Standard acrylamide was obtained from Sigma Aldrich (Diesenhofen, Germany). Methanol, formic acid, n-hexane, acetone, ethyl acetate and acetonitrile were of HPLC grade and obtained from Merck (Darmstadt, Germany). Sodium chloride, ammonium sulfate and potassium ferrocyanide were of analytical grade, and purchased from Tianjin Chemical Factory (Tianjin, China). Ultrapure water (18.2 mΩ·cm) obtained from a Millipore purification system (Billerica, MA, USA) was used throughout the experiment.

The microwave-assisted extraction was carried out in an X-100A microwave extraction device (Xianghu Instrumental Company, Beijing, China) with a microwave power of 1,000 W and a temperature monitor as well as microprocessor programmer software to control the performance parameters of the device, i.e., microwave power, temperature and running time.

A total 123 food samples were collected from different supermarkets and restaurants in Guangzhou, China. The collected samples were transported to the laboratory and stored at −20 °C. The samples (about 2 g) were accurately weighed into a 50 mL centrifuge tube, and then mixed with an appropriate amount of ultra-pure water (20 mL). For food products with high fat content (crisps, chips, roasted nuts, olive, hazelnut paste, peanut paste, traditional desserts, meals), a defatting step should be performed by using n-hexane extraction (20 mL, twice) at room temperature before adding the water. Two grams of ammonium sulfate and 1.1 g potassium ferrocyanide were added. The mixture was vortexed for 2 min, then irritated by microwave for 10 min at 400 W power. After centrifugation at 7,000 g for 10 min at 4 °C, the supernatant was collected. Then, 6.5 g of sodium chloride and 20 mL of ethyl acetate were added, and the mixture was vortexed for 2 min. After centrifugation at 3,500 g for 10 min at 4 °C, the upper ethyl acetate layer was collected. Then, the ethyl acetate extract was dried by nitrogen gas, and the residue was redissolved in 0.8 mL of ultrapure water in a 2.5 mL capped centrifuge tube. Twenty µL of the aqueous sample was injected into the LC-MS-MS system for the analysis.

LC-MS-MS analysis of acrylamide was performed with an Agilent 1100 high-performance liquid chromatography (HPLC) system (Santa Clara, CA, USA) consisting of a quaternary pump, an autosampler and a temperature-controlled column oven, coupled to an Agilent 1100 MS detector equipped with an electrospray ionization interface. The analytical column was a Kinetex C18 column (100 × 4.6 mm, 2.6 µm particle size) from Phenomenex (Torrance, CA, USA). The mobile phase was acetonitrile and 1/1,000 formic acid aqueous solution (30:70, v/v) with a flow-rate of 0.2 mL/min. The temperature of column was kept at 25 °C. Data acquisition was performed, with a delay time of 8 min, in a selected ion-monitoring (MRM) mode using the following interface parameters: a drying gas (N₂, 665 Pa) flow of 11 L/min, nebulizer pressure of 300 Pa, drying gas temperatures of 350 °C, a capillary voltage of 11 kV and a fragmenter voltage of 40 eV.

The statistical analyses were carried out with SPSS Window version 10.0 (SPSS, Chicago, IL, USA). Data for acrylamide levels of analysis of variance was performed to test the difference in acrylamide levels depending on food type.

Prior to analysis of acrylamide, the LC-MS-MS method was validated to ensure the quality of the data. The linearity of the calibration curve was checked by a series of standard solutions of acrylamide ranged from 20 to 200 µg/L at seven different concentrations. The recoveries and the relative standard deviation were studied by the prepared samples spiked with different concentrations of acrylamide (0.8, 1.0 and 1.5 µg/kg). The results are expressed in Table 1. The method showed a good linearity in concentrations ranging from 20 to 200 μg/mL with a correlation coefficient of 0.9993. The limit of detection was 2.4 μg/mL based on a signal/noise ratio of 3:1 [23]. The recoveries ranged from 91% to 97%, and the relative standard deviation was 3.5%.

The contents of acrylamide in several food samples were simultaneously determined by LC/MS/MS in this study and gas chromatography (GC) based on the National Standards of Peoples Republic of China GB/T 5009.204-2005. No significant difference between the results of two methods was observed (p < 0.05).

A total 123 samples classified into 15 categories were analyzed for amounts of acrylamide. The occurrence and levels of acrylamide in the samples are shown in Table 2, and the frequency distribution of acrylamide content in foods is illustrated in Figure 2. In total 115 out of 123 samples showed positive levels of acrylamide ranging from 0.41 to 4,126.26 µg/kg, and the levels of acrylamide in 34 samples were more than 100 µg/kg. That is, acrylamide was present in 93.5% of the total samples analyzed, and 27.6% of the total samples contained over 100 µg/kg of acrylamide.

There was a great variation between different food types. Four samples of soybean paste were found positive, and their acrylamide contents (4.08–24.40 µg/kg) were the lowest among the samples tested. The samples of processed and cooked meat, rice roll, noodle, roasted bread, wafer biscuit, and sauteed nuts contained high levels of acrylamide, with average values of 22.62–44.92 µg/kg for the 37 positive samples out of the 41 samples analyzed. Roasted cake and biscuits showed markedly high levels of acrylamide, with average values of 68.34–97.57 µg/kg in 36 positive samples out of 38 samples analyzed. In this study, the highest levels of acrylamide contents, with an average of 131.73–604.27 µg/kg, were found in 38 positive samples of processed and cooked crisps, fried flour, rice, prawns, and potato products, and only 2 of the 40 samples were below the LOD. Similar results were seen in the previous research, which might be accounted for by the differences in cooking and processing parameters, methods, type and quality of raw materials, as well as formulations [24]. The effect of conditions of cooking and processing on the occurrence of the contaminant was considerable, and research in the relationship between levels of acrylamide and those parameters would shed some light on the options for reduction of the acrylamide content in foods.

In the present study, the occurrence of acrylamide in protein-rich foods was also investigated. The results showed that all heated meat foods were found positive, with the largest value of 78.57 µg/kg. This indicated that the occurrence of the acrylamide contamination occurred in the daily cooking procedures, but the heated protein-rich foods usually contained lower levels of acrylamide than carbohydrate-rich foods. The previous reports showed that acrylamide mainly originates from the Maillard reaction of the amino acid asparagine with reducing sugars [15,25]. Some studies have demonstrated that sugars could play an important role in the formation of acrylamide in some foods, and glucose might be the most effective [26,27].

The distribution and contents of acrylamide in food in some previous studies are shown in Table 3. In the present study, the levels of acrylamide in samples were equal or comparable to the similar samples in the previous research. As reported in other countries and regions, acrylamide also occurred in the daily heated foods in Guangzhou City, such as heated protein-rich foods, rice cakes, instant noodles, and biscuits (Table 2), which made up a large part of the daily public food consumption and therefore showed some risk to public health. The highest contents of acrylamide are often present in other carbohydrate-rich foods heated at higher temperatured, such as the roasted and fried snacks, which show different consumption patterns for people of various ages.

Those kinds of foods have a heavy contribution to acrylamide intake due to the fact they are usually a part of the normal diet, breakfast or supplemental food for children and teens, as well as for persons at work. As a result, it is urgent to employ abatement strategies for acrylamide in food for public health.