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Alumnus writes about Food Energy in the Biofuel debate

Posted on 29.04.08Category: Development News, College News

Calculating Food Energy

Dr Chris Cragg, PPE, 1970

From: The European Energy Review, April 2008

In late February, amidst much self-promotion from Virgin Atlantic, a Boeing 747 flew from London to Amsterdam with one of its four tanks filled with a mix of 80% jet kerosene and a 20% mix of coconut and babassu palm oil. Of course, the minimal distance of 373 km meant that the lumbering giant had only just reached cruising altitude when it started coming down again, but no matter. This was supposed to be an example of ecological flying and a triumph for the humble coconut. Virgin Atlantic proudly explained that the oil of 150,000 coconuts was used. This provided less than 4.1% of the fuel on the trip. Presumably to fill all the tanks would have needed around three million of the things.

Given that pure coconut oil goes solid at 25Â°C, there must have been some technical skill involved here. However, it did prompt the thought that there might have been an alternative use for the produce of these palms, most notably for food. While clearly this is an issue for biofuels, it is curious how reluctant people are to make the rather obvious link between hydrocarbons and carbohydrates; the former being variations on the theme CH3 and the latter variations on Câ€¢H2O. Yet the link is staring straight at you when you look at a package of breakfast cereal in the morning. It is called the Calorie.

For reasons best known to themselves, the food people define the fattening Calorie as the amount of energy required to heat 1kg of water by 1Â°C. This is clearly designed to confuse it with the old energy unit, the calorie, which is the amount of energy required to heat 1gram of water by 1Â°C. One Calorie is thus 1,000 calories or a kcal. In the energy industry sadly, this has now been replaced by the pathetically tiny joule. However one kcal equals 4184 joules, or 1 joule equals 0.00023906 kcal.

It may be an old unit but the kcal does provide a fascinating link between the energy content of food and that provided by standard hydrocarbons. For example, a kilogram of diesel contains around 10,800 kcal, or 10.7 kcal per gram. In this respect it has almost twice the thermal content of peanuts, which weigh in at a fattening 5.5 kcal per gram. As a general rule of thumb, carbohydrates contain 4 kcal per gram, fats 9 kcal per gram and alcohol 7 kcal per gram.

Now it so happens that the recommended daily dose to keep body and soul together is 2,500 kcal for men and 2,000 kcal for women. The UN definition of under-nourishment is 1,800 kcal per day. At the other extreme, if you eat 3,500 kcal more than you use, you will put on almost half a kilo in weight.

According to Boeing, a 747 apparently does 12 litres of jet kerosene to the kilometre, although aircraft fuel consumption figures are notoriously difficult. One litre of this fuel contains 8,986 kcal, so a kilometre of flight will require 107,838 kcal of energy. If this jet kerosene were replaced directly by food Calories, it would thus amount to the food energy required to keep 60 people above the UN under-nourishment line for one day per kilometre. The entire trip would keep them alive for over a year. Alternatively, of course, the Jumbo would need the energy content of 7.3 tonnes of liquefied peanuts to complete the entire trip. Just an airline snack really!