Characterization of the physicochemical changes and volatile compound fingerprinting during the chicken sugar-smoking process.

Sugar-smoking contributes to improving flavor attributes of meat products. However, there is rather limited information concerning the relationship between sugar-smoking process parameters and volatile compound (VC) fingerprinting as well as related quality attributes of sugar-smoked chicken. In this work, the changes in VC across the whole sugar-smoking process were determined and analyzed and physicochemical properties, free fatty acid, thiobarbituric acid reactive substances values, and E-nose were also performed to characterize the quality properties of sugar-smoked chicken breast (CB) and chicken skin (CS). Results suggested that a higher amount (P < 0.05) of total VC was observed in CS compared with CB during the whole processing, which may be correlated with higher thiobarbituric acid reactive substances values, and higher polyunsaturated fatty acid/saturated fatty acid ratio. According to E-nose analysis, the volatile flavor is clearly separated in the sugar-smoking stage. Volatile fingerprinting results revealed that heterocycles were the characteristic flavor formed during sugar-smoking process and hexanal, nonanal, furfural, 5-methyl-2-furancarboxaldehyde, and 2-acetyl-5-methylfuran were the major volatiles of the CS, which was closely related to lipid oxidation and caramelization reaction. Above all, the flavor of sugar-smoked chicken was mainly derived from CS and sugar-smoked process improved the flavor of CS. This study could provide theoretical guidance for regulation of the color and flavor of sugar-smoked chicken and further promote the development of the industry.

Introduction

Chicken meat is the most commonly consumed meat source in many countries and simultaneously have an increase in consumption and production around the world (Takran et al., 2020). Chicken meat products are available as either sugar-smoked chicken products (Chen et al., 2013) or roast chicken products (Huang et al., 2020). The generation of characteristic flavor and aroma compounds of chicken meat is thermally derived, particularly those involved in the Maillard reaction and thermal degradation of lipids as well as their interaction between the 2 reactions (Chen et al., 2009; Jayasena et al., 2013). Chicken meat flavor mainly relies on several processing procedures and related factors including free fatty acids, cooking, pH, water content and salt content, etc (Jayasena et al., 2013), defining the flavor development and affecting the acceptability and volatile flavor component transformation of final products (Sañudo et al., 2000). Among them, traditional processing procedures for chicken meat include salting, saucing, smoking, and frying, etc (Shi and Ho, 1994). With regard to smoking technology, although using wood smoking to cure meat products has a very long history, the use of wood smoke incurs the formation polycyclic aromatic hydrocarbons (PAH) partially because of incomplete combustion (Yurchenko and Mölder, 2005). In recent years, evidence has shown that PAH contamination of smoked foods can be significantly reduced by replacing conventional (traditional) wood smoking with sugar smoking (Chen et al., 2013).

Sugar-smoking, a popular smoking method involving preprocessing of meat, such as salted and/or dried, and then exposed to sugar, has been used as it imparts a characteristic flavor to meat products favored by average consumers (Chen et al., 2013). Besides, sugar-smoking improves sensory properties and decreases moisture content (Pittia and Antonello, 2016). Previously, many researchers only focused on determining the flavor composition of wood-smoked, cold, or hot smoked meat products (Turan et al., 2008; Yu et al., 2008; Gómez-Estaca et al., 2011). For example, Yu et al. (2008) revealed that 48 volatile compound (VC) of Chinese traditional smoke-cured bacon using wood smoke. Gómez-Estaca et al. (2011) reported that cold-smoked sardine (Sardina pilchardus) and dolphinfish increased fish lipid oxidation stability and revealed the presence of typical phenol and carbonyl derivatives, as well as some oxidation products and PAH. However, there is rather limited information concerning the relationship between sugar-smoking process parameters and VC fingerprinting as well as related quality attributes. Clarifying these relationships is meaningful to modify and design the final flavor profiling by combining with sugar-smoking technology in a targeted way.

The aim of the present study was, therefore, to determine the change of pH values, water and salt content, fat oxidation, and VC during the processing of sugar-smoked chickens, which will provide more information related to the sugar-smoked products.

Materials and methods

Processing of Sugar-Smoking Chickens and Sample Preparation

Twenty-five three-yellow chickens (cold carcass weight 500 ± 40 g) were randomly selected 24 h postslaughter (conducted following the European Community, 1099/2099/EC 2009 guidelines). Animals were reared in a farm (Wenzhou, Zhejiang, China) and slaughtered at their market weight in a local commercial slaughter house. The facilities of the slaughter house met the requirements of the Institute of Animal Care and Use Committee. Immediately after slaughter, the carcasses were chilled at 4°C in a ventilated room for 24 h. Detailed process flow chart of sugar-smoked chickens is shown in the Figure 1.

Figure 1

Process flow chart of sugar-smoked chickens. Abbreviations: BC, BS: the end of baking of chicken breast and skin; DC, DS, the end of air-drying of chicken breast and ski; PC, PS: the end of pickling of chicken breast and skinn; RC, RS: raw breast and skin; SC, SS: the end of sugar-smoking of chicken breast and skin.

Twenty-five chicken breast (CB) and chicken skin (CS) were sampled at 5 different processing points: raw (RC, RS), the end of pickling (PC, PS), the end of air-drying (DC, DS), the end of baking (BC, BS), and the end of sugar-smoked (SC, SS). Immediately after sampling, all the samples were cut to small piece (about 1 cm × 1 cm × 1 cm) and were wrapped in aluminum foil, frozen, and stored at −40°C before analysis, except the analysis of moisture content.

Determination of pH Value, Moisture Content, and Salt Content

The moisture content was determined in duplicate according to GB/T5009.3-2003; the salt content was evaluated as chloride in duplicate and was assayed according to GB/T12457-2008. The results of the moisture and the salt content were both expressed as g per 100 g muscle. For pH determination, 5 g of samples and 20 mL of distilled water were mixed with DY89-I high-speed homogenizer (Ningbo, Zhejiang, China) at 10,000 rpm for 30 s at 4°C, and then pH was determined by a pH meter (Mettler Toledo Instruments Co., Ltd., Shanghai, China) in duplicate. The 5 replicates were considered as mean values.

Determination of TBARS Values

The thiobarbituric acid reactive substances (TBARS) values were measured according to the method of our previous study (Yang et al., 2017). The values were expressed as mg of malondialdehyde per 100 g of muscles.

Analysis of FAA

Lipid Extraction and Free Fatty Acids Purification

Lipid was extracted as described, and free fatty acids (FAA) were purified by our previous study (Yang et al., 2017).

Determination of FAA

Free fatty acids were quantified according to the method of our previous study (Yang et al., 2017) with slight modifications. The fatty acid methyl esters were analyzed using a TSQ8000EVO (Thermo Fisher, Waltham, MA) equipped with a flame ionization detector and a split injector. 1 μL of solution was injected in split mode (10:1) onto a RtxWax capillary column (Restek, Bellefonte, PA; 30 m  0.25 mm id  0.25 lm film thickness). The temperature of the column was programmed as follows: 3 min at 160°C, increments of 10°C/min to 200°C, and maintained at 200°C for 2 min, then increments of 2°C/min to 230°C and maintained at 230°C for 10 min. The flow rate of the carrier gas (He) was 1.2 mL/min. Identification of fatty acids was performed by comparison of the retention times with internal standards. The results are expressed as the absolute content of free fatty acid.

Analysis of Volatile Compounds

The extraction of VC of CB and CS was performed using solid-phase microextraction (SPME). The Supelco device containing (70 μm length) PDMS/DVB (Polydimethoxysilane/divinylbenzene) was used. For headspace SPME extraction, 4.5 g of ground sample was placed in a 20 mL vials. 2-Methyl-3-heptanone was used as internal standard. Analyses were performed using TSQ8000EVO. The VC were separated on TG-5MS (30 m × 0.25 mm × 0.25 μm) flexible quartz capillary column (ThermoFisher). The fiber was desorbed and maintained in the injection port at 270°C for 2 min. The GC oven temperature was first isothermal for 10 min and then raised to 200°C at a rate of 4°C/min, held for 2 min, next raised to 250°C at a rate of 20°C/min, and then kept for 7 min. Mass spectra were obtained using electron impact with ionization energy of 70 eV, scan mode ranging 33 to 350 m/z. The identification and quantity of compounds was achieved by comparing their mass spectra with those in NIST and Wiley eighth spectra libraries (minimum 90% accuracy).

E-Nose Analysis

The E-nose analysis was conducted using a commercial PEN 3.5 E-nose (Airsense Analytics GmBh, Schwerin, Germany) containing 10 metal-oxide semiconductors (Supplementary Table 1). Accurately weigh the 4.5 g of the minced CB and CS sample into a 40 mL headspace sample bottle, respectively. These kept at temperature (30°C) for 10 min for gas generation in the headspace. The headspace gaseous compounds were pumped into the sensor arrays. The responses of sensors were expressed as the ratio of conductance G/G0 (G and G0 represent the conductances of sensors exposed to sample gas and zero gas, respectively). An assessment time of 120 s was used to make sure the stable response signs. Each test was conducted in 5.

Statistical Analysis

The ANOVA was performed to compare means for pH value, moisture content, salt content, TBARS, FAA, and VC. When a significant difference (P < 0.05) was detected, the comparative analysis between means was conducted using Duncan's multiple range tests by SAS 8.0 software (SAS Institute Inc, Cary, NY). Data were presented as means ± SD. Regarding the E-nose analysis, the senor signals measured at 110 to 112 s were subjected to principal component analysis using the Winmuster software (Airsense Analytics Inc., Germany) to discriminate and classify samples. All graphs were draw up by using originPro 8.5 software (OriginLab Corporation, Northampton, MA).

Results and discussion

The Change of Physicochemical Properties

The change of physicochemical properties during processing, including pH values, moisture content, and salt content of the sugar-smoked chickens was summarized in Table 1. The pH values of CB increased, ranging from 5.91 to 6.39. The pH values of CS significantly increased (P < 0.05) in the pickling stages and then slowly decreased. Huang et al. (2014) reported that the pH values of smoked bacon were 5.73 to 5.96, which was lower than CB. The rise in the pH values was mainly related to the lipid oxidation and the generation of amino acids through protein degradation (Huang et al., 2014). Compared with other stages, the pH value of the sugar-smoking stage (SS) was the lowest, possibly because the citric acid and phosphate could be attached to the sugar-smoked CS in the pickling stage, which further resulted in relatively high content of acidic substances after sugar-smoking. Furthermore, some studies indicated that glucose more easily formed furfurals in the caramelization reaction process under acidic conditions (Ajandouz et al., 2010), which could further contribute to the improvement of flavor profiles of sugar-smoked chicken during the processing of sugar-smoking.

Table 1

Changes in pH value, moisture contents, and salt content of chicken breast (CB) and skin (CS) during processing.

Indicator	RC	PC	DC	BC	SC	RS	PS	DS	BS	SS
pH value	5.91 ± 0.05e	6.24 ± 0.01c	6.26 ± 0.04c	6.41 ± 0.07a	6.39 ± 0.04a,b	6.09 ± 0.02d	6.33 ± 0.02b	6.22 ± 0.04c	6.25 ± 0.01c	5.51 ± 0.06f
Moisture content (g/100 g muscle)	69.7 ± 1.21a	69.05 ± 0.72a	40.43 ± 0.16a,b	37.62 ± 0.17a,b	37.35 ± 0.48a,b	64.42 ± 2.17a	62.83 ± 3.49a	20.39 ± 1.64c,d	11.73 ± 1.37b,c	11.35 ± 1.51d
Salt content (g/100 g muscle)	1.12 ± 0.02e	3.88 ± 0.18c	6.42 ± 0.56b	6.78 ± 0.27a,b	7.34 ± 0.6a	0.82 ± 0.07e	2.65 ± 0.83d	2.44 ± 0.59d	2.06 ± 0.14d	2.04 ± 0.2d

a-fDifferent letters in the same row indicate that there is significant difference (P < 0.05, along the lines).

Abbreviations: BC, BS, baking chicken breast and skin; DC, DS, air-drying chicken breast and skin; PC, PS, pickling chicken breast and skin; RC, RS, raw chicken breast and skin; SC, SS, sugar-smoking chicken breast and skin.

Each value is expressed as mean ± SD.

Regardless of CB or CS, the moisture content kept decreasing during processing. Puangsombat et al. (2012) reported that the moisture level of fresh meat products ranged between 69 and 82%, except in the high-fat parts (bacon, breast skin, and thigh skin), which contained low moisture levels (approximately 37%). In our study, the average moisture content of the final CB and CS was 37.35 and 11.35%, respectively, which was lower than that of other cured meat products (Marra et al., 1999; Martın et al., 2001; Jin et al., 2010) and higher than that of the reported smoked bacon (Huang et al., 2014). It could be caused by osmotic dehydration in the early stages and the water evaporation in the mid-late stages of processing. The low moisture content is a typical property of Chinese traditional smoke-cured meat products and also one of the most important factors for prolonging shelf life under room temperature.

Simultaneously, the salt content of CB also significantly increased (P < 0.05) during processing, whereas it markedly increased (P < 0.05) in the first stages and then slowly decreased in CS. The rise of salt content for CB was related to salt diffusion of CS and great dehydration in the pickling and air-drying stages, whereas the last 2 stages was ascribed only to dehydration because of temperature (Huang et al., 2014). The average salt content of the final product was 7.34%, which was similar to that of a typical dry-cured ham (6–8%) (Motilva and Toldrá, 1993).

The Change of Lipid Oxidation

As shown in the Figure 2, TBARS of CB and CS gradually increased during processing and the oxidation degree of CS was higher than CB. This result was consistent with the report by Pikul et al. (1984). To our knowledge, CS contained the highest contents of fat, which was more susceptible to oxidation. It could be also seen that sugar-smoking process accelerated the speed of lipid oxidation. This was different from the result of Hobson et al. (2019) probably because of the difference in processing procedures. Heat treatment and oxygen are also factors which promote lipid oxidation (Pikul et al., 1984). As the process continued, the salt concentration enhanced and the moisture content declined, which could inactivate the neutral lipase and phospholipase and to some extent favored the activity of acid lipase (Toldrá, 2006). Huang et al. (2014) suggested that salt varied closely with both POV and TBARS. Pikul et al. (1984) reported that the increase of pH may be helpful for phospholipid hydrolysis and the TBARS numbers of roasted muscles and skin were further elevated because of water evaporation and loss of juiciness, which effectively increased the total lipid content of roasted samples, which could be the reason for further acceleration in the SS.

Figure 2

Evolution of TBARS values of chicken breast (CB) and skin (CS) during processing. Abbreviations: D, the end of air-drying; B, the end of baking; P, the end of pickling; R, raw materials; S, the end of sugar-smoking. a-eDifferent letters indicate that there is significant difference (P < 0.05).

The Change of FAA

Table 2 showed that the sugar-smoked CB and CS both contained 14 kinds of FAA. It was founded that palmitic acid (C16:0), stearic acid (C18:0), oleic acid (C18:1), and linoleic acid (C18:2) were the main free fatty acids in the samples, coinciding with the result of Huang et al. (2014). In relation to the saturated fatty acid, the content of CB increased firstly and then slowly decreased in the SC stages, whereas those in CS kept increasing. These differences were mainly attributed to the differences found for palmitic acid (C16:0), followed by myristic acid (C14:0) and stearic acid (C18:0). Concerning the monounsaturated fatty acid, the difference between CB and CS for oleic acid (C18:1), and palmitoleic acid (C16:1) was clearly reflected in the final product values, with total fraction values for CS be significantly higher. During the SS, polyunsaturated fatty acids sharply decreased (P < 0.05) in the CB and CS. This might be attributed to lipid oxidation, resulting in the formation of more intense volatiles (Zhang et al., 2019), such as Chinese traditional smoke-cured bacon (Huang et al., 2014). The increase of total FAA contents could also be because of the lipolysis of triglycerides and phospholipids (Buscailhon et al., 1994). Besides, some enzyme, including neutrallipase, phospholipase, and acid lipase could contribute to lipolysis in meat products (Zhang et al., 2019). It was founded that CB was higher than CS, which might be because of faster lipid oxidation rate under the condition of exposure to air (Ang and Lyon, 1990) and, to the extent, the lower degree of autooxidation of the internal meat, in line with the results of TBARS values.

Table 2

Changes in the free fatty acid of chicken breast (CB) and skin (CS) during processing (ng/g).

Kinds	RC	PC	DC	BC	SC	RS	PS	DS	BS	SS
C14:0	3.35 ± 0.02i	5.45 ± 0.3g	6.02 ± 0.05e,f	6.36 ± 0.06d,e	5.88 ± 0.26f	4.92 ± 0.16h	7.46 ± 0.08a	6.66 ± 0.2c,d	7.21 ± 0.14a,b	6.99 ± 0.17b,c
C16:0	61.52 ± 2.24d	70.34 ± 0.38c	75.3 ± 0.95a,b	76.25 ± 0.47a	62.03 ± 0.62d	60.8 ± 3.11d	64.37 ± 0.63d	62.22 ± 0.73d	61.78 ± 2.5d	72.65 ± 0.16b,c
C18:0	43.82 ± 1.12b,c	43.8 ± 1.02b,c	45.43 ± 0.64b	48.6 ± 1.68a	43.42 ± 2.62b,c	41.26 ± 1.19c	40.87 ± 0.3c	41.74 ± 0.35c	41.29 ± 2.04c	40.73 ± 0.84c
C16:1	15.12 ± 0.06f	18.02 ± 0.97e	25.22 ± 1.58b,c	20.84 ± 0.01d	21.15 ± 0.21d	26.2 ± 0.1b,c	19.4 ± 1.05d,e	24.07 ± 2.16c	33.34 ± 1.85a	27.39 ± 1.25b
C18:1	42.76 ± 0.02b	48.15 ± 1.83a,b	46.22 ± 0.86a,b	40.18 ± 0.87b	45.18 ± 7.32a,b	47.35 ± 0.19a,b	49.29 ± 0.15a,b	48.16 ± 1.61a,b	48.39 ± 10.94a,b	54.18 ± 2.09a
C22:1	0.49 ± 0.01c	0.56 ± 0.05b	0.57 ± 0.00b	2.32 ± 0.02a	0.53 ± 0.05b,c	0.31 ± 0.01d	0.34 ± 0.01d	0.31 ± 0.04d	ND	ND
C18:2	61.12 ± 0.62e	74.27 ± 4.14b	84.15 ± 1.56a	80.67 ± 0.9a	75.22 ± 5.7b	64.77 ± 1.33d,e	71.71 ± 0.78b,c	74.03 ± 0.16b	72.24 ± 1.26b,c	67.03 ± 0.39c,d
C20:2	7.61 ± 0.45c	10.57 ± 0.54a	11.28 ± 0.07a	8.82 ± 0.19b	6.29 ± 0.17d	ND	2.94 ± 0.6f	4.77 ± 0.01e	4.92 ± 0.41e	4.64 ± 0.04e
C20:3	13.62 ± 0.18a	13.18 ± 0.01a	11.48 ± 0.59b	12.79 ± 1.15a	9.05 ± 1.05c	3.14 ± 0.36d	3 ± 0.13d	3.12 ± 0.07d	2.79 ± 0.19d	2.96 ± 0.04d
C20:4	44.45 ± 0.18b	43.14 ± 1.19b	45.77 ± 0.12a,b	47.61 ± 0.12a	43.25 ± 1.99b	15.71 ± 2.74c	12.22 ± 0.02d	11.61 ± 0.09d	9.74 ± 1.29d	10.15 ± 1.06d
C22:4	13.88 ± 2.45b	18.03 ± 0.86a	17.14 ± 0.12a	18.77 ± 0.42a	16.9 ± 0.27a	1.09 ± 0.06c	1.3 ± 0.01c	1.25 ± 0.01c	1.17 ± 0.00c	1.16 ± 0.05c
C22:5	13.88 ± 2.45c	18.03 ± 0.86a	17.14 ± 0.12b,c	18.77 ± 0.42c	16.9 ± 0.27b	1.09 ± 0.06e	1.3 ± 0.01d,e	1.25 ± 0.01d	1.17 ± 0.00d	1.16 ± 0.05d,e
C22:6	11.17 ± 1.27a	12.3 ± 0.17a	11.79 ± 0.27a	11.56 ± 0.51a	10.11 ± 0.18b	3.17 ± 0.04c	2.81 ± 0.05c	3.94 ± 0.31c	3.58 ± 0.29c	3.14 ± 0.28c
∑SFA	112.1 ± 2.88d	129.32 ± 1.53b	133.41 ± 1.78a,b	136.3 ± 1.13a	117.14 ± 1.39c	106.99 ± 1.75b	112.7 ± 0.84b	110.63 ± 0.58b	111.56 ± 4.69b	121.41 ± 0.81a
∑MUFA	58.37 ± 0.07d	66.73 ± 0.9b,c,d	72.01 ± 2.43a,b,c	63.34 ± 0.9c,d	66.85 ± 7.47b,c,d	73.86 ± 0.28a,b	69.03 ± 1.21b,c	72.54 ± 3.73a,b,c	81.73 ± 9.09a	81.56 ± 3.34a
∑PUFA	171.29 ± 3.59d	200.74 ± 3.65b	209.22 ± 0.9a	205.57 ± 3.17a,b	187.4 ± 7.7c	93.47 ± 5.05f	101.72 ± 1.4e,f	107.19 ± 0.03e	106.85 ± 2.74e	97.59 ± 0.77f
Total	341.76 ± 0.78e	396.79 ± 1.21c	414.63 ± 5.12a	405.22 ± 3.41b	371.4 ± 1.62d	274.32 ± 3.02h	283.45 ± 3.45g	290.36 ± 4.28g	300.14 ± 7.14f	300.56 ± 1.76f

a-iDifferent letters in the same row indicate that there is significant difference (P < 0.05, along the lines).

Each value is expressed as mean ± SD.

Abbreviations: BC, BS = baking chicken breast and skin; DC, DS = air-drying chicken breast and skin; MUFA, monounsaturated fatty acid; PC, PS = pickling chicken breast and skin; PUFA, polyunsaturated fatty acid; RC, RS = raw chicken breast and skin; SC, SS = sugar-smoking chicken breast and skin; SFA, saturated fatty acid.

The Change of Volatile Compounds

As summarized in the Table 3, a total of 40 and 54 VC were isolated and identified using SPME-GC-MS technique in CB and CS, respectively. The HCA was given to search for sample distribution pattern according to overall comparison of the VC (Supplementary Figure 1). The HCA demonstrated a clear clustering tendency of the samples, including the PS; RS, BS, DS, and DC; RC, PC, and BC as well as SC and SS. Figure 3 showed that a higher amount (P < 0.05) of total VC was observed in the CS compared with CB, except the baking stages. Our research was in agreement with Pippen and Nonaka (1963) who founded that the overall yield of volatile material was greater and a more complex nature from skin and skin fat than from lean leg and breast muscle. During baking stages, this was not surprising, because CB contained an appreciable amount of protein, amino acids, etc, and high temperature during baking stages, which resulted in the faster speed of Maillard reaction and fat oxidation and could be expected to give rise to VC. Besides, Ang (1988) suggested that protein, water, and total lipid in various broiler tissues might affect the specific tissue stability by influencing the physical or environmental conditions for the oxidation process. It was concluded that physical and chemical characteristics could result in generating intense VC. In addition, the reason for CS also may be because of the directly exposure to the air (Ang and Lyon, 1990), which further promoted oxidation rate and, to the extent, slowed down the degree of autooxidation of the internal meat.

Table 3

Changes in volatile compounds (VC) of chicken breast (CB) and skin (CS) during processing.

Compounds	RT	RC	PC	DC	BC	SC	RS	PS	DS	BS	SS
Alcohol
3,7-dimethyl-1,6-Octadien-3-ol	11.12	ND	4.28 ± 0.67d	5.44 ± 1.42d	ND	ND	8.81 ± 0.96c	18.32 ± 1.59b	29.49 ± 1.64a	ND	ND
Terpinen-4-ol	13.03	ND	0.48 ± 0.06d	ND	ND	ND	1.62 ± 0.24b	8.09 ± 0.23a	1.05 ± 0.14c	1.59 ± 0.07b	ND
Isopinocarveol	13.1	ND	ND	ND	ND	7.27 ± 0.7b	ND	ND	2.31 ± 0.28c	2.45 ± 0.01c	14.36 ± 0.38a
Phytol	14.91	ND	ND	ND	ND	3.99 ± 0.04a	0.66 ± 0.04c	0.44 ± 0.05c	1.41 ± 0.62b	0.82 ± 0.06c	ND
4-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-Buten-2-ol	15.35	ND	ND	ND	ND	5.96 ± 0.15b	ND	ND	ND	ND	10.91 ± 1.71a
(Z)-9-Octadecen-1-ol	15.51	ND	ND	ND	ND	ND	ND	ND	0.42 ± 0.14	ND	ND
2-methyl-1-Hexadecanol	17.42	1.4 ± 0.02d	0.78 ± 0.53d	14.69 ± 0.26c	81.83 ± 7.3a	81.49 ± 18.27a	1.36 ± 0.05d	0.66 ± 0.09d	6.43 ± 3.17c	7.92 ± 0.37c	65.33 ± 1.23b
α-acorenol	18.37	0.14 ± 0.09d	0.33 ± 0.11d	4.87 ± 1.89b	5.16 ± 1.45b	ND	0.75 ± 0.03d	1.33 ± 0.34c,d	10.55 ± 2.01a	2.73 ± 0.8c	ND
3,7,11-trimethyl-1-Dodecanol	19.4	0.19 ± 0.06d	ND	7.14 ± 0.8c	11.93 ± 1.68b	17.55 ± 1.64a	0.44 ± 0.01d	ND	ND	2.00 ± 0.37d	ND
Behenic alcohol	21.06	0.85 ± 0.23d	ND	12.74 ± 3.07c	115.17 ± 4.04a	72.45 ± 0.54b	1.45 ± 0.1d	1.33 ± 0.4d	2.11 ± 1.43d	3.49 ± 0.68d	9.67 ± 0.35c
2-(octadecyloxy)-Ethanol	22.88	3.14 ± 0.98e,f	2.68 ± 1.81e,f	25.68 ± 4.61d	82.49 ± 4.86b	161.06 ± 1.28a	1.76 ± 0.19f	0.94 ± 0.03f	4.29 ± 1.51e,f	7.44 ± 1.87e	49.55 ± 1.49c
1-Tricosanol	23.92	5.69 ± 0.92d,e	3.37 ± 1.53e	36.29 ± 7.63c	64.53 ± 0.41a	31.17 ± 2.49c	2.29 ± 0.02e	2.2 ± 0.02e	5.76 ± 1.69d,e	11.07 ± 0.45d	51.58 ± 1.88b
2-hexyl-1-Decanol	24.95	1.02 ± 0.28d,e	0.65 ± 0.38e	6.16 ± 0.22b	12.05 ± 1.78a	5.43 ± 0.8b	0.4 ± 0.04e	0.38 ± 0.14e	3.55 ± 0.55c	2.52 ± 0.73c,d	6.39 ± 2.12b
2-Hexyl-1-octanol	28.9	0.61 ± 0.06c	0.57 ± 0.44c	1.61 ± 0.58b,c	5.27 ± 1.17a	4.8 ± 0.46a	ND	ND	ND	ND	ND
Total		13.01 ± 5.35c	13.17 ± 0.28c	115.28 ± 13.04b,c	378.42 ± 115.73a	391.09 ± 74.93a	19.76 ± 1.08c	33.59 ± 5.32c	62.2 ± 14.01c	41.75 ± 4.46c	212.67 ± 8.00b
Aldehydes
Hexanal	4.2	2.74 ± 0.00e	ND	1.32 ± 0.84e	ND	ND	4.82 ± 0.11e	206.89 ± 2.42a	31.38 ± 0.25b	16.23 ± 1.53d	24.68 ± 3.33c
Heptanal	6.24	ND	ND	ND	ND	ND	ND	ND	23.57 ± 1.59b	29.71 ± 0.01a	ND
Benzaldehyde	7.7	ND	ND	ND	ND	ND	ND	15.32 ± 5.2a	12.19 ± 2.81a	12.46 ± 3.26a	ND
(E)-2-Octenal	10.06	ND	ND	ND	ND	ND	ND	3.59 ± 1.11b	5.69 ± 0.03b	5.15 ± 0.05b	65.03 ± 3.4a
Nonanal	11.18	1.27 ± 0.43h	ND	11.99 ± 6.16g	58.92 ± 1.05e	212.36 ± 0.78c	ND	25.78 ± 1.06f	244.66 ± 1.32b	89.89 ± 0.08d	407.79 ± 0.94a
8-Octadecenal	18.95	0.29 ± 0.07d	2.49 ± 2.44d	14.79 ± 0.21c	109.36 ± 0.83a	78.27 ± 4.86b	0.91 ± 0.04d	0.4 ± 0.06d	2.4 ± 1.51d	3.23 ± 0.4d	12.59 ± 0.43c
5-Octadecenal	25.75	1.99 ± 0.49c,d	1.1 ± 0.74d	9.29 ± 0.48c	40.67 ± 8.84a	41.17 ± 4.32a	0.88 ± 0.01d	0.59 ± 0.04d	1.96 ± 0.93c,d	3.47 ± 0.05c,d	18.98 ± 0.95b
Total		5.89 ± 0.84e	3.58 ± 1.7e	40.36 ± 5.96e	208.95 ± 29.94c,d	331.8 ± 92.24b	6.61 ± 0.15e	255.18 ± 68.5b,c	328.5 ± 3.39b	160.26 ± 1.33d	531.02 ± 4.27a
Hydrocarbons
n-Hexane	2.02	21.57 ± 8.70g	17.81 ± 5.32g	306.14 ± 31.4e	369.56 ± 3.6d	190.85 ± 1.88f	24.39 ± 1.94g	15.41 ± 1.41g	529.67 ± 1.75c	1,188.75 ± 1.77a	626.43 ± 2.99b
p-Xylene	5.57	ND	ND	13.8 ± 0.82f	118.19 ± 2.43a	76.85 ± 3.85b	3.41 ± 0.16g	30.28 ± 0.74d	21.63 ± 0.61e	40.1 ± 0.42c	ND
Styrene	6.07	ND	ND	12.9 ± 4.23c	50.84 ± 3.47b	267.55 ± 6.75a	5.31 ± 0.64d	ND	13.42 ± 0.04c	54.65 ± 1.66b	ND
Undecane	11.02	ND	ND	6.8 ± 0.64b,c	11.95 ± 7.13b	81.81 ± 2.33a	1.94 ± 0.48c	ND	ND	ND	ND
9-methylheptadecane	12.72	ND	ND	ND	28.01 ± 1.95b	20.68 ± 6.19c	3.27 ± 1.02e	3.66 ± 0.08e	14.38 ± 0.45d	18.25 ± 1.16c,d	109.3 ± 1.51a
Dodecane	13.44	0.75 ± 0.22g	1.65 ± 0.14g	23.25 ± 1.07e	76.95 ± 5.02b	95.82 ± 8.77a	4.9 ± 0.12f,g	8.63 ± 1.08f	31.84 ± 0.49d	20.97 ± 2.38e	71.43 ± 1.67c
2,6,10-trimethyl-Tetradecane	15.29	ND	ND	ND	ND	ND	0.42 ± 0.03b	0.33 ± 0.1b	1.47 ± 0.37a	1.36 ± 0.04a	ND
Tetradecane	17.93	0.73 ± 0.24f	1.18 ± 0.72f	28.29 ± 1.86d	119.41 ± 4.06a	83.74 ± 3.85b	2.73 ± 0.43f	2.25 ± 0.29f	9.78 ± 3.56e	12.45 ± 0.01e	52.9 ± 1.56c
2,6,10,15-tetramethyl-Heptadecan	19.22	0.8 ± 0.18e	1.39 ± 1.07e	26.29 ± 11.79c,d	140.91 ± 39.73a	86.88 ± 1.45b	2.25 ± 0.12d,e	2.53 ± 0.58d,e	7.18 ± 1.97d,e	9.98 ± 0.56d,e	43.63 ± 1.36c
Nonadecane	20	5.92 ± 1.25g	12.53 ± 4.75f	74.43 ± 5.51d	167.88 ± 1.84b	288.97 ± 3.97a	6.81 ± 0.27g	7.41 ± 1.42g	11.05 ± 1.05f,g	22.34 ± 1.57e	110.46 ± 1.46c
Hexadecane	21.97	12.66 ± 6.31d,e	7.49 ± 4.6e	85.71 ± 15.47c,d	471.48 ± 142.24a	382.34 ± 4.25b	6.19 ± 0.52e	3.21 ± 0.2e	13.75 ± 3.49d,e	25.64 ± 1.42d,e	101.65 ± 2.48c
Heptadecane	23.83	11.05 ± 0.82e	4.27 ± 2.63f,g	48.44 ± 1.33d	111.98 ± 1.11b	237.79 ± 5.52a	2.67 ± 0.67f,g	2.01 ± 1.12g	4.9 ± 0.01f,g	7.22 ± 0.15e,f	81.97 ± 2.64c
Octadecane	25.6	6.11 ± 1.43e	2.08 ± 1.42f	20.01 ± 0.29c	78.7 ± 1.78b	92.95 ± 1.19a	1.12 ± 0.15f	0.63 ± 0.00f	2.77 ± 0.21f	2.69 ± 0.26f	11.41 ± 0.00d
9-hexyl Heptadecane	27.29	2.42 ± 1.00b	3.28 ± 1.81b	7.92 ± 1.08b	24.27 ± 5.06a	27.54 ± 3.91a	0.88 ± 0.09b	ND	1.94 ± 0.35b	1.78 ± 0.37b	31.52 ± 1.94a
Total		55.54 ± 1.95e	51.88 ± 5.91e	657.35 ± 100.9d	1,772.78 ± 258.75a,b	1,862.86 ± 361.31a	65.65 ± 9.93e	80.54 ± 10.07e	659.59 ± 12.11d	1,409.46 ± 104.49b,c	1,245.18 ± 36.95c
Esters
Hexanoic acid, methyl ester	6.8	ND	ND	ND	ND	ND	ND	ND	22.33 ± 1.69a	4.06 ± 0.55b	ND
1-methyl-4-(1-methylethenyl)-Cyclohexanol, acetate	9.3	ND	ND	ND	ND	ND	14 ± 1.64b	24.06 ± 1.02b	25.42 ± 5.91b	42.1 ± 6.49a	47.65 ± 9.18a
2-furoate Methyl	10.8	ND	ND	ND	ND	ND	ND	ND	ND	ND	462.65 ± 1.55
10,13-Octadecadiynoic acid, methyl ester	12.56	ND	ND	ND	ND	ND	ND	14.26 ± 0.66c	31.15 ± 0.05b	11.36 ± 1.4d	81.94 ± 0.55a
Lavandulyl acetate	14.78	ND	ND	ND	ND	ND	0.51 ± 0.08d	1.45 ± 0.01c	3.18 ± 0.17a	2.65 ± 0.39b	ND
9-Octadecen-12-ynoic acid, methyl ester	15.75	0.49 ± 0.08b	1.29 ± 0.48b	7.41 ± 2.73b	26.63 ± 9.49a	20.81 ± 13.29a	3.73 ± 0.21b	3.77 ± 1.00b	9.55 ± 0.98b	7.40 ± 1.00b	25 ± 2.39a
Geranyl isovalerate	16.94	ND	ND	ND	8.04 ± 2.05a	ND	0.83 ± 0.06b	1.32 ± 0.26b	1.05 ± 0.06b	1.62 ± 0.17b	ND
7-Methyl-Z-tetradecen-1-ol acetate	21.4	0.82 ± 0.50c	0.66 ± 0.12c	6.79 ± 0.83c	50.29 ± 11.69a	41.3 ± 4.7b	0.67 ± 0.12c	0.54 ± 0.07c	1.66 ± 0.78c	1.99 ± 0.54c	7.28 ± 1.11c
Total		1.02 ± 0.1e	1.95 ± 0.02d,e	14.19 ± 3.56c,d,e	84.95 ± 23.23b	62.11 ± 61.99b,c	19.72 ± 1.96c,d,e	45.39 ± 2.33b,c,d,e	95.91 ± 5.28b	55.16 ± 2.73b,c,d	620.31 ± 19.96a
Heterocycles
Furfural	4.87	ND	ND	ND	ND	929.43 ± 132.7b	ND	ND	ND	ND	18,087.59 ± 717.5a
5-methyl 2- Furancarboxaldehyde	7.8	ND	ND	ND	ND	167.74 ± 6.6b	ND	ND	ND	ND	4,039.33 ± 3.46a
2-Acetyl-5-methylfuran	9.59	ND	ND	ND	ND	ND	ND	ND	ND	ND	469.46 ± 1.6
5-Hydroxymethylfurfural	14.8	ND	ND	ND	ND	ND	ND	ND	ND	ND	244.3 ± 16.54
1-(2-furanyl)Ethanone	6.52	ND	ND	ND	ND	ND	ND	ND	ND	ND	687.3 ± 1.67
2-hydroxy-3-methyl-2-Cyclopenten-1-one	9.4	ND	ND	ND	ND	ND	ND	ND	ND	ND	407.75 ± 1.05
Total		ND	ND	ND	ND	1,097.18 ± 219.00b	ND	ND	ND	ND	23,422.99 ± 792.61a
Ketones
Acetoin	2.94	12.88 ± 2.00a	ND	0.27 ± 0.16b	ND	ND	17.96 ± 3.32a	ND	ND	ND	ND
2-methyl-3-Octanone	8.21	ND	ND	ND	ND	ND	ND	254.51 ± 1.13a	19.65 ± 1.16b	ND	ND
1-(1,2-Dimethyl-cyclopent-2-enyl)-ethanone	10.3	ND	ND	ND	ND	ND	ND	ND	ND	ND	248.02 ± 1.45
Total		12.88 ± 2.00d	ND	0.27 ± 0.16e	ND	ND	17.96 ± 3.32c	254.51 ± 1.13a	19.65 ± 1.16c	ND	248.02 ± 1.45b
Others
Estragole	15.62	ND	ND	ND	ND	ND	ND	ND	ND	ND	103.1 ± 2.75
3-hydroxy-Dodecanoic acid	13.64	0.15 ± 0.05c	ND	ND	ND	ND	ND	16.02 ± 1.32b	125.61 ± 2.88a	ND	ND
Total		0.15 ± 0.052d	ND	ND	ND	ND	ND	16.02 ± 1.32c	125.61 ± 2.88a	ND	103.1 ± 2.75b

a-gDifferent letters in the same row indicate that there is significant difference (P < 0.05, along the lines).

Each value is expressed as mean ± SD.

Abbreviations: BC, BS = baking chicken breast and skin; DC, DS = air-drying chicken breast and skin; PC, PS = pickling chicken breast and skin; RC, RS = raw chicken breast and skin; SC, SS = sugar-smoking chicken breast and skin.

Figure 3

The absolute content of the total volatile compounds of chicken breast (CB) and skin (CS) in the different stage (ng/g). Abbreviations: B, the end of baking; D, the end of air-drying; P, the end of pickling; R, raw materials; S, the end of sugar-smoking. a-fDifferent letters indicate that there is significant difference (P < 0.05).

Figure 4 showed that the content of heterocyclic compounds reached the highest compared with other kinds of compounds. The heterocyclic compounds were produced only in the SS and was greater in the CS than CB, in agreement with the result reported by Sung (2013) who observed that furan compounds (furfural, 5-methyl-2-furancarboxaldehyde,5-hydroxymethyl-2-furaldehyde) were the major volatile constituents found in the smoke condensates. And, these compounds were considered as characteristic flavor compounds of the sugar-smoking process. Among them, furfural, with a sweet, woody, bready, and caramel aroma (Kerth and Miller, 2015), could be produced as a flavor active intermediate present in the early stages of the Maillard reaction and a precursor of other heterocyclic compounds (Fors, 1983), with relatively low threshold. It was mainly produced by the sucrose pyrolysis of the smoked chicken (Sung, 2013). 5-Methylfurfural has a sweet, spicy, coffee, and caramel flavor (Fors, 1983). 2-Acetyl-5-methylfuran has a sweet, moldy, nutty, and caramel-like aroma note (Steen et al., 2017). 5-Hydroxymethyl furfural has a sweet, coffee, and caramel flavor characteristic. The formation of 5-Hydroxymethyl furfural was produced by an acid-catalyzed dehydration of fructose (Román-Leshkov et al., 2006), which was not detected in the CB. In particular, 1-(2-furanyl)-ethanone has strong sweet balsamic-cinnamic odor note and are founded in processed foods (Cho et al., 2010). However, its formation mechanism had not been clearly shown.

Figure 4

Changes in the absolute content of volatile components (VC) of hydrocarbons, aldehydes, esters, alcohols (A) and heterocycles, ketones, others (B). Heterocycles are not produced in the stage of RC, PC, DC, BC, RS, PS, DS as well as BS. Ketones are not produced in the stage of PC, BC, SC as well as BS. Others are not produced in the stage of PC, DC, BC, SC, RS as well as BS. Abbreviations: BC, BS: the end of baking of chicken breast and skin; DC, DS, the end of air-drying of chicken breast and skin; PC, PS: the end of pickling of chicken breast and skin; RC, RS: raw breast and skin; SC, SS: the end of sugar-smoking of chicken breast and skin. a-eDifferent letters indicate that there is significant difference (P < 0.05).

Figure 4 showed that the total content of aldehydes compounds gradually increased from the raw to the air-drying stages in the CS. When it reached the baking stages, the total content dropped significantly (P < 0.05). This may be because the flavor compounds produced by the oxidation of the CS were diffused inward during the baking stages, so that the content of the CB was significantly higher, indicating the dynamic change process of the flavor composition of CB and CS. According to Table 3, most aldehydes belong to the straight-chain aldehydes with more than 5 carbon atoms such as hexanal, heptanal, and nonanal. This finding was consistent with Domínguez et al. (2014) who reported that hexanal was the main VC of foal meat. Mottram (2007) suggested that all were produced from fat oxidative degradation and strecker amino acid reaction. The content of hexanal in the CS was higher than that CB, which was derived from ω-6 unsaturated fatty acid (Wettasinghe et al., 2001) and corresponded well with the higher TBARS values. Hexanal is described as intense grass-like aroma note, whereas heptanal has a fatty and oily aroma, and nonanal shows grassy, oily and bitter (Wettasinghe et al., 2001; Takakura et al., 2014). The presence of benzaldehyde in the processing of CS may be associated to the degradation of phenylalanine (Xie et al., 2008). Owing to low odor thresholds and distinctive odors, the above volatile aldehydes should belong to potent contributors to the chicken flavor (Xie et al., 2008). Besides, long-chain aliphatic aldehydes, for example 8-octadecenal and 5-octadecenal were also founded. However, these high-molecular weight aldehydes probably act as precursors of the volatile saturated and unsaturated aldehydes, because lower volatility makes them less important to meat flavor (Xie et al., 2008).

From the Figure 4, the total content of alcohol compounds kept increasing in the CB. And the content of CS decreased significantly during the baking stages. The phenomenon occurred may be related to the action of enzymes, which could reduce the aldehydes produced by the catabolism of fatty acids and amino acids to the corresponding alcohols (Chen et al., 2009). Among alcohol compounds (Table 3), 3,7-dimethyl-1,6-octadien-3-ol is commonly known as linalool and has a floral, citrus-like aroma (Adebo et al., 2018), of which the content was higher in CS. Terpinen-4-ol has a woody, earthy, and musty aroma (Adebo et al., 2018). It has been demonstrated that terpenoids including 3,7-dimethyl-1,6-octadien-3-ol, terpinen-4-ol, isopinocarveol, α-acorenol, and 9-octadecen-1-ol were the major sources of natural flavor additives in foods and fragrances (Singh and Sharma, 2015), with those content was slower despite in the CB or CS. The linear saturated alcohols have a higher threshold value, thus exerting little effect on the flavor perception (Dirinck et al., 1997). As shown in the Figure 4, the content of hydrocarbons in the CB and CS gradually increased. For CB, the content reached the maximum during the SS. Among them, the aromatic and aliphatic hydrocarbons might derive from the thermal degradation of lipid by thermal homolysis or autoxidation of long-chain fatty acid (Song et al., 2011). Nevertheless, as a result of high odor thresholds, they were generally believed to have little contribution to meat flavor (Xie et al., 2008). Simultaneously, some compounds may be important intermediates in the formation of heterocyclic compounds, which was beneficial to improve overall meat flavor.

The esters of C1-C10 fatty acids have a typical fruity aroma, while the long-chain esters of long-chain fatty acids show an oily taste (Flores, 2018). The presence of ethyl hexanoate produces the aroma of fruit (banana, green apple, etc.) and may be related to the alcohols added during the pickling process, as well as the fermentation of microorganisms (Tešević et al., 2009). It was reasonable to assume that aldehydes compounds in the SS may be oxidized to form acid, such as furfural, and further to form ester so that result in the existence of 2-furoate methyl. 3-Hydroxy-dodecanoic acid is a fatty acid that could act as a precursor to volatile ketones (2-undecanone) (Labows et al., 1980). Estragole is the main flavor of star anise and has the aroma of star anise (Sun et al., 2014). It only was detected in the CS. The ketones could arise from lipid degradation, and they have been implicated in the buttery aroma note of cooked meats (Peterson et al., 1975).

E-Nose

Examination of the score plot (Figure 5) in the area defined by the first 2 principal components (93.85%) revealed a clear separation between CB and CS. The predicted accuracy of this model was more than 85% (Guohua et al., 2012), indicating that PC1 and PC2 already contained a large amount of information of the samples. Figure 5 showed that the distribution of PC, DC, and BC; PS, DS, and BS were relatively concentrated, suggesting that the overall flavor had no significant in the PC1. This result was similar to the above-mentioned result of HCA. Besides, RC, SC, RS, and SS could be well distinguished in the PC1. These results demonstrated that the processing procedures greatly caused the shifts of the flavor precursors as the results of the biochemical reactions occurring during the different stages, especially including lipid oxidation, lipolysis, Maillard reaction, and caramelization reaction, thus leading to the differences in the contents and types of volatile flavor substances.

Figure 5

The PCA analyses of chicken breast (CB) and skin (CS) using E-nose. Abbreviations: BC, BS: the end of baking of chicken breast and skin; DC, DS, the end of air-drying of chicken breast and skin; PC, PS: the end of pickling of chicken breast and skin; RC, RS: Raw breast and skin; SC, SS: the end of sugar-smoking of chicken breast and skin.

Conclusion

The VC fingerprinting mainly included heterocyclic, aldehydes, alcohol and hydrocarbons compounds in sugar-smoked chicken. The flavor of sugar-smoked chicken was mainly derived from CS and sugar-smoked process improved the flavor of CS. The changes in physicochemical attributes and the composition of free fatty acids, as well as the increased of TBARS values, Maillard reaction and caramelization reactions favored the generation of intense flavor during the sugar-smoking CS. Hexanal, nonanal, furfural, 5-methyl-2-furancarboxaldehyde, 2-acetyl-5-methylfuran, and 1-(2-furanyl)-ethanone were the major volatiles of sugar-smoked chickens. However, the quantitative mechanism toward the flavor contribution of sugar-smoked technology needs further investigation.