Food inspection and food safety (subject)

The topics and materials are as follows. Wish you success!

Heavy metal pollution in food and its detection technology

This paper briefly introduces the present situation of heavy metal pollution in food, and briefly describes the regulations on the limit of heavy metal pollution in food at home and abroad. The detection technology of heavy metals in food was introduced emphatically, and its future development trend was discussed.

introduce

Heavy metals refer to metals with specific gravity above 5, such as copper, lead, zinc, nickel, cobalt, cadmium, chromium, mercury, bismuth, tin, antimony, niobium and molybdenum [1]. Heavy metals are widely distributed in the atmosphere, lithosphere, water and biosphere. Under normal circumstances, the natural background concentration of heavy metals will not reach harmful levels. However, with the rapid development of social industrialization, the mining, smelting, manufacturing and processing activities of heavy metals are increasing day by day, which leads to some heavy metals such as lead, mercury, cadmium and cobalt entering the atmosphere, water and soil environment, causing serious environmental pollution. What we usually call heavy metal pollution refers to the deterioration of environmental quality caused by the increase of harmful toxic heavy metals in the environment caused by human activities and beyond the normal range.

From the perspective of food safety, the most concerned heavy metals are mercury, cadmium, lead, chromium, metalloid arsenic and other heavy metals with obvious biological toxicity. Although arsenic is a nonmetallic element, its source and harm are similar to those of heavy metals, so it is usually classified as heavy metals for research and discussion. Heavy metals pollute food, drinking water and air, and finally threaten human health. Vegetables, fruits, grains, fish, etc. It is polluted by heavy metals and cannot be removed by soaking, cleaning and cooking. Most of the heavy metals in the environment cannot be biodegraded. On the contrary, they can be enriched thousands of times under the biomagnification of the food chain and eventually enter the human body. With the accumulation of more and more heavy metals in the human body, the human body will produce various reactions, which are harmful to health. Some heavy metals also have teratogenic, carcinogenic or mutagenic effects, endangering life safety. According to research, heavy metal pollution is mainly caused by chronic damage of food after it enters the human body. It takes a long time for heavy metals to show toxicity after entering the human body, so they are often not easy to be found early, and they attract enough attention before the onset of toxicity, thus increasing their harmfulness.

Minamata disease and pain in Japan in 1950s were found to be public hazards caused by mercury pollution and cadmium pollution, so the food-borne hazards caused by environmental pollution caused by heavy metals through the food chain attracted people's attention. In recent ten years, with the rapid development of China's economy, the imbalance between environmental governance and environmental pollution has become increasingly serious, leading to the increasingly serious problem of heavy metal pollution in food. For example, water pollution in China is serious. Nearly half of China's seven major water systems and many lakes are polluted, and the water quality of more than 80% urban rivers generally exceeds the standard, especially the problem of heavy metal pollution is very prominent. According to the incomplete statistics of 15 provinces and cities, 50% of small and medium-sized rivers with fishery value do not meet the fishery water quality standards, which leads to the decline of aquatic products quality. Agricultural pollution caused by heavy metal cadmium discharged from industrial wastewater and sewage can make the cadmium content in rice as high as 1.3-5.4 mg/kg, which greatly exceeds the national limit of 0.2mg/kg. The daily intake of cadmium in some polluted areas is several times or even dozens times higher than that in non-polluted areas. The pollution degree of soil heavy metals in China is also increasing, and the pollution area is expanding year by year. According to relevant statistics, at present, at least 20 million hectares of soil in China are polluted by heavy metals, and more and more soils, especially those in suburbs and sewage irrigation areas, are being polluted by heavy metals. Every year, the grain production is reduced by more than100000 tons due to heavy metal pollution, and the grain contaminated by heavy metals is as high as120000 tons, resulting in a total economic loss of at least 20 billion yuan. For example, in 2002, Nanjing Institute of Soil Research, Chinese Academy of Sciences conducted a key investigation on five vegetable bases in a suburb of southern Jiangsu. The results showed that the soil cadmium content in five vegetable bases exceeded the standard by 265,438+0-80%. In some places, the mercury in the soil exceeded the standard, reaching 44%. In addition, among the 20 vegetable samples collected according to the national pollution-free vegetable standard, the over-standard rate of chromium, cadmium and lead is 15%, 20% and 20% respectively. Generally speaking, the problem of food quality and safety in China is becoming more and more serious, which has also attracted the attention of governments at all levels and relevant departments. Paying attention to food safety means paying attention to health. To solve the problem of heavy metal pollution in food, we must first control pollution sources, conscientiously implement relevant environmental laws and regulations, and prevent environmental pollution. Secondly, it is necessary to establish and improve the early warning mechanism of food heavy metal pollution as soon as possible, expand and strengthen the monitoring of food pollution, and improve the detection technology level of food heavy metal pollution.

Special Exhibition of Instruments: Pesticide Standards

2. Sources and hazards of heavy metal pollution in food

2. 1 Sources of heavy metal pollution in food

There are several ways to pollute food with heavy metals.

(1) Some areas have special natural geological conditions and high content of heavy metals in the environmental background. In some special areas, such as mining areas and submarine volcanic activity areas, because of the high content of toxic metals in strata, the content of toxic metals in animals and plants is obviously higher than that in general areas.

(2) Man-made environmental pollution causes food pollution caused by toxic and harmful metal elements. Waste gas, waste water and waste residue containing heavy metals discharged from industrial production, and the use of agricultural chemicals such as pesticides and fertilizers containing heavy metals will cause environmental pollution of water and soil. For example, the concentration of cadmium in polluted water can reach 0.2-3 mg/kg, which is 1000-2000 times higher than that in normal water. The concentration of cadmium in polluted soil can be 800 times higher than that in normal soil. The cadmium content of plants planted in these soils increased obviously. It is worth mentioning that heavy metal pollution is different from that caused by general pesticides and fertilizers. Even if their concentration in the environment is very low, because the environment is not easy to purify, organisms can accumulate thousands of times in higher organisms through the biomagnification of the food chain after ingesting heavy metals in the environment, and then enter the human body through food, causing potential harm.

(3) Toxic metal elements contained in machinery, pipelines and containers used and contacted in the process of food processing, storage, transportation and sales, and food pollution caused by additives added due to technological needs.

2.2 Harm of heavy metal pollution

(1) Mercury has metallic element mercury in nature, commonly known as mercury, inorganic mercury and organic mercury. Mercury and its compounds are common and widely used toxic metals and compounds. Mercury enters the human body mainly through people's intake of contaminated fish, shellfish, grains and rice. Although there are still differences on the ways of mercury accumulation in fish and shellfish in the hydrosphere, a large amount of evidence shows that the mercury accumulated in fish and shellfish is almost organic regardless of human pollution or natural pollution. Mercury pollution in grains and rice may mainly come from pesticides and wastewater pollution. Mercury poisoning is mainly caused by organic mercury poisoning. Patients with mercury poisoning often show numbness of fingers, lips and tongue, slurred speech, narrowed vision, dyskinesia and nervous system damage. In severe cases, paralysis, limb deformation, dysphagia and even death may occur. Investigation shows that if the cumulative intake of methylmercury exceeds 500 mg, there will be symptoms such as numbness of limbs, narrowing of vision and dyskinesia. If the cumulative intake of methylmercury exceeds 1000 mg, acute symptoms such as spasm and paralysis may occur, and death will soon occur. For example, pregnant women can avoid getting sick by eating fish contaminated with mercury, but methylmercury in the body will enter the fetus through the placenta and cause neonatal occurrence? Congenital Minamata disease? .

(2) Cadmium is a blue-white metal, which is widely distributed in nature, but its content is very small. The causes of cadmium pollution mainly come from waste gas, waste residue and wastewater discharged from metal smelting, mining, electroplating, paint, pigment, ceramics, plastics and pesticides. Cadmium can be absorbed by plant roots and transferred to animals through drinking water and feed, so that livestock and poultry food contains cadmium. In cadmium polluted areas, the concentration of cadmium in food can be about 20 times higher than that in normal areas. Cadmium mainly accumulates in kidney and liver after entering human body, and cadmium poisoning mainly damages renal function, bones and digestive system. After cadmium damages renal proximal convoluted tubules, it can cause the loss of nutrients such as calcium and protein, decalcify bones, cause skeletal deformities and fractures, and lead to unbearable bone pain and death due to pain. Acute cadmium poisoning often causes vomiting, diarrhea, dizziness, salivation and loss of consciousness. In addition to acute and chronic poisoning, studies show that cadmium and its compounds have certain mutagenic, teratogenic and carcinogenic effects.

(3) Lead is a grayish white metal. Lead is mainly used to make batteries, pigments, glazes, etc. Alkyl lead, such as tetraethyl lead, is widely used as an explosion-proof agent for gasoline because of its good seismic performance. Lead pollution to the environment mainly comes from smelters, leaded gasoline exhaust gas and the use of leaded materials. Lead poisoning is a kind of cumulative poisoning, which is mainly caused by lead-polluted air, drinking water, soil and food entering the human body. After lead enters the human body, part of it can be excreted through the kidneys and intestines. Lead left in the body can replace calcium in bones and accumulate in bones. With the increase of accumulation, the body can show toxic reaction. Lead poisoning can lead to hematopoiesis, kidney and nervous system damage. Lead poisoning often manifests as mental retardation, slow response, anemia and other chronic poisoning symptoms. From the degree of harm, lead has the greatest impact on the growth and development of fetuses and young children, so the probability of lead poisoning in children is much higher than that in adults. At present, the lead pollution of children in China is serious.

(4) Arsenic is a nonmetal, but because many physical and chemical properties are similar to metals, it is often called metalloid. Arsenic compounds include inorganic arsenic and organic arsenic. Arsenic compounds are often used as pesticides and growth promoters in livestock and poultry, so pesticide and veterinary drug residues are the main causes of arsenic pollution in food. Arsenic has a strong inhibitory effect on many enzymes in human body, which can seriously interfere with the activities of many enzymes in human body and the respiration, division and reproduction of cells, causing metabolic disorders in human body. Arsenic poisoning can be divided into acute and chronic types. Acute arsenic poisoning is mainly manifested as gastroenteritis, which can lead to paralysis of the central nervous system and death in severe cases, and patients often bleed from seven orifices. Symptoms of chronic arsenic poisoning include general neurasthenia syndrome, skin pigmentation, excessive keratinization and peripheral neuritis. Now arsenic and its compounds have been identified as carcinogens.

3. Pollution limits of heavy metals in food at home and abroad

Due to the increasingly serious problem of heavy metal pollution in food, the awareness and attention of governments, related groups and organizations and many enterprises around the world are also increasing, and many corresponding policies and regulations have emerged. In recent years, China has also revised and promulgated relevant laws and regulations to adapt to the development of the international economic situation. The United States and other western countries have also revised or promulgated many corresponding laws and regulations. Table 1 lists some national standards (GB 2762-2005) for pollution limits of heavy metals such as mercury, lead, cadmium and arsenic in food implemented in China since June 2005 10 [2]. Relevant standards formulated by Codex Alimentarius Commission (CAC) of FAO/ WHO are also listed in this table for comparison [3].

As can be seen from the table 1, most quota indicators in China have reached international standards. However, due to the differences between China's national conditions and international standards, there are still gaps in some limits. For example, the lead limit index of fish in China is 0.5 mg/kg, and the CAC standard is much stricter, only 0.2 mg/kg. For example, the lead limit index of livestock meat in China is 0.2 mg/kg, and the CAC standard is 0. 1 mg/kg. It should be noted that with the changes and development of the domestic and international economic situation, the limit indicators of various pollutants in food will also change to meet the requirements of the situation. But in general, the limited indicators tend to be more stringent. According to the national standard of China 1994, the limit of lead in cereals, beans, potatoes and livestock meat was 0.4, 0.8, 0.4 and 0.5 mg/kg respectively, but in 2005, the limit was reduced to 0.2 mg/kg. In addition, different countries often have different limit standards. Therefore, it is important and necessary for departments and related enterprises engaged in international trade to know and master the limit standards of pollutants in related products of relevant countries in time. Table 1 only lists some limited indicators of heavy metal pollution in some foods. For more limited indicators, please refer to relevant literature.

The detection methods of heavy metal elements in food include photometry, turbidimetry, point comparison, chromatography, spectrometry, electrochemical analysis, neutron activation analysis and so on. The determination methods of heavy metals in food are stipulated in relevant national standards in detail. Listed below are the national standard detection methods for lead, cadmium, mercury and arsenic in food. Other determination methods of heavy metals can be found in relevant references.

(1) The common detection methods of lead in food are: graphite furnace atomic absorption spectrometry with the detection limit of 5 μ g/kg; The detection limit of flame atomic absorption spectrometry is 0.65438 0 mg/kg; Single sweep polarography with detection limit of 0.085 mg/kg; Dithizone spectrophotometry with detection limit of 0.25 mg/kg; The detection limit of hydride generation atomic fluorescence spectrometry is 5 μ g/kg.

(2) The common detection methods of cadmium in food are: graphite furnace atomic absorption spectrometry, with the detection limit of 0.1μ g/kg; The detection limit of flame atomic absorption spectrometry is 5 μ g/kg; Spectrophotometry, the detection limit is 50 μ g/kg; The detection limit of atomic fluorescence spectrometry is 65438±0.2μg/kg/kg.

(3) The common detection methods of total mercury in food are: atomic fluorescence spectrometry, with the detection limit of 0.1.5 μ g/kg; Cold atomic absorption spectrometry with detection limit of 0.4 μ g/kg (pressure digestion method) or 10 μ g/kg (other digestion methods); The detection limit of dithizone spectrophotometry is 25 μ g/kg. Methylmercury is usually analyzed by acid extraction, sulfhydryl cotton adsorption and separation, and then by gas chromatography or cold atomic absorption spectrometry.

(4) The common detection methods of total arsenic in food are: hydride generation atomic fluorescence spectrometry with detection limit of 0.01mg/kg; The detection limit of silver salt method is 0.2 mg/kg; The detection limit of arsenic spot method is 0.25 mg/kg; The detection limit of borohydride reduction spectrophotometry is 0.05 mg/kg.

4.2 Progress in detection technology of heavy metals in food

The progress of heavy metal detection technology in food is mainly manifested in three aspects. First, the technical progress of the testing instrument itself,

The continuous improvement of hardware and software functions; The second is sample treatment, including sample digestion and necessary enrichment methods.

Step; The third is the improvement and optimization of the determination method.

Atomic absorption spectrometry is one of the main detection techniques of heavy metals in food. Electrothermal atomization (graphite furnace), flame atomization or hydride generation can be used. The detection limits of these methods are very low. At present, the atomic absorption spectrometer mostly uses CCD solid-state detector instead of photomultiplier tube, which greatly improves the degree of automation and can realize the integration and automatic switching of flame and graphite furnace. The software function of the instrument has been greatly improved, and the operation is more flexible and convenient.

Inductively coupled plasma mass spectrometry can not only determine the concentration of metal elements, but also give isotope information, so isotope tracing research can be carried out. At present, the most striking development of ICP-MS is the dynamic reaction cell technology. This technology can greatly prolong the life of ICP-MS mass analyzer and improve the analytical sensitivity of ICP-MS. ..

Now inductively coupled plasma atomic emission spectrometry (ICP-OES) has also become a common method for the determination of heavy metals in food. After digestion, the sample can directly enter the high temperature plasma of 5000 ~ 7000 K, and the emission lines are observed and analyzed by a polychromatic instrument. The advantage of this method is that about 70 kinds of elements can be analyzed, and the sensitivity of each element is very high, and its detection limit can reach ppb level. However, the use of double monochromator optical system and double detector full-spectrum direct reading ICP-OES instrument can avoid the shortcomings of traditional full-spectrum direct reading ICP spectrometer, such as long preheating time, small incident light slit and short detector life, which should be the future development direction of full-spectrum direct reading ICP spectrometer.

Studies have proved that the forms of elements in food are closely related to their toxicity, for example, the toxicity of trivalent arsenic is much greater than that of pentavalent arsenic, so it will be a very important goal to separate and quantitatively determine the true forms of elements in samples. At present, the combination of high performance liquid chromatography and inductively coupled plasma mass spectrometry is the most commonly used speciation analysis method, accounting for more than 70% of speciation analysis research. In addition, the combination of capillary electrophoresis, supercritical chromatography and gas chromatography with ICP-OES or ICP-MS will be the development direction of speciation analysis in the future.

In practical work, it is often required to get the test results quickly, so the research and development of rapid detection technology will be one of the important development directions in the future. For the detection instrument itself, its development trend will be better and better stability and detection limit, easier to use, faster analysis speed, and more and more information can be obtained. The development of rapid detection technology of heavy metal pollution in food depends not only on the use of advanced instruments, but also on the pretreatment and preparation technology of samples to a great extent. With the continuous emergence of various efficient, rapid and sensitive analytical instruments and methods for metal pollutants, the traditional sample pretreatment and preparation technology can no longer meet the needs of practical work. The emergence and rapid development of microwave digestion technology for food samples provide favorable conditions for the development of rapid detection technology for heavy metal pollutants. Microwave digestion method has the advantages of simplicity, rapidity, high efficiency, safety, good reproducibility and wide application, and can fully meet the requirements of rapid detection of heavy metal pollutants in food when used in conjunction with corresponding instruments.

For most small and medium-sized food production and operation enterprises in China, there is generally no condition to use large and expensive advanced instruments.

Therefore, the more economical spectrophotometer has become the main instrument for the determination of heavy metal pollutants. However, the detection limit of general spectrophotometer is high, and sometimes it can not meet the requirements of determination. In this case, it is necessary to effectively separate and enrich the heavy metals in the sample in advance. For many years, we have been committed to the spectrophotometric research and application of heavy metals, and have designed and proposed many new detection methods [1 1]. In particular, our recently developed enrichment device for heavy metals and harmful elements such as lead, cadmium, mercury and arsenic can effectively and rapidly separate and enrich the above elements, such as 1 ~ 2 micrograms of measured elements in 200 ml solution, and rapid and accurate photometric determination can be realized by using efficient and sensitive new chromogenic reagent, such as1in drinking water. It can be completed in half an hour, which greatly improves the timeliness, sensitivity and selectivity of spectrophotometry, and can be compared with other instrumental analysis methods to a great extent.