One of our readers wrote to Sainsbury’s to ask that guideline daily amounts (GDAs) of energy shown on packaging should be shown in kilojoules, the SI unit, as well as or instead of kilocalories. He has received a reply that provides cause for optimism.
But why are there are two metric units in common use for food energy labelling? As is often the case, tradition and inertia are partly to blame. The calorie was the first unit used, over a century ago, to measure the energy we get from food . It is a metric unit, but has several drawbacks as explained at the end of this article. The modern international system of units (SI) uses in its place the joule, and the use of the calorie is declining in many countries around the world.
One of our readers is closely following the switch from calories to joules wherever these units compete for our attention. He wrote recently to Sainsbury’s on the matter of food labelling, and this is their reply:
“Thanks for contacting us regarding the display of Guideline Daily Amounts (GDA) energy values of our mini shredded wholegrain wheat cereal. We appreciate you taking the time to contact us.
The Food Information Regulation (FIR), which you refer to in your e-mail, has a transition period ending in December 2014 by when pre-packed foods sold in the UK will be required to show the energy content of a food on drink product on the front of the packaging in kilojoules. I can confirm between 2012 and December 2014 we will be changing all our packaging to incorporate the labelling changes required by the FIR.
GDAs are guidelines for healthy adults and children about the approximate amount of calories (kcal), fat, saturated fat, carbohydrate, total sugars, protein, fibre, salt and sodium required for a healthy diet. GDAs were developed by the Institute of Grocery Distribution as voluntary, back-of-pack nutrition labelling guidelines in 1998, and are widely used in the UK in their current format which at present does not include kilojoules. Food Drink Europe (FDE) are currently consulting with the European Commission to establish what format GDA labelling should take under the new FIR legislation. Once the European Commission has provided guidance on the inclusion of kilojoule information on front-of-pack multiple traffic light labelling and back-of-pack GDA labelling, we will update our labelling accordingly.
Thank you again for contacting us, I hope this answers your query. If there’s anything else I can help you with, don’t hesitate to contact me.
Jo Macintyre, Customer Manager
Sainsbury’s Supermarkets Ltd, 33 Holborn, London EC1N 2HT”
Sainsbury’s is a leader in clarity of food labelling, and follows the Government’s preferred ‘traffic light’ labelling for GDA information, unlike many of its competitors.
Don’t delay your weekly shop, however. We have been here before:
The calorie (cal)
The unit of the energy that is favoured by the UK and US food, fitness and slimming industries is the Calorie. In reality it is 1000 calories, sometimes written as kcal. The calorie approximates to the amount of heat energy required to raise 1 gram of pure liquid water through 1 degree Celsius. However, it suffers from three drawbacks:
- The amount of heat required to raise 1 g of pure liquid water through 1 °C depends upon the initial temperature of the water. Therefore, in reality the unit is meaningless unless the initial temperature of the water is also stated.
- Not being part of any integrated system of measurement, the unit has little practical application in wider physical calculation.
- It is not a recognised by the Units of Measurements Regulations that govern the units to be used in the UK.
The joule (J)
Historically, it was thought that there were several different forms of energy: heat energy (often measured in calories or British Thermal Units), work energy (measured in ergs, kilogram metres or foot pounds) and electrical energy (measured in joules or kilowatt hours). About two centuries ago, scientists realised that all these forms of energy are convertible with varying degrees of difficulty but at fixed rates of exchange. Thus one calorie is about 4.2 joules.
SI recognises only one unit of energy – the joule. It is used to measure all forms of energy, including the energy content of food. The joule is defined in terms of the units of distance in metres, mass in kilograms and time in seconds. Power in watts is energy in joules divided by time in seconds. So, for example, the energy content of sugar is about 17 MJ/kg and that of petrol (gasoline) about 35 MJ/L; the engine of a Mini Cooper has a maximum output of about 90 kW.