Further Reading
4.3 Collection of Diet Samples
seasons. The samples representing the grazing season will be collected in September when the animals are still pasturing, and representing indoor feeding in February.
Whole milk, standardized approximately to 4% fat content and packaged into 0.5 L containers ready for consumers should be collected. Five packages are to be taken over a one week period and repeated every second week for a month totaling to 10 samples.
The samples must be frozen immediately after collection. The packaged milk should be thawed at room temperature, then heated up to 40 °C and shaken to homogenize the fat. Aliquots of 50 mL are then transferred from each 0.5 L container into a 750 mL acid-washed polyethylene bottle to make one pooled 500 mL sample representing each dairy. The 24 units of 500 mL samples should be collected and then these will be divided into two 12-sample groups and the samples are pooled to make two final sam- ples of 6 L each. These samples are then freeze dried, after which samples of approx- imately 750 g dry weight are obtained. Each will be used for trace element analysis.
The winter samples will be prepared similarly, resulting in a total of 4 final samples.
4.2.10 Fresh Eggs
If the egg producers feed the birds differently in the summer than in the winter, then seasonal variation should be taken into consideration in the sampling. Otherwise, the samples can be collected during any season.
Samples should be collected from large wholesale companies. It is recommended that for every million kg of eggs produced, 5 egg producers should be chosen among the largest producers. Thus, if the packaged amount of eggs in that region or country is 6 million kg per year, then 30 individual producers’ samples should be collected. The suitable size of the “producer sample” is 12 eggs (one dozen). Label the egg samples with tags describing sampling date, wholesale company, farmer and farm location. In the laboratory, pool the eggs in such a way that one egg per carton of 12 eggs is removed and pooled, according to region, into an acid-washed glass or plastic container.
The structure of the eggs is destroyed and mixed to avoid excessive foaming.
Excessive mixing with stainless steel, egg beaters or whisks must be avoided. Place a 100 g sample each into frozen-food containers and freeze dry. Pool freeze dried samples to compose at least 4 final samples of 0.5 kg dry weight representing major production regions.
4.2.11 Other Staple Foods
Besides staple foods mentioned above, granulated potato and other staple foods impor- tant for a given country or region should also be collected for trace element analysis.
Therefore, it is important that the diets, which are to be analyzed, are representa- tive of the country or region in question. To obtain nationally representative food consumption data, it is required that the sample amount should be sufficiently large and be selected by taking into consideration such variables as age, sex, race, income, region, urbanization and season. For example, in the United States, a recent national food consumption survey sampled over 20 000 persons for 3 days using 24 h recalls and food diaries. Although a smaller number of persons might have been adequate, the U.S. Food and Drug Administration pointed out that the totals should be a sub- stantial segment of the total population.
4.3.1 Collection and Preparation of Foods for Composition of Representative Mixed Total Daily Diets, Market Basket Method
It is obvious that such large-scale food consumption survey is not an easy task.
However, national food consumption or disappearance data are often available, which can be used to define the quantities of commonly consumed foods. It is there- fore suggested that this approach be accepted as the principal method of preparing the diets to be analyzed for trace elements.
If data are available, diet composition should be based on the food consumption of the healthy adult population aged 25–40 years of each country or region. If these data are not available, adult population with a wider age range may be taken as a basis. However, in some cases the national food consumption data may not be very detailed and/or up to date. In such occasions or as an alternative to the market bas- ket method, “duplicate portion method” can be employed.
If results of a recent national food consumption survey are available, they should be utilized, otherwise data on national food availability can be estimated from the OECD food consumption statistics,4 which provides per capita consumption (in gram per day) of some 50 food commodities for most of the countries. The latest available statistics should be used.
Ideally, all foods whose daily consumption is 0.1 g or more should be included in the food list. In Western countries this would result in approximately 230 food com- modities.3Household disappearance studies have shown that out of a total of 120 food items most commonly consumed, only 22 types of foodstuff were consumed in excess of 10 kg per year.5As representative purchasing and handling of 230 food items would be rather tedious, it is suggested that the following guidelines should be followed:
(a) All foods consumed in amounts of 1.0 g day⫺1or higher should be included.
(b) If a food item or ingredient is consumed less than 1.0 g day⫺1, it should be included only if it contains any of the 14 essential trace elements in concen- tration levels higher than 100 times that of a mean concentration level of the same elements in foods consumed in amounts of 100 g day⫺1or more.
Food items which certainly would fall in the category (b) are many condiments, con- fectioneries, instant coffee and tea. A recent compilation of data on mineral element
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concentration levels in 456 food commodities is available in order to define which food items should be included on the basis of category (b).6
In this connection, it should be mentioned that OECD Food Consumption Statistics4do not include food items such as coffee, tea and condiments, which are very rich in many trace elements. Moreover, only total refined sugar consumption is indicated, although much of it is consumed under the category of confectioneries, many of which again are very high in some of the trace elements. Therefore, it is pro- posed that the above-mentioned items be included in food lists to be developed.
The next step in the preparation of a food list is to group the foods according to major food types to be studied. Individual foods within these types should be ranked according to a decreasing order of daily consumption. It is recommended that foods be grouped into the following categories:
1. Cereals and bakery products
2. Dairy products (excluding butter and margarine) 3. Meat, poultry, eggs and meat products
4. Fish, crustaceans, mollusks and fish products 5. Potatoes and root vegetables
6. Leafy and legume vegetables, nuts and mushrooms 7. Fruits and berries
8. Non-alcoholic and alcoholic beverages, confectioneries, sugar and condiments 9. Food fats (butter, margarine, vegetable oils, lard).
The next step after grouping the foods according to major types would be to group food items under different categories,e.g.peaches as, fresh, frozen, canned, jammed and dried.
Unfortunately, as this type of aggregation is not largely applicable to national food disappearance data, the person responsible for the preparation of food lists should approximate the consumption of the food packed or processed differently.
The next step should be to aggregate foods that are similar in type but are con- sumed in small amounts,e.g.confectioneries. If the food consumption data are not very precise, the purchaser may decide which confectioneries ate to be included in the aggregate.
The shopping guide is developed from the food list. Special care must be exer- cised in the following points:
1. Purchasing foods of seasonal nature. Therefore, shopping should be done twice both in summer and winter
2. Similar foods should be purchased in such a way that they represent the most important manufacturers and brands
3. Packaging (fresh, canned, cartons)
4. Type of food transport and storage prior to kitchen preparation
5. The amount of food item to be purchased must be large enough to be repre- sentative and to allow losses during kitchen preparation.
Examples of shopping guides are available in Annex V a and V b in the FAO/WHO Guidelines.1
4.3.2 Collection of Food Samples
The purpose of the study should be defined very clearly. For example, if market bas- ket diet study is applied to a whole country, then at least 4 cities representing differ- ent areas of the country should to be selected for purchasing of foods. In each city, all the foods are to be collected according to the same shopping guide. The first col- lection should take place in late summer or early fall, thus representing the summer season, and the second collection preferably in late winter to represent the winter season. During each collection (4 cities⫻2⫽8) enough foods should be purchased to represent altogether 10 total daily diets in terms of amount of food, representing 80 total daily diets.
4.3.3 Duplicate Portion Technique
Duplicate portion technique is another way to collect the total diet samples.7The most convenient means to do this would be to collect 3–7 days duplicate diets from different population groups. One can select sampling sites, considering parameters such as socio-economical, cultural, regional variations and different age population groups. In all the cases, the subjects are not supposed to alter their eating habits dur- ing the collection period.
For a general survey of a country or a region, duplicate diet from major central hospitals may be most representative. Hospital food does not greatly differ from typ- ical local food consumed in most countries and is thus supposed to be rather close to the national average.8 Moreover, central hospitals have employed professional dietitians, therefore the sample and data collection will be done properly. Finally, from the financial point of view this method is attractive, as the hospitals may be willing to donate the diet samples provided that the research results are made avail- able to the hospital authorities.
The easiest way to collect the duplicate diet will be to give each subject a 5-L and a 1-L pre-cleaned polyethylene container for food collection. Samples will be col- lected for three, preferably seven consecutive days from every subject. During meal time, while the serving is done, the amount of food equal to the amount consumed by the person will be placed on a separate plate. Then the duplicated food on the plate will be transferred to the 5-L polyethylene container with the utensils used by the subjects. The subjects should take the 1-L container to their work or wherever they go during the collection period. Everything the subjects had consumed during this period will be duplicated and collected. This will include all the meals and drinks. The inedible parts, skin of fruits, seeds, bones will not be collected, as these parts are not eaten by the subjects. Containers immediately should be transferred into a deep freezer.
4.3.4 Homogenization and Freeze Drying
Homogenization is one of the most important steps for the trace element analysis of the total diet samples. Since the collected samples consist of the various food items such as fats and vegetables, they must be homogenized with great care before
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analysis. The samples should be homogenized with a homogenizer having Ti blades in order to prevent contamination. The homogenized samples are then freeze dried and powdered in an all Teflon cylindrical mill. Samples should be weighed before and after homogenization and freeze drying in order to find their water content.
Now the samples are ready for further handling as described in the following sections.