salmon was wild. Pollan
further claims that grass fed beef is a better source omega 3s than
farmed salmon, which is what most consumers eat now.
It seems as if how the salmon, as well as the cattle, are raised is significant in
each case.

What Michael Pollan does say on pp. 268-269 of his book is that, like industrial cattle, farmed salmon are fed on grain, and they therefore contain less omega-3s than wild salmon, which eat small creatures that have accumulated omega-3s from the oceanic equivalent of grass, the tiny phytoplankton. He then speculates (my italics) that "if the steer is fattened on grass and the salmon on grain, we might actually be better off eating the beef," and that "the species of animal you eat may matter less than what the animal you're eating has itself eaten."

In fact, farmed salmon are primarily raised not on grain but on fish meal, a feed which is problematic in its own way, but which is plenty rich in omega-3s. A thorough survey published in 2005 found that salmon farmed in various regions throughout the world have consistently higher omega-3 levels than wild salmon, mainly because they are consistently fattier. And beef? The long-chain omega-3s in grassfed beef are present at around 20 milligrams per 100 grams (about a quarter-pound) of beef. The levels in farmed salmon are around 3 grams per 100 grams of fish: more than a hundredfold higher. Even salmon raised experimentally on vegetable oil for three-quarters of their life (to begin to address the issue of sustainability) have 1 gram of omega-3s per 100 grams fish: 50 times more than grass-fed beef. These are huge differences!

So species matters a lot. Ocean-going creatures live in a cold environment and need highly unsaturated fats that won't congeal at temperatures that can approach the freezing point; mammals are warm-blooded and need saturated fats that won't be too fluid at body temperature. Even grass-fed beef still comes from a warm-blooded steer, and farmed salmon is still a cold-blooded fish. And when it comes to the highly unsaturated omega-3s, we're far better off eating salmon.

Of course this is just one small piece of a large and complicated picture. There is plenty to be said in favor of grass-fed beef, plenty of problems with salmon aquaculture, and there's more to a healthy diet than omega-3s, which we can also get from other fish and shellfish. But it's good to have each piece of the picture in the right place, right-side up, however small it is.

Hamilton, M.C. et al. Lipid Composition and Contaminants in Farmed and Wild Salmon. Environ. Sci. Technol. 2005, 39, 8622-8629.
Torstensen, B.E. et al. Tailoring of Atlantic salmon (Salmo salar L.) flesh lipid composition and sensory quality by replacing fish oil with a vegetable oil blend. J. Agric. Food Chem. 2005; 53:10166-78.

Good Fats In Grass-Fed Beef?

My friend Daniel Patterson, the chef and owner of Coi in San Francisco, recently told me that some of his meat suppliers have been promoting grass-fed beef as a good source of healthful omega-3 fatty acids, both the linolenic acid found in walnuts and canola oil, and the very long chain fatty acids characteristic of fish oils. Beef, the archetype of foods laden with cholesterol-raising saturated fats, as a health food? I took a fresh look at the numbers.
Sure enough, grass-fed beef has substantially more omega-3 fatty acids than conventional beef raised on grain or a combination of grain and other concentrated nutrients. This makes sense: green grass doesn’t have much fat or oil, but its membranes are rich in linolenic acid. When the grass is cut and turned into hay, the linolenic acid tends to be oxidized and converted into other molecules, so hay is a poorer source, and grains contain little or no linolenic acid.
Once it gets into cattle, the grass linolenic acid meets several fates. Much of it is hydrogenated by microbes in the animal’s rumen, and turned into more saturated, less valuable fatty acids. Some of it enters intact into the circulation, and ends up being deposited in both muscle and milk. And a small portion of it is converted by the cattle tissues into the longer-chain fatty acids found in abundance in fish.
So is grass-fed beef a meaningful source of omega-3s? No. An entire grass-fed beefsteak contains hundredths of a gram of long-chain omega-3s, and less than a quarter of a gram of linolenic acid. You can get the same quantities from a couple of walnut pieces and a few grams—a very small bite—of salmon or oyster. Beef is wonderful stuff, and grass-fed beef is especially lean and flavorful, but it’s still beef.

Dannenberger, D. et al., J. Agric. Food Chem. 2004, 52 (21) 6607

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.