What is the most energy efficient method of cooking a ‘British’ roast dinner?
What is the most energy efficient method of cooking a ‘British’ roast dinner?
Friday, 17 April 2009
An easy question to ask, but a surprisingly difficult one to answer from reliable published information. Steve James looks at the energy efficiency of cooking.
There is, of course, no standard definition of a ‘British’ roast dinner. Most people would accept the fact that it will have a piece of roast meat as its core ingredient. Traditionally this was likely to be beef, pork or lamb but increasingly it will be based on a whole chicken. The meat will be sliced hot and presented with a mixture of roast and boiled vegetables; a gravy, sauce or both and in many cases a stuffing. If beef, it will traditionally be accompanied by a Yorkshire pudding.
In the past we have carried out studies on the energy requirements in the cooking of a 2.25 kg rolled rib of beef. Taking the joint straight out of the freezer and cooking it absorbed up to 13,729 kJ (3.8 kWh). If joints were cooked from ambient (20°C) then as little as 6700 kJ (1.9 kWh) could be required.
Very little of the energy used ends up in the cooked piece of meat (see graph below). So where does it all go? During the cooking process a meat joint will lose moisture and shrink. Some of the moisture will evaporate directly into the air in the oven and the rest will end up in the roasting pan. Heat energy is absorbed in the evaporation process and raising the steam produced to the operating temperature of the oven. In addition heat is required to raise the temperature of the oven to its operating temperature and maintain it at that temperature.
The energy used to keep the oven hot is the major component of the total energy used especially in a poorly insulated oven (see graph below). Consequently the much shorter cooking time required when the meat starts off from ambient substantially reduces the total energy requirement during roasting.
Energy used in cooking 2.25 kJ beef joint
A combination microwave oven offers an alternative method of roasting a chicken to using a conventional oven. Studies carried out by Intertek have shown that using a combination microwave oven results in a 30% energy saving to that used in an electric oven when cooking a medium chicken (1.02 kWh compared with 1.32 kWh). Cooking time was also reduced from 1.5 hours to 27 minutes but the skin was not so crisp.
If roast vegetables are a critical part of your ‘British’ roast then adding them to the roast chicken in a conventional oven is likely to add little to the energy already required to roast the chicken. However, cooking them in the combination microwave will require a separate operation and use probably a further 0.8 kWh of energy. Using the conventional oven is likely to produce a superior product but consume 1 kWh of energy.
If you are happy to forgo the roast potatoes and opt for healthy new potatoes then there are more options. Cooking a 600 g four person portion in a conventional or combination microwave will typically consume 0.15 to 0.16 kWh, which is approximately 30% of the 0.5 kWh required on an electric hob.
It is a similar story if you opt for frozen mixed vegetables to accompany the meat and potatoes. Cooking a 450 g four person portion will use 0.14 kWh in a standard microwave oven, 0.17 kWh in a combination microwave or 0.4 kWh on the hob.
Heating up 800 g of gravy in either a microwave or on the electric hob is likely to consume about the same amount of energy, between 0.19 and 0.2 kWh.
Totalling it all up then for a ‘healthy’ roast dinner for 4 of chicken, new potatoes, vegetables and gravy the microwave oven is the green option and consumes 1.5 kWh of energy (see table below)
Energy (kWh) to cook ‘Healthy’ roast dinner
However, for a real roast dinner of roast vegetables, frozen vegetables and gravy the best bet is to use the oven for the chicken and roast vegetables and the microwave for the frozen vegetables. The total energy used increases to 1.75 kWh (see table below) but you could probably include your apple pie if you cooked it with the roast.
Energy (kWh) to cook ‘Real’ roast dinner
However, it is very difficult to generalise because much of our work has shown that there is no such thing as a typical microwave or conventional oven. Much more work is required in this very energy intensive area to produce real ‘facts’.