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Study of the flavor volatiles in double-fried pork: Identification and analysis of their changes during frozen storage using headspace solid-phase microextraction and gas chromatography-mass spectrometry

Aiquan Jiao1,2,3, Xueming Xu2,3, Zhengyu Jin1,2,3*

1State Key Laboratory of Food Science and Technology, Jiangnan University, Wuxi, 214122, PR China

2School of Food Science and Technology, Jiangnan University, Wuxi, 214122, PR China

3Synergetic Innovation Center of Food Safety and Nutrition, Jiangnan University, Wuxi, 214122, PR China

*Corresponding Author:
Zhengyu Jin
Food Science
State Key Laboratory of Food Science and Technology
Jiangnan University
PR China
Tel: +0086-510-85919189
E-mail: [email protected]

Accepted date: April 04, 2017

Citation: Jiao A, Xu X, Jin Z. Study of the flavor volatiles in double-fried pork: Identification and analysis of their changes during frozen storage using headspace solid-phase microextraction and gas chromatography-mass spectrometry. J Food Technol Pres. 2017;1:47-55

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Abstract

The flavor volatiles present in double-fried pork and their changes during frozen storage were investigated for the first time using head space-solid phase micro extraction (HS-SPME) and gas chromatography-mass spectrometry (GC-MS) methods. We identified total 91 types of volatiles including eighteen heterocyclic compounds (6.9%), fifteen alcohols (11.4%), thirteen aldehydes (51.2%), twelve esters (7.7%), eleven hydrocarbons (2.3%), seven sulfur compounds (9.4%), six ketones (7.0%), and four acids (1.4%). Both the content and the types of heterocyclics and esters significantly increased post cooking. Principal component analysis showed three principal components (PC) with the first one (PC1) representing majority of the flavor compounds (73.65%), identified as the characteristic flavors. We could distinguish between the samples with different frozen time from the distinct flavors: (i) 1-penten-3-ol, 3-methyl-1-butanol and octanal in one month frozen samples; (ii) nonanal, trans-2,4-decadienal, and phenylethyl alcohol in 2-3 months frozen samples, and (iii) hexanal and ethanol in the 5-6 months frozen samples.

Keywords

Double-fried pork, Volatile compounds, GC-MS analysis, frozen storage, PCA analysis.

Introduction

Pork, an important nutritional ingredient in the human diet, is rich in high-quality protein, essential fatty acids, and some vital trace elements such as calcium, phosphorus, iron, and thiamin [1]. Double-fried cooking, a characteristic processing technique for pork in China, has also been popular all over the world in recent years. Flavor of double-fried pork is considered as the primary palatability factor and is shown to be of great importance. The number of flavor volatile compounds, identified in the extracts of livestock and poultry meat, is over one thousand; those include aldehydes, ketones, esters, ethers, alkanes, alkenes, alcohols, carboxylic acid, and some heterocyclic compounds (oxygen, nitrogen and sulfur) [2]. The flavor of pork is not only influenced by breed, age, gender, feed environment of animals, but also affected by the physical and chemical factors such as processing methods (cooking, baking, or fermentation process), storage conditions, or the addition of different pickled formulations and spices [3,4]. Many researchers have studied the flavor of pork processed by different methods, for example, Zeng et al. [5] investigated the compositions of flavor compounds of pork, boiled in clear soup or braised in soy sauce; Chen et al. [6] studied the aroma of pork slices cooked in fermented mustard greens; Du [7] determined the flavor of fermented pork, while Qin et al. [8] investigated the variations of aroma compounds in roasted Rongchang pork under different temperature conditions. However, little information available on the flavor compounds of pork, processed by double-fried cooking. Therefore, the objective of this study was to identify the flavor compounds in double-fried pork and to investigate the changes in these flavors during frozen storage. To the best of our knowledge, this is the first report, where the flavor volatiles present in double-fried pork were extracted using head spacesolid phase micro extraction (HS-SPME) technique -a rapid, inexpensive, and solvent-free method for extracting volatile and semi-volatile [9].

Materials and Methods

Materials

Streaky pork, green onion, ginger, garlic and pepper were purchased from local market. Thick broad-bean sauce, flavor bean sauce and mushroom sauce were supported by PIXIAN Watereress Co., Ltd. (Sichuan, China), Chuannan Fermentation Co., Ltd. (Sichuan, China), and Foshan Haitian Flavoring and Food Co., Ltd. (Guangdong, China), respectively. Salt (food grade), white granulated sugar, and rice wine were purchased from Jiangsu Salt Industry Group Co., Ltd. (Jiangsu, China), Hangzhou Luzhibao Food Co., Ltd. (Zhejiang, China), and Huzhou Laoheng Fermentation Co., Ltd. (Zhejiang, China), respectively. Ajinomoto was purchased from Taitaile Food Co., Ltd. (Shanghai, China). All other chemicals were of analytical grade.

Sample preparation

Figure 1 presents the flow chart of preparing double-fried pork. Following the preliminary practice of meat processing, water was boiled with powdered ginger and garlic, scallions, pepper, rice wine and salt for approximately 30 s. The streaky pork was rolled to 60-80% mature so that chopsticks can be inserted through into the pork, taken out to form “lamp socket” more easily, the key is to have a little meat and a little blood in the middle area), and cooled in the refrigerator for 3 min (while high temperature facilitates the separation of fat and lean meat, low temperature is not conducive for preparing pork slices). Finally, the cooked pork was cut down to fillet (~5 cm long, 3 cm wide and 0.2 cm thick). The double-fried cooking of pork was carried out in an induction cooker at 180°C for 3.5 min. A little vegetable oil was poured on the pre-heated pan (40% heating); when the oil temperature reached a certain degree (90°C), the precooked pork was put into the pan, and stir-fried for 1.5 min (the oil temperature was maintained at about 90°C by keeping the heating temperature at 180°C for 1.5 min. The pork fat curled up, forming “lamp socket”. At this point, thick broad-bean sauce and flavor bean sauce were added and cooked for 1 min. Next, green onion, ginger, garlic, white granulated sugar, Ajinomoto, mushroom sauce and rice wine were added and again cooked for 1 min to obtain the final product of doublefried pork, which was stored at -20°C for further analysis.

food-technology-manufacture

Figure 1: The flow chart of the manufacture of double-fired pork.

Headspace solid-phase microextraction (HS-SPME) for the volatiles of double-fried pork

A 75 μm carboxen/poly (dimethylsiloxane) (CAR/PDMS) coated fiber (Supelco, Inc., Bellefonte, PA, USA) was used for the HS-SPME of the volatile compounds from the doublefried pork sample. Each pork sample (7 g) was cut, mashed and put into a 15 ml glass bottle. The glass bottle was then tightly capped with a silicon septum. The samples were equilibrated at 65°C for 10 min, and the volatile compounds were extracted at the same temperature (65°C) with stirring for 30 min in a multipurpose sampler with SPME capability (MPS2, Gerstel, Germany). Immediately after extraction, the fiber was inserted into the injection port of GC (gas chromatography) for 5 min at 250°C (in split mode) to desorb the analytes. All experiments were carried out in duplicate.

Gas chromatography-mass spectrometry (GC-MS) analysis for the volatiles of double-fried pork

Analysis of the flavor compounds of double-fried pork was performed on a 1200 L GC/MS-MS system (Finnigan Co., USA) equipped with a 1079 middle injector and a mass selective detector (MSD). The MSD was used to identify unknown compounds. The separation of volatile and semivolatile compounds was performed using a fused silica capillary column (specification: 50 m × 0.32 mm i.d., coated with BPX-5 at 0.5 μm film thickness, SGE Ltd.). The temperature of both the injector and the detector was set at 250°C. Following was the GC program for identification: an oven temperature of 40°C held for first 2 min, then heating to 100°C (5°C/min) held for 0 min, and finally, heating to 250°C (10°C/min) held for 20 min. Helium was used as the carrier gas with a flow rate of 70 μl/ min and a split ratio of 50 ml/min. Mass spectra were recorded in the centroid scan mode at an ionization voltage of 70 eV, an emission current of 50 μA for filament, and an ion source temperature of 200°C. Electron impact (EI) mass spectra were recorded in the 33-450 amu range with a scan time of 0.400 s.

Volatiles were identified by comparing their mass spectra with the mass spectra from MS libraries (NIST/WILEY/REPLIB/ MAINLIB) [10]. The volatiles were identified according to the matched-degree as well as the purity (>800). The relative percentage content of each compound was calculated by area normalization method. Principal component analysis (PCA) was performed to determine the respective contribution of various flavor compounds on the total aroma, and the synthesis score of principal components was calculated as follows:

Where PCn is the principal component n, fn is the score of PCn, λn is the characteristic value of PCn, and PC is the total score or synthesis score.

Statistical analysis

Statistical analysis was performed using SPSS 17.0 for Windows (SPSS Inc., Chicago, Illinois USA). The results were analyzed by performing the analysis of variance (ANOVA) procedure. Correlation analysis was performed to determine the significant differences in the double-fried pork samples with different frozen storage times [11].

Results and Discussions

Tables 1 and 2 show that the amount of the main flavor compounds (such as aldehydes, hydrocarbons, and alcohols) is higher in raw pork than those in double-fried pork. However, we found that the relative amount of some trace volatiles (ketones, esters, acids, heterocyclics and sulfurs), especially the esters and heterocyclics, in double-fried pork increased significantly. Moreover, the types of the volatiles of esters, heterocyclics, sulfurs increased in pork after cooking, while other flavor substances reduced in the number of species. The largest changes in relative amounts occurred in the ester and heterocyclic contents. The relative content of esters dramatically increased from 0.21% to 7.74% with increase in the types from six to twelve, while the heterocyclic content increased from 0.36% to 6.87% with eighteen types. Furthermore, the aroma of boiled specimens was soft and downy to some extent, mainly exhibiting a flavor profile comprising of grass aroma, fat aroma and fruit aroma. On the other hand, the aroma of cooked specimens was nutty, butyric, fruity, and burnt. During the frozen storage processing, we did not find any significant changes in the relative contents and types of volatiles in double-fried pork. Among heterocyclic compounds, we found mainly furans, pyrazines and pyrroles in double-fried pork. Furans contribute grain fragrant and sweet smell to the overall aroma of double-fried pork. The furans, we identified in the double-fried pork, were mainly 2-amyl furan, 2-acetyl furan, furfural and furfuryl alcohol. Pyrazines, found in double-fried pork, are mainly alkyl pyrazines. It forms in the Strecker degradation, with dicarbonyl compounds and amino acids forming alpha amino ketones and alpha amino ketones through condensation and oxidation, forming pyrazine. The threshold of methyl pyrazine is relatively high (>1 × 10-6 g/g) with little impact on pork flavor. However, if one or more of the methyl group is replaced by the ethyl, the corresponding smell thresholds will be greatly reduced with important effect to the flavor of food. In this study, 2-ethyl-5-methyl pyrazine, 2-ethyl- 6-methyl pyrazine, 2,5-dimethyl-3-ethyl pyrazine and 2-methyl- 3-ethyl pyrazine in double-fried pork had smell thresholds and significantly affected the flavor. Pyrrole compounds, similar in structure of furans as well as their formation mechanism, is mainly generate from the Maillard reaction. It is formed via 3-deoxidization ketone sugar reacted with amino compounds and ammonia or after dehydration condensation and cyclization reaction. 2-Acetyl pyrrole has grains aroma, but alkyl pyrrole and acyl pyrrole will produce bad flavor for double-fried pork.

Table 1: Result of GC-MS analysis for the relative content (%) of volatile flavors of double-fried pork during cooking process and frozen storage.

Number Volatile compounds Aldehydes A B C D E F G H
1 Pentanal 2.500 2.251 2.006 2.265 2.132 2.238 2.284 2.216
2 Butanal, 3-methyl- nd 2.926 3.168 2.899 3.322 2.879 2.934 2.949
3 Hexanal 46.665 37.632 37.159 37.717 38.001 37.829 37.776 37.883
4 2,4 Pentadienal 1.817 nd nd nd nd nd nd nd
5 Heptanal 2.401 0.691 0.736 0.668 0.753 0.726 0.772 0.719
6 2-Hexenal 0.063 nd nd nd nd nd nd nd
7 Octanal 1.848 0.631 0.649 0.592 0.561 0.662 0.661 0.672
8 2-Heptenal, (Z)- 0.589 nd nd nd nd nd nd nd
9 Nonanal 3.421 2.252 2.139 2.366 2.347 2.402 2.306 2.282
10 2 Octenal 0.466 0.298 0.269 0.113 0.109 0.101 0.097 0.088
11 Benzaldehyde nd 1.292 1.269 1.402 1.438 1.532 1.623 1.701
12 2-Nonenal, (E)- nd 0.099 nd nd nd nd nd nd
13 Benzeneacetaldehyde nd 1.816 1.944 1.809 1.854 1.914 1.918 1.859
14 Decanal 0.052 nd nd nd nd nd nd nd
15 Octadecanal 0.016 nd nd nd nd nd nd nd
16 d-Myrtenal 0.049 nd nd nd nd nd nd nd
17 Z-Citral 0.377 0.501 0.543 0.467 0.475 0.502 0.484 0.457
18 E-Citral 0.415 0.739 0.792 0.673 0.657 0.715 0.651 0.624
19 2,4-Decadienal, (E,E) 0.059 0.159 0.164 0.179 0.168 0.172 0.165 0.159
Alcohols
20 Ethanol 0.758 2.02 2.11 2.36 2.85 3.754 4.265 4.195
21 Carveol, dihydro-, cis- nd 1.397 1.379 1.210 1.143 1.154 1.162 1.166
22 2-Propen-1-ol 0.043 1.381 1.352 1.326 1.039 1.034 1.042 1.025
23 1-Butanol 0.205 0.182 0.237 0.255 0.243 0.241 0.237 0.319
24 1-Penten-3-ol 0.117 0.231 0.226 0.168 0.124 0.154 0.169 0.176
25 1,8-Cineole 1.589 0.388 0.388 0.343 0.288 0.276 0.253 0.296
26 1-Butanol, 3-methyl- nd 0.701 0.679 0.519 0.466 0.419 0.503 0.685
27 3-Buten-1-ol, 3-methyl- 0.063 nd nd nd nd nd nd nd
28 1-Pentanol 4.492 1.371 1.319 1.299 1.581 1.657 1.391 1.310
29 1-Hexanol 0.979 0.658 0.450 0.524 0.381 0.2 0.148 0.138
30 1-Octen-3-ol 2.938 1.932 1.858 1.881 2.001 1.944 1.848 1.907
31 1-Heptanol 0.688 nd nd nd nd nd nd nd
32 Thujyl alcohol 0.156 nd nd nd nd nd nd nd
33 2-Decanol 0.034 nd nd nd nd nd nd nd
34 Cyclohexanol, 1-methyl-4-(1-methyletheny 0.126 nd nd nd nd nd nd nd
35 p-Menth-8-en-1-ol, stereoisomer 0.084 nd nd nd nd nd nd nd
36 2-Cyclohexen-1-ol,1-methyl-4-(1-methylethyl)-, trans- 0.098 nd nd nd nd nd nd nd
37 1-Hexanol, 2-ethyl- 0.067 nd nd nd nd nd nd nd
38 Linalool 0.207 0.247 0.304 0.225 0.263 0.317 0.266 0.266
39 1-Octanol 0.506 0.152 0.141 0.134 0.128 0.120 0.117 0.165
40 2,3-Butanediol 0.093 nd nd nd nd nd nd nd
41 Terpinen-4-ol 0.373 0.210 0.170 0.054 0.093 0.102 0.086 0.083
42 2-Octen-1-ol-(E) 0.418 0.189 0.138 0.105 0.091 0.085 0.064 0.058
43 alpha-Terpineol 0.204 nd 0.146 nd nd nd nd nd
44 Borneol 0.115 nd nd nd nd 0.07 nd nd
45 Nerol 0.225 nd nd nd nd nd nd nd
46 Geraniol 0.329 nd nd nd nd nd nd nd
47 Phenylethyl Alcohol 0.130 0.353 0.322 1.095 0.827 0.156 0.126 0.130
48 Beta-Fenchyl alcohol nd nd nd nd 0.09 nd nd nd
Hydrocarbons
49 Octane 0.051 nd nd nd nd nd nd nd
50 Heptane, 2,4-dimethyl- 0.057 nd nd nd nd nd nd nd
51 Decane 0.031 nd nd nd nd nd nd nd
52 Undecane nd nd nd nd nd nd nd nd
53 Tridecane nd nd nd nd nd nd nd 0.094
54 Toluene 3.989 0.405 0.427 0.526 0.581 0.665 0.956 0.840
55 Camphene 0.222 0.826 1.073 0.673 0.391 0.146 0.134 0.211
56 a-Curcumene nd nd 0.157 nd nd nd nd nd
57 Styrene nd 0.112 0.107 0.102 0.096 0.078 0.056 0.08
58 Camphene hydrate 0.097 nd nd nd nd nd nd nd
59 Sabinene 0.135 nd nd nd nd nd nd nd
60 Benzene, ethyl- 0.123 0.068 0.070 0.065 0.058 0.052 0.064 0.054
61 p-Xylene 0.076 0.078 0.074 0.070 0.071 0.066 0.062 0.068
62 beta.-Pinene 0.113 nd nd nd nd nd nd nd
63 alpha.-Copaene 0.083 nd nd nd nd nd nd nd
64 l-Phellandrene 0.030 0.171 0.162 0.167 0.156 0.155 0.168 0.152
65 beta.-Phellandrene nd 0.825 0.731 0.538 0.533 0.156 0.217 0.277
66 beta.-Myrcene 0.257 0.700 0.136 0.141 0.098 0.074 0.068 0.073
67 1,3,6-Octatriene, 3,7-dimethyl-, (Z) nd 0.143 nd nd nd nd nd nd
68 trans-Caryophyllene nd 0.161 nd nd nd nd nd nd
69 Cyclohexene, 1-methyl-4-(1-methylethylid 0.089 nd nd nd nd nd nd nd
70 .alpha.-Terpinene 0.056 nd nd nd nd nd nd nd
71 l-Limonene 0.312 0.541 0.425 0.494 0.455 0.502 0.607 0.509
72 gamma.-Terpinene 0.083 nd nd nd nd nd nd nd
73 Benzene, 1-methyl-3-(1-methylethyl)- 0.068 nd nd nd nd nd nd nd
74 o-Cymene 0.089 nd nd nd nd nd nd nd
75 Cyclopentane, (2-methylpropylidene)- 0.100 nd nd nd nd nd nd nd
76 Limonene oxide 0.135 nd nd nd nd nd nd nd
77 beta.-Bisabolene 0.252 nd nd nd nd nd nd nd
78 Cyclohexene, 3-(1,5-dimethyl-4-hexenyl)- 0.159 nd nd nd nd nd nd nd
79 Benzene, 1-(1,5-dimethyl-4-hexenyl)-4-me 0.252 nd nd nd nd nd nd nd
Ketones
80 Acetone nd 3.704 3.682 3.655 3.469 3.124 2.766 2.183
81 2-Propanone, 1-hydroxy- nd 0.256 0.283 0.246 0.275 0.297 0.313 0.355
82 2-Heptanone 0.176 nd nd nd nd nd nd nd
83 2-Butanone, 3-hydroxy- 0.251 0.172 0.189 0.197 0.175 0.179 0.179 0.231
84 1-Octen-3-one 0.077 nd nd nd nd nd nd nd
85 4-Octen-3-one 0.083 nd nd nd nd nd nd nd
86 2,3-Octanedione 2.928 2.529 2.595 2.665 2.732 2.714 2.803 2.842
87 5-Hepten-2-one, 6-methyl- 0.204 0.205 0.232 0.251 0.379 0.401 0.366 0.419
88 Bicyclo[2.2.1]heptan-2-one, 1,7,7-trimet 0.075 nd nd nd nd nd nd nd
89 p-menth-1-en-3-one 0.303 nd nd nd nd nd nd nd
90 Xanthoxylin 0.353 nd nd nd nd nd nd nd
91 Ethanone, 1-(2-hydroxy-4,6-dimethoxyphen nd 0.117 nd 0.107 nd nd nd nd
92 2-Cyclohexen-1-one, 2-methyl-5-(1-methyl) nd nd nd nd 0.335 nd nd nd
93 2-Propanone, 1-(acetyloxy)- nd nd nd nd nd nd 0.087 0.095
Esters
94 Formic acid, 2-propenyl ester nd 0.733 nd nd nd nd nd nd
95 Methyl laurate nd nd nd nd nd nd nd 0.097
96 2,2-Dimethyl-6-oxoheptanoic acid methyl ester nd nd nd nd nd nd nd 0.077
97 1-Butanol, 3-methyl-, acetate nd 0.071 0.064 0.060 0.051 0.045 0.043 0.047
98 Hexanoic acid, ethyl ester nd 0.110 0.201 0.168 0.309 0.351 0.269 0.252
99 Propanoic acid, 2-hydroxy-,ethyl ester 0.069 0.172 2.045 2.407 2.284 2.248 2.404 2.488
100 Octanoic acid, ethyl ester nd 0.065 nd nd nd nd nd nd
101 Linalyl acetate nd 2.687 2.428 2.401 2.359 2.363 2.454 2.428
102 Propanoic acid, 2-methyl-, 3,7-dimethyl- nd 0.713 0.72 0.693 0.801 0.776 0.719 0.721
103 Benzoic acid, methyl ester nd 0.159 0.242 0.152 0.169 0.172 0.164 0.159
104 2(3H)-Furanone, dihydro- nd 0.191 0.382 0.231 0.465 0.325 0.372 0.368
105 Benzoic acid, ethyl ester nd 2.552 2.683 2.646 2.776 2.808 2.683 2.641
106 Benzeneacetic acid, ethyl ester nd 0.062 0.079 0.081 0.073 0.068 0.074 0.072
107 Butanedioic acid, diethyl ester nd 0.221 0.219 0.223 0.213 0.216 0.215 0.218
108 Acetic acid, butyl ester 0.039 nd nd nd nd nd nd nd
109 Acetic acid, cyano-, 2-ethylhexyl ester 0.043 nd nd nd nd nd nd nd
110 Glycerol 1,2-diacetate 0.038 nd nd nd nd nd nd nd
111 (3S)-1,3-Diacetoxy Butane(impure) 0.015 nd nd nd nd nd nd nd
112 3,6-Octadecadiynoic acid, methyl ester 0.008 nd nd nd nd nd nd nd
Acids
113 Acetic acid 0.126 0.861 0.898 0.935 0.987 1.047 1.095 1.109
114 Butanoic acid nd 0.201 0.198 0.192 0.183 0.187 0.184 0.179
115 Pentanoic acid 0.104 nd nd nd nd nd nd nd
116 Hexanoic acid 0.231 0.173 0.162 0.168 0.157 0.172 0.174 0.168
117 Octanoic acid 0.036 nd nd nd nd nd nd nd
118 Nonanoic acid 0.020 nd nd nd nd nd nd nd
119 Decanoic acid 0.022 nd nd nd nd nd nd nd
120 Hexadecanoic acid 0.087 0.198 0.186 0.191 0.182 0.186 0.197 0.198
Heterocyclic compounds
121 Furan, 2-ethyl- nd 0.558 0.553 0.562 0.545 0.554 0.531 0.488
122 2-Furancarboxaldehyde nd 0.536 0.704 0.806 0.903 0.929 0.943 0.901
123 2-Furanmethanol nd 0.817 0.831 0.811 0.904 0.907 0.921 0.935
124 2-Furanmethanol, 5-methyl- nd 0.077 0.072 0.079 0.071 0.068 0.071 0.057
125 2-Furancarboxaldehyde, 5-methyl- nd 0.101 0.122 0.137 0.148 0.189 0.191 0.194
126 Furan, 2-pentyl- 0.357 0.191 0.198 0.193 0.199 0.261 0.272 0.277
127 Pyrazine, methyl- nd 0.669 0.648 0.633 0.679 0.646 0.668 0.714
128 Pyrazine, ethyl- nd 0.098 0.086 0.091 0.096 0.092 0.085 0.088
129 2,5-dimethyl pyrazine nd 1.220 1.201 1.325 1.287 1.352 1.393 1.294
130 Pyrazine, 2,6-dimethyl- nd 0.591 0.536 0.573 0.552 0.529 0.598 0.532
131 Pyrazine, 2-ethyl-3-methyl- nd 0.280 0.268 0.275 0.272 0.261 0.246 0.237
132 Pyrazine, 2-ethyl-5-methyl- nd 0.073 0.071 0.062 0.060 0.057 0.053 0.051
133 Pyrazine, 2-ethyl-6-methyl- nd 0.078 0.072 0.069 0.068 0.061 0.044 0.057
134 Pyrazine, 3-ethyl-2,5-dimethyl- nd 0.201 0.193 0.189 0.191 0.186 0.173 0.182
135 Pyrazine, trimethyl- nd 0.324 0.317 0.319 0.338 0.305 0.296 0.291
136 Ethanone, 1-(1H-pyrrol-2-yl)- nd 0.554 0.518 0.505 0.548 0.402 0.432 0.472
137 Ethanone, 1-(2-furanyl)- nd 0.316 0.324 0.341 0.295 0.268 0.226 0.228
138 1H-Pyrrole, 1-methyl- nd 0.189 0.184 0.176 0.175 0.162 0.154 0.151
Sulphur-Containing Compounds
139 2-Propen-1-thiol 5.637 4.15 4.291 4.186 3.925 3.893 3.911 3.868
140 Disulfide, methyl 2-propenyl 0.625 0.214 0.224 0.231 0.216 0.245 0.253 0.254
141 Diallyl sulfide 0.546 0.241 0.239 0.235 0.217 0.214 0.218 0.207
142 Diallyl disulphide 0.529 3.174 3.269 3.225 3.204 3.266 3.342 3.264
143 Propanal, 3-(methylthio) nd 0.201 0.194 0.192 0.185 0.188 0.191 0.186
144 Propane, 1-(ethynylsulfinyl)- nd 1.028 1.025 1.034 1.021 1.01 1.018 1.093
145 Propanoic acid, 3-(acetylthio)-2-methyl- nd 0.389 0.394 0.401 0.358 0.362 0.322 0.317
Others
146 Benzene, chloro- nd nd nd nd nd 0.095 0.080 0.095
147 Cyclotrisiloxane, hexamethyl- 0.041 nd nd nd nd nd nd nd
148 Cyclotetrasiloxane, octamethyl- 0.378 nd nd nd nd nd nd nd
149 Cyclopentasiloxane, decamethyl- 0.096 nd nd nd nd nd nd nd
150 Cyclohexasiloxane, dodecamethyl- 0.161 0.088 0.123 nd 0.115 nd nd nd
151 Hydroxylamine, O-decyl- 0.026 nd nd nd nd nd nd nd
152 4-nitrophthalamide 0.369 nd nd nd nd nd nd nd
153 Silanediol, dimethyl- 1.611 nd nd nd nd nd nd nd
154 1-Heptadecanamine 0.041 nd nd nd nd nd nd nd
155 Phenol, 3-methyl- 0.026 nd nd nd nd nd nd nd
156 N-benzylidene-dimethylammonium chloride nd 1.06 1.083 nd nd nd nd nd
157 Phenol, 2,4-bis(1,1-dimethylethyl)- nd nd nd nd nd 0.439 0.209 0.165
158 Phenol, 2,6-bis(1,1-dimethylethyl)-4-met nd 0.115 0.126 nd nd nd nd nd
159 Phenol, 4-methyl- nd 0.111 0.048 nd nd nd 0.059 0.049
160 Phenol, 4-methoxy- nd nd nd 0.055 nd nd nd nd
161 Caffeine nd 0.192 nd 0.173 nd nd nd nd
162 Phenol, 2-methyl-5-(1-methylethyl)- nd nd 0.655 nd nd nd nd nd
163 Thymol nd nd 0.467 nd 0.224 nd nd nd
164 Phenol, 2-methoxy- nd nd 0.043 nd nd nd nd nd
165 1-Methoxy-2-propoxyethane nd nd nd nd nd nd nd 0.160
166 1,3-Dioxolane, 4,5-dimethyl-2-pentadecyl nd nd nd nd nd nd nd 0.082

Table 2: Relative content of volatile compounds of double-fried pork during cooking processing and frozen storage.

A (%) (101) B (%) (91) C (%) (89) D (%) (83) E (%) (84) F (%) (81) G (%) (84) H (%) (89)
Aldehydes 60.738 (15) 51.287 ((13) 50.838 (12) 51.150 (12) 51.817 (12) 51.672 (12) 51.671 (12) 51.609 (12)
Alcohols 15.037 (26) 11.412 (15) 11.219 (16) 11.498 (15) 11.608 (16) 11.683 (16) 11.677 (15) 11.919 (15)
Hydrocarbons 6.859 (24) 4.030 (11) 3.362 (10) 2.776 (9) 2.439 (9) 1.894 (9) 2.332 (9) 2.358 (10)
Ketones 4.45 (9) 6.983 (6) 6.981 (5) 7.121 (6) 7.365 (6) 6.715 (3) 6.514 (6) 6.125 (6)
Esters 0.212 (6) 7.736 (12) 9.063 (10) 9.062 (10) 9.500 (10) 9.372 (10) 9.397 (10) 9.568 (12)
Acids 0.626 (7) 1.433 (4) 1.444 (4) 1.486 (4) 1.509 (4) 1.592 (4) 1.650 (4) 1.654 (4)
Heterocyclic compounds 0.357 (1) 6.873 (18) 6.898 (18) 7.146 (18) 7.331 (18) 7.229 (18) 7.297 (18) 7.149 (18)
Sulphur-Containing Compounds 7.337 (4) 9.397 (7) 9.636 (7) 9.504 (7) 9.126 (7) 9.178 (7) 9.255 (7) 9.189 (7)
Others 2.749 (9) 1.566 (5) 2.545 (7) 0.228 (2) 0.339 (2) 0.534 (2) 0.348 (3) 0.551 (5)

Apart from heterocyclic compounds, esters also increased significantly. We found that post cooking, the relative contents of esters as well as their types significantly increased from 0.212% to 7.736%. However, we did not find ethyl caproate, ethyl methyl benzoic acid, benzoic acid, benzene acetic acid ethyl ester, 4-hydroxy butyric acid lactone, linalyl acetate, isobutyric acid orange flower ester in the boiled specimens, but found in the cooked specimens, indicating that these important flavor compounds were formed during the process of cooking, influencing the flavor of double-fried pork. Most of these compounds have a pleasant aroma of fruits and flowers; some have elegant and sweet, pure and fresh and powerful flavor. We performed PCA analysis for flavor volatile compounds of double-fried pork during frozen storage to determine the principal component of volatiles in them and to understand the influence of frozen storage processing on these compounds. We selected total 80 kinds of flavor compounds for double-fried pork with different frozen storage times; Table 3 shows the results of PCA. According to the characteristic value and variance contribution of the principal component (PC) we found that the cumulated variance contribution ratio of the first three PCs was 89.87%, and the average values of the first six PCs was more than 1. Moreover, according to the two principles of the determination of the numbers of PCs -cumulative variance contribution rates reaching up to 85% and the characteristic value higher than one -we chose three PCs (PC1, PC2, and PC3) with contribution rates of 73.65%, 9.91% , and 6.30%, respectively. Table 4 presents the PC factor loading matrix; there are many flavor compounds - including benzaldehyde, geranial, dihydrogen celery alcohol, 1-octene-3-alcohol, linalool, p-xylene, 3-hydroxy-2-butanone, 2,3-symplectic diketone, methyl heptene ketone, ethyl caproate, ethyl lactate, ethyl methyl benzoic acid, benzoic acid, furfural, furfuryl alcohol, 2-amyl furan, 2,6-dimethyl pyrazine, 3-ethyl- 2-2-acetyl methyl pyrazine, pyrrole, 2-acetyl furan, N-methyl pyrrole and allyl methyl sulfide - exerting very high loading in PC1. We found total of 63 volatile compounds positively related to PC1, including three aldehydes, twelve alcohols, seven hydrocarbons, ten esters, four ketones, three acids, seventeen heterocyclics, and seven sulfurs- the characteristic flavor compounds of double-fried pork. In addition, hexanal, nonanal, trans-2,4-sebacic olefin aldehyde and benzene ethanol had positive effect on PC2, whereas myrcene, caproic acid and 2-ethyl pyrazine had positive effect on PC3. In order to reflect the various compounds accounted for the proportion of each principal component clearly, we projected the points on three-dimensional load diagram (Figure 2) to the projection plane, thus achieving the two-dimensional load diagram (Figure 3). In Figure 3, PC1 represents the information of the majority of flavor compounds with few of them obtained from PC2. According to the synthesis score of double-fried pork.

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Figure 2: The dimensional loading diagram for the three principal components (PC1, PC2 and PC3) factors.

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Figure 3: The dimensional loading diagram for the two principal components (PC1 and PC2) factors. Figure

Table 3: Eigenvalue, percentage of variance and cumulative of six principal components.

Principal component Eigenvalue Extraction Sums of Squared Loadings
Variance (%) Cumulative (%)
1 58.921 73.651 73.651
2 7.929 9.912 83.563
3 5.042 6.302 89.865
4 3.693 4.616 94.481
5 2.399 2.999 97.480
6 2.016 2.520 100.000

Table 4: Loading matrix of the first three principal component factors.

Component
1 2 3
Pentanal Q1 -0.338 0.297 0.493
Butanal, 3-methyl- Q2 0.176 0.157 -0.225
Hexanal Q3 -0.617 0.575 0.530
Heptanal Q4 0.486 0.244 -0.253
Octanal Q5 -0.366 -0.825 -0.001
Nonanal Q6 -0.481 0.762 0.170
2-Octenal Q7 0.881 -0.440 0.066
Benzaldehyde Q8 0.886 0.252 -0.087
Benzeneacetaldehyde Q9 0.511 -0.347 -0.722
Z-Citral Q10 0.540 -0.481 -0.492
E-Citral Q11 0.743 -0.362 -0.372
2,4-Decadienal, (E,E) Q12 -0.033 0.753 -0.528
Ethanol C1 -0.889 0.110 -0.373
Carveol, dihydro-, cis- C2 0.875 -0.481 0.017
2-Propen-1-ol C3 0.927 -0.121 -0.115
1-Butanol C4 0.962 -0.126 -0.007
1-Penten-3-ol C5 0.659 -0.706 0.052
1,8-Cineole C6 0.921 -0.258 -0.013
1-Butanol, 3-methyl- C7 0.452 -0.706 0.313
1-Pentanol C8 0.954 -0.127 0.247
1-Hexanol C9 0.962 0.233 0.103
1-Octen-3-ol C10 0.987 -0.136 0.063
Linalool C11 0.935 0.095 -0.193
1-Octanol C12 0.990 -0.025 0.082
Terpinen-4-ol C13 0.725 -0.518 0.238
2-Octen-1-ol-(E) C14 0.957 -0.133 0.187
Phenylethyl Alcohol C15 0.381 0.809 -0.152
Toluene T1 -0.927 -0.139 0.070
Camphene T2 0.921 -0.192 -0.234
Styrene T3 0.946 0.220 -0.065
Benzene, ethyl- T4 0.942 -0.125 -0.161
p-Xylene T5 0.955 0.001 0.170
l-Phellandrene T6 0.595 0.009 0.007
beta.-Phellandrene T7 0.948 -0.054 0.081
beta.-Myrcene T8 0.735 -0.133 0.608
l-Limonene T9 0.943 -0.242 0.147
Acetone K1 0.874 0.356 -0.272
2-Propanone, 1-hydroxy- K2 0.917 0.040 0.068
2-Butanone, 3-hydroxy- K3 0.960 0.220 -0.064
2,3-Octanedione K4 0.973 0.017 -0.109
5-Hepten-2-one, 6-methyl- K5 0.960 0.131 0.170
1-Butanol, 3-methyl-, acetate E1 0.974 -0.099 0.107
Hexanoic acid, ethyl ester E2 0.990 -0.068 -0.089
Propanoic acid, 2-hydroxy-,ethyl ester E3 0.973 -0.041 0.197
Linalyl acetate E4 0.791 -0.201 0.518
Propanoic acid, 2-methyl-, 3,7-dimethyl- E5 0.943 -0.113 0.103
Benzoic acid, methyl ester E6 0.985 -0.026 -0.039
2(3H)-Furanone, dihydro- E7 0.955 0.134 0.089
Benzoic acid, ethyl ester E8 0.971 0.138 -0.055
Benzeneacetic acid, ethyl ester E9 0.958 0.238 -0.113
Butanedioic acid, diethyl ester E10 0.955 -0.119 0.090
Acetic A1 0.918 -0.243 -0.032
Butanoic A2 0.937 -0.183 -0.106
Hexanoic A3 0.408 0.176 0.822
Hexadecanoic acid A4 0.897 0.336 0.287
Furan, 2-ethyl- Z1 0.754 0.422 -0.334
2-Furancarboxaldehyde Z2 0.974 0.149 -0.147
2-Furanmethanol Z3 0.892 -0.102 0.202
2-Furanmethanol, 5-methyl- Z4 0.734 0.439 -0.214
2-Furancarboxaldehyde, 5-methyl- Z5 0.895 0.319 -0.006
Furan, 2-pentyl- Z6 0.958 0.227 -0.025
Pyrazine, methyl- Z7 0.940 -0.244 -0.120
Pyrazine, ethyl- Z8 0.464 0.517 0.612
2,5-dimethyl pyrazine Z9 0.914 0.262 0.032
Pyrazine, 2,6-dimethyl- Z10 0.962 0.202 -0.047
Pyrazine, 2-ethyl-3-methyl- Z11 0.863 0.465 -0.049
Pyrazine, 2-ethyl-5-methyl- Z12 0.982 -0.110 -0.057
Pyrazine, 2-ethyl-6-methyl- Z13 0.924 0.245 -0.013
Pyrazine, 3-ethyl-2,5-dimethyl- Z14 0.919 0.218 0.047
Pyrazine, trimethyl- Z15 0.908 0.387 -0.055
Ethanone, 1-(1H-pyrrol-2-yl)- Z16 0.952 0.012 0.277
Ethanone, 1-(2-furanyl)- Z17 0.937 0.251 -0.162
1H-Pyrrole, 1-methyl- Z18 0.977 0.130 -0.049
2-Propen-1-thiol S1 0.873 -0.137 -0.376
Disulfide, methyl 2-propenyl S2 0.957 -0.249 0.130
Diallyl sulfide S3 0.975 -0.008 -0.132
Diallyl disulphide S4 0.973 0.162 -0.041
Propanal, 3-(methylthio) S5 0.993 0.040 0.064
Propane, 1-(ethynylsulfinyl)- S6 0.945 0.090 -0.041
Propanoic acid, 3-(acetylthio)-2-methyl- S7 0.987 -0.029 0.006

The initial period of double-fried pork, before frozen storage, achieved the highest score. The score for double-fried pork decreased with increasing frozen storage times with the lowest score obtained for the double-fried pork sample stored for six months. Figure 3 illustrates the score plot of PC factor (PC1 and PC2), clearly distinguishing the flavor volatile compounds with different frozen storage times. As evident from Figure 4, the samples with frozen time of zero or one month, two months, three or four months and five or six months fall in the fourth quadrant, in the first quadrant, in the second quadrant and in the third quadrant, respectively. The main volatiles that affected the double-fried pork sample frozen for one month were 1-pentene-3-alcohol, isoamyl alcohol, caprylic aldehyde; the main volatiles that affected the double-fried pork sample frozen for two or three months were nonanal, trans-2,4-sebacic olefin aldehyde, benzene ethanol; and the main volatiles that affected the double-fried pork sample frozen for five or six months were hexanal and ethanol.

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Figure 4: Score plot of principal component factor. B=frozen storage for 0 month; C=frozen storage for 1 month; D=frozen storage for 2 month; E=frozen storage for 3 month; F=frozen storage for 4 month; G=frozen storage for 5 month; H=frozen storage for 6 month.

Conclusions

We identified a total of 91 types flavor volatile compounds in double-fried pork - aldehydes (51.3%), alcohols (11.4%), sulfurs (9.4%), esters (7.7%), ketones (7.0%), heterocyclic compounds (6.9%), hydrocarbons (4.0%), and acids (1.4%). This study showed that heterocyclic compounds, esters, and aldehydes had a significant contribution to the overall flavor of doublefried pork, while alcohols, ketones and hydrocarbons had little effect. Post cooking, the most significant changes occurred in heterocyclic and esters contents. We found that the relative contents of heterocyclic compounds and esters increased from 0.36% to 6.87% and 0.21% to 7.74%, respectively and their types also increased from 1 to18 and 6 to 12, respectively. While the flavor profile of boiled specimens was dominated by grass aroma, fat aroma and fruit aroma, the aroma of cooked specimens was nutty, butyric, fruit and burnt. PCA analysis of the double-fried pork samples with different frozen storage times showed that PC1 represented the information for the most volatile flavors (73.7%), while the PC2 and PC3 represented little information for the volatiles. PC1 represented the characteristics of double-fried pork flavor; the synthesis score gradually decreased with increasing frozen storage time. The frozen storage time affected flavor compounds of the doublefried pork samples. During a frozen storage time of one month, the following flavor compounds were affected: 1-pentene-3- alcohol, isoamyl alcohol and caprylic aldehyde. Increasing the frozen storage time to two to three months affected mainly nonyl aldehyde, trans-2,4-sebacic olefin aldehyde and benzene ethanol. Furthermore, a frozen storage time of six months affected hexanal and ethanol.

Acknowledgements

We acknowledge the financial support from the Natural Science Foundation of China (No. 31501418), the National Key Technology R&D Program for the 12th Five-Year Plan (No. 2016YFD0400304) and the independent research project program of the State Key Laboratory of Food Science and Technology of Jiangnan University (No. JUSRP1029).

References