Systematic study of the quality and safety of chilled pork from wet markets, supermarkets, and online markets in China

Abstract

Background: With increasing media coverage of food safety incidents, such as that of clenbuterol residues in pork, food safety has become a major public health concern in China. Rapidly developing online markets attract increasing numbers of Chinese consumers to purchase food on the Internet. However, the quality and safety of food sold online are uncertain and are less reported on. Objective: This research aimed to systematically study the quality and safety of chilled pork from wet markets, supermarkets, and online markets in China. Results: The chilled pork samples from online markets were fresher than those from wet markets and supermarkets based on the surface redness (a* value). Chilled pork contained high levels of nutritional elements, especially the magnesium and phosphorus levels in samples from online markets. The levels of heavy metal element residues and veterinary drug residues in all chilled pork samples were within the standards limits. In addition, huge differences existed in the quality and freshness of the chilled pork samples from online markets according to principal component analysis (PCA). Conclusions: Most chilled pork sold in Chinese markets was qualified and safe. It is necessary to establish an effective online market supervision system for chilled pork.

1. Introduction

Pork meat is widely consumed around the world, especially in Europe and Southeast Asia. China, the largest consumer of pork in the world, accounts for approximately half of global pork consumption. In 2017, pork consumption in China reached 54.8 million tons, twice as much as that in the European Union (20.8 million tons) and five times as much as that in the United States (9.5 million tons) (United States Department of Agriculture and Foreign Agricultural Service, 2018). Pork consumption per capita in China grew steadily between 1975 and 2012 (Verbeke and Liu, 2014). Pork not only possesses a pleasant favor and taste but also contributes to the daily intake of protein, fat, and important micronutrients, such as essential trace elements (de Smet and Vossen, 2016). Because of the high levels of protein and fat and the contamination by microorganisms, pork is easily corrupted during storage, transportation, and sales, resulting in low quality or even safety problems. On the other hand, the residue of harmful substances has become a big challenge to the safety of pork, including heavy metal elements and veterinary drugs. In 2011, clenbuterol, a kind of β-agonist class veterinary drug, which is banned in China, was found in fresh pork produced by Shuanghui Group, the largest Chinese meat producer. It has been reported that β-agonists cause adverse effects to human health, such as cardiovascular and central nervous diseases (Shao et al., 2009). Nevertheless, these detrimental compounds are still used illegally as feed additives for their functions in enhancing growth and reducing the body fat of pigs (Wang et al., 2010). Therefore, the safety of pork is receiving more and more attention from consumers. With the increasing exposure of food safety incidents, media coverage left consumers with a high level of uncertainty and wariness regarding the safety of food. Food safety has become a major public health concern in China.

Chinese consumers are used to purchasing pork from wet markets and nearby supermarkets. As an important part of traditional sale channels, wet markets provide fresh and cheap food for consumers, and so still function well in most cities. The majority of outdoor wet markets have been moved to indoor operations in cities (Bai et al., 2008). Moreover, the urban wet markets usually possess large scale and considerable numbers of consumer groups. In wet markets, pork is put on the table in a big hunk at room temperature, and sellers will cut off a part of it and sell according to consumers’ request, which makes pork exposed to the potential environmental hazards and cross-contamination. Therefore, supermarkets have become the other choice for consumers to buy pork. Supermarkets, where the pork is under the scrutiny of the supermarket’s food merchandising department, are considered as a safer and better source than wet markets, especially the multinational supermarket chains (Qing et al., 2014). In supermarkets, pork is cut into small pieces, packed, and stored in the refrigerator for sale. With the wide diffusion of internet technologies into businesses and households, ecommerce industry and online markets are growing rapidly. More and more Chinese consumers prefer to shop on the Internet because of its convenience. The total sales of online shopping in China reached up to 308 billion dollars in 2013, accounting for 7.9% of total sales (Ding and Lu, 2015). However, research regarding the quality and safety of food sold in online markets is limited. The food in online markets, whose sources are more varied intricate than the traditional sale channels, is difficult to regulate and supervise.

The objective of this research was to systematically study the quality and safety of chilled pork from wet markets, supermarkets, and online markets in China. The information obtained from our study might roughly reflect the current situation of the Chinese pork market. Hangzhou, the city where we chose to sample, is one of the most renowned and prosperous cities in China and the location of the headquarters of Alibaba, the world’s largest retailer. Hangzhou possesses a developed economy and logistics industry. The chilled pork samples from wet markets and supermarkets were purchased in Hangzhou, while samples from online markets were purchased on the Internet and sent to Hangzhou by logistics.

2. Materials and methods

2.1. Chilled pork samples

A total of 143 different chilled pork (longissimus lumborum muscle) samples were collected from wet markets, supermarkets, and online markets. Among them, 57 and 33 samples were purchased from wet markets and supermarkets, respectively, in six main districts in Hangzhou, Zhejiang, China (Fig. 1). The wet markets where we chose to sample possess large scale and stable consumer groups. The supermarkets where we chose to sample mostly belong to multinational retail corporations, including Walmart, Tesco, Carrefour, etc. At least three chilled pork samples (approximately 400 g/each sample) from three different disperse stalls were purchased at each wet market or supermarket. Then the samples were immediately transferred into sterile bags and put into a bubble chamber with ice packs. The refrigerated samples were transported to the laboratory within 1 h and were immediately measured for color, pH, and total volatile basic-nitrogen (TVB-N). The remaining chilled pork samples were stored at −20 °C for further analysis.

Fig. 1.

Map of six main districts in Hangzhou with locations of sampled wet markets and supermarkets

All 53 chilled pork samples were purchased at online markets located in 12 different provinces in China, including Anhui (1 sample), Beijing (4 samples), Guangdong (1 sample), Hainan (4 samples), Hebei (3 samples), Henan (1 sample), Jiangsu (3 samples), Shandong (5 samples), Shanghai (17 samples), Sichuan (2 samples), Tianjin (1 sample), Zhejiang (11 samples). These samples were transported to the laboratory in cold chain with modified atmosphere packaging and then immediately measured for color, pH, and TVB-N. The remaining chilled pork samples were stored at −20 °C for further analysis.

2.2. Determination of color, pH, and TVB-N

The surface color value of chilled pork samples was measured by CIE L ^* a ^* b ^* system using a Minolta chromameter (CR-10 Plus, Minolta, Japan), with measurements standardized with black and white calibration plates. The pH of samples was measured at different sites on the slice of chilled pork using a pH meter (PH-STAR, Matthäus, Germany). The TVB-N content of samples was determined according to Chinese National Standard GB 5009.228-2016 (NHC, 2016). Briefly, 10.0 g of the chilled pork sample was homogenized with 100 mL of distilled water and filtered through filter paper. Ten milliliters of the filtrate were added to 5 mL of magnesia (10 g/L) and then distilled for 5 min using a Kjeldahl distillation unit. The distillate was absorbed by 10 mL of boric acid (20 g/L) and titrated with 0.01 mol/L hydrochloric acid. The TVB-N (mg/kg) content was calculated by the following equation:

,

where V ₁ is the titration volume of the tested sample (mL), V ₂ is the titration volume of blank (mL), c is the actual concentration of hydrochloric acid (mol/L), and m is the weight of the chilled pork sample (g).

2.3. Determination of element contents

The element contents of samples were determined according to Chinese National Standard GB 5009.268-2016 (NHC and SAMR, 2016) with slight modifications. In order to determine the contents of elements, the chilled pork samples first underwent a digestion procedure. In brief, approximately 10.0 g of the sample was homogenized using a tissue homogenizer. The homogenate (1.0 g) was placed in a quartz pressure reaction vessel and 5 mL of nitric acid was added. Then the vessel was removed to a microwave oven (MarsXpress, CEM, USA) for digestion. The digestion procedure was conducted by applying a program as follows: 0–5 min, 20–120 °C; 5–10 min, 120 °C; 10–15 min, 120–150 °C; 15–25 min, 150 °C; 25–30 min, 150–180 °C; 30–45 min, 180 °C. After the vessel had cooled, the digest was transferred to a 50-mL volumetric flask and diluted with distilled water. The element contents (Ca, Fe, Zn, Mg, Cu, and P) in samples were determined by inductively coupled plasma optical emission spectrometry (Optima 8000, Perkin Elmer, USA). The other element contents (Pb, As, Hg, Cd, and Cr) were determined by inductively coupled plasma mass spectrometry (ELAN 9000, Perkin Elmer, USA). The limit of detection was as follows: Ca, 0.2 mg/kg; Fe, 1.0 mg/kg; Zn, 0.5 mg/kg; Mg, 5.0 mg/kg; Cu, 0.2 mg/kg; P, 1.0 mg/kg; Pb, 0.02 μg/kg; As, 1.0 μg/kg; Hg, 1.0 μg/kg; Cd, 0.1 μg/kg; Cr 10 μg/kg. The intra-assay coefficients of variation (CVs) of element content determination ranged from 0.53% to 3.73%, whereas the inter-assay CVs ranged from 3.67% to 6.94% (Table S1).

2.4. Determination of veterinary drug residues

2.4.1 Tetracyclines

The residues of tetracyclines (tetracycline, oxytetracycline, and chlortetracycline) were detected by high-performance liquid chromatography (HPLC; Agilent 1260, Agilent, USA) and quantified by commercial standards according to Chinese National Standard GB/T 21317-2007 (SAMR, 2007) with slight modifications. Briefly, a chilled pork sample was homogenized using a tissue homogenizer. Five grams of the homogenate was weighed into a 50-mL plastic tube, added 40 mL of ethylenediaminetetraacetic acid (EDTA)-McIlvaine buffer solution, and then extracted by ultrasound for 30 min. The extraction was centrifuged at 3000 r/min for 5 min. The collected supernatant was diluted to 50 mL and purified by solid-phase extraction cartridge. Finally, the extraction was obtained with an injector, filtered through a 0.22-μm membrane filter, and injected into a HPLC system equipped with Inertsil C18-3 column (4.6 mm×250.0 mm, 5 μm). The mobile phase, consisting of methanol (A), acetonitrile (B), and distilled water (C) with 10 mmol/L trifluoroacetic acid, was pumped at 1.5 mL/min into HPLC system with the following gradient elution program: 0–5 min, 1%–6% A, 4%–24% B; 5–9 min, 6%–7% A, 24%–28% B; 9–12 min, 7%–0% A, 28%–35% B; 12–15 min, 0% A, 35% B. The total running time was 15 min. The absorbance was detected at 350 nm. The column temperature was maintained at 30 °C and the injection volume was 100 μL.

2.4.2 β-Agonists

The residues of β-agonists (clenbuterol, ractopamine, and salbutamol) were detected by the liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS; Agilent 6460, Agilent, USA) according to Bulletin 1025-18-2008 of the Ministry of Agriculture of China (MARA, 2008a) with slight modifications. Two grams of the homogenized samples were weighed into a 50-mL plastic tube and 10 mL of sodium acetate buffer solution and 50 μL of β-glucuronidase/arylsulfatase were added. Then the solution was incubated at 37 °C for 12 h. After that, the solution was acidified to pH 1.0 with perchloric acid and centrifuged at 5000 r/min for 10 min. The supernatant was transferred to another tube and adjusted to pH 4.0 with sodium hydroxide. Next, the solution was centrifuged at 5000 r/min for 10 min and purified by solid-phase extraction cartridge. Finally, the extraction was filtered through a 0.22-μm membrane filter and injected into a LC-MS/MS system equipped with Waters Atlantics C18 column (2.1 mm×150.0 mm, 5 μm). The mobile phase, consisting of acetonitrile (A) with 0.1% formic acid and distilled water (B) with 0.1% formic acid, was pumped at 0.2 mL/min into the LC-MS/MS system with the following gradient elution program: 0–2 min, 4% A; 2–8 min, 4%–80% A; 8–21 min, 80%–23% A; 21–22 min, 23%–95% A; 22–25 min, 95% A; 25–25.5 min, 95%–4% A. The total running time was 25.5 min. The column temperature was maintained at 30 °C and the injection volume was 20 μL.

The mass spectrometer was operated in positive electrospray ionization mode (ESI+). The following parameters were optimal: capillary voltage, 3.2 kV; cone voltage, 35 V; multiplier voltage, 650 V; desolvation gas flow rate, 600 L/h; cone gas flow rate, 50 L/h; source temperature, 110 °C; desolvation temperature, 350 °C. The radio frequency (RF) lens was set at 0.5 V. Ion energies 1 and 2 were both held at 0.5 V. The quantitative ion-pairs were as follows: clenbuterol, 277.1>203.3 m/z; ractopamine, 302.3>164.3 m/z; salbutamol, 240.2>148.2 m/z. Detection was carried out in multiple reaction monitoring (MRM) mode. Argon was used as the collision gas.

2.4.3 Sulfonamides

The residues of sulfonamides (total) were detected by LC-MS/MS (Agilent 6460, Agilent, USA) according to Bulletin 1025-23-2008 of the Ministry of Agriculture of China (MARA, 2008b) with slight modifications. Five grams of the homogenized sample were weighed into a 50-mL plastic tube and 30 mL of ethyl acetate was added. The mixture was vortexed for 2 min and centrifuged for 10 min at 5000 r/min. Hydrochloric acid (0.1 mol/L) was added to the collected supernatant. Then the mixture was evaporated at 45 °C to remove the ethyl acetate. The remaining mixture was purified by solid-phase extraction cartridge. Finally, the extraction was filtered through a 0.22-μm membrane filter and injected into a LC-MS/MS system equipped with Waters Atlantics C18 column (2.1 mm×150.0 mm, 5 μm). The mobile phase, consisting of acetonitrile (A) with 0.1% formic acid and distilled water (B) with 0.1% formic acid, was pumped at 0.2 mL/min into the LC-MS/MS system with the following gradient elution program: 0–5 min, 10%–25% A; 5–20 min, 25%–55% A; 20–30 min, 55%–10% A. The total running time was 30 min. The column temperature was maintained at room temperature and the injection volume was 10 μL.

The mass spectrometer was operated in ESI+. The following parameters were optimal: capillary voltage, 3 V; desolvation gas flow rate, 440 L/h; source temperature, 80 °C; desolvation temperature, 300 °C. Detection was carried out in MRM mode. Argon was used as the collision gas. The intra-assay CVs of the veterinary drug residue determination ranged from 2.02% to 4.60%, whereas the inter-assay CVs ranged from 3.18% to 9.17% (Table S1).

2.5. Statistical analysis

Data were expressed as mean±standard deviation (SD) from at least three independent experiments. One-way analysis of variance (ANOVA) with the multiple range significant difference (Duncan) test (P<0.05) was carried out using SPSS 22.0 to determine the significant differences among the chilled pork from three different sources. Principal component analysis (PCA) was carried out using the Unscrambler X 10.3 to reduce the dimensional space and visualize the data trends. For PCA, the input data were standardized (1/standard deviation) and mean centered (mean=0).

3. Results and discussion

3.1. Color, pH, and TVB-N

Color (L ^* value, a ^* value, b ^* value), pH, and TVB-N of chilled pork samples from three different sources were presented in Fig. 2. Color is one of the important quality traits of chilled pork for consumers in the purchase decision. As we can see from Fig. 2, L ^* value and b ^* value showed no difference in chilled pork samples from three different sources, while the a ^* value of chilled pork from online markets was significantly higher than that from offline markets (wet markets and supermarkets). The difference of a ^* value was related to the accumulation of metmyoglobin on the chilled pork surface. After pigs were slaughtered, oxygen diffused to a deep level in meat and oxidized the myoglobin to oxymyoglobin in the first few hours. Afterwards, the cherry red oxymyoglobin was further oxidized to grey-brown metmyoglobin, and thus the a ^* value of meat increased in the first 24 h and declined gradually during storage under refrigerated conditions (O'Sullivan et al., 2002; Estévez et al., 2003). The higher a ^* value of chilled pork from online markets than that from offline markets indicated a better freshness of pork samples procured from online markets. The pH value of meat gradually increased during storage due to the decomposition of protein and accumulation of alkaline substances (ammonia and amine compounds). No significant difference was observed in pH values among the chilled pork samples from three different sources. As for the TVB-N value, one of the important reference indices for evaluating meat freshness, the chilled pork samples from supermarkets and online markets showed significantly lower TVB-N values than those from wet markets. Similar to pH value, the TVB-N value increased gradually during storage under refrigerated conditions (Huang et al., 2017). The TVB-N value remained below 150 mg/kg, which suggested that the meat was still fresh (Huang et al., 2014). Most of the chilled pork samples (132/143) in this investigation were fresh according to this criterion. Collectively, our results suggested that the chilled pork samples from wet markets, supermarkets, and online markets were mostly fresh and acceptable. Furthermore, the chilled pork samples from online markets were the freshest, which might be attributed to the cold chain system and the modified atmosphere packaging.

Fig. 2.

Physicochemical parameters of chilled pork samples from three different sources

Data are expressed as mean±standard deviation. Different lowercase letters in the same parameter indicate significant differences (P<0.05) obtained by ANOVA and Duncan test. TVB-N, total volatile basic-nitrogen

3.2. Nutritional elements

The contents of nutritional elements (Ca, Fe, Zn, Mg, Cu, and P) in chilled pork samples from three different sources were presented in Table 1. As an important kind of red meat, pork provides nutritional elements for the human diet in an organic and well-absorbable form (de Smet and Vossen, 2016). Pork is a good carrier of calcium. The Ca levels of chilled pork samples from wet markets, supermarkets, and online markets were (0.23–330.60), (0.57–85.42), and (1.51–787.84) mg/kg, respectively. There was no detectable variation in Ca levels among the chilled pork samples from the three different sources. Zhu et al. (2007) compared the levels of Ca between boar pork and conventional pork, and no significant difference was observed. The dietary composition and rearing regime also showed no effect on the Ca level of pork, indicating that the level of Ca might be a stable value in pork (Zhao et al., 2016). Similarly, no significant difference in Fe levels was observed among chilled pork samples from the three different sources. The total Fe level of pork was not influenced by the rearing system and storage time (Karwowska and Dolatowski, 2013). Zn levels in chilled pork samples (10.25–51.44 mg/kg) were in the range reported by Jablonska et al. (2013) in 24 selected meats (4.3–87.3 mg/kg). Meat contributed to 26% of the dietary zinc intake in the Polish diet (Jablonska et al., 2013). As shown in Table 1, the chilled pork samples from wet markets possessed a higher Zn level than those from supermarkets. Observed levels of Mg in chilled pork samples ranged from 203.3 to 385.0 mg/kg, and the samples from online markets showed the highest Mg level. Our results for Zn and Mg levels in chilled pork samples were slightly higher than those reported by Djinovic-Stojanovic et al. (2017) in samples collected from Serbian markets. No significant difference in Cu level was observed among the chilled pork samples from the three different sources. The Cu levels of pork samples obtained in our study (e.g. (2.89±1.82) mg/kg in samples from supermarkets) were almost five-to ten-fold higher than those from Swedish supermarkets and butcheries ((0.41±0.23) mg/kg) (Gerber et al., 2009). As a growth promoter, Cu was incorporated into feed at a concentration exceeding the physiological requirements of pigs. However, the excessive dietary Cu cannot be absorbed by the pigs (Zhao et al., 2016). The efficiency of dietary Cu absorption was related to the genetic variation (Andrée et al., 2010). Regarding P levels, the chilled pork samples from online markets ((2.58±0.33) mg/kg) showed significantly higher values than those from wet markets ((2.30±0.24) mg/kg) and supermarkets ((2.41±0.21) mg/kg).

Table 1.

Contents of nutritional elements in chilled pork samples from three different sources

Source	Ca (mg/kg)		Fe (mg/kg)		Zn (mg/kg)
	Mean±SD	Range	Mean±SD	Range	Mean±SD	Range
Wet market	39.81±43.62a	0.23–330.60	7.20±3.57a	3.59–16.21	21.38±9.04b	12.58–51.44
Supermarket	40.62±27.23a	0.57–85.42	6.97±4.33a	3.51–22.33	16.02±3.23a	10.25–25.49
Online market	66.15±124.45a	1.51–787.84	6.36±2.49a	2.87–12.57	19.08±7.22ab	11.73–47.43
Source	Mg (mg/kg)		Cu (mg/kg)		P (g/kg)
	Mean±SD	Range	Mean±SD	Range	Mean±SD	Range
Wet market	281.72±27.73ab	217.70–331.40	3.18±1.51a	0.94–7.32	2.30±0.24a	1.97–3.71
Supermarket	274.55±23.41a	228.60–314.80	2.89±1.82a	0.47–11.13	2.41±0.21a	2.18–3.05
Online market	294.19±38.70b	203.30–385.00	3.68±2.31a	0.55–8.47	2.58±0.33b	1.68–3.25

^{a, b} Means in the same line with different lowercase letters are significantly different (P<0.05) according to ANOVA and Duncan test. SD: standard deviation. The measurements of chilled pork samples were based on wet weight

3.3. Heavy metal elements

The contents of heavy metal elements (Pb, As, Hg, Cd, and Cr) in chilled pork samples from three different sources were presented in Table 2. The maximum residue limits of heavy metal elements in pork were based on the Chinese National Standard GB 2762-2017 (NHC, 2017). Apart from the nutritional elements, the heavy metal element residues in meat have received increasing attention in recent years. The heavy metal elements were accumulated and concentrated in the organs and tissues of animals from feed, pasture, polluted air or water (Andrée et al., 2010). Owing to the reduction of Pb emissions, the Pb level in food decreased significantly (Pilarczyk, 2014). As shown in Table 2, the Pb level of chilled pork samples from online markets ((11.27±12.74) μg/kg) was nearly half of those from wet markets ((22.82±20.48) μg/kg) and supermarkets ((25.55±21.29) μg/kg). The levels of Pb observed in our study were similar to the previous study reported by Gerber et al. (2009) in pork loin ((18.0±1.0) μg/kg). As for As and Hg levels, these heavy metal elements were mostly found in seafood, while in meat they were often detected at marginal levels (Andrée et al., 2010). Expectedly, the As levels of all chilled pork samples were lower than the standard limit (0.5 mg/kg), and the samples from wet markets showed the lowest As levels. There was no detectable variation among the Hg levels of chilled pork samples from the three different sources. The Hg levels of 44 samples (14 samples from wet markets, 15 samples from supermarkets, 15 samples from online markets) were below the limit of detection (1.0 μg/kg). Cd, a human carcinogen (Group 1), is efficiently retained in the liver and kidney after absorption by the human body and has a long biological half-life (10–30 years). For the general non-smoking population, food is the main source of Cd exposure (Tomović et al., 2011). The Cd levels in chilled pork samples were extremely low, ranging from 0.10 to 2.25 μg/kg. The Cd levels in samples from online markets ((0.47±0.39) μg/kg) were higher than those from wet markets ((0.21±0.31) μg/kg) and supermarkets ((0.18±0.19) μg/kg), but much lower than those reported by López-Alonso et al. (2007) in pork samples obtained from north-west Spain ((9.0±7.0) μg/kg). No significant difference in Cr levels was observed among chilled pork samples from the three different sources. The heavy metal element levels of all chilled pork samples were in line with the Chinese National Standard, indicating that the chilled pork sold in Chinese markets was qualified and safe in heavy metal element residues and contaminants.

Table 2.

Contents of heavy metal elements in chilled pork samples from three different sources

Source	Pb (μg/kg)		As (μg/kg)		Hg (μg/kg)
	Mean±SD	Range	Mean±SD	Range	Mean±SD	Range
Wet market	22.82±20.48b	2.70–138.00	7.58±6.72a	ND–31.70	1.61±1.78a	ND–7.50
Supermarket	25.55±21.29b	7.00–102.20	14.05±8.24b	4.40–43.30	1.48±2.10a	ND–6.20
Online market	11.27±12.74a	0.10–80.60	11.49±4.91b	4.50–31.40	1.38±2.28a	ND–13.00
Limit	≤200		≤500		≤50
Source	Cd (μg/kg)		Cr (μg/kg)
	Mean±SD	Range	Mean±SD	Range
Wet market	0.21±0.31a	ND–2.25	46.16±57.17a	ND–335.90
Supermarket	0.18±0.19a	ND–0.69	72.01±31.78a	ND–130.40
Online market	0.47±0.39b	ND–1.85	77.82±58.27a	ND–182.20
Limit	≤100		≤1000

^{a, b} Means in the same line with different lowercase letters are significantly different (P<0.05) according to ANOVA and Duncan test. ND: not detected, <1.0 μg/kg for As, <1.0 μg/kg for Hg, <0.1 μg/kg for Cd, <10.0 μg/kg for Cr. The measurements of chilled pork samples were based on wet weight

3.4. Veterinary drugs

The veterinary drug residues of chilled pork samples from the three different sources were presented in Table 3, including three groups: tetracyclines (oxytetracycline, chlortetracycline, and tetracycline), sulfonamides (total), and β-agonists (clenbuterol, ractopamine, and salbutamol). The maximum residue limits of veterinary drug in pork were based on the Chinese Agricultural Industry Standard NY/T 632-2002 (MARA, 2002). The levels of detected veterinary drugs in chilled pork samples were all below the limit of detection, indicating that all samples were in line with the industry standard in the veterinary drug residues. Tian et al. (2016) developed a novel ultra-HPLC-MS/MS approach for detection of 46 veterinary drugs and analyzed 30 pork samples and 20 beef samples obtained from Beijing local markets using this method. No veterinary drug residue was detected among the 50 commercial samples, except enrofloxacin in one pork sample and sulfadimethoxine in one beef sample. Regarding the veterinary drug residues of pork from two provinces in Vietnam, Hung Yen and Nghe An, tetracyclines were not present in pork samples, but 50% (9/18) and 5% (1/18) samples were positive for sulfonamides and β-agonists, respectively (Tuyet-Hanh et al., 2017).

Table 3.

Contents of veterinary drugs in chilled pork samples from three different sources

Source	Oxytetracycline (μg/kg)	Chlortetracycline (μg/kg)	Tetracycline (μg/kg)	Sulfonamides (μg/kg)
Wet market	ND (<50)	ND (<50)	ND (<50)	ND (<0.5)
Supermarket	ND (<50)	ND (<50)	ND (<50)	ND (<0.5)
Online market	ND (<50)	ND (<50)	ND (<50)	ND (<0.5)
Limit	≤100	≤100	≤100	≤100
Source	Clenbuterol (μg/kg)	Ractopamine (μg/kg)	Salbutamol (μg/kg)
Wet market	ND (<0.5)	ND (<0.5)	ND (<0.5)
Supermarket	ND (<0.5)	ND (<0.5)	ND (<0.5)
Online market	ND (<0.5)	ND (<0.5)	ND (<0.5)
Limit	Negative	Negative	Negative

ND: not detected. The measurements of chilled pork samples were based on wet weight

3.5. Principal component analysis

PCA was applied to individual color, pH, TVB-N, and elements of chilled pork samples for reducing the dimensional space built on the dissimilarities among samples from different sources. To visualize the data trends and discriminate the efficiency of detected parameters, a scatter plot of chilled pork samples was obtained using the first two principal components from PCA (Fig. 3). In PCA graphics, points, which are geometrically close to each other, possess similar properties. As shown in Fig. 3, the chilled pork samples from online markets were highly dispersive, while those from wet markets and supermarkets were relatively concentrated on the left side of the graphic, indicating that the chilled pork from online markets possessed huge differences in quality and freshness. These chilled pork samples were purchased from online markets that were located all over China. The breed and feed of pig or sanitary conditions of abattoirs might vary greatly, which could result in the huge differences of the pork. Therefore, the safety and quality of chilled pork sold in online markets needed to be paid more attention.

Fig. 3.

PC-1 vs. PC-2 scatter plot of the chilled pork samples from the three different sources

(a) Scores plot of PCA on individual color (L ^*, a ^*, b ^*), pH, TVB-N, and elements of chilled pork samples from three different sources (57 samples from wet markets, 33 samples from supermarkets, 53 samples from online markets); (b) Loadings plot. PCA, principal component analysis; TVB-N, total volatile basic-nitrogen

The influence of different parameters that described the chilled pork samples from the three different sources can be evaluated from the scatter plot (Fig. 3). Most samples from wet markets located on the left side of the graphic showed high TVB-N value. In wet markets, the chilled pork was sold on a cutting board at room temperature. The cutting board, knife, hands of seller, or air might contaminate the pork, leading to the multiplication of microorganism and the increase of TVB-N value. Several chilled pork samples from wet markets showed high Zn and Fe levels. A large number of chilled pork samples from online markets were located on the lower-right side of the graphic, indicating the high levels of Ca, Mg, P, and Cd, which was consistent with the results given by Duncan’s test.

4. Conclusions

A total of 143 different chilled pork samples from wet markets, supermarkets, and online markets were evaluated in our study. Levels of color (L ^*, a ^*, b ^*), pH, TVB-N, nutritional elements, heavy metal elements, and veterinary drug residues in chilled pork samples were determined and assessed by comparing with the levels in standards and data in the literature. The results showed that all chilled pork samples were in line with standards in heavy metal element residues and veterinary drug residues, indicating that the chilled pork sold in Chinese markets was qualified and safe. In addition, chilled pork samples from online markets were fresher than those from wet markets and supermarkets. However, huge differences existed in the quality and freshness of chilled pork samples from online markets according to PCA analysis. Therefore, it is necessary to establish an effective online market supervision system for chilled pork.

List of electronic supplementary materials

Table S1

Intra-assay and inter-assay coefficients of variation(CVs) of element content determination

Table S2

Intra-assay and inter-assay coefficients of variation(CVs) of veterinary drug residue determination