Urea (Aladdin, 99.5%); Thiourea (Aladdin, 99%); Tris (Aladdin, 99%); distilled water (Watsons); Bradford protein assay kit (Biosharp, BL524A); sinapic acid (Aldrich, 98%); trifluoroacetic acid (Alfa Aesar, 99.5%) were used as received.
Four species of meat, including chicken, pork, beef and duck, are common edible meat, were selected in this study. Totally, 103 pure meat samples and 81 mixed-meat samples were collected and the different tissue parts of each species shown in Table S1 were used. Pork, chicken and duck were chosen to mix with beef meat in a binary mixture at the mass percentage of 0.0%, 25.0%, 50.0%, 75.0% and 100.0% for mimicking beef meat adulterated with other species. Moreover, the ternary mixture samples containing 50% beef meat mixed with different percentage of chicken meat (50–0%) and pork meat (0–50%) were used for investigating the ability of the approach in differentiating the ternary mixtures. As the price of duck meat is the lowest among many different meat species, it is likely to partly substitute beef products with duck meat in adulteration. Therefore, duck meat was mixed with beef meat in 3% and 5% to evaluate the detection limit of the approach. All of these meats in fresh were purchased at local markets or supermarkets. Five market samples for testing were purchased in a supermarket and a hotpot restaurant. All the meat samples were stored at -20℃ until use.
For each sample, 5 g of muscle tissues was chopped into paste, fascia and fat were discarded. Then, 0.5 g of meat paste was suspended in 10 mL protein extraction reagent (6M Urea, 1M Thiourea, 50mM Tris, at pH8.2) (Bargen et al. 2014) and homogenized with a homogenizer operated at 8000rpm for 2 min. The homogenate was centrifuged at 12000g for 1 min at 4℃. The supernatant was then filtered with a 0.45µm (polyvinylidene fluoride, PVDF) membrane filter, followed by dialysis in distilled water for 8 hours.
To minimize the effect of non-biological factors (such as different extraction efficiency and the protein content of different individuals) on the protein profiles, the protein content normalization is applied (Wu and Li 2016). The protein concentration of each dialyzed sample was measured using Bradford method with a microplate reader (Synergy H1 H1MF, Agilent, USA). The sample was diluted with 0.1% trifluoroacetic acid solution to 0.35 mg/ml (protein concentration). Quality control (QC) samples were prepared by mixing 20uL protein solution of each diluted sample for evaluating the stability of this method. The MALDI matrix solution, saturated sinapic acid (SA) solution, was prepared by dissolving SA in 30% acetonitrile solution with 0.1% trifluoroacetic acid. The sample solution (15uL) was mixed with saturated SA solution in 1:1 ratio. Then, 2.5µl of each mixture was applied on the MALDI sample plate (n = 5) and dried at room temperature, which was then analyzed with a MALDI-TOF mass spectrometer. Triplicated analyses were performed to ensure the intraday and interday reproducibility.
The Autoflex Speed MALDI-TOF/TOF mass spectrometer (Bruker Daltonics, Germany) was employed in this study. All the analyses were conducted in the positive ion mode with a mass range of 3,000 − 22,000 Da in a linear mode. The MS conditions are as follows: laser power: 99.6%; frequency: 500Hz; shot number: 20000 operated at random walk mode; delayed extraction time: 350 ns.
The MALDIquant package (Gibb and Strimmer 2012) of R programming language was used for data preprocessing, including spectra smoothing, baseline correction, alignment, technical replicates averaging and peak detection. Finally, a feature matrix was created and then a normalization method (MS total useful signal, MSTUS) (Wu and Li 2016) built in Microsoft Excel was used to normalize this feature matrix.
Multivariate data analysis was performed with SIMCA-P14.1 software (version 14.1; Umetrics AB, Umeå, Sweden). PCA was applied to reveal the general clustering and grouping trends among four meat species. PLS-DA was used to maximize the separation between samples, and the feature peaks responsible for the separation were found based on the corresponding V + S-plots (VIP ≥ 1, |p(corr)| ≥ 0.5, |p| ≥ 0.05) of PLS-DA (Almeida et al. 2013; Bao et al. 2016; Li et al. 2019; Peerbhay et al. 2013). Then, these feature peaks of four meat species were further confirmed in the MALDI-MS raw spectra based on a sufficient signal-to-noise ratio (S/N > 3) and a good reproducibility, and then identified through Uniprot (the protein database). To build a linear regression prediction model for determining the adulteration ratios in binary mixtures, PLS method is applied, in which, samples among four meat species were separated into training (2/3) and validation set (1/3). (Table S2).
To evaluate the prediction capability of our approach, blind test and market-sample test were adopted. Seven binary mixture samples of beef mixed with chicken, duck or pork in random ratios (sample 1 − 7) were prepared as the blind samples, and five beef products (M1, M2, M3, M4 and M5) purchased from a supermarket and a hotpot restaurant were used as the market samples. The same sample preparation procedures and MS data acquisition conditions mentioned above were adopted for the sample analysis. Moreover, the same multivariate data analysis, including PCA, PLS-DA and the linear regression prediction model based on partial least squares (PLS) method were applied to the data analysis.