Migration behavior of non-volatile UV absorbable materials in microwave sensitive materials for microwave popcorn packaging bags

Abstract: The commercial microwave popcorn packaging bags were immersed in simulated food corn oil by Microwave heating equipment, and the non-volatile ultraviolet absorbable substances were extracted under simulated microwave heating conditions. The non-volatile ultraviolet absorbable substances migrated into corn oil were extracted and concentrated by n-hexane-acetonitrile, and the volume of N, N-dimethylacetamide was fixed to 2 mL, then separated and quantified by high performance liquid chromatography.

The results showed that the average migration amounts of the four samples were 93.3, 134.1, 38.4 and 11.8 ug/in 2, respectively. Only one sample met the FDA limit of <20 ug/in 2.

With the development of science and technology and the improvement of people's requirements for food packaging, a variety of leisure food packaging machines with special functions and effects are emerging. The popularity of microwave oven and the development of new microwave packaging materials also make microwave food packaging gradually popularized and applied. Microwave food packaging materials refer to packaging materials which can be processed directly by microwave oven and food together. They are mainly divided into microwave penetrating materials, absorbing microwave energy materials and reflecting microwave energy materials. The material that absorbs microwave energy is also called microwave sensing material. Metallized polyethylene terephthalate (PET) film is the most common microwave sensing material. One of the most important properties of this material is its surface resistance. When the thickness of the metal coating is very large, the surface resistance is zero, and the microwave energy transmitted to the coating surface is all reflected back.

As the thickness of metal coating decreases, the surface resistance increases gradually and the amount of microwave absorbed increases gradually. When the coating thickness is optimal, it absorbs 50% of microwave energy. The film is laminated with kraft paper with thermal stability to form a package for certain foods. The instant temperature of these foods can be as high as 250 degrees when they are cooked by microwave heating. Under such high temperature conditions, the possibility of the migration of residual monomers, oligomers, additives, reaction byproducts and other unknown substances to packaged foods in the process of injection molding and processing of polyethylene terephthalate (PE T) is greatly increased. Moreover, in the process of microwave heating, PET can also be depolymerized to produce monomers, endangering human health. PET mineral water bottles and beverage bottles were detected. Four kinds of cyclic oligomers were determined by mass spectrometry and nuclear magnetic resonance. Dihydroxyethyl terephthalate (BHE T) and cyclic trimer of PET were determined by high performance liquid chromatography. The migration experiments using simulated food and real food were compared. It was concluded that simulated food in official standards was still an important medium for migration testing, but the migration differences between simulated food and real food were also existed, so further research was needed.

Metallized polyethylene terephthalate (PE-T) microwave sensing materials are often used in the packaging of microwave foods such as potato chips, popcorn, fish chips, waffles and pizzas. The migrating substances of the materials under high temperature conditions can be divided into two categories: non-volatile substances and volatile substances, among which non-volatile substances include ultraviolet absorbable substances and violet. Non absorbable substances outside. Non-volatile UV absorbable substances are non-volatile substances that can be detected under UV conditions and released from microwave food packaging under certain simulated conditions.

At present, there is no national standard for the detection of hazardous substances which can be migrated at high temperature. A method for the determination of total migration of harmful substances in microwave-heated containers was established in the previous study. The migration behavior of non-volatile ultraviolet migrating substances in popcorn packaging to corn oil was studied and some useful data were obtained.

Main raw materials: microwave popcorn, supermarket purchase; goldfish corn oil, dihydroxyethyl terephthalate (BHE T), purity greater than 93.0%, diethyl terephthalate (DE T), standard, purity greater than 99.0%, dimethyl terephthalate (DM T), standard, purity 99.5%, n-hexane, analytical purity, acetonitrile, chromatographic purity, N, N-dimethyl Acetamide (DMAC), chromatographic purity, dichloromethane, analytical purity.

Main Equipment and Instruments: Microwave-heated extraction dish, made from polytetrafluoroethylene (PTFE) with reference to ASTM F1349, has an inner diameter of 8.2 cm and an outer diameter of 10.2 cm; microwave oven, high performance liquid chromatography (HPLC), CBM-20A, nitrogen blower, electronic balance, Fourier transform infrared spectrometer, etc.

The experimental principle: the sample was put into a custom-made microwave heated extraction dish, added corn oil, heated in a microwave oven, cooled, stirred and weighed a small amount of corn oil, dissolved by n-hexane, extracted by acetonitrile, acetonitrile layer, dried by nitrogen blowing, dissolved in DM AC, and determined by high performance liquid chromatography. Sample blank experiments were conducted according to the same method. According to the area concentration curve of DM T, the content of migration in corn oil extract concentrate was calculated (QA). Then, according to the formula (1), the migration of non-volatile UV-absorbent substances in microwave sensing materials was calculated: M = 6.4516 *QA *TO OS *V A (1) - the migration of non-volatile UV-absorbent substances in microwave sensing materials, the content of migrant substances in microwave/in 2 QA -, the content of migrant substances in corn oil, and the addition of microwave/mL TO -. The quality of the oil in the dish heated by wave (1 G oil per square centimeter of contact area), the quality of the oil sample taken from the dish heated by microwave, the final volume of the concentrated extract, 2.0 mL

Non-volatile Ultraviolet Absorbable Substances Migration Behavior of Microwave Sensitive Materials Used in Microwave Popcorn Packaging Bags Sample Processing: Before each experiment, the microwave heated extraction dish was washed and dried with water, then the residual oil was washed and dried with dichloromethane. The paper packaging bag of microwave popcorn was cut open, the microwave sensing material was cut, the cream and other substances on the surface were wiped off with filter paper, and the diameter was 10.2 cm.

Selection of microwave heating conditions: First take a bag of microwave popcorn, microwave heating according to the label instructions, while recording the temperature changes, draw a time-temperature change map. Three microwave popcorn bags were taken and the samples were processed. Then the samples were put into a clean microwave heated extraction dish. Corn oil was added to the extraction dish according to the ratio of 1.0 g corn oil per square centimeter of contact area (52.8 g). Then the heating dish was put into the microwave oven. Other conditions are the same, by changing the microwave power, microwave heating time, or in the microwave oven into a cup of a certain volume of distilled water (with a little zeolite), you can adjust the temperature changes during the simulation heating extraction process. An optical fiber temperature probe is inserted into the hole of the microwave oven to ensure that the probe has good contact with the sample surface. The temperature changes were recorded every 5 s and the time temperature curve was drawn. Comparing the time-temperature curve of simulated heating with that of popcorn heating, a group of experimental conditions with the closest temperature change were selected as the conditions for migration measurement.

Heating extraction experiment: Take three identical samples, sample processing, and then put into the microwave heating extraction dish, and the food contact side up, so that it contacts with the simulated food. 52.8 g corn oil was added to the extraction dish, and then the extraction dish was put into the microwave oven. A temperature probe is inserted into the hole of the microwave oven to ensure that the probe has good contact with the sample surface. Then microwave heating was conducted according to the selected test conditions, and the temperature changes were recorded every 5 s. After heating, take out the microwave heating dish, cool, stir the oil, take 3.0 g (+ 0.03 g) to 50 mL beaker, add 25 mL hexane, shake the oil and transfer to 125 mL liquid funnel. The beaker was washed with 25 mL n-hexane and 25 mL acetonitrile respectively, and the lotion was transferred to the funnel. The liquid separation funnel is strongly oscillated, and the layer is separated, and the acetonitrile layer is removed. The beaker was washed with 25 mL acetonitrile and transferred to the liquid separating funnel. After shaking, the beaker was layered statically. The acetonitrile layer was merged with the first acetonitrile. The 50 mL acetonitrile solution was evaporated to 0.4-0.5 mL at 50 C under the protection of nitrogen, and then cooled to 2 mL with DM AC. In the microwave heating dish, no sample was added, the rest procedures were the same, blank test was conducted.

Standard curve plotting and sample testing conditions of HPLC: Shimadzu Inertsil C8 column (5 micron, 4.6 mm x 250 mm); mobile phase A: water + acetonitrile + acetic acid = 85% + 15% + 0.25% (volume fraction, the same below); mobile phase B: water + acetonitrile = 15% + 85%; detection wavelength 254 nm; sample size 20 microL; gradient program: mobile phase B within 0-8 minutes 4%-60%, 60%-70% in 8-17 minutes, 70%-100% in 17-24 minutes, 100% in 24-35 minutes, 100%-4% in 35-40 minutes and 4% in 40-45 minutes.

The standard substance was qualitatively judged: mixed standard solution of BHE T, DM T and DE T was prepared by DMAC.

The concentration of each substance is about 10 g/mL, and after 0.22 m filtration, HPLC is measured. Calibration curves: DM T solutions with concentrations of 1, 2.5 and 5 ug/mL were prepared by DM AC, filtered by 0.22 UG membrane and determined by HPLC. The calibration curve was drawn with DM T concentration as abscissa and peak area as ordinate. The sample and the blank sample of the constant volume were filtered with a 0.22 M filter membrane and then HPLC was measured. The peak area of each peak is replaced by the standard curve, and the total concentration is C1. The same method was used to integrate and calculate the blank samples, and the total sum of concentration obtained was C2. QA = C1-C2, and the non-volatile ultraviolet absorbable substance (M) of the microwave sensing material can be obtained by substituting the formula (1).

Results and discussion:

Infrared analysis of microwave sensing materials: Four brands of microwave popcorn samples were purchased in supermarkets, the inner packaging paper bag of microwave popcorn was cut, the microwave sensing materials between two layers of paper were removed, and the samples were washed and dried and analyzed by infrared spectroscopy. The infrared characteristic absorption peaks of the four samples are in agreement with the standard PE T sample, indicating that the main components of the four samples are PET.

Total Mobility of Nonvolatile Ultraviolet Absorbable Substances: Data show that the main component of microwave-sensitive materials is PE, while the residual monomers, oligomers, additives, reaction byproducts and other unknown substances in PET injection and processing may migrate to food during heating and cooking. The factors affecting the migration of these substances mainly include the molecular structure of the migrants, the influence of multicomponent and temperature, the most important of which is the influence of temperature. Therefore, the most important basis for choosing simulation conditions is also the consistency between simulated heating process and actual food heating temperature changes. The curves of microwave power, time, volume of water and temperature-time variation for microwave heating of four samples are shown in Fig. 3-6.

From figs. 3 to 6, we can see the trend of time and temperature during the heating process of popcorn and simulated food, and compare the maximum temperature of each test.

From the simulation curves, the heating time and the temperature change trend are the closest to the popcorn heating curve, and the conditions are chosen as the experimental conditions for simulating food heating. It can be seen from the pictures that the optimum microwave heating conditions of sample 1-4 are: sample 1, microwave power 890 W, time 195 s, water 100 mL; sample 2, microwave power 890 W, time 200 s, water 75 mL; sample 3, microwave power 890 W, time 185 s, water 100 mL; sample 4, microwave power 890 W, time 170 s, water 75 mL.

Migration of non-volatile substances in simulated food heating process: Studies have shown that DET, DM T and BHET are three common monomers in the process of PET synthesis and decomposition. Therefore, the standard solution is used to qualitatively determine whether there are migration of these 3 substances in the samples. The retention time of BHE T, DM T and DET are 9.5, 15.7 and 20.5 minutes respectively. The retention time of these three monomers can be used to determine whether they migrate from the sample to the simulated food. According to the results of HPLC, the calibration curve was drawn with the concentration of DM T as abscissa and the peak area as ordinate, and the regression equation was obtained as shown in equation (2). Y = 57782x-4082.9, r = 0.9999 (2) in the Y - peak area X - DM T standard solution concentration

It can also be seen from Fig. 7 that there are many smaller peaks in the sample chromatogram from 8 to 18 minutes, but no BHET, DM T, DET were detected. There are some distinct peaks in 20-28 min, which may be cyclic oligomers of PET, and trimer is the main component. Some researchers specialize in the study of PE T oligomers in microwave-sensitive materials. Several migrations of oligomers were detected in microwave popcorn heated by PET packaging, mainly trimer. The migration of PE T from microwave-sensitive packaging materials to food and analogues was also studied. The total migration amount of oligomers and the migration of various oligomers. ASTM F1349 takes the migration of PET oligomers as the detection index, and the peak of signal-to-noise ratio (> 3 times) is calculated in this paper. This is because there may be some other substances besides PE T oligomers, such as monomers, plasticizers and so on. At present, there is no specific research on the types of these migrants and migration conditions. Therefore, the calculation method in this paper can better reflect the migration characteristics of non-volatile UV absorbable substances. The specific types and hazards of migratory materials need to be further studied. The non-volatile ultraviolet absorbable substance (M) of the microwave sensing materials can be obtained by integrating the peaks (> 3 times the signal-to-noise ratio) in the chromatograms of each sample and calculating them.

The migration limits of non-volatile substances in four samples ranged from 11.2 to 143.0 ug/in 2, while the FDA's migration limits for PE T materials in contact with food were less than 20 ug/in 2 when the temperature of baking, baking, cooking and heating was higher than 250 F (121 C). Therefore, only one imported brand met the requirements of this limit. The results of the 3 brands tested exceeded the limit. This is because there is no special standard for the migration test of microwave sensing materials in China, and the relevant research results are few, which makes the implementation of the market access system for this kind of packaging materials lack of strong technical support. Therefore, such products may potentially harm people's health.

Conclusion:

(1) Microwave sensing materials and other food contact materials suitable for high temperature conditions such as microwave heating or oven baking, which contain oligomers, monomers and some additives will migrate at high temperatures.

(2) Corn oil was used as a simulated food to simulate the migration of microwave sensing materials in microwave popcorn bags at high temperatures. The migration of non-volatile substances was determined by high performance liquid chromatography (HPLC).

(3) Four brand samples on the market were relocated, of which only 97.1 imported Chinese plastics brands in July 2010 met the relevant FDA quota.