Heat-retention cooking

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Other names: heat retention box, thermal cooker, vacuum cooker, magic box, heat retention bag, heat retention cookers, hay box, hot bag, food warmer etc.

Heat-retention cooking is an age-old method that can be used to conserve energy not only during times of crisis, but anytime. Depending on the food item and amount cooked, the use of a haybox or insulated cooker saves between 20% and 80% of the energy normally needed to cook a food. The longer an item usually takes to cook normally, the more fuel is saved.

In conventional cooking, any heat applied to the pot after it reaches boiling temperature is merely replacing heat lost to the air by the pot. In retained-heat cooking, food is brought to a boil, simmered for a few minutes depending on the particle size (see table below), then put into the retained-heat cooker to continue cooking. Since the insulated cooker prevents most of the heat in the food from escaping into the environment, no additional energy is needed to complete the cooking process.

Retained-heat cooking has many other advantages in addition to energy and water conservation. It makes timing less critical, since it keeps meals hot until serving time (up to 6 hours).

Example of cooking times:

Food:           Simmering time:     Haybox time:

White rice      5 minutes           1-2 hours
Brown rice      10 minutes          2 hours
Dried beans     10-15 minutes       3-4 hours
Pasta           5 minutes           25 minutes
Potatoes        10 minutes          30-40 minutes

Source:

Cooking times

How to make a heat-retention box, or bag

The retained-heat cooker itself is any kind of insulated container that can withstand cooking temperatures and fits relatively snugly around the pot.

Examples of insulating materials:

hay

sawdust

cotton waste

dried leaves

waste paper

straw

wool

polystyrene beads

vermiculite

The insulation is placed between the rigid walls of a box, within a double bag of material. The most effective insulating materials create many separate pockets of air, which slow down the movement of heat. 5-10 cm of thickness (depending on the material) is necessary for good insulation. Some materials, such as aluminum foil or mylar, actually reflect heat back toward the pot.

“Instant hayboxes” are possible by wrapping a sleeping bag, blankets, and/or pillows around a pot. We have tried this method, with good results.

     ______________________
     \  i n s u l a t e d /  ^
      \   c u s h i o n  /  6cm
       \________________/    v
   ___                    ___
   |  \         ^        /   |
   |   \       6cm      /    |
   |    \       v      /     |
   |     | ||||||||||| |     |
   |     | |||P O T||| |<6cm>|
   |     | ||||||||||| |     |
   |     |_____________|     |
   |                         | ^
   |   i n s u l a t i o n   |6cm
   |_________________________| v
   (_________________________)

          if basket is too high, or pot too low, lower section of basket
          can be filled up with a densely-packed insulating material.

Gemini graphic by Rekka Bellum. Based on raster graphic by Bernd Müller. Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0)

Original graphic:

https://energypedia.info/wiki/File:Fireless_Cooker_Sketch_Bernd_M%C3%BCller.jpg

Advantages of heat-retention cooking

Slow cooking retains the food’s nutrients and vitamins more than if prepared on a constantly hot fire.

Food can be kept warm for a long time.

Fuel savings.

Food cannot burn.

Food can be left unattended in the hot bag.

Can reduce the frequency of cooking by preparing enough food for two meals and putting half of it in the hay box to keep warm.

Heat retention bags and boxes are cheap to build.

Adapting recipes to heat-retention cooking

Content below sourced from:

Turn any recipe into a thermal cooker recipe

Leave food inside the thermal cooker for ~1.5x the recipe length.

The food inside haybox cooks at the average temperature of its insulated contents. This average temperature drops off over-time, meaning that cooking time must be extended slightly. Depending on what you are cooking, cooking for even longer might be better.

Keep in mind that after ~8 hours, the temperature inside the haybox may drop below 60˚C. For food safety reasons you should keep food above this temperature.

Tips

It is important that food inside the pot is mostly covered in liquid/sauce to ensure that heat can be transferred or conducted to all the other food. Some recipes may need to be adjusted for less liquid. Normally, these recipes would rely on some of the sauce being 'boiled off' during the simmering. This will not happen inside the thermal cooker as it is completely sealed.

Make sure all food is heated through before placing inside of the haybox.

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