Dear Alice,

My sister just made her first visit to Japan. Before she went back to California, she made me promise to ask you about what she disparagingly referred to as "bento turf" — the little strips of plastic grass that come in practically every bento (boxed lunch) you buy in Japan. We figure the grass is there to add color and visual appeal, but to her, it's crazy for a country with too few resources and too much garbage to use plastic for garnish. Why not use something edible or at least biodegradable? I've been in Japan long enough to know there's usually a reason things are done the way they are, but I can't begin to guess on this one. Can you tell me why the heck there's plastic grass in bento boxes?

Carol S., Toyama Prefecture

Dear Carol,

Those strips of green plastic, cut on one end so they do look a lot like grass, are called haran (sometimes baran). They are indeed intended to add color, an essential part of food presentation in Japan, but as you guessed, there's more to it than that.

I took your question to Ayao Okumura, a chef, food consultant and cookbook author with a keen interest in the history of cooking. He explained that a variety of dividers, including plastic grass, are used when making bento to prevent assertive flavors from seeping from one tidbit into another. Separating foods also slows bacterial growth, thus extending the shelf-life of these highly perishable prepared meals.

It turns out Okumura is no fan of fake food foliage. He was quick to point out that haran were originally made from fresh leaves, which he said offer a distinct advantage over the modern plastic variety.

"Certain plants, when damaged, release very active antimicrobial substances called phytoncides that prevent the plant from rotting," Okumura explained. "If you use the leaves from such plants to wrap or divide foods, the phytoncides in the leaves inhibit the growth of bacteria in the food. So meals packed with real leaves stay safe longer than meals packed with plastic."

Although Japanese people have long had simple ways to carry prepared foods away from home, such as wrapping cooked rice in leaves, the bento as we know it, with small amounts of many foods presented attractively in a compact container, has its roots in the Edo Period (1603-1867).

Playful presentations

In those days, well-to-do people ordered elaborate meals for outdoor parties and other excursions. The food needed to be packed tightly to prevent shifting during transport, but also had to look good because playful, attractive presentations were then prized.

Okumura showed me how to fashion a fresh haran, using a leaf from his own garden.

"Making multiple cuts like this," he said while deftly wielding a knife to serrate one edge, "releases more of the phytoncides, making the leaf even more effective as a preserving agent."

In the Kansai area, which includes Kyoto and Osaka, the plant of preference was a member of the lily family, Aspidistra elatior. In Edo (present-day Tokyo), cooks tended to use sasanoha (the leaves of the bamboo grass plant), particularly for sushi.

Plastic substitutes came into use around the mid-1960s according to Okumura.

"This was the time when supermarkets were getting their start in Japan, and the big stores were looking for ways to cut costs so they could offer lower prices," he explained. "Using plastic instead of leaves saves labor."

In more recent years, with the internationalization of sushi, the plastic variety has spread overseas where it is usually marketed as "sushi grass."

If you stop to think about it, it's perhaps surprising that manufacturers haven't added some kind of antimicrobial agent to the plastic grass strips. After all, there are all sorts of products in Japan with built-in kokinryoku (antibacterial properties), ranging from kitchen sponges to luxury pantyhose. But while I couldn't find microbial beast-beating sushi grass, I did learn that some of the clear films used to cover commercial bento meals incorporate a compound derived from wasabi root that inhibits the growth of microorganisms.

Knowing that Okumura has read many old texts about food preparation, I was curious whether he'd found any indication that people in earlier times understood that it was something inside the leaves that helped preserve food.

Empirical observation

"I don't think they understood how it worked," Okumura replied. "They only knew, from empirical observation, that the leaves somehow kept food fresher. It was only in the postwar period that scientists had the tools to figure out the science behind old practices. Basically," he said, cutting another leaf, "there's nothing new under the sun."

For a peek at the history of Japanese bento, including traditional techniques to improve food safety, stop by the Ajinomoto Foundation for Dietary Culture by Feb. 29 to catch the small but informative o-bento exhibition on the second floor.

There is limited explanation in English with more extensive information in Japanese. The center's permanent exhibit, which reproduces typical Japanese kitchens from various modern times, is also worth a look.

The Ajinomoto Foundation for Dietary Culture is at 3-13-65 Takanawa, Minato-ku, Tokyo, a 3-min. walk from Takanawadai Station on the Asakusa Subway line; open 10 a.m.-5 p.m., Mon.-Sat. (Closed Sun. and holidays); admission free. For more information call (03) 5488-7319 or visit www.syokubunka.or.jp