Application and Progress of Microwave Technology in Food Analysis

- May 08, 2019 -

With the rapid development of science and technology, the automation of food ingredient determination is getting higher and faster. For example, the simultaneous determination of more than ten or even dozens of elements can be completed in a few minutes by plasma emission spectrometer. However, the pretreatment of food samples often requires several or even dozens of hours of manual operation, which is very incongruous. Since the 1970s, microwave technology has been gradually developed at home and abroad. It can be used to digest various geological, metallurgical, biological, food and cosmetics samples, which greatly shortens the sample pretreatment time and satisfies the requirements of analysis and testing. The present situation and development of microwave digestion technology and its application in food analysis were described in this paper. Current status of microwave digestion technology

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1.1 Atmospheric pressure microwave digestion technology

The initial microwave-assisted acid digestion was carried out in an open container in a household microwave oven at atmospheric pressure. In general, corrosion-resistant coatings are sprayed in the furnace chamber, and acid mist is continuously discharged during operation. Microwave power and reaction time are easy to monitor and control. In 1975, Abu-Samara et al. first used ordinary microwave oven to digest the standard substances such as fruit leaves and calf liver successfully with HNO 3-HClO 4. Subsequently, swordfish, tuna, oyster, shrimp, spinach, tomato leaf wheat flour, rice flour and other samples were treated by atmospheric pressure microwave digestion technology. Xu Liqiang and Wang Daning refitted the household microwave oven and developed a sample dissolving method combining microwave technology with traditional heating technology of electric heating plate. The sample was first digested in an open container with mixed acid in microwave oven, then further processed on the electric heating plate or evaporated to nearly dry. Satisfactory results were obtained in the digestion of honey, milk powder, pig liver, wheat flour and other samples. The result. Atmospheric pressure microwave digestion has the advantages of large capacity and good safety, but the digestion time is about 20-30 minutes, which is difficult to digest some high-fat samples completely. Most of the digestion containers used are open containers, vulnerable to pollution or volatilization loss, and the temperature can not exceed the boiling point of acid.

1.2 Pressurized microwave digestion technology

P>1983, Matther et al. put forward the method of microwave sample dissolution in closed container. It has the advantages of microwave heating and high pressure digestion tank technology, but the materials used in digestion tank must be able to pass through microwave. Because of the sealing of the reaction tank, the temperature in the tank rises rapidly, which makes the pressure in the tank rise sharply and increases the boiling point of the test solution. Some samples which can not or are difficult to be digested by acid at atmospheric pressure may be quickly digested. Sample dissolvers usually have closed polypropylene tanks, polycarbonate bottles and polytetrafluoroethylene cups. In the early 1980s, CEM and its National Bureau of Standards (NBS) of the United States had the products of microwave sample dissolving instrument, which had the functions of temperature control, pressure control and timing. Now it has been developed to be controlled by computer. China has commercialized instruments in the 1990s, but they only have the function of timing voltage control. At present, there are more MDS type of American CEM company, MK type of Shanghai Xinke company and WR type of Beijing Meicheng instrument in the literature.

Foreign microwave sample dissolving devices usually attach temperature and pressure sensors to the sealed digestion tank to transmit the changes of temperature and pressure in the container to the monitoring system in time. The relationship between the basic parameters of microwave and the presence of some commonly used acids alone or in different proportions can be determined by experiments. These data lay a foundation for designing safe and accurate optimal dissolution methods for some unknown samples. The latest product of American CEM company, MARS intelligent control microwave digestion system, uses computer monitoring system to control automatic frequency conversion power, accurate temperature and pressure process control and other means to ensure the safety of sample dissolution process. The former Shanghai Xinke Microwave Sample Dissolution Research Institute used optical fiber pressure automatic control system to monitor and control pressure, and adopted three safety measures to ensure the safety of sample dissolution process.

1.3 Other microwave sample processing technologies

P>In recent years, a series of new instruments and devices have been developed, such as microwave muffle oven, microwave high temperature ashing oven, microwave high pressure digestion and extraction system, microwave volatile content measurement system, microwave Kai-type nitrogen determinator and so on. If the microwave muffle oven is used, the sample can be ashed or melted within 10 minutes (only 20-30 minutes for domestic instruments). The temperature of ashing or melting can be reached (500-1000 degrees Celsius). CEM's MAX high temperature ashing furnace is composed of microwave and built-in electronic balance. The sample is rapidly programmed to rise from 100 ~1200 C under insulation conditions. Direct ashing without carbonization, oxygen-free and smoke-free can complete the determination at one time. Used for analysis of burning residue and burning weight reduction ashing. For example, CEM's SMART automatic microwave volatile content measurement system, built-in electronic balance, three-phase temperature detection, precise process temperature control volatilization, within 5 minutes can test the percentage of volatile at each temperature point of the sample 0.01%-99.99%. It can be used for the determination of water and solid content. Application of microwave digestion technology in food analysis

2 Microwave digestion technology has become one of the indispensable means for sample digestion because of its characteristics of energy saving, time saving, less pollution and complete decomposition. Previously, it was mainly aimed at normal and trace metal elements such as Pb, As, Hg, Se, Gr, Fe, Mn, Ca, Mg, etc. The suitable detection methods were AAS, AFS, ICP-AES or ICP-MS. With the development of microwave technology, the application field is more and more extensive. Li and others used microwave digestion GC to determine organochlorine pesticides in fish, and microwave-assisted derivatization GC-MS to determine fatty acids in edible oil. Liu Chuanbin et al. reported the preparation of trehalose samples from yeast by microwave digestion and high performance liquid chromatography. Xiong Guohua et al. introduced the extraction of chloramphenicol from cooked meat by microwave extraction and the determination of drug residues in meat by HPLC or GC. Li Hai et al. used microwave digestion spectrophotometry to determine arsenic and manganese in pork, carp, rice, fruit, ice sugar and other samples; He Sheng et al. used microwave digestion-oscillopolarography to determine germanium in Gynostemma pentaphyllum; Xu Wenguo et al. used microwave heating technology to replace the traditional burning method for gravimetric analysis. Up to now, microwave digestion technology of trace elements in food such as lead, cadmium, mercury, chromium, antimony and germanium has been listed as the national standard test method.

In the 1980s, commercialized microwave sample preparation system has been developed in foreign countries. It has good functions, time, temperature and pressure control devices, and can handle multiple samples simultaneously at one time, but it is expensive. CEM's MDS series sample preparation system can process up to 12 samples at one time. They recommend that 1 g of beef and sausage products be simultaneously digested by heating in 6 tanks in HNO 3 system, 0.5 g of milk powder, soybean products, soybean oil and grain oil in 12 tanks in HNO 3-H 2 system, and 2 g of fish tissue in 4 tanks at the same time, all of which can be completely digested within 20 minutes. Sun Yuling et al. used MILESYONE-7200MG microwave digestion instrument of Milston Company of Italy to digest cereal and vegetable samples respectively. ICP-MS was used to determine trace rare earth elements in digestion solution. The results showed that the measured values were in agreement with the recommended values of the standard. The technical indexes of the method were satisfactory.

In the late 1980s, domestic scholars modified domestic microwave ovens to make reactors with different materials, and carried out microwave digestion of various samples. The results were good. Xu Liqiang et al. used the reformed domestic microwave oven and the self-made PTFE autoclave to digest pig liver, wheat flour and mixed diet samples. Ca, Mn, Mg, Fe, Zn, Cu and Se were determined by AAS or ICP-AES or AFS. The results were basically consistent with the recommended values. Wang Daning et al. improved the household microwave oven, used the polyethylene plastic barrel with cover on the market as reactor, digested corn, rice, sorghum, tomato paste and other samples under atmospheric pressure, measured the copper, zinc, Cd, lead, As by AAS method, the recovery rate was 90%-110%; digested the wheat flour, cabbage and other standard substances, and determined the above elements by AAS method, the results were basically consistent with the recommended standard values. Kong Xianghong and other household microwave ovens decomposed dehydrated vegetable carrots in closed polytetrafluoroethylene tank with HNO 3-H 2. The relative standard deviations of copper, iron, Mg, zinc and Ni were 012%-115% by AAS method, and the recovery was 95.0%-110.6%.

In the 1990s, after the commercialization of domestic microwave sample-dissolving system, all manufacturers set up safety protection measures. This promotes the rapid development of microwave sample dissolution technology in China. Jiuhua et al. used domestic MK microwave digestion system to digest rice grain, rice powder, Auricularia auricula powder, milk powder, cooked egg yolk (white), spinach, beef, brown sugar and other food samples. The microwave power of digestion, digestion time, control pressure, types and quantities of digestive reagents were discussed. Compared with traditional wet and dry methods, Fe was used as the observation index. There was no significant difference. The digestive time was saved by 40%-90%. The amount of digestive reagent was half or less of that of wet digestion. Peng Qian et al. established a microwave digestion AAS method for the determination of Ni in margarine and hydrogenated oil, which had no significant difference compared with the national standard method. Li Fang and others used MK microwave digestion system to digest newly picked vegetable samples. Nineteen elements such as Al, Fe, Pb, P and As in the digestion solution were determined by ICP-AES. The results showed that the recoveries of each element ranged from 87% to 104%. Some volatile elements or easily contaminated elements obtained good recoveries, indicating that microwave digestion was feasible. Liu Husheng et al. used WR microwave sample processing system made in China to digest tea samples. Trace rare earth elements in digestion solution were determined by ICP-MS. The results were in good agreement with the recommended values. The technical indexes of the method were satisfactory. Fan Chunyue, Luo Jianbo, Liang Chunsui, Guo Heming, Haolin and Cao Yi were used to determine Al, Pb, Cu, Cr, Cd, Zn, Sr, Se in food and health products by microwave digestion-AAS method, while Yang Fang and Luan Yan were used to determine As and Se in food, seafood and health products by microwave digestion-AFS method. Microwave digestion technology has achieved good results in food analysis pretreatment, and now it has become one of the indispensable means for sample pretreatment. However, it is a new technology after all, and there are still some areas to be improved and perfected. For example, when batch samples are processed, they must be of the same type in order to be digested at the same time at one time, otherwise, they must be digested separately. Foreign equipment allows up to 12 samples, while domestic equipment allows up to 10 samples or less. Moreover, the different positions of digestion tank in the furnace chamber and the different microwave energy intensity obtained will affect the accuracy of the results to a certain extent. In addition, the residual acid in digestive juice sometimes interferes with the determination of trace elements. For example, the amount of HClO4 or HNO3 residues has an effect on the determination of Pb and Cu by AAS.

3 Prospect

Modern food analysis puts forward higher requirements for sensitivity, precision, trace, valence, morphology and multielement analysis. In microwave technology, sample digestion, drying, extraction, protein hydrolysis and other methods have been used abroad. Microwave protein hydrolysis technology can be combined with high performance liquid chromatography to achieve simultaneous determination and determination of multi-component. Because the extraction liquid can be heated to the boiling point above the normal pressure quickly in the sealed high pressure vessel, the boiling point of the solvent can be improved without decomposing the extract to be measured, and the recovery and efficiency of the extraction can be improved, thus greatly improving the accuracy and efficiency of the modern GC/HPLC method. also

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