Safety concerns when using solar cookers for cooking and water pasteurization are summarized in the sections below. Some are common sense based to protect the user, and others require careful temperature monitoring to keep the cooking food safe to eat.
Cooking temperaturesHarmful food microbes, including bacteria and viruses, are killed when heated to 65°C (150°F).
With all cooking methods, certain bacteria produce heat-resistant spores that germinate after food has been cooked. Therefore, cooked food should be kept at temperatures above 52°C (125°F). If cooked food is allowed to drop to temperatures between 52°C (125°F) and 10°C (50°F) for a period of time, these bacteria can spoil the food and lead to food poisoning. Food that stays in this temperature range for more than four hours should be heated again to cooking temperatures before consumption. (Even after reheating there is still a risk of illness. If you are unsure you should discard the food.)
Frozen and uncooked food
It has been carefully documented with regard to solar box cookers that it is safe to place raw refrigerated or frozen food, even chicken or other meat, in a solar cooker in the morning several hours before the sun begins to cook it. Refrigerated food placed in a cooker remains sufficiently cold until the sun starts to heat the SBC. Once the full sun is on the oven, the heating of food proceeds quickly enough so that there is no danger of food poisoning. Uncooked grains, beans and other dried raw foods can also be placed in a cooker in advance. Both of these methods facilitate absentee cooking.
Keeping cooked food
There are three main points at which caution is required: it is dangerous to keep cooked food more than three or four hours in an unheated or cooling solar cooker unless both the cooker and food have been cooled rather quickly to below 10°C(50°F) in which case the cooker is serving as a cool box; it is dangerous to let cooked food remain overnight in a cooker unless it is likewise cooled; and it is dangerous for food to partially cook and then remain warm in the cooker when temperatures are not sustained as might occur on a poor solar cooking day, at the end of the day or when clouds move in. Cooked or partially cooked food should either be cooled to below 10°C(50°F) or cooking should be finished with an alternate fuel. If food has remained in the temperature danger zone for 3 to 4 hours it should be considered spoiled and should be discarded. Reheating the food does not correct the problem as heat does not inactivate all toxins.
Food does not have to be visibly spoiled in order to be toxic and cause illness evidenced by nausea, vomiting and diarrhea. Even if food has not been at the incubating temperatures of the danger zone for the full 3 to 4 hours, absolutely discard food that is bubbling, foaming, has a bad smell, is becoming discolored, or gives any other indication of spoilage. Discard it out of reach of animals and children and thoroughly wash the pot. Discard it without tasting it as even small amounts can make an adult very sick.
If temperatures below 10°C(50°F) cannot be obtained, it is still valuable to drop food temperatures as low as possible and as quickly as possible rather than allowing food to remain warm since bacteria grow more slowly at lower temperatures.
An alternative method of holding cooked food is to reliably maintain the temperature of the entire food mass above 53°C(125°F). This can be achieved by first heating the food to boiling, simmering for a few minutes to allow heat to penetrate to the center of each particle and for a pocket of steam to collect under the lid. Then proceed as for retained heat cooking. This provides the level of temperature needed throughout the food, whereas leaving a pot of food on a very small flame may allow food at the edges to remain in the danger zone. Where neither of these methods can be used, it is best to cook amounts of food that will be consumed in one meal relatively soon after being cooked.
Important considerations when solar cooking
- Solar cookers generally require direct sunlight to function properly. Shadows, clouds and inclement weather limit their effectiveness, unless the cooker has incorporated some thermal mass to help hold the cooking temperature during brief cloudy periods. Solar cookers should be used on mostly sunny days, in locations where shadows are not a concern.
- In most regions of the world there are a few months when simple solar cookers have limited usefulness, due to low solar radiation intensity. In general, you can solar cook when the length of your shadow on the ground is shorter than your height. This is an indicator that the sun is high enough in the sky to cook. Some solar cookers with tall rear reflectors however, are efficient enough to be used year-round.
- You can typically solar cook two meals per day — a noontime meal and an evening meal. You typically cannot cook early in the morning or after sunset. The sun is most intense between 10:00 a.m. and 2:00 p.m., which is when breads and pastries should be baked if possible.
- And, of course, always wash your hands before and after handling food, and use clean utensils and pots.
Eye specialists have noted that individuals exposed to direct or highly-reflected sunlight for long periods of time, such as sailors and fisherman, have an increased risk of developing cataracts as a result of receiving excess amounts of UV radiation. Most users of solar cookers, especially panel and box-type cookers, spend relatively little time in the sun since food needs only be placed in the cooker and left, usually without any stirring, until finished. Also, the intensity of sunlight reflected by these types of cookers is somewhat less than that of direct sunlight. Certain curved concentrator-type cookers (parabolics) are of a more concern since they are designed to multiply available solar radiation. And given the higher temperatures reached, stirring of the food is often required, resulting in more time spent near the cooker. Most of the best parabolic cooker designs have their focal point inside of the dish rather than project the sunlight to a point outside the dish where it might come in contact with the cooks eys. With a little common sense, however, solar cookers can be used and enjoyed safely.
Panel and box cookers
- When inserting or removing food, place your body between the sun and the cooker, creating a shadow across the reflective area. Alternatively, rotate the cooker away from the sun briefly.
Curved concentrator cookers
- When available, opt for a cooker with a low focal point. (i.e. below the rim of the cooker)
- When available, opt for a cooker with a device - such as rod on which the pot slides or a "swing arm" on which the pot sits - to allow for pot access without having to lean over the cooker.
- Use the cooker in a fenced area to prevent unwanted access. Alternatively, the cooker may be raised on a platform or used on a rooftop if feasible.
All cooker types
- Do not stare at the sun's glare.
- If children will be in the vicinity, explain to them not to stare at the glare.
- Wear sunglasses if available.
- Safe internal meat temperatures
- Why solar cook
- Where solar cook
- How solar cookers work
- Solar water pasteurization
- Solar cooking and health - Dar Curtis
- Barbecue with a Side of Chemicals, Try Solar Cooking Instead - Janice Stanger Ph. D.
- WHO global strategy for food safety: safer food for better health - World Health Organization
- A 2002 audio interview with microbiologist Bob Metcalf where he explains the science behind food safety and water pasteurization - There is also a transcript of the interview.