Sampling of Major Staple Foods

This is one of the most important steps in trace element analysis. The quality of the results depends on the care taken in sampling. This subject is thoroughly discussed by Crosby and Patel.¹It is of utmost importance to design and facilitate sample collection protocol to ensure that the final pooled samples truly represent the entire regional or national production of the selected foods or consumption of daily diets.

For the sampling of a certain staple food, one has to take into account the main production areas and most important varieties for human consumption. Wholesale and retail marketing channels must be thoroughly known. In terms of animal products, variables such as animal breed and type(s) of fodder used should be addition- ally taken into account. Furthermore, seasonal variation definitely affects the origin of vegetables and fruits and has to be taken into account when purchasing foods from retail stores for representative total daily diets. Finally, the size and population den- sity of the country in question set certain requirements to the sample collection in order to obtain representative samplers.

The following methods for collection and preparation of staple foods for chemical constituent determinations are presented using partly some of the recommenda- tions mentioned in the Joint FAO/WHO Guidelines document.²

4.2.1 Wheat

The best way of collecting representative whole wheat samples is to make sampling arrangements with state granary stores and commercial mills. The samples obtained through these sources should represent the most important wheat-growing areas in a country, or a region. The samples should be taken from the “silo-samples” of each granary store. A silo-sample is a sample representing the contents of one silo and should be collected continuously during binning. One silo-sample should be about 3–5 kg in size.

Again it is very important to confirm that the grain to be sampled is all of domestic origin and intended for human consumption. Spring and winter wheat should be sampled separately. Thus, assuming that 15 mills or granaries are available for sampling,

the following sampling system is recommended: two spring and two winter wheat silo-samples of 3–5 kg each will be collected from each of the 15 mills or granaries resulting in a total of (2⫹2)⫻15⫽60 samples and weighing 200 kg in total.

A sub-sample of 100 g will be taken from each silo-sample and combined to represent 2 winter wheat and 2 spring wheat samples for each mill (60⫻100 g⫽6000 g). A final sub-sample of 100 will be taken to represent each mill, totaling 15⫻100

⫽ 1500 g. Silo-samples with obvious soil contamination will be discarded and replaced by new samples. These final samples (1–5 kg) will be carefully mixed in a glass or plastic-mixing chamber and approximately 500 g of each sample should be used for analysis.

4.2.2 Wheat Flour

The type of wheat flour, which is called baker’s flour, should be collected; this prod- uct should contain a minimum 0.5% of residue after ashing. This figure is a meas- ure of mineral content of flour. Samples should be collected from the leading commercial milling companies.

The following sampling example is for 5 mills: arrange 1 kg flour samples to be taken weekly during 8 successive weeks from all of the 5 mills. This will result in 40 units of 1 kg samples. Divide the samples randomly into four 10 sample sets and pool 100 g sub-samples within each set to make a 1 kg final sample. Mix these samples carefully and take 500 g sample for analysis.

4.2.3 Bread

In terms of ash content, the most commonly consumed type of white bread should be collected. It is suggested that regionally representative samples should be the tar- get of sample collection. It is very important that an adequate number of the major bakeries are represented in the final samples of each region.

Similarly, the type of whole wheat bread that is consumed the most is to be collected. Whole rye bread should be collected in those countries where it has a signif- icant role in the average bread consumption of the population. In some countries, mixed wheat and rye bread may be important cereal products, and consequently, the major types of such breads should be collected. It is recommended that five loafs totaling at least 2 kg should be collected and placed in clean polyethylene bags from each of the bakeries.

In the laboratory, combine the bread samples in such a way that half of each loaf is to be sliced for a pooled sample. Allow the slices of bread collected to dry over a period of time at room temperature, protected from dust under a loosely fitted polyethylene wrapping. When the slices are about half dry, they will be easier to homogenize in the food processor compared to a completely dry sample that is too hard for effective grind- ing. If the slices are fresh, on the other hand, it becomes too difficult to grind them because they will become mushy, which is difficult to work with. It is advisable to experiment first to determine the optimal sample moisture for homogenization. The samples should be sliced with a stainless-steel knife, however, the final homogenization in a food processor having Ti blades. After homogenization, the samples are either

freeze dried or oven dried in a cleaned and cellulose-lined oven at 60 °C. The final amount of sample material for analysis should be about 500 g dry weight per sample;

at least four samples per region should be collected from several regions of the country.

4.2.4 Pasta

Collect all types of pasta samples from large wholesalers. Collect the best-selling pastas so that one sample represents each pasta’s 10% unit of its total turnover. For example, if Company X sells 60% elbow, 30% spaghetti and 10% lasagna, then the amounts of samples to be collected are: 6 kg elbows, 3 kg spaghetti and 1 kg lasagna.

Pool pasta samples in the laboratory are collected from different companies in a representative way so that the various types of pasta are pooled together to form regionally representative samples. Mix thoroughly these samples and homogenize a 4 kg mix sample using a food processor equipped with Ti blades. At least 4 pasta samples each weighing 0.5 kg dry weight should be taken for the final analysis.

In addition, the corresponding pasta flour samples should be collected. Pack pasta flours into polyethylene bags, then re-pack into a larger polyethylene bag to avoid contamination. The sample sizes are: consumer packages for macaroni and 1 kg sample for macaroni flours. Label the samples with tags containing sampling dates and, in terms of flours, the proportion of imported wheat, if known, and major production areas and crop year. The flour samples should be pooled to form regionally representative samples. Ti-blade food processors or acid-washed glass cylinders may be used for mixing the flour samples. Again, at least 4 flour samples each weighing 0.5 kg should be taken for final analysis.

4.2.5 Rice

Both white and converted rice samples should be collected. It is important to carefully determine the origin of the rice. Some European countries such as France, Italy and Spain have domestic rice production. The main emphasis is on the collection of domestic rice samples, if available, but for purposes of comparison it is advisable to collect imported rice from the major rice-producing countries. To guarantee adequate representativeness of samples, the best method is to collect samples from major wholesalers. Retail-sized packages should be collected over a period of time.

Pool the samples in the laboratory and carefully mix them in large, acid-washed polyethylene, polypropylene or glass cylinders. Use the pooling procedure suggested for wheat earlier in the text. The total amount of final samples should reflect the ratio of domestic to imported production. The final number of samples should be at least 4, representing the most important production area, or 4 imported samples if there is no domestic production. The final samples should be placed in polyethylene bags, with at least 0.5 kg per sample.

4.2.6 Potato

The most convenient way of collecting representative potato samples would be to make a sampling agreement with large potato wholesale companies or potato-processing

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factories, which already have long-term agreements with the potato farmers to purchase their products. It is very important to confirm that the potato samples collected are varieties used and grown for human consumption only. Potatoes grown for the chemical industry or for fodder purposes are not acceptable.

A sampling system could easily be arranged so that the 2–3 kg samples taken would be a good representative of growing areas. Each final sample to be analyzed for trace elements should weigh a total of 50 kg (20⫻(2–3) kg sub-samples). After washing to remove soil, the two ends of each potato are removed and the potato flesh is sampled with a titanium corer (20 mm diameter). This would result in a sample of 2⫻5 cm and weighing 20 g. Each final sample would thus weigh approximately 12 kg fresh weight and provide about 1.5–2.0 kg dry material. Preferably, freeze drying should be used. If freeze drying is not possible, use oven drying overnight at 80 °C.

The individual 1.5–2.0 kg samples should then be ground in approximately 100 g portions each in a blender equipped with Ti blades. The ground portions will be combined and carefully mixed in a plastic or glass containers. Approximately 500 g of each final sample should be used for trace elements determination.

4.2.7 Meat

Tenderloin and pork shoulder are the best parts for the sample collection because they are not very expensive cuts, and thus will be easier to obtain at a reasonably low cost. In terms of broilers, the entire carcass will be collected. It is preferable that the samples be collected both in winter and in summer so as to reflect different feeding regimes.

Beef and pork samples should be collected from large slaughterhouses in such a way that each slaughterhouse collects the samples over a long period of time to guarantee adequate material representation of the samples,e.g.30 large beef farms are represented by a slaughterhouse. Two 0.5 kg beef samples should be collected per beef farm in each slaughterhouse. The individual samples must represent different carcasses. Stainless-steel knives can be used for cutting the meat samples, as well as for the other meat samples.

Broilers should also be collected from large slaughterhouses similarly as for beef and pork samples, except that whole bird carcasses will be collected. For example, collect 40 broilers per slaughterhouse so that the material represents as many large broiler farms as possible.

Pack the samples immediately into polyethylene bags, coded with tags and then freeze. The tags should contain sampling dates and, if possible, the approximate locations of the meat producers (farms) to allow the exploration of the regional representation of the samples. The samples should be transported to the laboratory in a frozen state. They should be thawed and cut into 1 cm³pieces, transferred into frozen food containers and freeze dried. Only the edible parts will be utilized; discard skin or bones as well as subcutaneous fat. Poultry shears or stainless-steel knives may be used for cutting the meat samples.

For all sorts of meat samples, only a food processor equipped with Ti blades can be employed for the final sample homogenization. At least four final samples per one major production area are to be prepared for analysis.

4.2.8 Vegetables and Fruits

Collect vegetables during different seasons,i.e.in spring, summer and autumn so as to represent different growing conditions. If there are imported vegetables in the market, they should be collected separately and the country of origin of the produce should be established. Imported samples are usually available only during the winter season.

Taking cabbage as an example for collection of vegetables, cabbage samples should be obtained from a wholesale lot consisting of 10 heads; then 10 such wholesale containers should be randomly selected for sampling and one cabbage from each container is sampled. Then, the cabbages sampled as described above should be transferred into an empty container. The above principle should be used for all vegetables and fruits to ensure the necessary material representation of the samples. In terms of domestic and imported vegetables and fruits, the most commonly consumed varieties must be represented in the collection scheme according to their market share. The samples should be aggregated within varieties so that the final samples to be composed represent regional or national production or importation. Prepare four final samples per variety for analysis.

Here are some examples for sampling of carrot, cabbage, tomato, lettuce and apple. One could apply similar collection scheme for other vegetables and fruits.

Pre-wash carrot samples carefully using a scrub brush. Wash them in some place other than in the actual laboratory space where the final preparation of the samples will be performed. This is a very important precondition in order to avoid soil contamination that will totally destroy the possibility of obtaining meaningful results. Then rinse carrots with distilled water before peeling them. On a colourless polyethylene or Teflon cutting board, peel and cut them into small cubes, about 1 cm³, using acid-washed knives. Mix the cubes and pack into acid-washed frozen-food containers, then freeze.

Remove the outermost injured leaves of cabbages and then rinse with distilled water. Cut cabbages into small pieces, mix and pack into frozen-food containers and then freeze.

Rinse tomatoes with distilled water and cut into quarters for the final sample.

Transfer the sample into frozen-food containers and then freeze.

Remove non-edible parts of lettuce with a knife. Cut samples into small pieces with a knife and transfer into frozen-food containers and then freeze.

Freeze dry all the frozen vegetables, followed by homogenization using a food processor equipped with Ti blades.

Peel apples with acid-washed stainless-steel knives then cut them into 1 cm³ pieces and transfer into frozen-food containers, freeze and freeze dry. Remove cores as well as stems.

4.2.9 Milk

The most practical way of collecting representative milk samples is to plan a sampling system with the leading dairy companies representing the most important milk production areas. As the trace element composition of milk is likely to be affected by the type of fodder the animals are fed, the seasonal variation may be important.

Therefore, samples should be collected during both the grazing and indoor feeding

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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 samples of 6 L each. These samples are then freeze dried, after which samples of approximately 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 important for a given country or region should also be collected for trace element analysis.

Dalam dokumen Trace Element Analysis of Food and Diet (Halaman 72-77)