HOT BOX. Basic solar oven can be a breakthrough for the countries that depend on wood fuel for cooking. COOKING BY THE SUN

EACH day millions of third-world families waste valuable time chopping down precious forests in a desperate struggle to find fuel for cooking. The women spend hours tending hot fires in smoky, windowless rooms. The wood shortage is reaching crisis level in many parts of the world, as forests are being depleted at a rate of 27 million acres a year.

By the year 2000, according to the United Nations Food and Agriculture Organization, 2 billion people will be affected.

Robert Metcalf, a bacteriologist at California State University, Sacramento, believes there's a simple solution to much of this problem: Let the sun do the cooking.

Dr. Metcalf and the organization Solar Box Cookers International are promoting the use of solar ovens around the globe. ``Sunshine is free, non-polluting, and virtually inexhaustible,'' Metcalf says.

He's convinced that his simple box made of cardboard or wood, aluminum foil, and one pane of glass can retard the destruction of forests and free third-world people from hours of searching for wood.

For years, solar ovens suffered from poor design. But in the mid-'70s, two creative grandmothers from Tempe, Ariz., Barbara Kerr and Sherry Cole, designed one that could retain heat longer, was larger, and could cook on cloudy days.

In January, Metcalf had an opportunity to test his theory that solar ovens could help solve the problem of deforestation. Under the auspices of the Pillsbury Foundation and Foster Parents Plan, he and Dr. William Sperber, a microbiologist, conducted workshops in El Progreso, Guatemala.

When Metcalf arrived in El Progreso, he was struck by the daily struggle to collect firewood. ``Men, women, and children are out all times of the day with machetes, collecting and hauling firewood,'' he says.

In the last three years, the cost of fuel wood has doubled in El Progreso. Those who have to buy it spend about 30 percent of their income on it.

``If the residents continue collecting wood at the present rate, in 10 years the area will resemble the desert areas of Africa,'' says Mo Tejani, field director with Foster Parents Plan there.

Metcalf held daylong workshops in five villages, which were attended by 145 people. The workshops taught participants how to use and construct a solar oven. Everyone helped make one.

Throughout the workshops, Metcalf admonished everyone to ``get it on early,'' ``don't worry about overcooking,'' and ``try everything.'' At the beginning of each workshop, the senoras prepared food that was cooked in the ovens.

After three hours, they went out to see how the food had cooked in the winter sun. Steam came out of the pots as they lifted the lids with hot pads to reveal baked chicken, rice, corn, and potatoes.

In contrast, alternative meals cooked over wood took all day to cook and were more expensive.

The participants were able to compare the taste of meals cooked, using the two methods, and found they tasted better cooked by the sun. As no water was added to most of the food, it retained its natural flavor.

Six months after the workshops, Metcalf returned to El Progreso to evaluate how successful the solar ovens had been.

He says, ``The challenge always has been after they learn about solar ovens, will they use them?'' He was thrilled to find that in one area, 56 families were continuing to use them.

One woman said, ``When I arrived at the workshop and I saw you cooking, I said, `It is kind of weird to cook with the sun.' Now I realize it is possible.''

Another woman responded with, ``I couldn't believe I could cook corn, beans, all that food, without wood.''

In that area, most women do not have ovens, so they were extremely pleased to make cakes and pies in the estufas solares, as they call them.

The women were especially grateful that they no longer had to hunt for wood. ``My husband had to go a distance of about eight kilometers during the dry season to collect firewood,'' said one. ``It's up in the hills. It's very difficult.''

Another woman said, ``My husband and children used to spend all day Sunday collecting wood so we would have enough for the week.''

Metcalf believes these successes can be repeated in other areas of the world. ``My personal goal is to have the message of solar cooking reach 50 countries in the next five years,'' he says. Toward this effort, he has also conducted workshops in Bolivia and Baja California, Mexico.