Cooking With A Thermos
I must admit that cooking with a thermos flask was not something that had occurred to me. As I did my research for this article it became clear that cooking in vacuum flask is something that had been around for a while, and that many people do it. If you think about it, it makes sense that the unique heat retention characteristics of a vacuum flask should allow easy, portable cooking.
There are accounts of soldiers and civilians during WW2 using vacuum flasks to cook food, although I imagine that the practice is as old as the flask itself. Thermos cooking has been used for many years by travelers, hunters, campers and hikers. There’s an entire movement dedicated to information and recipes for cooking in vacuum flasks.
The father of survivalism, Kurt Saxon advocated saving money by cooking with a thermos (Saving Money With A Thermos Bottle), and thermos cooking is ideal for emergencies where power, gas, or other fuel may be hard to come by.
There are several factors that make cooking in a thermos attractive:
- It saves time since you don’t have to watch the food or stir it
- It’s energy efficient, apart from getting water (or ingredients) to boil
- It saves money, by saving energy and time
- It’s hard to overcook or burn food
- It seals in and enhances the flavours
- It simplifies cooking beans, rice, wheat and other grains
In fact a thermos can be a great way to cook a wide variety of slow cooking foods. Of course, you can use a slow cooker, but the advantage of thermos cooking is that it is portable and doesn’t require electricity.
How it works
The idea of cooking in a thermos is pretty simple. Thermos plus boiling water plus dried ingredients plus time equals hot cooked food.
The food you want to cook is heated in a pot, expending only enough energy for it to reach cooking temperature. It is then sealed inside the vacuum flask for several hours. The food is cooked by the stored heat of the flask which, because of it’s vacuum insulation, retains it’s heat for hours. As much as 10 to 20 times less energy is required to cook this way than using conventional methods.
An alternative method is to pour boiling water into the thermos and then add your ingredients. This method is best used when there is no way to bring the ingredients to boil on a stove. Adding the cold ingredients to the hot water will immediately effect the overall heat in the system, and make cooking times longer. This method is only really practical with single ingredients that can easily cook through like rice and oats, or with dehydrated food.
Selecting your thermos
Key to successfully preparing food like this is a using a good quality thermos. You need to consider the following:
- It needs to have excellent heat retention and insulation properties
- It must be large enough
- A wide mouth makes it easier to get food in and out of the flasks (but also reduces heat retention)
Some thermoses recommended by online “thermos chefs”
|Thermos Nissan 61-Oz flask w/ handle||The large capacity of this flask makes it ideal for cooking for families but it's small mouth makes it hard to get food out and to clean.||61 Oz/1.8 L||Full||$42.21|
|Thermos Nissan 34-Oz flask w/ handle||Smaller version of the 61-Oz||34 Oz/ 1 L||Short||$37.29|
|Thermos Stainless King 40-Oz flask||Recommended by many thermos chef's but review found nasty QC problems.||40 Oz/ 1.18 L||Short||$26.99|
|Stanley 2QT Classic Vacuum Bottle||Recent versions highly recommended by thermos chef's. Past version suffers bad quality.||64 Oz/1.89 L||none yet||$42.88|
|Thermos Compact Stainless Steel Beverage Bottle||Recommended by thermos chef's. 3 sizes.||16 Oz/ 473 ml||none yet||$23.51|
|Thermos Stainless King 16-Oz Food Jar||The only food jar we found that we can recommend for thermos cooking. See tips below.||16 Oz/ 473 ml||Short||$26.37|
* Prices are from Amazon.com and are indicative only.
What to cook
Most foods that can normally be cooked in liquid can be cooked in a thermos. Stay away from anything that can be easily overcooked or foods that cannot be brought to a full boil. Also avoid foods that require multiple ingredients and those with cheese (hard to clean out).
These are all suitable for thermos cooking:
- Rice or Beans
- Noodles or spaghetti
- Cubed potatoes
- Soups, stews and chilli
- Spaghetti sauce
- Meats like Beef and Chicken cut into cubes
- Oats or dried and freeze-dried foods
- Eggs in their shells
Tips for thermos cooking
Wide mouth vs narrow mouth
The wide mouth of a food jar makes it look like an attractive choice to use for thermos cooking but if you look at the specifications on jars and then on traditional coffee flasks you will notice something. Most food jars are rated to keep food hot for about 7 hours while traditional flasks can stay hot for more than 12 hours. This is a definite consideration when using a thermos to cook, but it doesn’t mean that you can’t use a food jar. With items that need shorter cooking times like dehydrated and freeze dried foods, noodles and pasta, a jar will do the trick.
Planning ahead is essential as the time factor is the most important. You need to give enough time for the food to cook properly.
Preheat the bottle
Preheating before adding your ingredients will help maximize heat retension. This ensures that the hot food you are sealing inside does not lose heat by having to heat the stainless steel of the inside of the flask.
To preheat, fill the thermos with boiling hot water and let it sit for five minutes. When you’re ready to add your hot food or ingredients, simply empty the water.
Some foods wneed to be precooked such as meat or beans which require high amounts of heat. These should be cooked on the stove to the point where you would normally turn down the heat to simmering before being added to added to the thermos. A best practice is to bring food to a rolling boil then put it in the flask. This keeps it hottest longest.
Cooking rice and grains
To make sure the rice and grains are cooked perfectly, allow enough time for the them to absorb enough water.
An useful piece of equipment to have is a preserving funnel. This helps avoids dangerous splashes when filling the flask with hot food.
Lay the thermos on it’s side
While a sealed thermos is effectively a closed unit, the food inside is not necessarily heating evenly. This may lead to convection currents in the liquid (see article). Since gravity has an effect on these currents, laying the flask on it’s side leads to more even convection, and more even heat distribution and cooking.
Insulate the thermos
While the flask is itself insulated, if the outside air is very cold, it will speed up heat transfer at the lid. (see article) Insulate the flask from cold outside air with a blanket, cloth, jersey or the suchlike to reduce heat loss.
Something to remember when cooking with a thermos
If a large part of the cooking time is spent at temperatures lower than 76F (60°C), a danger exists of food poisoning due to bacteria, or toxins produced by bacteria. This is especially true of foods with multiple ingredients including meats (like stews), and so it is essential to heat food sufficiently on the stove before sealing it in the thermos. A temperature of at least 76F (60°C) throughout the food for 10 minutes will serve to kill most harmful bacteria, effectively pasteurizing the food.
Recipes for thermos cooking
Recipes that are designed for slow cookers/crock pots can be scaled down to fit in a thermos. Typical cooking times will vary depending on your thermos and its ability to retain heat. Here are some general guidelines. The first number is how long to boil the food before putting it in the thermos, the second is the amount of time required to cook the food in the thermos. You may need to conduct a few experiments to get your own times right.
- Rice: 5m/1.5 hr.
- Cubed Beef: 15m/4hr.
- Chicken: 8m/3h.
- Beans: 10m/4h.
- Cubed potatoes: 5m/2h.
- Oats: 5m/2h
For a brilliant short article on thermos cooking you can visit Carolyn Shearlocks site theboatgalley.com. Carolyn has also produced a recipe book for cooking in a boat galley that contains many recipes that can be prepared in a thermos.
These videos are both great short demonstrations of cooking with a thermos. There are links to more resources below the videos.