Sampling, transport and storage of samples for analysis

- Lot

A lot is defined as an amount of food of the same composition and physical, chemical and sensory characteristics, produced and handled in one and the same production run and under exactly the same processing conditions. In practice, a lot generally is the quantity of food produced within a certain time interval during an uninterrupted period of processing of a production line.

-Lot sample and  sample unit

A lot sample is a fraction of the total amount produced, withdrawn randomly, to evaluate the conditions of the lot. In the case of foods filled into individual packages, a lot sample is composed of n individual packages. In the case of bulk foods, which are not filled into individual

packages, a lot sample is composed of  n aliquots of a measured volume or weight of the product. Individual packages or aliquots are called sample units and – for the purpose of assessing the lot – are examined individually. From the combined results of analysis relative to n sample units, it is possible to infer the characteristics of the lot as a whole, although the result of the examination of one single sample unit may never be taken as representative of the lot.

In Salmonella tests the criterion for foods is absence in any of the sample units examined. In such a case it is common to composite (mix together) sample units to perform one single analysis. The presence of Salmonella in the composite sample is unacceptable, irrespective of how many or which sample units are contaminated.

-Lot  sampling plans

Whenever the goal is to evaluate lots or batches, the taking of  n sample units must follow a statistically adequate sampling plan. The most commonly used are the two- or three-class plans established by the International Commission on Microbiological Specifications for Foods.

1-The two-class  sampling plan

The two-class sampling plan classifies lots into two categories, acceptable or unacceptable, depending on the analysis results of  n sample units. Two-class sampling plans are used more in the case of presence/absence tests, such as Salmonella, for example, in which absence is acceptable and the presence in any of the  n sample units is unacceptable.

2- The three-class  sampling plan

The three-class sampling plan classifies the lots into one of three categories, i.e. (1) acceptable; (2) intermediate quality but marginally acceptable; and (3) unacceptable. Three-class sampling plans are recommended for quantitative tests, for which the goal or standard is not the absence, but values that fall within a range between m and M. The parameters used in these plans for making decisions regarding the lots tested are: n: is the number of sample units that need to be randomly taken from one and the same lot and which are to be examined individually. The  n sample units constitute the representative sample of the lot. As for non-quantitative and presence/absence tests (Salmonella or Listeria monocytogenes, for example) the sample units may be composite or pooled and subjected to a single analysis. However, when pooling or compositing samples the instructions and guidelines described in the chapters dealing with the specific tests in question should be consulted and strictly adhered to.

m: is the microbiological limiting criterion established for a given microorganism, in a given food. In a three-class sampling plan, this value separates an acceptable lot from a lot of intermediate but marginally acceptable quality.

M: is a tolerable limit, above the microbiological limiting criterion m and which may be reached by a certain number (c) of sample units, but may not be exceeded by any of these. In a two-class plan, M separates an accept-able lot from an unacceptable one. In a three-class plan, M separates a lot of intermediate but marginally accept-able quality from an unacceptable lot.

c: among the n sample units that constitute a representative sample of the lot, c is the maximum number of units that may be accepted with counts above the limit-ingcriterion  m, provided none of these units exceeds the M limit. In those cases in which the microbiological criterion is absence, c is equal to zero, and consequently, the two-class sampling plan is to be applied.

-Analytical unit

A sample unit generally contains a quantity of product greater than necessary for performing the analysis, for the simple reason that, when collecting a sample unit, it is important to collect great enough quantities to allow for storing counter-samples and preventing accidental losses. The analytical unit is the amount of food that is actually used to perform one or more tests on the sample unit. The number of analytical units necessary for the analysis depends on the number and types of tests that will be performed on one and the same sample unit, that is: one for general quantification tests (total aerobic mesophilic counts, yeast and mold counts, total coliforms/ fecal coliforms/  E. coli counts,  S. aureus counts, B. cereus counts, C. perfringens counts), one for each presence/absence test (Salmonella, Listeria monocytogenes and all the tests requiring enrichment in specific broth) and one for any other test that requires the sample to be subjected to a differentiated treatment (counts of spore forming bacteria, counts of heat-resistant molds and others).

References

SILVA, N.D .; TANIWAKI, M.H. ; JUNQUEIRA, V.C.A.;  SILVEIRA, N.F.A. , NASCIMENTO , M.D.D. and GOMES ,R.A.R .(2013) . MICROBIOLOGICAL EXAMINATION METHODS OF FOOD AND WATE A Laboratory Manual . Institute of Food Technology – ITAL, Campinas, SP, Brazil.