I’ve completed the first build of my solar oven design. I’ll go over the specs quickly, before we discuss cooking with the oven.
The oven is made from cardboard boxes, aluminized mylar (emergency ‘space’ blankets), cotton for insulation (Bonded Logic’s UltraTouch™ Denim Insulation), and greenhouse glazing for the window. The reason for using commercial insulation and commercial glazing is so that the R-values and transmittance (how much light gets through the window) would be known factors. Later on, I plan to replace these materials will less expensive and more available materials, so as to compare the performance.
The interior box of the oven is cardboard, lined with a layer of aluminized mylar, and then an innermost layer of heavy black cotton cloth. The interior is 4 feet long, by 12 inches high, by 16 inches wide. The interior is surrounded by 4″ boxes filled with insulation and wrapped with the mylar. Then the outer box is 4 feet 8 inches long, by 16 inches high, by 24 inches wide, and is also lined with the mylar. Weather stripping is used between the edges of the greenhouse glazing and the rest of the oven. Some photos of the build:
The boxes containing the insulation, inside the outer box:
Assembled with the foil and inner black cloth:
The completed solar oven with window:
Cooking with the Solar Oven
I was not happy with my initial design of the reflectors for the oven (aluminized mylar over cardboard). So I will have to re-think that part of the design. However, the oven worked well enough without reflectors.
In order to cook at low temperatures, you need to raise the interior temperature of the food above 140 degrees Fahrenheit. Bacteria begin to be killed in the 130′s, but it takes a long time to kill enough bacteria at those temperatures.
Once you raise the temperature above 140, your cooking time to pasteurize the food is less than half an hour. See this PDF from the USDA for the cooking times, at various temperatures, needed to make food safe. [Pork steaks, chops, and roasts need to be cooked to 145 degrees for safety. But I would not recommend cooking pork at all in a solar oven.]
These temperatures are the interior temperature of the food, not the oven temperature. To be sure of the interior temperature, I used a digital meat thermometer, similar to this model.
Pasteurizing the food kills the bacteria, but it does not necessarily mean that the food is cooked. For a guide to low-temperature cooking, I’ve found that ‘sous vide‘ cooking texts work well. The type of cooking called ‘sous vide’ (under vacuum) is used in fine restaurants to cook food thoroughly, but at low temperatures, just as we would in a solar oven. But they cook food vacuum-packed in plastic and immersed in a water bath with a precisely controlled temperature. This PDF has some recommendations (on the last page) about how long to cook food at low temps. Basically, most red meat and poultry are cooked through if they are held at temperatures above 140 degrees for at least an hour. You can hold the temperature in that range for longer, in order to make the food more tender, but an hour is sufficient.
My first attempt at cooking with the solar oven was a large boneless chicken breast (14.5 ounces). Once the interior temperature of the food went above 140 degrees, I held the food at that temperature for over an hour. The chicken was thoroughly cooked; it was tender, but not rubbery or mushy. The texture and taste were excellent. However, with solar cooking at low temperatures, you don’t get a browned crust on the exterior of the food. So a browned the exterior over an alcohol stove. The results were delicious: