Basic steps in food inspection in the laboratory

Basic steps in food inspection in the laboratory

The basic steps of food inspection are: sample collection; sample processing; sample analysis and detection; analysis results recording and processing in four stages.

1 sample collection

The collection of samples, also known as sampling and sample preparation, refers to the extraction of a representative sample for analysis and testing. Sample collection generally consists of three components: sampling, sampling, and sample preparation. Attention must be paid to the date of manufacture, lot number, representativeness and uniformity of the sample. The number of samples should meet the requirements of the test item for the sample volume. The sampling container should be made of hard glass bottles or polyethylene products according to the inspection items.

The general steps of sampling are: 1 acquisition of the original sample; 2 mixing of the original sample; 3 shrinking the original sample to the required amount. Different methods should be used for sample collection for different samples.

Liquid sample collection: For large barrels and canned samples, 0.5L of upper, middle and lower samples can be taken by siphon method, and 0.5~1.0L after mixing. For large pool samples, 0.5L can be sampled at the four corners of the pool and at the upper, middle and lower layers of the pool. After mixing well, take 0.5~1.0L.

Collection of solid samples: The original sample of each part of the sample should be sufficiently uniform to make the sample uniform and representative. For large samples, it should be cut into small pieces or crushed, sieved, pulverized, and there should be no loss or splash of material when sifting, and all sieved, then the original sample is thoroughly mixed, and then the quadruple method is used for shrinking. The amount of sample, up to the required amount, is generally 0.5~1.0kg.

The operation of the quadruple method is as follows: the sample is thoroughly mixed and then stacked into a conical shape, and then pressed downward from the top of the cone to make the sample be pressed to a thickness of 75 px, and then uniformly “10” from the center of the top of the sample. The ground is divided into four parts, and the two parts of the diagonal sample are mixed. If the amount of the sample reaches the required amount, it can be used as an analysis sample. If the amount of the sample is still greater than the required amount, continue to shrink as described above and continue to shrink to the sample requirement.

Immediately after sampling, close the plug, label it, and fill in the sampling record carefully. The sample record shall indicate the name of the sample, the sampling unit, the address, the date, the sample lot number or number, the sampling conditions, the packaging conditions, the number of samples, the inspection items, and the sampler. Samples should be properly packaged and kept according to different inspection items.

General samples should be kept for one month after the end of the test, in case they need to be re-examined. Deteriorating foods are not retained. It should be sealed and kept as it is when it is preserved. In order to prevent the sample from being damp, air-dried and deteriorated during storage, the appearance and chemical composition of the sample are not changed, and it is generally required to be stored in a cold and protected from light. The test sample is generally taken from the edible portion and is calculated from the sample being tested. Samples that are unsatisfactory in sensory judgment need not be subjected to physical and chemical tests, and are directly judged as unqualified products.

Foods imported from other places should be combined with the manifest, the veterinary health personnel certificate, the health inspection authority of the commodity inspection authority or the health department, the production license and the inspection certificate or laboratory test list to understand the date of departure, source location, quantity, quality and packaging. In the case of sampling in a food factory, warehouse or store, the batch number, date of manufacture, factory test record and on-site hygiene status of the food should be known. At the same time, attention should be paid to the transportation, storage conditions, appearance, packaging container, etc. of the food.

2 sample processing

Samples often contain certain impurities or other components that interfere with the analysis, affecting the correctness of the analytical results. Therefore, before the analysis and inspection, the characteristics of the sample, the principle and characteristics of the analytical method, and the properties of the measured object and the interferent should be used. Differences, using different methods, separating the analyte from the interferent, or separating and removing the interferer, so that the analytical assay yields the desired result.

Common methods for sample processing are:

• Solvent extraction method: The principle is to separate the analytes from the interference properties of the interferents. For the determination of bacillus toxin, the aflatoxin is extracted with a common organic solvent and then determined by high performance liquid chromatography. This method is simple in operation and good in separation effect, but the extractant is often volatile, flammable, explosive and toxic, so care should be taken during operation.
•  organic matter decomposition method: the principle is to use high temperature treatment to oxidize and decompose the organic matter in the sample, in which C, H, O elements escape with CO2 and H2O, the measured metal elements and other components are released for further determination. Specific methods include dry ashing and wet digestion.
• Dry ashing is to place the sample in a crucible, first carbonize it under low temperature and low heat, remove moisture and black smoke, and then ash to a black carbon-free particle at a high temperature of 500-600 ° C in a high temperature furnace. If the sample is not easily ashed, the sample may be wetted with a small amount of HNO3, and then ashed after evaporation and, if necessary, ashing with NH4NO3, NaNO3 and other auxiliary ashing agents to promote ashing and shortening ash. Time to reduce the loss of volatile metals such as Hg. The ash after ashing should be white, light grayish white. This method has complete organic destruction, simple operation, small blank value, and is often used for the determination of ash in samples, but the operation time is longer.
• Wet digestion is carried out in a strong acidic solution. The oxidizing ability of H2SO4, HNO3, H2O2 and other oxidizing agents is used to decompose the organic matter. The metal to be tested is finally left in the solution in an ionic state, and the solution is cooled and made up for measurement. This method is carried out in solution, the heating temperature is lower than the dry ashing temperature, the reaction is mild, and the metal volatilization loss is less, which is commonly used for the determination of metal elements in the sample. A large amount of harmful gases are generated during the digestion process, so the digestion should be carried out in a fume hood or in a well ventilated area. Since a large amount of reagents are added during the operation, it is easy to introduce more impurities, so at the same time of digestion, a blank test should be performed to eliminate the error of impurities introduced by reagents and the like.
• Distillation method: The distillation method is a method in which the difference in volatility of each component in the substance to be tested is used for separation. The interference component can be removed, and the component to be tested can be distilled off and the distillate can be collected for analysis. For example, the constant Kjeldahl method for measuring protein content is to digest the protein into volatile nitrogen, then distill it, absorb the distilled ammonia with HBO3, and then measure the ammonia content in the absorption liquid, and then convert it into protein. The content.
• The method of heating during distillation can be determined according to the boiling point and characteristics of the substance to be distilled. When the substance to be distilled is stable in nature, is not easily exploded or burned, it can be directly heated by an electric furnace. For the distillate having a boiling point of less than 90 ° C, a water bath can be used; for a liquid having a boiling point higher than 90 ° C, an oil bath, a sand bath or a salt bath method can be used. For some of the components to be tested, the atmospheric pressure heating distillation is easy to decompose, and vacuum distillation can be used, and the vacuum pump or the water jet pump is generally used for decompression.
• For some organic components with a certain vapor pressure, it is usually separated by steam distillation. For example, in the determination of volatile acids in liquor, in the steam distillation, the volatile acid and the steam are distilled together from the sample solution in proportion to the pressure, thereby accelerating the distillation of the volatile acid.
•  salting out method: by adding a certain inorganic salt to the solution, the solubility of the solute in the original solvent is greatly reduced, and precipitated out from the solution, this method is called salting out. For example, in a protein solution, a large amount of a salt, especially a heavy metal salt, is added to precipitate the protein from the solution. When performing the salting-out operation, it should be noted that the substance to be added in the solution should be selected so as not to destroy the substance to be precipitated in the solution, otherwise the purpose of salting out extraction cannot be achieved.
•  chemical separation methods mainly have the following methods:
•  sulfonation and saponification: commonly used to treat oil or fat-containing samples. For example, in the analysis of pesticide residues and fat-soluble vitamins, the oil is sulphonated by concentrated H2SO4 or saponified by alkali, and becomes hydrophilic by hydrophobicity, so that the non-polar substances to be detected in the oil can be easily non-polar. Or a weakly polar solvent is extracted.
•  Separation separation method: a method of separating by a precipitation reaction. Adding an appropriate amount of precipitant to the sample causes the test substance to precipitate out or remove the interference precipitate to achieve the purpose of separation.
• Masking method: The interference component is converted into a non-interfering component by using a masking agent and an interference component in the sample liquid, that is, masked. This method can eliminate the interference effect and simplify the analysis step under the operating conditions without separating the interference components, and thus is widely used in food analysis, and is commonly used for the determination of metal elements.
• Clarification and Decolorization: Clarification is used to separate the turbid material from the sample to eliminate its effect on the analytical determination. A clarifying agent is usually used to precipitate the turbid substance and act to remove the turbid substance. The clarifying agent should not interfere with the component being tested or affect the analysis of the component being tested. Decolorization is a method of removing colored substances in a sample that easily interfere with the measurement results to eliminate interference. It is usually carried out using a decolorizing agent. Commonly used decolorizing agents are: activated carbon, white clay and the like.
•  Chromatography (also known as chromatographic separation): is a general term for a method of separating substances on a carrier. According to the principle of separation, it can be divided into adsorption color layer separation, distribution color layer separation and ion exchange color layer separation. The separation effect of this kind of method is good, and its application in food analysis is gradually wide.
•  Concentration: After the food sample is extracted and purified, sometimes the volume of the purified solution is large, and it needs to be concentrated before the measurement to increase the concentration of the component to be tested. Commonly used concentration methods are atmospheric pressure and reduced pressure concentration. The main principle is to use the vapor pressure of the water in the substance under specific conditions to be greater than the partial pressure of the air, so that the moisture escapes from the sample, thereby concentrating the sample.

3 sample analysis and detection

There are many methods for the analysis and detection of samples. The same test items can be measured by different methods. When selecting the test method, the most suitable analysis should be based on the nature of the sample, the content of the tested components, and the interference components. The method is both simple and accurate. The main object of food testing is the identified components to be tested in the sample. Analytical methods in juice production are generally fixed. The specific test methods will be introduced later.

4 Recording and processing of analysis results

The results of the analysis should be accurately recorded and processed according to the prescribed methods, and the correct way to ensure the final correctness of the analysis results, the specific method will be described in detail later.

For the expression of the results, the measured values ​​of the parallel samples are reported as the arithmetic mean. The number of significant figures of the general measured values ​​should meet the requirements of the hygienic standard, even higher than the requirements of the hygienic standard. The reported result should be one more effective than the hygienic standard. The number, such as the lead content, is 1 mg/kg; the reported value should be 1.0 mg/kg.

The unit of sample measurement should be consistent with the hygiene standards. Commonly used units are: g / kg, g / L, mg / kg, mg / L, μg / kg, μg / L and so on.