Food Composition 281Variability in foods: regional and
other differences
Foods are biological materials and, as such, have a
naturally variable composition. Even processed foods
produced under highly controlled circumstances
show some variability. Therefore, a database must be
able to predict the composition of a single sample of
food within the limits defi ned by its natural variabil-
ity. Variability may be contributed by one or more of
the following factors: brand, cultivars or species,
season, climate, geographical location (e.g., soil type),
fertilizer treatment, method of husbandry, harvesting,
preservation state, stage of maturity, enrichment/for-
tifi cation standards, preparation methods, food color,
variation in recipes and formulations, distribution
and marketing practices, and other factors. For
critical components variability may affect the suffi -
ciency, defi ciency, or excess of the intake of a givenTable 11.4 Sampling process of foods for food composition data
1 Prioritizing foods for inclusion 4 Defi nition of sampling size
May be based on: Amount of material required:
● type ● objective of analyses
● frequency and ● analyses of individual samples or composite samples of the food
● amount of specifi c foods or products consumed
● quality and quantity of existing data
● number of components to be measured; determine the number and
weight of aliquots needed as required by the chemical methods
● appropriateness of prior analytical methods or
● perceived benefi t/risk of particular foods as sources of
components of interest
May be affected by:
● changes in the forms of foods or
● levels of components, including reformulation or fortifi cation
Levels of available resources will impact on the process of setting
priorities
2 Defi ning prioritized foods
Within the context of the objective, defi ne the specifi c
characteristics of the food that may contribute to the variability of
the estimate:
● uncooked or raw foodstuffs versus cooked forms of the food
● composition of prepared or multicomponent foods (i.e. mixed
dishes)
● individual brands or cultivars or generic value
3 Defi nition of sampling unit
Collection of units (packages, bunches, or items) representative of
the total population of food units:
● sample units must be taken from the available types and forms
of the food for which the composition estimates are being
determined. Production, consumption or sales statistics may be
used. The population of items may be supplied to or distributed
through an entire nation or region or be only typical of a
particular subpopulation (e.g. ethnic group or tribe)
● select sample units from all the various types of food and
geographical or manufacturing locations of food consumed by
the population of interest. The units may be selected according
to the relative importance (e.g., frequency of consumption) for
given types
● sample units that are collected can be analyzed as individual
units or may be combined together or composited and analyzed.
The analysis of composite samples reduces the costs associated
with the analysis of individual samples, but information about
the variability of the component in that food will be lost
● policy for saving reserve or archive aliquots5 Protocol for sample collection
Foods should be typical of the usual preparation and consumption
practices
Correct units of foods should be selected
Protocols should be tested for the adequacy of food storage and
transportation facilities, sample unit documentation and labeling,
and packaging and short-term preservation requirements
Policies for the substitution of units should be in place in the event of
unavailable sample units
Sampling among ethnic or native populations may impose additional
restrictions owing to cultural or religious customs
Samples should be clearly coded for identifi cation. Documentation
should start from the planning stage, throughout purchasing,
transporting, preparation and the combining of samples, to analyses
including storage condition, use of reference samples, recording of
data obtained (duplicate or triplicate values), as well as manipulation
of the data, e.g. expression of the data on a wet (as eaten) basis, as
opposed to the content of a freeze-dried sample. Correction factors
applied or calculations (e.g. N × Jones factor = protein) should be
recorded for each foodstuff analyzed
Documentation and handling of sample units should be under the
careful control of the principal co-coordinator and all laboratory
personnel should be informed before the start of the project of the
reasons for handling the samples in a specifi c manner. The samples
should preferably be marked with three-digit random codes for
analysts to ensure that analyses are unbiased. Values should only be
decoded as results become available, by the principal investigator.
This will improve the reliability of the results if performed on a
double-blind basis