Thursday, 4 October 2012

Efficiency of different ways to cook potatoes

In my book I reported on my experiments to see how much energy was used cooking potatoes either by boiling in a saucepan or baking them in an oven. (The oven took 6 times as much energy as the hob.) Recently I posted about cooking with the microwave - when it is more efficient than the hob and when not. A reader queried on how pressure cookers compared, so I borrowed one from a friend and did some more experiments. Finally, I tested out an electric steamer which we sometimes use (though mainly for greens) and a microwave. Here are the results.


NB. It is very difficult to get consistent results from cooking because the cooking time depends a good deal on the size and shape of the potatoes - in this case largish pieces - and for cooking on the hob the precise heat settings. Also the equipment is important - the size, shape and material of pans, the efficiency of your microwave and the power used by your steamer will vary. My experimental results are only a rough guide to what you might get in your kitchen.

Headline results

Here are the 5 cooking methods with approximate energy consumption and associated carbon emissions for cooking 400g potatoes (2 servings), starting with the most efficient in terms of carbon emissions. The pressure cooker and hob tie for first place.

Energy kWhFuelkg CO2Notes
Pressure cooker0.3gas0.065 minutes steaming at 12 psi
Hob0.3gas0.0610 minutes simmer (lid on)
Steamer0.2electricity0.0920 minutes at 0.55 kW
Microwave0.2electricity0.1010 minutes at nominal 800W
Oven1.5gas0.2860 minutes at gas mark 5

For cooking larger quantities the pressure cooker would win out over the saucepan because it does not need more water to cover the potatoes (or, for even larger quantities, a larger pan). Also, with the oven, larger quantities will need very little extra energy.

In general, for all cooking methods, smaller pieces cook quicker and save energy and for cooking on the hob it is essential to set the heat as low as possible to maintain the temperature without unnecessary water loss.

If you have an induction (electric) hob instead of a gas hob then you will need considerably less energy - probably around a third to a half less. From my experiments I estimate that my hob was heating the pans with about 50% efficiency, which is quite high. With an induction hob you can expect at least 80% efficiency (see theinductionsite.com). This improvement is not quite enough to offset the higher cost and carbon emissions of electricity, but as we get more renewable energy on the electricity grid the carbon emissions at least will improve.

Pressure Cooker

I was surprised that the pressure cooker did so well with even such a small amount of potatoes in such a large pan (the pressure cooker was nearly 4 times the mass of the saucepan). The key is that in a pressure cooker you actually steam your food rather than boil it which means you need less water. A pressure cooker will save much more when cooking larger quantities because you don't need to add any more water unless there is a longer cooking time.

To get the best from your pressure cooker it is important to avoid putting in more water than you need and to set the hob low while steaming to reduce water loss.

The pressure cooker operated at 12 psi above the ambient air pressure which means the water boiled at around 118C. The pressure cooker and lid weighed 3.2 kg. I added 250ml water to start with and 80ml evaporated away. It took 0.2 kWh gas to come up to steaming temperature and another 0.1 kWh to maintain this for 5 minutes cooking. 

Boiling in a saucepan on the hob

With this method you have to heat a lot of water to start with and there is a bit of a trade off between using a large pan which is more efficient on the hob or a small pan which requires less water to cover the potatoes and itself has a lower heat capacity. In this case my pan and lid came to 0.9 kg and I used 500ml water.

It is critical to keep the lid on and avoid simmering more vigorously than necessary to keep up the temperature.

Here is a comparison of where the heat went in using the pressure cooker versus the hob. These calculations are based on available data for the heat capacity of potatoes, aluminium and water from www.engineeringtoolbox.com. The energy used for the potatoes is similar and the water loss by evaporation was also similar in this case. The larger pressure cooker pan offsets the smaller amount of water to heat.

Where the energy goes: cooking 400g potatoes 2 different ways

Steamer (Electric)

My steamer has no control beyond on/off and it uses 550 W when on. It is good for cooking bulky stuff especially leafy greens which are quick to cook - this potato task was definitely not its raison d'etre. Although the steamer used less energy than cooking on the hob since this was electricity it generated more carbon emissions and cost more.

Microwave

My microwave is nominally 800W but actually uses 1.2 kW so it is only 65% efficient. It is a cheap and cheerful device - I dare say you get better ones but they would have to be a lot better to win over the hob in terms of cost or carbon emissions. However, the microwave would have done a great deal better with smaller pieces of potato because the microwaves only penetrate a cm or so and the heat takes a while to conduct through to the centre. Perhaps I should have cooked the potatoes on the defrost setting which allows the food to 'rest' at intervals for the heat to spread through the food.

Oven

Even our small (gas) oven uses 0.4 kWh just to come up to temperature and nearly 1 kWh to maintain that for an hour without anything in it. The potatoes themselves use very little of the heat and so larger quantities would be relatively much more efficient.  You can get more efficient ovens - our one is 11 years old.

Conclusions

From the point of view of an epicure it is scandalous to compare these cooking methods as though they were equivalent because clearly they are not. For example, the texture and taste of baked potatoes is nothing like that from boiled or steamed. Still, as with most energy uses, until you know the energy requirements you can't make an informed choice between recipes, or evaluate advice on reducing energy for cooking. It would be interesting to run more experiments with larger quantities and different foods but even what I have done here is enough to demonstrate the basic principles:
  • Avoid using the oven unless you have a lot to cook at once
  • The steamer is also best used for larger quantities
  • The microwave is good only if you avoid heating a lot of water and the food is in small pieces
  • Control over water loss is important so keep the heat low and keep the lid on saucepans


8 comments:

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  2. Recently I posted about cooking with the microwave - when it is more efficient than the hob and when not. A reader queried on how pressure ... microwavepressurecooker.blogspot.com

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  3. A reader queried on how pressure cookers compared, so I ... powercooker.blogspot.com

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  4. That's a good comparison, Nicola, thanks. One additional option you might like to consider is using a hotbox / hotbag for retained heat cooking. I don't do potatoes any other way: heat for 2 minutes, put pan in the bag for 20. I'd guess the hotbag reduces your hob figures by about 50%. If you really want to save energy put the pressure cooker into a hotbag the moment it comes up to pressure!

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    1. I don't have a hot box but one day I must get or make one. I note that McGee (http://www.amazon.co.uk/McGee-Food-Cooking-Encyclopedia-Kitchen/dp/0340831499) recommends cooking potatoes at 80-85C rather than boiling, to reduce the tendency for the outside to overcook before the middle is done. It will take a little longer at the lower temperature but the hot box should be able to maintain that temperature for quite a while.

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    2. That's why my potatoes are perfect every time! :-)

      You should consider a hot bag as opposed to a hot box. A hot bag can fold up and be put in a cupboard, and works for cooking AND keeping food warm (amazing for coordinating dishes for a dinner or Christmas lunch for instance). Every kitchen should have one (or two or three)!

      Mine comes from here: http://www.hotbag.co.za/pages/home/ (I have no affiliation with the company, but I have done lots of work on efficient cooking in Africa).

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    3. Thanks for the link. I have just sent off for a quote. Assuming I get one I will report back how I get on with it.

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