What is the calorific value of gravy?

I vaguely remember a topic in my Secondary School science around calorific values and how ‘calories’ are measured. There is an old definition: one calorie is the amount of energy needed to raise the temperature of one gram of water by one degree centigrade. One gram of water (at standard temperature and pressure) is one millilitre. I seem to remember burning a cube of cheese and measuring the temperature rise of a test tube of water.

So when I consume a day’s food with a calorific value of 2,000 kilocalories that food, if totally burnt, could heat 20 litres of water from freezing to boiling points. (Impossible to achieve that perfect machine and process, but the number illustrates the potential energy stored in food.)

Every day in every school we throw away food – either from over-preparation, over-ordering or as plated waste. It is a topic that has concerned me over and over, from at least two viewpoints – healthy eating and environmental impact.

Every school meal menu meets or exceeds the national nutritional standards, but these only reach the child’s plate if they take a full range of what is on offer. Too often salad, veg and fruit is not taken and so is left at the end of service. To ensure a full menu offer for every child at the point of service, despite children ordering each morning from the menu, kitchens prepare more meals than ordered. When plates return to the trolley they are scraped off – it saves on hot water and soap for washing up if they are mostly scraped. The food waste is collected – somewhere in the archive of this blog series, you will find the calculation for what that amounts to over a year and how many skips it would fill.

Children do not necessarily consume the healthy standard meal that was prepared for them, fresh that day.

Energy is wasted or lost, in production and in disposing of waste. We could be sorting waste and sending it off to a bio-generator of course, but this is the third of the ‘R’s – recycle. If we start thinking that energy generation from ‘recyclables’ is the aim we will try to increase food waste and cheer ourselves when we do so. It is the wrong aim entirely.

We should focus instead on the first of the ‘R’s – reduce. If we cook only as much as is served, and serve only as much as is eaten, and eat what we ask for, we will reduce energy used in producing, transporting, storing, preparing and serving food, and in dealing with the waste. We will eliminate any greenhouse gasses produced by that waste as it degrades naturally.

I know the problem quite well – I just need to find some answers that can be applied in real life.

The calorific value of gravy, by the way, is 13 calories (or do they mean kilocalories?) per serving – it gets scraped off plates as well. The calorific value of 200 plates of gravy would heat a bath of water.

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