Cooking in the Box — Reducing Energy Expenditure by Half
In the episode “Schonend garen, Energie sparen — Die Kochkiste” (“Cook gently, save energy — The cooking box”), the Austrian National Radio station ORF presented recently an interesting way to cook, featuring a book by Margot Fischer entitled “Kochen in der Kiste: Der energiesparende und entspannte Weg zu köstlichen Gerichten” “Cooking in the Box — an Energy Saving and Relaxing Path to Delicious Meals”. Cooking in a box refers to an ancient way of finishing off the cooking process in an insulated container after an initial heating-up phase on a stove. This story tries to convey my understanding how it works and why it can save energy, supported by measurements and calculations. The result: the author’s claim to reduce energy expenditure by 60–80% is indeed justified.
Background
Before diving into the experiments, a brief history is perhaps in order as well as a short explanation of the cooking process in general.
Short History
Performing certain stages of the cooking task in an insulated container or at least keeping food for later consumption goes back quite some time. For instance, it is an ancient Jewish tradition to prepare a meal on Shabbat eve as it not permitted to work (and cook) on Shabbat. Later a similar approach was used by German inventor Karl Drais to produce a “Kochmachine” in 1834, one of the earliest known boxes insulated with hay, hence hay-boxes. At the Paris world exhibition in 1867 Scandiavian exhibitors demonstrated a similar concept, known in France to this day as Marmite norvégienne. This lead eventually to the hay-day of hay-box cooking before and around World War I, for various reasons: saving energy as well as time. Publications include “The idle Hour Cookbook” by John F. Chambers in 1910 or “Meals that cook themselves and cut the costs” by Christine Frederick in 1915. With fossil fuels having powered the rise of our current civilisation, hay-box cooking lost its appeal, also being associated with rationing and war. Perhaps these days, with worries about energy security, hay-box cooking might make a comeback.
The Process
From my own cooking experience and from what I understand about the physics of cooking (e.g.: “Die Genussformel: Kulinarische Physik”) cooking involves certain stages for meals such as stews and casseroles:
- fry perhaps some onions, garlic, vegetable, or meat in the stock pot at high temperature, at least 140°C; the purpose is to trigger the Maillard reaction which produces the chemical compounds that carry the distinctive flavour; add spices, too;
- add liquid, mostly in form of water, vegetable or meat stock, or watery vegetables such as diced tomatoes; then bring food to a temperature, usually close to boiling point at 95°C;
- maintain food at a certain temperature, 95°C or below, simmer for usually quite some time, perhaps about one or two hours.
Whereas the first two steps are usually short and consume some amount of energy, the third and much longer phase has an important task: break down organic chemical compounds to make them easier to digest in the first place, but also to let flavours seep out into the stew. For food stuff containing carbohydrate such as grains and potatoes, the starch starts to bind water above 50°C when it starts to well up and begins to gelatinize at 62.5°C for potato starch and 67.5°C for wheat starch. This is what makes your risotto creamy.
The protein compounds in meat also are subject to transformations, too; they start to break down above a certain temperature. Collagen, the fibres made up of three strands of protein that hold muscles together but also make it tough, break down and make food tender: pork and chicken at 65°C, beef, duck and goose at 75°C. This process of “decomposition” neither consumes energy, nor produces energy. All these processes happen at a constant temperature in the pot.
In summary, the premise of cooking meals such as stews is that, after an initial heating-up phase, a certain temperature has to be maintained for as long as it takes to render the food tender.
The Experiments
The objective of the experiments and subsequent investigation is twofold: a) show that putting a hot pot in an insulated box does maintain the temperature needed to break down food stuff as long as needed without adding energy as heat, and b) in comparison with a traditional way of simmering on a hob, estimate the amount of energy that can be saved.
There is some commonality between the experiments. The phase of frying and heating up is needed in both. As a simplified version, a system of known material and weight was used, a 1.5kg stock pot plus 2 litres of water (2kg). The pot was heated up on an induction stove to the target temperature of 95°C in both cases, then covered in a cling film to prevent water vapour to escape (which has a cooling effect).
The initial heating up energy E₀ needed to heat pot and water from an initial temperature of 21°C to a target temperature of 95°C is estimated to be 700 kJ in total for the 3.5kg assembly, or a specific energy of 200 kJ/kg¹.
Cooking in the Box
Once the water in the pot had reached its target temperature, the pot was immediately put into a make-shift cooking box, made up of an old cardboard box stuffed with lambs wool:
The assembly was left on the wooden kitchen floor with the temperature measurement of the water periodically taken over the course of 10 hours using a kitchen thermometer. Curve fitting was then employed to approximate an exponential decrease of the internal temperature².
The diagram shows how the temperature drops exponentially; after 3 hours 75°C are reached, and after about 5.5h 65°C, the temperatures above which beef and chicken proteins break down, respectively. Since most cooking-in-the-box recipes require about that length of cooking, 3–5 hours, the requirement is met: the temperature in this box is kept high enough for food to cook, i.e. become tender, for the amount of time needed, even in this simple arrangement with potential for improvement.
Cooking on the Hob
The second experiment is about finding out how much energy is required in a traditional setup to maintain the temperature and, by extension, how much less is needed in the alternative cooking box arrangement. The difference is an indication of efficiency gain.
In a conventional setup, maintaining the temperature of a stock pot is possible by adding the same amount of energy that is lost through convection, radiation and conduction in form. The most important question now is: how much energy has to be added, or more precisely what is the power needed to compensate the heat loss? A very easy way to answer that question is to leave the heated stock pot to cool on the hob on its own. The initial gradient of the temperature decrease is an indication of the heat loss on the hob Qₕ³.
The stock pot was left on the hob with the temperature measurement of the water taken periodically over the course of 5 hours using a kitchen thermometer. Again, curve fitting was employed to approximate an exponential decrease of the temperature.
Similarly to the above diagram, the temperature drops exponentially; already after 40 minutes 75°C are reached, and after about 70 minutes 65°C; this is not enough to fully cook the food. The initial temperature gradient of 34.6 °C/h determines the heat loss at the initial temperature of 95°C which amounts to 91 W for this assembly, or a specific heat loss of 26 W/kg, or in more practical terms: 94 kJ/kgh.
Comparing with the initial heat-up energy, 94 kJ/kgh is about half of the initial 200 kJ/kg to bring the pot to the target temperature; stewing for say 2 hours would then incur about ~200kJ, the same about as for heating up. Not having to spend that energy by using a cooking-box entails saving 50% of energy. Stewing for 4 hours yields an energy saving of 400/(200+400) -> 66%. Therefore, the promise is held: energy savings up to a range of 60–80%.
Conclusion
Through a simple experiment it could be demonstrated that a cooking box achieves what a stove does: maintaining temperature in order to “cook” food, i.e. breaking down its constituents, but without additional energy expenditure. It has also been shown that this method of finishing off a meal could save in excess of 50% in energy. Above that, much better insulation material than lambs wool (R-value of 0.6 m²K/W) such as Polyurethane (R-value of 1.2 m²K/W) would maintain the temperature twice as long. Perhaps it can be used to construct a wood — cladded modern version of traditional early 20th century hay-boxes.
Appendix
A few notes on how the results were achieved, necessitating only secondary school mathematics.
- Experiment Parameters
The stock pot is made of aluminium with an internal steel disk to work on an induction hob; 2 litres of water was used.
Water: mass = 2 kg, specific heat capacity = 4.2 kJ/kg°C
Steel: mass = 0.5 kg, specific heat capacity = 0.42 kJ/kg°C
Aluminium: mass = 1.0 kg, specific heat capacity = 0.7 kJ/kg°C
Total mass = 3.5 kg, average specific heat capacity = 2.7 kJ/kg°C
Temperature: initial = 21 °C, target = 95 °C - Exponential Model
Similar to any system that stores energy, the decrease of energy can be modelled like an exponential decay, in mathematical terms with time t, amplitude A, time constant τ and initial temperature T₀:
T(t) = A * exp(-t /τ) + T₀
Using the data set obtained in the experiment, the parameters can be estimated using a non-linear least squares method (curve_fit). - Heat Loss Model
A simple model for the thermal energy in matter, assuming both mass and specific heat capacity being constant, is:
E(t) = m * cₚ * T(t)
with the temperature T [°C], the specific heat capacity cₚ [kJ/kg°C] and the mass m [kg]. The heat loss is equal to the time derivative of the thermal energy:
Q(t) = dE/dt = m * cₚ * dT/dt = — m * cₚ * A/τ * exp(-t /τ)
The initial loss at Q(0) = — m * cₚ * A/τ.
