Introduction to Preparation of sample for analysis

The preparation of samples for analysis involves three steps: homogenization of the content and withdrawal of the analytical unit, preparation of the first dilution of the analytical unit and the preparation of serial decimal dilutions, for inoculation into or onto culture media.

Before starting procedures certain precautions are recommended, to ensure that all activities be conducted under aseptic conditions:

Make sure that the work area is clean and that all doors and windows are closed to avoid air currents.

Disinfect all working surfaces with an appropriate disinfectant (ethanol 70%, 500 ppm benzalkonium chloride solution, 200 ppm sodium hypochlorite solution or any other chlorine-based compound are adequate).

Wash and disinfect your hands with a disinfect-ant appropriate and safe for skin contact. Verify the necessity or not to use gloves in the chapters specifically dealing with pathogen tests.

Prefer working inside vertical laminar flow cabinets to prevent contamination of the sample by the environment and contamination of the environment and the analyst by the sample. In case a vertical laminar flow cabinet is not available, work in an area located as close as possible to the flame of a Bunsen burner, which, when working well, will produce a steady blue flame. When handling powdered samples, it is not recommended to work very close to the flame of a Bunsen burner. ISO 7218:2007 stipulates the use of a separated area or a laminar flow cabinet.

Avoid the formation of aerosols when opening tubes, flasks or plates after agitating or releasing the content of pipettes or flame-sterilizing inoculation loops. Never use a pipette by mouth, but use mechanical pipettes instead. After use, place the pipettes and other utensils in disposable trays and not directly onto the surface of the bench.

All instruments and utensils used to open packages and withdraw analytical units (scissors, tweezers, knives, spatulas, etc.) must be previously sterilized (in an auto-clave or sterilization oven) or immersed in ethanol 70% and flame-sterilized at the time of use.

Before opening the packages, disinfect the external area with ethanol 70%, maintaining contact until the alcohol has fully evaporated. In the case of flexible pack-ages, cut open with a sterile pair of scissors. In the case of rigid packages with a screw cap, unscrew and remove the cap aseptically. In the case of cans that come with an “easy open” lid with wide opening, open the can aseptically and remove the lid. In the case of cans without an “easy open” feature, use a sterile can opener. In the case of cans, glass containers, boxes and other packaging intended to be subjected to the commercial sterility test. The objective of these procedures is to ensure the integrity of the sealing system, for later analyses of the package, if necessary. Observe and note any abnormality concerning either the package itself or its content, such as blowing, leakage, off-odors and/or strange or atypical appearance, the presence of foreign objects, and so on.

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.