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by Larry Schlussler PhD on December 23rd, 2022








I recently watched a webinar ( )which looked at the potential of using cleaner cooking techniques for school breakfast and lunch programs in Rwanda. Food is typically cooked with wood fuel which is problematic because of the negative impacts on air quality and the forest ecosystem.

Here are my comments on the webinar.

I appreciated the webinars holistic approach.. I found much of the information presented highly valuable. The webinar was a catalyst for these comments.

Retained Heat Cooking
Cooking large quantities of food provides some unique opportunities for clean cooking. Larger volumes of food have less surface area per unit volume. The length of time food can be kept hot is proportional to it’s volume and inversely proportional to it’s surface area, as a consequence retained heat cooking becomes much more effective when large quantities of food are cooked. With retained heat cooking food can remain at simmering temperatures for many hours. From my experience presoaked beans can be brought to a boil, the heat turned off and the beans will be cooked in one to two hours. This cooking method can work with any fuel. An insulated pot can be designed to work as both as an insulated cooking device and a retained heat cooker, I have used a number of these devices.

Precooked Beans
Using retained heat cooking it will take about the amount of energy to cook precooked beans as it does to cook dried beans since cooking either only requires energy to bring the food to a boil.

Solar Preheater
The energy required for cooking could be reduced further by incorporating a solar thermal preheater. The preheater could also supply pasteurized and hot water for cleaning. With retained heat cooking a preheater could easily cut the energy required for cooking in half.

EPC’s (electric pressure cookers)
Both EPC’s and retained heat cookers are fairly efficient, however from my analysis retained heat cooking uses less energy. When cooking in an EPC the food must be heated to a higher temperature in addition the added structural elements needed to support the pressurized chamber must also be heated. Heating more material to a higher temperature requires additional energy. Compared to a pot when cooking a given volume of food the size of the EPC must be increased by about 30%, this prevents the vents from being clogged, however this also increases the amount of mass which must be heated, further increasing energy consumption. Another consideration with EPCs is the wattage they consume, this point is particularly important if multiple EPCs are used at the same time, the electric service may not be able to supply the large amount of power required. With retained heat cooking a smaller wattage cooker could be used, cooking will take longer, however no attention, need be paid to the food while cooking, it could even cook overnight.

When cooking with retained the cooking process requires little attention, there is no need for stirring to keep food from burning at the bottom of a pot also with no burnt food cleaning is much easier.

Water Use and Presoaking
In the webinar it was mentioned that presoaking beans would require additional water, cooking with the soak water will eliminate the need for additional water. It is sometimes claimed that cooking with the soak water effects the taste of the beans, a blind taste will show whether this idea is valid.

Wood Storage
To minimize rain exposure and maximizing drying, stored wood could form a narrow north south running wall. The wall could be held in place by wooden stakes. A rain cap on the top of the wall would also help keep the wood dry. When sunny the east side of the wall would receive the morning sun and the west side afternoon sun, accelerating drying. Breezes from the east would also speed drying.

Testing Retained Heat Cooking
It would be beneficial and straight forward to test this technique. I suggest using a pot 20 liters or larger. When cooking beans presoak them overnight. When heated the beans will stay hot longer if the pot is fairly full. The insulating cover could be made by first resting the pot on an insulating layer. This layer could be made by first placing a piece of Al foil, the size of the bottom of the pot on a table, three small wooden blocks would then be placed on the foil to support the pot. The blocks could be one or two cm thick. The pot would than be covered with a thick layer of blankets and towels. Do not cover the pot before steam stops escaping from the cover, since moisture from the steam will degrade the insulation. Check the beans one and two hours later. After an hour the beans still should be above 95 Deg C. A more convient cooker could be made which would insluate the beans while they are being heated and also act as a retained heat cooker when the heat is turned off. I have built a number of cookers of this variety. With retained heat cooking food can cook overnight with no attention and remain hot enough to eat the next morning.

I would be glad to discuss these concepts and how they may be implemented.

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