Perk Up Food Flavors with. . .Black Plastic? (with pesto recipe)

Pesto, a zesty sauce for pasta and spread for crusty breads, typically derives much of its flavor from basil. The fresher this herb, the richer the pesto’s flavor–which is why many people with a pesto passion keep a basil patch outside the kitchen door.

USDA scientists compare the yield and quality of identical crop cultivars raised over different colored plastic mulches. USDA/ARS

Work by Michael J. Kasperbauer’s team at USDA showed that tomatoes planted over red plastic mulch grew bigger and matured earlier than those grown over bare soil or other colored mulches. USDA/ARS

The flavor of basil, an herb that plays a big role in pesto, can be heightened or dampened depending on the color of plastic mulch over which it’s grown.

Though most herb gardeners grow their basil clumps in bare earth, new research from scientists with the U.S. Department of Agriculture suggests that cooks will get richer flavored greens by laying a swath of plastic mulch over the ground prior to planting. But not just any color plastic. Basil plants developed more aromatic flavor constituents–typically families of chemicals known as aliphatics and terpenoids–when the plastic was black, green, or yellow, and not white or blue. However, gardeners who care less about aromatic potency and more about maximizing yield should mulch their basil plot with red plastic, the new data indicate.

These findings, slated for publication soon in the Journal of Agricultural and Food Chemistry, are just the latest to emerge from ongoing studies of how the color of sunlight reflected off the garden floor affects crop growth and quality.

Mulch science

Although farmers first warmed to the idea of plastic mulches some 45 years ago, until recently most of them found that the range of available hues was limited to what Henry Ford offered 1914 Model T buyers: “Any color, so long as it’s black.”

In fact, black was sufficient as long as the purpose of these sheets was only to preheat spring soils, inhibit weeds, or hold moisture in irrigated crop rows. But today, growers are increasingly adorning their fields with long sheets of plastic in a rainbow of colors. It’s not that farmers suddenly have a new interest in landscape design. Rather, it’s a testament to the value of years of research on plants’ light sensors by the Coastal Plains Soil, Water and Plant Research Center of the U.S. Department of Agriculture.

Green plants tap the energy in sunlight to drive photosynthesis and other important internal activities. However, more than 4 decades of work by plant physiologist Michael J. Kasperbauer and his colleagues at the center have demonstrated that the color of light can play almost as important a role in plants as the overall quantity of light available.

The scientists have focused on phytochrome proteins. These leaf sensors, discovered in 1959, measure the intensity of various light wavelengths, or colors, and plants actually alter their behavior in response to what they sense. For instance, when the ratio of incoming light begins to shift away from photosynthesis-powering red wavelengths–toward far-red wavelengths that typically reflect off green leaves–a plant recognizes that it’s getting crowded. It responds by putting more of its energy into growing taller than its encroaching neighbors.

In the past decade or so, studies at the USDA center have shown that farmers can control the chemical composition of plants by altering the light available to leaves–principally by covering the ground with plastic that reflects sunlight in given colors. White mulch, for instance, increased a plant’s wax content, which helped leaves conserve water and discouraged insect bites. Other color selections modified a plant’s flavors. Turnips growing over blue mulch, for instance, tasted sharp, whereas those raised over green mulch grew sweet.

In ruminating about the latter findings a few years back, Kasperbauer and his colleagues realized that mulch-color management might offer a nongenetic means of controlling a plant’s production of economically important constituent chemicals–like the aromatic ones that manufacturers distill into flavoring liquids. To test this idea, the researchers turned to basil.

Back to black?

Initially, they set young plants in fields that had first been covered with plastic in any of five colors or black or white. When the plants matured in June, the scientists harvested the leaves, dried them, and finally measured their size, abundance, and content of aromatic chemicals.

Kasperbauer’s group had already demonstrated that tomatoes and certain other crop plants develop bigger fruit when grown over red mulch. The new study showed that basil was no exception; it too produced more and bigger leaves when grown over red mulch. However, the foliage had relatively puny quantities of the flavor chemicals. It was almost, Kasperbauer says, as if the basil apportioned the same amount of these volatile, aromatic compounds to each plant, but the greater number and volume of leaves in red-mulched plants diluted the impact of those flavoring agents.

On the other hand, basil grown over green or yellow mulch did produce more of the volatile aromatic compounds per gram of leaves. Black mulch produced next most potent leaves, and the white and blue plastics were next. Red was dead last.

However, these data were for dried basil, leaves that had already lost much of their volatile aromatic compounds to the air. So Kasperbauer and the chemist he works with, John H. Loughrin, repeated the experiment a couple years later, this time analyzing aromatic-chemical yields from fresh leaves picked in August. Sure enough, the concentration of the mix of target chemicals was some 50 times higher than in dried leaves.

Moreover, the performance of the colored mulches varied somewhat between the two studies. In the new one, black-mulched plots yielded basil with the most aromatics per gram, followed by plots decked out in green and yellow, then those in white or blue. Red-plastic mulch again trailed the others, yielding plants with only about half as much of the flavor compounds as was produced by basil grown over black, green, or yellow plastic.

Because the black mulch reflected almost no light back onto the plants, the scientists acknowledge, the higher aromatic yields associated with it probably trace almost solely to the plastic’s greater warming of the plants–to the point of heat-stressing them.

It’s also important to keep in mind that the scientists charted mulch-induced changes for 26 different compounds. Although the mix tended to rise or fall as a group in response to the mulch used there was considerable variability in the concentrations of particular elements within each plant’s aromatic profile.

For instance, although basil grown over black plastic produced a whopping 40 percent more n-hexanol and about 17 percent more alpha-pinene and bornyl acetate than plants grown over green mulch did, it produced only 1.5 percent more linalool. Similarly, plants grown over red plastic yielded only 48 to 58 percent as much linalool and alpha-pinene as basil mulched in black, and their production of bornyl acetate was a mere 26 percent as high as black’s–yet their production of n-hexanol exceeded the black-mulched plants’ by 44 percent. Nevertheless, all of the plants still tasted like basil.

“You can guess where all this is taking us,” Kasperbauer told Science News Online. “It opens up that door to optimizing medicinal-type products from plants”–agents increasingly referred to as nutraceuticals (see http://www.sciencenews.org/sn_arc97/2_15_97/food.htm). That’s a ways off, the scientist concedes.

Using colored plastic mulches to enhance potentially bioactive agents in plants such as ginseng, garlic, Echinacea, or St. John’s wort will require scientists to first identify those chemicals from among hundreds of possibilities, says Kasperbauer. Then, researchers at USDA and elsewhere might look into colored-mulch management to selectively boost the target compounds. It will be difficult, but “it’s what we’ll be looking into,” Kasperbauer says.


RECIPE. . .

Pesto to make at home

In Genoa, Italy, pesto’s reputed home, basil accompaniments typically include olive oil, garlic, pine nuts, butter, and grated cheese. Sicilians have developed recipes that include cherry tomatoes.

Though many stores offer prepared pesto, the sauce will be fresher and healthier when prepared at home. The American Institute for Cancer Research shares one such recipe, below.

The group’s nutritionists point out that many culinary herbs–such as the oregano and mint used here–contain natural antioxidant substances “that help prevent and repair cell damage linked to cancer or heart disease.” Cooking with herbs is also good for weight control, the institute notes, because the flavor and richness offered by savory condiments allow cooks to cut back or eliminate added fat and salt.

If tomatoes aren’t in season, make your pesto plain green. It’ll still be great.

Besto Pesto

  • 3/4 cup loosely packed flat-leaf parsley
  • 1/3 cup loosely packed fresh basil leaves
  • 1/4 cup loosely packed celery leaves
  • 1/4 cup loosely packed mint leaves
  • 1/4 small red onion, chopped
  • 6 blanched whole almonds
  • 1 Tbsp. capers, rinsed and drained*
  • 1 garlic clove, chopped
  • 1/2 tsp. dried oregano
  • 12 cherry tomatoes
  • 3 Tbsp. extra virgin olive oil
  • Salt and freshly ground black pepper, to taste
  • 6 cups cooked pasta or 6 slices bruschetta

Place the parsley, basil, celery leaves, mint, onion, almonds, capers, and garlic in a food processor or blender. Pulse 6-8 times to chop them. Add oregano and tomatoes and chop coarsely. With the motor running, gradually drizzle in olive oil. (Herbs and vegetables can also be hand-chopped, then placed in blender to purée with oil.)

Transfer pesto to a bowl. Season to taste with salt and pepper. Let sit 30 minutes to allow flavors to meld. Toss with hot, cooked spaghetti or other pasta, or spread on bruschetta (toasted slices of Italian bread).

Store leftover pesto in the refrigerator, tightly covered. Use within 24 hours.

*Salt-cured capers are preferable to those preserved in vinegar, but rinse in tepid water to remove salt crystals.

Makes 6 servings.

Per serving: 279 calories, 9 g total fat (1 g saturated fat), 43 g carbohydrate, 8 g protein, 3 g dietary fiber, 67 mg sodium.

Janet Raloff

Janet Raloff is the editor of Science News for Students, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer.

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