Felicity Savage-King and Ann Burgess

Print publication date: 1993

Print ISBN-13: 9780192622334

Published to Oxford Scholarship Online: September 2009

DOI: 10.1093/acprof:oso/9780192622334.001.0001

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(p.433) Appendix 4 Calculating nutrients in foods

Source:
Nutrition for Developing Countries
Publisher:
Oxford University Press

A4.1 WHY WE CALCULATE THE NUTRIENT CONTENT OF FOODS

• To compare the nutrient content of different foods, and to decide which foods are good sources of nutrients and which give best value for money.

• To find out the nutrient value of different weights of raw and cooked foods, and meals.

• To find out the amounts of nutrients a person or group of people eats, and whether this is enough to cover their nutrient needs.

• To plan meals and diets which provide particular amounts of nutrients.

A4.2 Calculating the Nutrient Content of Foods

To find out the amounts of energy and nutrients in foods we use food composition tables. Food composition tables list the amounts of nutrients in 100g portions of different foods. You can use the tables in Appendix 1. But it is better to use reliable ones prepared for your own country.

Before you use food composition tables you need to understand:

• how food is measured;

• the meaning of ‘as purchased’ weight, ‘edible’ weight, and ‘per cent edible portion’

• how the proportion of water alters the proportion of other nutrients;

• cooking conversion factors.

Measuring foods

You can measure food:

• by weight—this is most accurate if you have reliable scales;

• by volume using cans, cups, handfuls, etc.;

• Fig. A4–1 Weighing is the most accurate way to measure a quantity of food.

• by items—for example, numbers of eggs, piles of tomatoes.

To use food composition tables you need to know the weight of food in grams. If you cannot weigh the food, you can estimate the weight by converting volume or item measures into grams using conversion tables like Table A4.4.

‘As purchased weight’, ‘edible weight’, and ‘per cent edible portion’

These terms are used in food composition tables.

‘As purchased’ weight of food

This is the weight of a food as we buy or harvest it—for (p.434)

Fig. A4–2 ‘As purchased’ food = ‘edible’ food + waste.

example, the weight of a bunch of unpeeled bananas, a whole fish, or unshelled groundnuts.

The ‘as purchased weight’ includes the weight of ‘waste’. Waste is the part of the food we do not eat such as skin, peel, or bones. ‘As purchased’ may be shortened to ‘AP’. In Fig. A4–2 the ‘as purchased weight’ of a banana is 100g.

‘Edible’ weight of food

This is the weight of food with waste removed—in other words, the weight of food which can all be eaten. For example, the weight of peeled bananas, fish flesh, or shelled groundnuts. In Fig. A4–2 the edible weight of banana is 63g.

The ‘weight of waste’ is the weight of the waste in the ‘as purchased’ weight. In Fig. A4–2 the weight of waste is 37g.

The edible portion means the part of the food which is edible.

Most food composition tables give the amounts of nutrients in 100g of the raw edible food—for example, in 100g of shelled groundnuts or in 100g peeled bananas. So to use food composition tables you must know the ‘edible weights’ of the foods.

‘Per cent edible portion’

This means the percentage of the ‘as purchased’ food which is edible. The per cent edible portion is the edible weight of food in 100g of ‘as purchased’ food. Look at Fig. A4–2. What is the weight of edible banana that we get from 100g ‘as purchased’ banana? The answer is 63g. We say the ‘per cent edible portion’ is 63 per cent.

‘Per cent edible portion’ can be shortened to ‘%EP’.

If you know the ‘as purchased weight’ and the %EP you can work out the edible weight of any food.

For example you buy 200g of unpeeled bananas. What is the edible weight of these bananas? The answer is

The formula we use to find out the edible weight from the ‘as purchased weight’ and the %EP is

Here is another example. If you are teaching, you might want to give this to your trainees.

Example

Sofie buys 1kg of sweet potatoes. After peeling them, what is the edible weight of potatoes? The food composition tables in Appendix 1 tell you that the %EP for sweet potato is 79.

So, using Formula 1,

(p.435)

For some foods, for example flour and milk, there is no waste. All the food is edible. The ‘as purchased weight’ equals the ‘edible weight’. The ‘per cent edible portion’ is 100 per cent.

Most food composition tables give values for %EP. Some give per cent waste

%EP = 100%−% waste,

e.g. Fig. A4–2 shows that:

% waste for bananas = 37%

So %EP for bananas =

100%−37% = 63%

However, %EPs can vary a lot depending on the quality of the food and how it is prepared. So it is better to work out local values if possible. Box A4.1 shows you how to do this. It is even more accurate to weigh the edible part of the food each time you want to know the nutrient content.

Water content

The water content of a food may vary. For example, dried beans contain much less water than fresh or cooked beans. It is important to understand how the amount of water in food affects the proportion of the other nutrients. The more water in the food, the lower the proportion of other nutrients. This is because the water dilutes the other nutrients.

Figure A4–3 explains this using fish as an example. The figure shows a small fresh fish which weighs 100g. The fish contains about 12g protein and 75g water as well as other nutrients and waste. The per cent protein in this fresh fish is 12 per cent.

Then the fish is dried. This removes 60g of water so the fish weighs only 40g. But it still contains 12g protein. Let us calculate the per cent of protein in the fish now.

40g fish contains 12g protein. So:

So the per cent protein in dried fish is 30 per cent.

Cooking conversion factors

Most food composition tables give the nutrient content for only a few cooked foods such as bread. This is because cooking methods, and particularly the amount of water used, vary so much. So to calculate the nutrient content of cooked food you may need to convert a cooked weight of food into its raw weight.

For example, to know the nutrient content of maize porridge you need to know how much maize flour it contains.

Sometimes you want to know how much cooked food (p.436)

Fig. A4–3 How water content alters the proportion of other nutrients.

you get from a certain weight of raw food. For example, how much porridge you can make from a given weight of maize flour.

To do these calculations you need to know the cooking conversion factors for the particular food.

There are two cooking conversion factors for each food.

1. 1. ‘Cooked to raw’ factor. This is the weight of raw food in 1g of cooked food, e.g.

• 1g thick porridge contains 0.29g maize flour.

The ‘cooked to raw’ factor is used to convert a given weight (or portion) of cooked food into its equivalent weight of raw food.

2. 2. ‘Raw to cooked’ factor. This is the weight of cooked food which contains 1g of raw food, e.g.

• 1g flour gives 3.5g porridge.

The ‘raw to cooked’ factor is used to convert a given weight (or portion) of raw food into its equivalent weight of cooked food.

It is best to calculate local conversion factors (see Box A4.2) because they vary with local cooking methods. If this is not possible you can use the ones in Table A4.5 at the end of this appendix.

(p.437) A4.3 COMPARING THE NUTRIENT CONTENTOF DIFFERENT FOODS

If you want to compare the nutrient values of different foods and find out which are good sources of particular nutrients:

1. 1. List the foods you want to investigate.

2. 2. Find each food in the food composition tables.

3. 3. Find the column of the particular nutrient you are interested in and write down the amount of the nutrient in 100g of the food.

4. 4. See which food gives the largest amount of the nutrient.

Example

You want to find out which local foods are good sources of fat. So:

1. 1. List local foods likely to contain fat.

2. 2. Find each food in the food composition tables.

3. 3. Look under the fat column and write down the amount of fat in 100g of each food.

g fat in 100g edible food

Groundnuts, dried

45.0

Milk, fresh

3.7

Margarine

83.0

Maize flour, 95% extraction

4.5

4. 4. Compare the fat contents. The richest source is margarine.

You can find out which foods are good sources of energy, protein, minerals, and vitamins in the same way. You can use this information to find out which foods give best value for money (see Chapter 9).

A4.4 CALCULATING THE NUTRIENT CONTENT OF FOODS AND MEALS

This section explains how to find out the nutrient content of different weights (or portions) of foods and meals.

Nutrients in the edible weight of raw foods

This is how you calculate the amount of energy, protein, or other nutrients in a given weight (or portion) of raw edible food. For example, calculate the amount of protein in 50g dried beans.

1. 1. Find the food in the food composition tables and then the value for the particular nutrient. This tells you the amount of the nutrient in 100g of the edible food, e.g.

• 100g edible dried beans contains 22g protein.

2. 2. Calculate the amount of the nutrient in 1g of the food, e.g.

3. 3. Calculate the amount of the nutrient in the given weight or portion of the food, e.g.

So 50g dried beans contains 11g protein.

When you understand this calculation it is quicker to use the following formula:

(Formula 5)

So, using Formula 5,

Nutrients in ‘as purchased’ raw foods

To calculate the nutrients in ‘as purchased’ foods:

1. 1. Convert the ‘as purchased weight’ to the ‘edible weight’ using Formula 1.

2. 2. Then calculate the amount of the nutrient in the edible weight using Formula 5.

Example

Find the Calories in 250g of unpeeled bananas

1. 1. Convert the ‘as purchased weight’ to the ‘edible weight’.

‘As purchased weight’ = 250g.

%EP for bananas is 63% (from food composition tables)

So, using Formula 1;

2. (p.438)
3. 2. Calculate the Calories in 158g of peeled banana.

100g edible portion of banana contains 82kcal (from food composition tables).

So, using Formula 5,

So 250g unpeeled banana contains 130kcal.

If you are calculating the amounts of energy and nutrients in several foods, you can write the values in a table like Table A4.1. Table A4.1 also shows how you can add together the nutrient values for several ingredients to find out the nutrient content of simple meals and snacks (e.g. bread and margarine).

Nutrients in cooked foods

When calculating the nutrient value of cooked foods, you may need to adjust values for some micronutrients, especially vitamin C and folate, to allow for losses during cooking (see Section 6.2).

Nutrient content of single cooked foods

To find out the nutrient content of a single cooked food (e.g. maize porridge, boiled rice):

1. 1. Weigh the cooked food.

2. 2. Find out the weight of raw food in this weight of cooked food. To do this:

• Weigh the food before cooking. This is the most reliable method but it is not always possible to do this.

or

• Use the ‘cooked to raw’ cooking conversion factor (explained above).

3. 3. Calculate the nutrient content of this weight of raw food using Formula 5.

Example

To calculate the Calorie content of thick maize porridge

1. 1. Weigh the cooked porridge.

Weight 700g.

2. 2. Find out the weight of maize flour in the porridge. You can do this by:

• Weighing the maize flour before cooking.

200g flour was used.

or

• Multiplying the cooked weight by the ‘cooked to raw’ cooking conversion factor in Box A4.2.

‘Cooked to raw’ factor = 0.29.

So

700g porridge × 0.29 = 203g flour (or about 200g).

3. 3. Calculate the Calorie content of 200g flour using Formula 5.

So the Calorie content of 700g thick porridge is 690kcal.

The nutrient content of cooked foods with several ingredients

To calculate the nutrient content of a meal or recipe which contains several ingredients:

1. 1. Weigh all the raw ingredients;

2. 2. Calculate the nutrients in each—see example in Table A4.2.

3. 3. Add up the quantities of Calories and each nutrient. This gives the nutrient content of the whole meal or recipe.

Table A4.1. Nutrient content of different foods

Food

Edible weight (g)

Energy (kcal)

Protein (g)

Iron (mg)

Vitamin A (RE)

Bananas

158

130

2

2

31

Beans

50

160

11

4

0

Snack:

210

504

16

4

0

Margarine

15

112

0

0

0

Total snack

225

616

16

4

0

(p.439) You may want to know the nutrient content of a portion of the meal, for example, a child’s portion. To do this:

1. 1. Weigh the total quantity of the cooked meal before serving. In our example, the cooked maize, beans, and onions weighed 3000g.

2. 2. Weigh the amount given to the child, e.g. 300g.

3. 3. Calculate the percentage of the total meal which was given to the child, e.g.

4. 4. Use this percentage to calculate the amounts of nutrients in the child’s portion, e.g.

If you want to know the nutrient content of the food the child actually ate, you must subtract the weight of food not eaten (this is called ‘plate waste’).

The nutrient content of a person’s or a family’s daily diet

To do this:

1. 1. Weigh (or estimate the weight) of all the food eaten by a person’s or a family’s diet over a particular period (usually 24 hours or longer)—see below.

2. 2. Calculate the nutrient content of each food.

3. 3. Add together the values for Calories, for protein, and for other nutrients.

There are several methods for estimating the amounts of foods people eat. These include:

• weighing or estimating the weight of all the foods eaten;

• asking people to record the amounts of all the food they eat;

• recording the amounts of food in a home at the beginning of the weighing period, all the food which is bought, and the food left at the end of the period.

You may need to:

• convert volume measures or numbers of items to weights;

• convert ‘as purchased weights’ to ‘edible weights’

• convert cooked weights to raw weights;

• measure plate waste.

In practice all the methods for finding out the weights of foods eaten are very time-consuming and difficult. People may change their eating habits if someone is watching what they eat, some family members may eat away from home, and it is hard to record everything eaten during 24 hours. Most nutrition workers will not need to do complicated dietary surveys like these. If you do, the references at the end of the chapter will help you.

A4.5 COMPARING THE NUTRIENT CONTENT OF MEALS OR DIETS TO NUTRIENT NEEDS

You may want to know if a group of people are eating enough to cover their needs—for example, if children are eating enough vitamin A. Or you may be asked to say what proportion of daily needs are covered by a particular meal or diet—for example, what proportion of energy needs are covered by a school meal or whether the rations provided to refugees cover their vitamin needs.

Table A4.2. Nutrient content of a simple meal

Food

Weight (g)

Energy (kcal)

Protein (g)

Fat (g)

Vitamin A (RE)

Wholegrain maize, raw

300

1035

28

13

0

Cowpeas, raw

400

1280

92

6

Onions, raw

100

38

1

0

Total in raw ingredients

2353

121

19

Total cooked weight

3000

Child’s portion

300

235

12

2

(p.440) To answer these questions you need to:

1. 1. Calculate the nutrient content of the meal or diet eaten by an average individual in the group.

2. 2. Find out the average individual nutrient needs for the group (see Appendix 2).

3. 3. Compare the nutrients in the food to nutrient needs.

Example

A pre-primary school gives a mug (300g) of soft sweet porridge as an early morning snack to 150 children aged 3–5 years. The head teacher wants to know what proportion of the children’s daily energy needs this provides. To find out:

1. 1. Calculate the energy in one mug of the soft porridge.

1. a. Find out the weight of raw maize flour and sugar in each mug of porridge.

The quantities used to make 150 mugs of porridge are 9kg maize flour and 1.5kg sugar

So the amounts of maize flour and sugar per child are:

2. b. Calculate the energy in 60g flour and 10g sugar using food composition tables.

100g flour gives 345kcal.

100g sugar gives 400kcal.

The total energy in 1 mug of porridge = 247kcal.

2. 2. Find out the average individual energy needs of pre-primary school children (ages 3–5 years) from Appendix 2.

Energy needs of children = 1510kcal/day.

3. 3. Calculate the percentage of energy needs supplied by a mug of porridge.

So a mug of soft porridge supplies 16 per cent of the children’s daily energy needs.

The formula for calculating the proportion of nutrient needs supplied by a meal or diet is:

(Formula 6)

You can write the results of these calculations on a table like Table A4.3, especially if you are examining more than one nutrient.

A4.6 Planning Meals and Diets

You may need to plan a diet for a particular group of people (e.g. children at boarding school, hospital patients) or a meal containing a certain quantity of Calories, protein, and other nutrients (e.g. a school lunch). To do this prepare a blank table like Table A4.3 and then:

1. 1. Write the amounts of each nutrient which should be supplied to each individual by the diet or meal (see Appendix 2).

Table A4.3. Percentage of daily energy needs supplied by porridge

Food

Weight (g)

Energy (kcal)

Protein (g)

Iron (mg)

For 150 children

Maize flour

9000

Sugar

1500

For 1 child

Maize flour

60

207

6.0

1.5

Sugar

10

40

0

0

Total

247

6.0

1.5

Average individual needs

1510

26

14

% of needs supplied by porridge

16

23

11

(p.441)

Table A4.4. Equivalent weights and volumes of foods. The volume of foods can vary according to how tightly it is packed, the shape of the container, etc. So check these values using local foods and measures.

Weight of food(g)

Volume (ml) of 100g of food

Food

In 250 ml

In 100 ml

Cereal flours

150

59

169

Stiff porridge

100

100

100

Soft porridge

100

100

100

Rice, raw

210

85

118

boiled

165

66

152

Cassava flour

130

53

189

Potato, raw diced

160

63

158

cooked diced

180

71

140

Sweet potato, raw diced

140

56

178

cooked mashed

265

107

94

Beans/Peas, raw

200

80

125

cooked

180

72

139

Groundnuts, shelled

whole raw

160

64

157

flour

120

48

210

paste

265

106

95

Cabbage, raw shredded

75

30

338

Green leaves, raw chopped

75

31

329

cooked

140

55

183

Onions, raw chopped

170

68

148

Tomato, sliced

190

77

131

paste

275

111

91

Banana, mashed

235

95

106

Mango, chopped

170

69

146

Orange sections

190

75

134

Pawpaw, chopped

150

59

169

mashed

250

100

100

Sugar

205

83

121

Meat, ground/minced

240

95

105

Fish, flaked

255

102

99

Milk, fresh

250

100

100

dried skimmed

105

43

236

dried whole

140

55

183

evaporated

265

106

95

condensed sweetened

350

139

72

Margarine

235

95

106

Oil

220

88

114

From FAO (in press). Calculated from values in Home Economics Research Report, no. 41, Agricultural Research Service, US Department of Agriculture (1969) and unpublished estimations.

(p.442) To use Table A4.4 you may need to know the volume of local containers. To find out:

• Fill container to the top with water and carefully measure the volume of water with a measuring container from a laboratory, or

• Weigh the container empty and full. The weight of water in grams equals its volume in millilitres, or

• Use the following volumes.

Volumes of some local measures (to top of container)

Check these volumes if possible

• 1 teacup or 1 glass holds about 200 ml.

• 1 eating spoon holds about 10 ml.

• 1 teaspoon holds about 5 ml.

• 1 Coke bottle (labelled 193 ml) holds about 200 ml.

• 1 beer bottle (labelled 440 ml) holds about 500 ml.

• 1 can for margarine/cooking fat (labelled 250g) holds about 250 ml.

1. 2. List the foods you want to use—take into account availability, cost, acceptability, and resources needed for preparation.

2. 3. Adjust the amounts of the main foods until together they supply enough Calories and the meal is not too bulky. These amounts usually supply enough protein.

3. 4. Add other foods as necessary to supply enough micronutrients.

Example

A project manager plans to give a meal of thick maize porridge and beans to young male labourers working on a tree-planting project. He wants the meal to provide 40 per cent of their daily energy needs.

1. 1. Average daily individual energy needs (from Appendix 2) are about 3000kcal. So 40 per cent of needs is 1200kcal.

2. 2. The ingredients of the meal are wholemeal maize flour and dry beans.

3. 3. The amounts of maize and beans to supply 1200kcal are:

• 260g maize gives  897kcal.

• 100g beans gives  320kcal.

• So the meal gives  1217kcal.

Table A4.5. Estimates for cooking conversion factors.

(Check values using local foods and cooking methods)

Raw food/cooked food

Conversion factor

‘Cooked to raw’(g raw in 1g cooked)

‘Raw to cooked’(g cooked from 1g raw)

Maize flour/thick porridge

0.29

3.5

Maize flour/thin porridge

0.14

7.0

Dry rice/boiled rice

0.4

2.5

0.77

1.3

Wheat flour/chapatti

0.63

1.6

Raw potato/boiled potato

1.0

1.0

Dry beans/boiled beans

0.4

2.5

Dry lentils/boiled lentils

0.33

3.0

Dry groundnuts/Boiled groundnuts

0.55

1.8

Raw green leaves/boiled drained leaves

1.67

0.6

Raw cabbage/boiled drained cabbage

1.25

0.8

(p.443) REFERENCES

Cameron, M. E. and Hofvander, Y. (1983). Manual on feeding infants and young children. Oxford University Press, Delhi.

Cameron, M. E. and van Staveren, W. A. (1988). Manual on methodology for food consumption studies. Oxford University Press, Oxford.

FAO (in press). Food and nutrition in management of group feeding programmes, Food and Nutrition Paper, no. 23. FAO, Rome.

Fig. A4–4 ‘You can calculate the nutrient values of local foods.’