However, in practice, we often encounter the following situations:
The test result of the sample is abnormal, and there is no deviation during the test. The results showed that the samples were contaminated during sampling.
For the above problems, it is necessary to start with the standardized sampling operation.
At the same time, due to different sample types, sampling operations are also different to ensure the scientific nature of sampling. Different sampling operations should be performed for different samples, and the classification of different sample sampling operations will be described below.
0 1 liquid sample
Generally speaking, it is easier to obtain representative samples from liquid samples. Liquid food is generally stored in large tanks, which can be stirred continuously or intermittently when sampling. For smaller containers, the liquid can be inverted before sampling to make it completely mixed.
The obtained samples should be sent to the laboratory in sterile containers, and the laboratory should thoroughly mix the liquid again before sampling and testing.
02 solid sample
Different tools are used according to different sample materials. Common sampling tools for solid samples include scalpels, spoons, cork drills, saws, pliers, etc. , must be disinfected before use.
For example, the food that has been stirred, such as flour or milk powder, is of uniform and stable quality, and a small amount of samples can be taken for testing. However, bulk samples must be taken from multiple points, and each point should be treated separately and thoroughly mixed before testing.
Meat, fish or similar foods should be sampled not only in the epidermis, but also in the deep layer. Be careful not to be contaminated by the surface when sampling in depth.
Some foods, such as fresh meat or cooked meat, can be sampled with sterilized scalpels and pliers;
Frozen food is usually drilled at an oblique angle with a saw, a wood drill or an electric drill without thawing to obtain deep samples;
When sampling powder samples, the sterilized sampler can be inserted into the bottom of the box at an inclined angle, and the sample can be taken out of the box after it is filled, and then samples can be taken from the upper, middle and lower parts of the sampler with a sterilization spoon.
03 water sample
When taking water samples, it is best to choose a jar with a dust-proof grinding bottle stopper. For water treated with chlorine, after sampling, add 0. 100 ml of water sample with 2% sodium thiosulfate solution.
Special attention should be paid to prevent the pollution of the sample when sampling, and the sample should be completely filled with the sampling bottle. If the sample is taken from the faucet, the inside and outside of the faucet mouth should be wiped clean.
Turn on the faucet to let the water flow for a few minutes, turn off the faucet and light it with an alcohol lamp, then turn on the faucet to let the water flow for 1-2 minutes, then take the sample and fill the sampling bottle.
This sampling method can ensure the quality of bacteriological analysis of water supply system, but if the purpose of testing is to trace the source of microbial pollution, it is suggested to take samples before disinfection of the faucet and apply cotton swabs inside and outside the faucet to detect the possibility of pollution of the faucet itself.
When collecting water samples from reservoirs, ponds, wells and rivers. Take out the bottle with sterile instruments or tools and open the cork. When sampling from flowing water, the bottle mouth should directly face the water flow.
The official sampling procedures in most countries clearly specify the instruments used for sampling. If there is no suitable sampling instrument or temporary sampling tool, it can only be operated by hand. However, when sampling, you should be especially careful to prevent your hands from touching the water sample or the inside of the sampling bottle.
04 packaged food
Small packaged food for direct consumption should be packaged as well as possible, and then opened after testing to prevent pollution;
Liquid or solid food in barrels or large containers should be sampled from several different parts by aseptic sampler and put into aseptic containers together. Don't be too humid to prevent the inherent bacteria in food from multiplying.
For barreled or large-capacity frozen food, samples should be taken from several different parts with sterilization tools to make them fully representative, and the samples should always be kept frozen before being sent to the laboratory. If the sample melts, it can't be frozen any more, just let it cool.
05 surface sampling
Microorganisms on surface samples can be transferred to suitable culture medium through inert carriers for microbial detection, which will neither cause the death of microorganisms nor make them proliferate. Such carriers include clean water, cotton swabs, etc.
It is difficult to achieve neither death nor proliferation of microorganisms, so it should be detected as soon as possible after sampling. Surface sampling technology can only transfer bacteria directly, and can't do serial dilution. It is only applicable when the number of bacteria is small, and its greatest advantage is that the sampling process will not destroy the sample.
Commonly used surface sampling techniques include cotton swab method, washing method and adhesive tape method. Take the cotton swab method as an example here.
In qualitative detection, all surfaces that need to be detected can be smeared. When quantitative detection is carried out, the area to be detected must first be determined with a sterile sampling frame. Use a 4cm long wooden stick or stainless steel stick with a diameter of 1- 1.5mm to wrap dry cotton to make a cotton swab (disposable cotton swab can also be used for convenience), put it in an alloy test tube, cover it and sterilize it.
When sampling, first soak the cotton swab in the diluent, then slowly rotate the cotton swab on the surface of the sample to be tested and smear it twice. When smearing, cotton swabs should be placed in the sampling frame. After sampling, put the cotton swab back into the test tube containing 10mL sampling solution.
The above are the relevant operation procedures and precautions for aseptic sampling of relevant samples. Aseptic sampling is of great significance, and the standardization of aseptic sampling is the premise to ensure the accuracy of sample detection.
Therefore, it is necessary to standardize the operation when sampling to ensure that pollution is eliminated from the source.