Beans on toast - a liquid lunch!

This is the Living Field web area for Sarah Doherty's Beans on toast project. Here she will explain the things that go into the making of this well loved dish. She will look inside the tin of beans, the loaf of bread and the tub of spread, asking questions of how much water is used in growing the crops and processing the harvest. 


More than just beans ....

Ten crops and a few bathfulls of water ....




In the simple calculation above, the beans straight out of the tin, plus two slices of bread, plus a layer of spread make up the fresh weight (in grams). The three constituents were then dried in an oven at 70oC to get their dry weight. The water content is the difference between fresh and dry weights and was 180 g. The percentage water content was calculated as weight of water over the fresh weight multiplied by 100.



Have you ever stopped to look at the packaging on your food? How is it that so many ingredients are used, even to make some of the simplest items? When you stop and think about just how many components there are, where they’ve come from and how they’ve been made - it’s really quite staggering!

[Back to top]


The tin of beans

Although at first glance beans on toast may be a relatively simple meal, breaking it down into its individual ingredients can be quite complex. We will now trace the many ingredients back to where they came from and show how water is used in the production process.




The diagram below shows the breakdown of the contents in a tin of baked beans into the primary components from which it originated. The first step was to note down the ingredients on the label of the tin. Different brands have slight variations, but for the purpose of this research, the ingredients are based on a standard tin of beans.



The diagram is split into two main sections: the beans (a primary crop) and the tomato sauce. The sauce contains eight ingredients, six of which came from crops, and two from other primary sources (salt and acetic acid).

In order to reach the tinned state, the primary components have undergone many industrial processes, most of which use water along the way. The taps therefore highlight the key areas of water use during processing.

[Back to top]


The slice of bread

White bread - sliced, ready to toast. What goes into an average loaf of white bread?



The diagram below shows what goes into an average loaf of white bread. There are lots of added ingredients compared to a homemade version aren’t there? Apart from the wheat (for the flour), two other crops are used: soybean and oil palm.



Soybean is used in this case to produce soy flour, a component which improves the moisture, texture and strength of the bread. The oil palm produces an oil which is used to achieve a good texture and softness in bread.

Many of the industrial processes needed to make the other components are difficult to find out, but the key, known areas for water use can be seen by the tap symbols.

[Back to top]


The slither of spread

Some people use pure butter, some have nothing at all. Most people, however use some sort of spread with their beans on toast.



One of the more common brands was used for the ingredients in the diagram below. Although this spread contains butter, there are plenty of spreads that do not contain animal products; these are normally based on a mixture of vegetable oils. The main oil that is in this particular spread is from oilseed rape, used to improve spreadability.



Common oils used for spreads include soybean, oilseed rape (rapeseed or canola), sunflower, olive, cottonseed, and corn oil. Oils tend to be chosen depending on what overall type of spread is wanted: for example, health spreads (low fat, dairy-free, cholesterol lowering, salt-free etc.), cooking spreads, functional spreads (e.g. those with added benefits such as omega-3), or those chosen simply to bring out different tastes.

[Back to top]


The water footprint for beans on toast

What is in the food and how much does it weigh?

An estimate of water footprint starts with the dry weight of each crop or crop product in the meal. The dry weight of each of the beans, bread and spread was found by spreading the products on trays and drying them in an over at 70oC to constant weight. From the dry weights of the beans, bread and spread, the weights of the main crops and crop products they contained were estimated from information on the labels.

What crops and other products go into beans on toast?

The diagrams above show that at least 10 different crops are used in the brands of beans, bread and spread that were selected for this study. The crops were haricot bean, soy bean, wheat, maize, sugar, tomato, paprika (or other spice extracts) and onion (or other herb extracts), oilseed rape and oil palm. In addition, there was a range of other ingredients, including microbial cultures such as yeast. The type of crop differs a little between different brands, but 10 crops is typical, assuming the spread is based on vegetable oil rather than dairy – if dairy products are used, then various types of grass and animal feed (and the crops from which the animal feed was made) would need to be added.

Estimating the water footprint

The water footprint – the volume of water used per unit weight of crop or product – was taken from values in the scientific literature (see reference below). Then by knowing the weight of each main ingredient in the meal and the water footprint of each, the water footprint was calculated for the whole meal. 

For a typical serving of beans on toast, with a dry weight of 120 g, the water footprint was about 450 litres, half in the beans and the other half in the bread and spread. If the meal and the water are both compared in terms of weight, then the water used was about 3,800 times the weight of the meal. We can see what this means in bathfulls. If a bath holds 140 litres, then more than three bathfulls of water are used to grow the crops that go into a single plate of beans on toast.

But what about all the other water used?

This figure of water footprint is very much an under-estimate of the total water used because it does not include all the water that is spent washing and processing the produce between the field and the tin or package. You can see from the flow charts for the beans, bread and spread that water is used at many points. It is difficult to get from the companies, or from other sources, full details of how much water was used in all these steps in processing. In addition there is all the water used in transportation of the products and by all the people who had anything to do with the products. There are many uncertainties therefore in estimating the full water footprint, but it most likely adds quite a few more bathfulls.

And we did not include the water used in the washing up and in flushing waste down the toilet.

And finally, there is nothing particular about beans on toast that gives it this large water footprint. Most meals based on a range of products will have a similar green and blue footprint that is several thousand time the weight of the meal, but there is no question that eating food grown locally that has used minimum water in processing and packaging will have a much lower total water footprint than highly processed food grown thousands of miles away.

Sources for water footprint estimates

Mekonnen MM, Hoekstra AY. 2011. The green, blue and grey water footprint of crops and derived crop products. Hydrol. Earth Syst. Sci 15, 1577-1600.

A pdf file of this paper is available at the following link:

The link 'Water footprint' in the menu to the left has further information on green, blue and grey water footprints.


[Back to top]