COOKING with the sun

As meals go, it was as simple as they come -- hamburger with some accompanying vegetables. Even so, Robert and Mary Metcalf of Sacramento, Calif., recall vividly that evening meal of three years ago. It was one of the most flavor-filled, succulent meals they had eaten, and it largely changed the way the Metcalfs cooked from then on.

An hour or so earlier, Dr. Metcalf, an associate professor in biological sciences at nearby California State University, had carried a simple plywood box with a glass top out onto the front lawn and faced it toward the southwest.

Then he turned his attention to other work, wondering if the afternoon sun would cook the meal as well in practice as he knew it should in theory. It did and then some.

In fact, the whole operation was so incredibly simple and the results so amazing that the Metcalfs decided solar cooking was the only way to go whenever the sun shone.

Since then they have done "more of our cooking and baking outdoors with the sun than we have indoors wtih our electric stove and oven."

Once, in an impressive demonstration of solar cooking power for the neighborhood, they used five of the box cookers to prepare 114 pounds of food for 75 people. The menu included two turkeys, a ham in orange glaze sauce, beef stroganoff, rice, bean casserole, six loaves of homemade bread, and a dozen chocolate cakes. The entire meal was cooked exclusively by the sun.

Ah, but that's sunny California. Would a hot-box cooker work in less sun-blessed regions? The answer is yes, but presumably less often. If solar hot-water systems work in your region, a solar cooker will as well.

Marshall Logvin, a former student of Professor Metcalf and now involved in solar-box-oven design and manufacture, has cooked "many a meal" in his hometown, Buffalo, N.Y. Other hot-box cookers known to Professor Metcalf work effectively in Minnesota and Illinois.

Obviously, you don't try to cook a meal on a heavily overcast day.Professor Metcalf says. On the other hand, a box oven will work very well even on a partly sunny day.

The bottom limit appears to be 15 minutes of sun an hour. At that rate vegetables and meats cook well at slow-cooker-type temperatures, but the higher temperatures required for baking bread, etc., are not forthcoming unless the day is largely free of clouds.

The Metcalfs list these reasons for preferring to cook with the sun:

* Solar cooking with a box cooker is easier than conventional cooking, because it requires less total time and effort.

* Foods cooked in a solar box cooker have a superb flavor because the oven cooks with an even heat flow; no water is ever added that can dilute the flavor of vegetables and meats.

* You save money on utility bills by not using a conventional stove and oven. You also avoid pumping unwanted heat into your house during the height of summer , allowing the home to remain more comfortable. If you normally run an air conditioner, you will use it less often when using a solar cooker, resulting in still further savings on the utility bill.

* Besides being a practical household appliance, the solar box cooker is also an outstanding educational tool, showing you quickly when and how the sun works in your location. The principles of the box cooker are the same as those for passive solar homes, solar greenhouses, and solar hot-water heaters.

* The solar box cooker is fun to use and a continuing source of amazement. It leaves you with a feeling of optimism about the future, no matter what emerges from the debate at OPEC meetings.

When Professor Metcalf praises solar cookers, he refers to the simple hot box , not those devices that employ parabolic reflectors and concentrating lenses in one way or another. These are expensive and difficult for the amateur to build, and can cook very little food at a time.

As a result of those inventions, solar cooking has been branded as impractical by many in the solar field. Those who do so don't know their history. Fortunately, however, two Arizona women. Barbara Kerr and Sherry Cole , did understand and, as a result of their experiments, the modern box cooker came into being.

In the late 1700s a Swiss scientist, Horace de Saussure, wanted to find out why the sun's rays seemed hotter when they passed through the glass windows of a carriage. So he built a small box, insulated the bottom and sides, and placed a glass lid on top. The inside of the box registered temperatures as high as 228 degrees F.

"Fruits . . . exposed to this heat were cooked and became juicy," he wrote in his journal.

In 1830 a British astronomer, Sir John Herschel, during an expedition to the Cape of Good Hope at the tip of Africa, built a solar hot box and used it to cook many of his meals. Temperatures under the South African sun rose to 240 degrees F. in the box.

Sir John cooked a variety of meats, vegetables, and eggs in the hot box, often surrounded by interested spectators.On one occasion, he wrote, ". . . a very respectable stew of meat was prepared and eaten with no small relish by the entertained bystanders."

Samuel Pierpont Langley, who later became director of the Smithsonian Institution, read of Sir John's solar cookouts and built a similar hot box so that he could study solar energy more effectively. On an ascent up Mt. Whitney in California, he found that at altitudes where the temperatures were so low the ground was frozen, the temperature in the hot box "rose above the boiling point of water."

Barbara Kerr and Sherry Cole, the modern-day developers, or rather, rediscoverers, of the hot-box solar oven cooker, found that temperatures in oven of their design will reach 275 to 300 degrees F. That degree of heat is sufficient to do all one's cooking and baking.

As Dr. Metcalf explains it, "We are so used to conventional oven cooking temperatures that we overlook optimum cooking temperatures." Meats, rice, and vegetables will cook whenever temperatures exceed 200 degrees F. Even lower temperatures are acceptable. (A slow-cooker pot, for instance, cooks at around 190 degrees F.; microwave ovens never raise food temperatures above the boiling point of water). Breads and cakes bake "to a crusty brown" whenever temperatures top 250 degrees F.

Obviously, at these lower temperatures, cooking times are increased.

"One has to plan earlier in the day with solar cooking, but late in the afternoon you can relax," the Metcalfs point out.

"If you're going to have pot roast with vegetables, scalloped potatoes and ham, or chicken and rice, you prepare the foods in the morning and place them in the box cooker, which is positioned so that it faces the southwest. In this position it will begin to receive the sun between 12 noon and 1 p.m. and will get the maximum sun between 2 and 3 p.m.

"If you return from work at 5 o'clock, your dinner will be fully cooked and ready for serving. If, on the other hand, you don't get home until later, the insulated cooker will keep the food hot for you for some time." With the exception of green vegetables, it is difficult to overcook foods in a solar hot box, the Metcalfs say.

What, then, is a hot-box cooker?Very simply, it is a box within a box with insulation in between the boxes (to prevent the heat from escaping), topped by double glazing.

The inside walls are covered with aluminum foil to help reflect the radiant heat from the sun onto the dark (cast iron, dark porcelain, or brown glass) cooking pots. A hinged lid, also covered with reflective material, is raised vertically when the box is in use so that it reflects additional rays into the oven.

This reflector lid, together with the improved insulating materials that are available today, is the principal reason the modern solar box cooker reaches higher temperatures than the hot box that performed so well for Sir John herschel more than a century ago.

The modern cooker is also larger (2 by 2 feet and 1 foot high) than those early models. The boxes must be relatively shallow to prevent the hot air from stratifying at the top of the box.

According to DR. Metcalf, a hot-box solar oven can be readily built using all new materials for $60, but much less if scrap materials are recycled. It is even possible to build a cooker using cardboard lined with aluminum foil, he says. Step-by-step plans for the Plywood Patio Solar Stove are available for $3 .75 from Kerr Enterprises Inc., PO Box 27417, Tempe, Ariz. 85281.

Meanwhile, Professor Metcalf and Marshall Logvin have written a 36-page energy primer and guide to solar cooking and hot water, entitled "Solar Now." It is available for $2 from Bob Metcalf, 1324 43rd Street, Sacramento, Calif. 95819 .