Guidelines for the preservation of official samples for analysis

Last updated:
1 June 1999
This project aims to publish guidelines, containing details about transport and storage conditions for official food samples.
Study duration: June 1999 to November 2000
Project code: E01031
Contractor: Campden and Chorleywood Food Research Association

For any enquiries concerning this research project, please contact the relevant Programme contact or email


The Food Safety Act 1990 empowers authorised officers to take or purchase samples which they have reason to believe may be required as evidence in proceedings under any of the sections of the Act, or the regulations or orders made under it. If they consider that the samples should be analysed then they must be submitted to a public analyst for the appropriate analysis.

One of the most common reasons for food enforcement actions to fail is doubt about the validity of analytical data presented. Furthermore, these criticisms often relate to the condition and history of the 'official food sample' when it is submitted for analysis.

In 1999, questions addressed to 12 local authorities indicated the lack of any clear, practicable and technically valid guidelines for the preservation of official samples for analysis. Furthermore, a series of codes of practice and international standards, which refer to food sampling, have little or no guidance relating to sample storage.

Research Approach

The overall objective of this project was to produce a set of guidelines which can be used as a best practice guide for the medium to long term storage of official samples by enforcement authorities. The guidelines would also provide information for retailers on how to store their part of the sample.
The project objectives were:

  • to identify appropriate storage conditions for sample transport;
  • to identify appropriate conditions for the long-term storage, of official samples
  • to provide clear advice to ensure that all three parts of an official sample remain representative and reflect the state of the product at the time of sampling
  • to be applicable to the food industry's own samples taken for routine or due diligence purposes

The guidelines apply to samples for analysis but not to samples for examination. As part of the project, a review of local authority practices was carried out and reported. Also, storage trials were carried out on formal food samples that were stored either frozen or chilled.

This was to simulate the storage that would occur to the part of the sample that is retained, possibly for long periods (a year) for analysis in cases of dispute by the referee analyst. Samples of sausages (sulphur dioxide), rancid chicken (free fatty acids in fat and protein breakdown), whisky (alcohol) and fruit juice (vitamin C) were stored and analysed at varying intervals.


To establish the need for detailed guidance on the conditions for transport and storage of official samples, 12 Local Authority Trading Standards department were asked questions relating to their current transport and storage practices. The summary of the responses from the 12 local authorities involved clearly demonstrated the need for more specific information and guidance on storage conditions for official samples. Based on these responses, two objectives were set, namely to provide guidelines on the storage of official samples and to carry out experiments in order to validate the storage conditions for problem analytes.

The guidelines document includes information on: legal issues; recommended containers; security and labelling; sample tracking records; sample transport conditions; factors affecting food sample deterioration; effective storage of samples at ambient; chilled and frozen temperatures; monitoring of storage temperatures; recommendations for long-term storage of samples and recommendations for storage relating to sensitive analytes.

Over a period of six months the experimental work addressed: the examination of the rate of temperature increase in a 'cool box' under a variety of conditions and using a range of food types, and the subsequent analysis of unstable analytes. The experimental work established that:

  • After approximately eight hours storage of frozen chicken (in a 'cool box' with ice packs), at both summer and autumn outside temperatures, the samples were still frozen. It was considered that this was a suitable mode of storage for short-term transportation of frozen food samples.
  • The temperature in the cool box containing chilled chicken (initial temperature, 3.6°C) reached 5°C after 2 hours.
  • There was no significance difference between the levels of sulphur dioxide preservative (SO2) in frozen sausages after zero, three and six months storage.
  • There was a significant increase in the levels of free fatty acids (FFA) in the fat from 'off' chicken over the three sampling times, when stored frozen.
  • There was no significant difference in the levels of total volatile nitrogen (TVN) in 'off' chicken over the three sampling times, when stored frozen. This test was used to measure protein breakdown in meat products.
  • The levels of vitamin C in a frozen blackcurrant drink fell significantly during the frozen storage period.
  • There was no significant decrease in the alcohol content of whisky samples between zero and six months storage in a fridge at 3°C to 5°C. However, there was a statistically significant decrease in the alcohol levels in the whisky samples, between month zero and month three, but this difference was well within the precision of the method, and therefore cannot be considered of practical significance.

The work in this project has provided information which, if followed, will increase the chances of an official sample remaining valid during transport and storage, but the work requires extending to be more comprehensive than has been possible in this study.


The guidelines developed through this project are used and referenced within the FSA's 'Practical sampling guidance for food standards and feedingstuffs' available from the FSA website ( These guidance documents on food standards and feedingstuffs sampling have been developed through the Sampling Co-ordination Working Group (SCWG) and has been endorsed by the Enforcement Liaison Group (ELG). The guidance offers advice on the issues local authorities may wish to consider when setting their sampling programmes each year and go on to give practical guidance in taking samples for food standards and feedingstuffs, complementing LACORS' existing guidance on microbiological sampling.
The guidelines developed through this project are also available from CCFRA as CCFRA Guidelines No. 36 and a copy of this document (published in 2001) is in the British Library.