SAMPLING

The sampling procedure depends on the aim of the study, e.g. should the sample be representative for the whole country or only for a specific area or project or should the sample cover different seasons or be collected during one growing season. The following points highlight the important aspects of the sampling procedure.

• Where is the food consumed and by how many?
• Are market statistics available? This provides information on the importance of the food stuff in the food chain.
• The population (total amount) of food items may be supplied to or distributed through an entire nation or region or be only typical of a particular sub-population group (e.g. ethnic group or tribe).

Different aspects influence the decision on where to sample

• Select sample units (single food items, packets, bunches, leaves) of all the various types of foods, from geographical or manufacturing/outlet locations, consumed by the population of interest.
• The units may be selected according to the relative importance (e.g. frequency of consumption, market share, food composition database requirements) for given food types.
• Sample units should preferably be randomly selected.
• Convenient sampling (e.g. purchasing samples at only one shop with the closest proximity) is  sometimes practised when analytical costs or other factors prevent random sampling of sample units from more than one sampling point (e.g. shop, geographical area)

Several factors are important in determining and identifying the food items that should be collected.

• What is the name of the food? (e.g. Tuna)
• Description of the food (e.g. Tuna, canned in water).
• Classification of the food (e.g. Medium fat fish).
• What forms/types/brand names of the food are consumed?
• Is the food in the natural state/processed food?
• How is the food prepared?
• Where is the food produced?
• When is the food produced?
• Is the food prepared from a recipe or formulation?

The following factors may contribute to the variability of the sample and influence the sampling protocol.

• geographic location (e.g. soil type, climate)
• seasonality
• harvesting procedures
• stage of maturity, age or ripeness
• food colour
• breed, brand or cultivars
• method of husbandry
• fertilizer treatment
• preparation methods
• preservation state
• enrichment/fortification standards
• variation in recipes and formulations
• distribution and marketing practices
• estimates of variability must be based upon sampling and analyses, specifically planned to yield such data 

The aim of the study and the funds available for analysis are the determining factors in deciding which nutrients will be analysed.

A specific analytical laboratory may not be able to conduct all the required nutrient analyses and therefore may need to sub-contract some of the analyses to other analytical laboratories. Upon choosing a laboratory which has to sub-contract to another, establish the turn-around time for the analysis sent to the sub-contracted laboratory. Ensure that the sub-contracted laboratory is aware of the preferred analytical methodology for your analysis. Contact the relevant laboratories to ensure that they meet requirements regarding valid/appropriate/ accredited methodology. Cost may also play a role in selecting the appropriate laboratory, but should not be the main consideration. Read more.

The field sample size is determined by the nutrients identified for analysis and it is best to consult with the analytical laboratory responsible for the analyses to determine how much of the field food sample should be collected. The field sample refers to the first initial sample taken from the general population.  Generally 100 to 500 g reduced from the representative field sample will be enough for the laboratory sample.

Definition of a sample
A sample is a single unit or a collection of units (e.g. packages, bunches, number of roots, fruits or items) representative of the total population of the food.

• Sample units must be taken from the available types and forms of the food for which the nutrient composition estimates are being determined.
• Production, consumption, or sales statistics to determine the importance of the food, may be used. 

Sample size
The amount of material required depends on:

• objective of analyses
• analyses of duplicate/triplicate samples individually or composite samples of the food
• number of nutrients/components to be measured, determine the number and weight of the laboratory sample from which the analytical aliquots will be taken as required by the chemical methods
• requirements and policy regarding storing/saving some of the field sample and/or saving/storing aliquots for later analysis.

Minimum number of samples
Most sampling schemes adopt a standard of at least 10 food sample units and 3 analytical samples.

• Available funds are often a limiting factor in the number of samples that can be done
• South African nutrition labelling regulations requires 12 sample units.

Calculation of number of samples
An example of how to calculate the optimal sample size is provided in Greenfield and Southgate, 2003, pp 214-215. Read more.

• Information on the mean and standard deviation of the nutrients in food is required.
• Information required for calculating the sample size can come from pilot studies, the literature or informed guesses.
• If there is great variance in the nutrient value the sample size will be larger compared to if the variance is small.

The analytical laboratories will provide information regarding the analytical sample amount (i.e. the portion prepared from the laboratory sample) required for the analysis of specific nutrients according to standard operating procedures in the laboratory. The representative field sample reduced to the laboratory sample (i.e. the sample sent to or received by the laboratory) and then further reduced to the analytical sample should therefore be large enough to meet requirements and this should be negotiated with the laboratories before the primary sample is collected. Read more.

• The steps in the sampling of a food sample for analysis should be documented before the sample is collected for quality control and reference purposes. The information available in the schematic overview on Sampling, additional reading recommended and consultation with experts on sampling where required could serve as a guideline for drawing up the study specific sampling protocol.

Read more in Greenfield and Southgate, 2003. Chapter 5, pp 62-83.

Collect raw food, bulk foodstuffs, multi-ingredient food prepared from standard recipes or fast foods from the food supply or source. The areas of collection could be, the land, market place, manufacturing/food outlets or from the homes of the members of the study population
The nature of the foods impact on how food samples are collected and the following factors have to be taken into account:

• cooking method (raw, boiled, fried, grilled)
• part of plant or livestock (leaf, leaf plus stem, growth tips, pulp, leg, chop, liver, with/without visible fat etc.)
• origin (cow, elephant, season, e.g. winter, cultivated)
• edible portion (with or without flowering parts/seeds/skin)
• preservation method (fresh, canned, smoked)
• preservation medium (brine, oil, drained solids or with preserving liquid/sauce)
• physical state (whole, dried, diced)
• colour and form (green, long, big size)
• special descriptors (low fat, unsweetened, coated, flavour added, low alcohol, enriched with calcium).

Follow the standard sampling protocol for the correct procedure to collect a representative random sample of the identified food item. Sometimes random sampling is not possible and a convenient sample, e.g. collecting samples at only one shop with the closest proximity, is collected.

Read more in Greenfield and Southgate, 2003. Chapter 5, pp 62-83.

It is important to ensure that the samples are clearly marked for identification, transport and storage purposes.  

• name of the food item
• name of fieldworker/collector
• date of sampling
• place of sampling (village, district, province)
• point of sampling (field, garden, supermarket)
• food code
• unit price of food, if applicable
• any other important detail to identify the food sample

The necessary sample handling procedures to be followed in the laboratory should be forwarded to the laboratory staff to ensure proper handling.

Collected samples should be transported to the selected laboratories with minimum delay whilst maintaining the cold-chain, if required.

If cooked or multi-ingredient food is analysed the product should be prepared according to standard preparation methods or from standard recipes under controlled conditions. Collecting multi-ingredient food at household level to represent food commonly eaten is a challenge.

Proper handling and transport of the samples is important to prevent nutrient losses. The use of the correct containers e.g. brown bags, glass containers, etc. is essential and should be clarified with the relevant laboratories when the sampling protocol is compiled. The mode of transportation, e.g. fresh or frozen is also important and should be clarified with the laboratories to prevent nutrient losses. Read more.

Before samples are analysed, the field sample needs to be reduced to the laboratory sample. It usually includes processes that change the structure of the food e.g. grinding, disintegration, homogenization (food blender), liquidizing to a pulp. Should it be necessary to store the field food samples or the processed laboratory sample, these samples have to be kept below -40 ºC or lower. Fat can be stored between -20 ºC or -30 ºC.

Read more in Greenfield and Southgate, 2003, pp 216-220.

For further reading the following Web site can be visited.

More information is given elsewhere in this website to provide guidelines on the requirements and appropriate methodology for the analysis of food.

• It is best to have nutrient values for at least 10 individual food samples as this provide some information on the range of values for a specific food item.
• Factors such as the cost of analysis may prevent data generators from analysing several food samples. In this situation the following could serve as a guideline:            
• Prepare three composite samples, each containing at least 3 or 5 units, and analyse in duplicate. The mean of duplicate determinations obtained for each of the three composite samples can then be averaged to provide a mean value and some indication of the range of values.
• The analysis of composite samples reduces the costs associated with the analysis of individual samples, but information about the variability of the component between samples will be lost, so that is what you sacrifice. Read more.

Guidelines regarding the reporting of data should be stipulated in the sampling protocol and examples are given below:

• data should be reported in specified units per 100 g of food analysed
• the moisture basis at which data should be reported
• the unit of measure, e.g. fat in g/100 g and not as percentage composition
• the number of decimal points for all the nutrients analysed
• whether an official laboratory report of analysis is provided or whether results are presented on an official letterhead of the company
• whether the data should be provided in a pre-designed Excel spreadsheet
• who the contact person at the laboratory is