Marked fluctuations due, for example, to the season and/or length of storage, may occur in the moisture content of foods, particularly starchy roots, tubers and fruits, leafy green vegetables, cereals, and processed fish. The difference between the percentages given in the table and the actual percentages in the foods can lead to considerable error in the use of the table.
For example, the analyses used for making the present table have shown that moisture content in raw cassava tubers varies from 45.9 to 85.3 percent, the corresponding food energy going from 215 to 52 calories per 100 grams edible portion. Themoisture content of raw yam varies from 56.3 to 78.6 percent, sweetpotato from 59.0 to 78.5 percent, plantain from 52.9 to 77.6 percent. Fish considered as dried may contain from 5.2 (butter fish) to 33.5 (mullet) percent moisture.
There is no doubt, therefore, that representative determination of moisture content should be made wherever possible, at least for starchy roots and tubers, leafy green vegetables, cereals, and processed fish, to allow for current local conditions.
Two simple methods of calculation of a conversion factor to apply to a food composition table, taking into account deviation from average moisture content, are given below:
First Method. The conversion factor F is calculated, using the following formula:
For example: Yam, attoto (Item No. 262) in this table reads thus:
|Representative Values Per 100 Grams of Edible Portion|
|Moisture (percent)||Food Energy (calories)||Calcium (mg)|
|Yam, attoto, tuber, raw||81||71||36|
Now, assuming that in a given country a representative series of analyses of this yam tuber has shown a different moisture content1, for example, 70 instead of 81 percent, the conversion factor F to be applied to each nutrient content, for 100 grams of edible portion, is:
The calcium content for a yam containing 70 percent water will then be determined as follows: 36mg×1.58 = 57 mg; and food energy of this yam will be: 71 × 1.58 = 112 calories.
This method should be used only when very precise data are needed, e.g., preparation of a national food composition table, or conversion of chemical composition of raw leaves, raw fish, etc., into dried leaves or dried fish.
1 The moisture content is usually determined by keeping the food in an oven at a temprature of from 100 to 105°C until constant weight is reached. The food is weighted before and after, and the loss in weight is expressed as a percentage of the original weight.
Second Method. The nomogram on the left can be used. For the example cited on page 227, a very straight ruler, preferably transparent, is placed between the two points 81 at the left (percentage as shown in table) and 70 at the right (actual moisture content), and the conversion factor is read directly along the middle line.
This quick method may be used for data recorded in food consumption surveys. In this case, the conversion factor F is to be directly applied to the weight of the food. Example: for 15 kg of yam containing 70 percent moisture, the figure of 15 × 1.58 (for the example cited on page 227), or 23.7 kg, will be recorded.
In other words, 23.7 kg of yam containing 81 percent moisture (i.e., 19 percent total solids) has the same composition as 15 kg of yam with 70 percent moisture (i.e., 30 percent total solids).