The method of calculation is as follows. The weights of the raw ingredients are used to calculate the total amounts of nutrients in the dish. A correction for wastage as a result of ingredients left on utensils and in the vessels used in preparation is made at this stage. The weight of the raw dish is then measured, using a scale weighing to about 1 g (a less accurate scale may be used if the total weight of ingredients is over 500 g). The dish is then cooked and the dish reweighed. (A minor correction to allow for the difference between weighing the dish hot and at room temperature is not usually necessary.) The difference in weight is taken as being accounted for by water, and the composition of the cooked dish is calculated as follows. Divide the total nutrients in the dish calculated from the raw ingredients by the weight of the cooked dish and multiply by 100. The water content of the raw ingredients less the loss in weight on cooking divided by the weight of the cooked dish gives the water content of the cooked dish if it is required. The detailed procedure for calculation of the nutrient content of a multi-ingredient food is outlined below.
1.Select or develop an appropriate recipe.
2.Collect the weight and nutrient content data for each ingredient.
3.Correct the ingredient nutrient levels for weight of edible portions where appropriate.
4.Correct the ingredients for the effects of cooking:
5. Sum the weights of the ingredients to obtain the weight of the recipe.
6. Sum the nutrient values of the ingredients to obtain the nutrient value of the recipe.
7. Adjust the recipe weight and nutrient levels to reflect changes in fat/water contents when the whole mixture is cooked; make any additional refuse adjustments; apply retention factors if available for the whole recipe.
8. Determine the quantity of prepared food produced by the recipe.
9. Determine the final values per weight (e.g. per 100 g), volume (e.g. per cup) or serving portion, as desired.
Source: Rand et al., 1991.