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We have shown that the time taken to cook has a strong influence on the fuel consumption performance of the stove. In particular certain users would like to get, for example, water for tea/coffee heated fast. Then the design needs to cater for that. Table 1 brings together information about cooking times required for a few commonly used foods.

Table 1: Simmer times for some common foods
Type of food Time in minutes
Ragi mudde to, ugali, etc. 15
rice 30
Maize and beans 120 +
Potatoes and other vegetables Size dependent
Fish and fowl 30
Meat Depends on the cut and recipe

The table only presents the simmer times and excludes the time required to bring the water food mixture to boil. Since the latter times determines the design power of the stove, this really has to be found on the basis of the time used by a traditional stove.

We will consider two examples to show how important this question of time can be in designing a stove. As we have learnt the time for cooking can for many foods be split into two parts - the time to bring the food mixture to boil, tb and the time to simmer, ts. Consider a manufacturer with a stove of 5 kW capacity. He mounts a sales campaign to sell his stove with the claim that the new design will bring 5 kg of water to boil water in 10 minutes as against 30 minutes that the traditional fire takes. This sales pitch will go down very well with large families that use rice as a staple. But women who cook only 1kg of rice - water mixture will see that for their task the new stove takes 31.2 minutes as against 36 minutes. At the other end of the spectrum women who cook beans will find that the new stove takes 130 minutes for their 5kg mixture as against the 150 minutes. In both these cases the new stove is not such a dramatic change. The market for the stove will shrink.

The second example comes from Addis Ababa in Ethiopia (Sielcken 1985). The staple food eaten here is called Injera and is eaten with a sauce. Household energy studies have shown that 80% of all fuel used goes to bake injera. Injera is a pancake like fermented batter made out of a flour of a special millet locally called tef(botanical name?). The pancake is baked on special clay plates called ``matat'' with very little grease. The specialty of injera is that traditionally it is baked in diameters of 60cm and cooked in batches of 20 once in 2 or 3 days. The time to cook each injera comprises of 2 minutes pouring and 3 minutes of baking. Thus the total time to complete the task is of the order of 100 minutes.

A little reflection will show that most of the stoves will be unsuitable to handle such a large matat. One way of solving this problem is the suggestion: why not cook the injera in sizes of 30cm? While this sounds reasonable in a superficial sense, if one calculates the time the suggestion is really preposterous. If we note that the baking time is independent of the size and the pouring time is reduced by half, the total time per injera is 4 minutes. For equal amounts of food one requires to bake 80 injeras of the smaller size and the time for the task will be 320 minutes. It is obvious that the design of the type is a nonstarter for baking injeras. The designs require modification and we shall consider it later. According to George and Vingle (1990) a similar type of problem is encountered with a type of bread called "rotla" in Gujerat. One presumes a similar situation prevails for the entire family of breads mentioned earlier.

Many an engineer will find it somewhat unreasonable that policy makers and field groups cry hoarse that s/he should take into account so-called social considerations. While the present author is not necessarily in sympathy with the latter group, it has to be conceded that there have been many examples of suggestions of the type indicated above. Not only that, there have been also instances of ridicule of traditional cooking practices. It is important for a good designer to define the tasks to be performed by the design with the utmost care, if his desire is to make a product and sell it in large numbers.