Furong Wangab,
Hailiang Shencd,
Xi Yangab,
Ting Liuab,
Yali Yang*ab,
Xueru Zhouab,
Pengtao Zhaoab and
Yurong Guo
ab
aCollege of Food Engineering and Nutritional Science, Shaanxi Normal University, Campus Chang'an, No. 620, West Chang'an Avenue, Chang'an District, Xi'an 710119, PR China. E-mail: yangyali@snnu.edu.cn; Fax: +86 029 85310471; Tel: +86 029 85310471
bNational Research and Development Center of Apple Processing Technology, PR China
cCitrus Research Institute, Southwest University, Chongqing, PR China
dCitrus Research Institute, Chinese Academy of Agricultural Science, Chongqing, PR China
First published on 16th August 2021
Most research concerning pyrazine formation in the Maillard reaction is mainly focused on free amino acids (FAAs), but limited information is available on the effect of peptides and proteins. In this study, three Maillard model systems (i.e., glucose and native sunflower seed protein, hydrolyzed peptides or FAAs, respectively) were prepared, and their effect on the formation of volatiles were further compared at different heating conditions by using of headspace solid-phase microextraction equipped with gas chromatography/mass spectrometry (HS-SPME-GC/MS). It was found that pyrazines were the characteristic volatile compounds in tested Maillard models, and with increasing heating temperature and time, the varieties of pyrazine formation significantly increased. The optimum reaction condition for pyrazine formation was at 140 °C for 90 min, which was subsequently applied to all sets of Maillard models. Further analysis showed that the short chain peptides generated by hydrolyzing sunflower seed protein (SSP), especially the molecular weight ranging from 1.2 to 3.0 kDa, significantly promoted the formation of pyrazines, which highlights the important role of peptides in the Maillard reaction models and is expected to intensify aroma promotion in sunflower seed oil.
Temperature and time are two key parameters that affect the formation of volatile compounds in MRPs. Several studies reported that formation of pyrazines increased with increased temperature.8,9 In glycine–glucose Maillard model system, more volatile compounds are produced at 180 °C than at 120 °C, which was probably because higher temperature is favorable for promoting the reaction rate between the sugar and amino groups.10,11 Zhang et al. showed that the system of xylose and soybean peptide exhibited a lower bitterness as temperature increased from 100 to 140 °C.12 Similar results were also reported by Lan et al., who found that high temperatures changed the pathway of the Maillard reaction and reduced the content of bitter amino acids.13 On the other hand, heating time has an important effect on MRPs. It was reported that the MRPs from Chinese shrimp waste hydrolysates (CSWHs) and xylose heated for 60 min exhibited a strong meaty aroma and umami taste as well as seafood aroma.14
So far, researchers have established numerous Maillard reaction models to study the pathways of pyrazines formation,6,15–18 where amino acids and carbonyl compounds were used as the main precursors. However, in the food systems, the content of FAAs is much lower than that of peptides or proteins, whereas the related reports regarding the formation of pyrazines in protein–carbohydrate and peptide–carbohydrate models are very limited. Previously, it was experimentally demonstrated that the peptides from whey protein hydrolysis played an important role in pyrazine formation.19–21
Although peptides are recognized as the precursors for MRPs, their contribution to formation of typical volatile compounds in sunflower seed oil processing has not been well understood.22–25 Therefore, this study was to explore the effect of heating temperature and time on the formation of pyrazines, with the aim to reveal the potential roles of FAAs and hydrolyzed sunflower seed peptides in the Maillard reaction model.
Amino acid | Content (mg/100 mg of hydrolyzed SSP) |
---|---|
a Numbers are represented as the means ± standard deviations. Mean values with different superscript letters have significant differences (p < 0.05). | |
Lysine | 0.2036 ± 0.0025a |
Histidine | 0.2014 ± 0.0010a |
Arginine | 0.2028 ± 0.0081a |
Tyrosine | 0.1586 ± 0.0011b |
Phenylalanine | 0.1188 ± 0.0023b |
Isoleucine | 0.0054 ± 0.0006e |
Leucine | 0.0328 ± 0.0007d |
Serine | 0.0312 ± 0.0009d |
Glutamic acid | 0.1202 ± 0.0097b |
Valine | 0.0844 ± 0.0008c |
Glycine | 0.0334 ± 0.0010d |
GC-MS analyses were performed using an Agilent 8890 GC coupled with an Agilent 5977B quadrupole mass selective detector (MSD, Agilent Technologies, Diegem, Belgium) with a Varian DB-1701 capillary column (30 m length × 0.25 mm i.d.; 0.25 μm film thickness). The working conditions of GC-MS were as follows: the transfer line to MSD was maintained at 250 °C; the carrier gas (He) flow rate was 1.0 mL min−1; the electron ionization (EI) was 70 eV; the scanned acquisition parameter was ranged from 30 to 550 m/z; the initial oven temperature was 40 °C, held 2 min; the temperature increased from 40 to 100 °C at a rate of 10 °C min−1 and held for 5 min, and then raised to 220 °C at a rate of 10 °C min−1 and held for 15 min; the equilibrium time was 0.5 min; the injection port was in split mode and the split ratio was 30:
1. The volatile components were identified by comparison of the mass spectrum with mass spectral libraries (NIST 2017). The formation of pyrazines was calculated by the absolute peak area of each individual pyrazine in a semi-quantitative way.26
Table 2 presents the molecular weight distribution of SSP and hydrolyzed SSP products. The Mw (weight-average molecular weight) of SSP was 15.38 kDa, and 90% of Mw was larger than 3.0 kDa. However, the Mw of SSP hydrolysates was 2.47 kDa, mainly in the range of 1.2–3.0 kDa, which accounted for 88.9% of the hydrolyzed SSP, and the Mw below 2.0 kDa accounted for 42.1%. These results suggested that after trypsin hydrolysis, SSP was hydrolyzed into the short chain peptide fragments.
Molar mass (kDa) | SSP | Hydrolyzed SSP |
---|---|---|
Mw (kDa) | 15.38 | 2.46 |
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The range of molecular weights (%) | ||
>26.15 | 5.3 | 0 |
14.2–26.15 | 22.4 | 0.3 |
8.0–14.2 | 42.2 | 0.8 |
3.0–8.0 | 26.9 | 9.5 |
1.8–3.0 | 2.4 | 46.8 |
1.2–1.8 | 0 | 42.1 |
Substances | 30 min | 60 min | 90 min | 120 min | 150 min |
---|---|---|---|---|---|
a Numbers are represented as the means ± standard deviations. | |||||
The model was heated at 100 °C | |||||
Furan compounds | 5.94 ± 0.05 | 16.47 ± 0.11 | 12.63 ± 0.09 | 22.44 ± 0.23 | 28.79 ± 0.44 |
Aldehydes and ketones | 9.41 ± 0.02 | 8.10 ± 0.08 | 6.53 ± 0.15 | 7.74 ± 0.07 | 5.83 ± 0.05 |
Pyrazine compounds | 0.24 ± 0.01 | 0.77 ± 0.03 | 1.11 ± 0.02 | 1.86 ± 0.14 | 2.18 ± 0.08 |
Pyrazines (% of total GC-MS peak area) | 2.00 | 3.00 | 5.00 | 6.00 | 6.00 |
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The model was heated at 110 °C | |||||
Furan compounds | 5.39 ± 0.03 | 9.39 ± 0.06 | 11.89 ± 0.09 | 11.42 ± 0.56 | 12.15 ± 0.97 |
Aldehydes and ketones | 4.54 ± 0.02 | 3.87 ± 0.10 | 3.29 ± 0.09 | 2.88 ± 0.22 | 2.89 ± 0.74 |
Pyrazine compounds | 0.11 ± 0.005 | 0.46 ± 0.03 | 0.63 ± 0.04 | 0.81 ± 0.03 | 0.81 ± 0.05 |
Pyrazines (% of total GC-MS peak area) | 1.00 | 3.00 | 4.00 | 5.00 | 5.00 |
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The model was heated at 120 °C | |||||
Furan compounds | 22.06 ± 1.02 | 32.22 ± 2.34 | 34.49 ± 0.99 | 18.35 ± 0.86 | 32.49 ± 1.11 |
Aldehydes and ketones | 6.59 ± 0.55 | 6.74 ± 0.25 | 6.43 ± 0.46 | 6.52 ± 0.05 | 8.92 ± 0.95 |
Pyrazine compounds | 0.77 ± 0.03 | 1.32 ± 0.07 | 1.61 ± 0.03 | 2.30 ± 0.06 | 2.13 ± 0.06 |
Pyrazines (% of total GC-MS peak area) | 2.60 | 3.30 | 3.80 | 8.50 | 4.40 |
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The model was heated at 130 °C | |||||
Furan compounds | 8.14 ± 0.34 | 10.67 ± 0.09 | 11.15 ± 0.97 | 15.25 ± 1.44 | 15.2 ± 0.85 |
Aldehydes and ketones | 2.40 ± 0.03 | 2.88 ± 0.08 | 3.00 ± 0.32 | 4.46 ± 0.32 | 4.87 ± 0.53 |
Pyrazine compounds | 0.32 ± 0.01 | 0.48 ± 0.08 | 0.87 ± 0.07 | 1.84 ± 0.02 | 1.01 ± 0.03 |
Pyrazines (% of total GC-MS peak area) | 2.90 | 3.40 | 5.80 | 8.50 | 4.80 |
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The model was heated at 140 °C | |||||
Furan compounds | 23.7 ± 0.34 | 24.62 ± 1.29 | 6.64 ± 0.06 | 6.85 ± 0.55 | 30.83 ± 1.87 |
Aldehydes and ketones | 7.48 ± 0.45 | 10.08 ± 0.47 | 2.20 ± 0.01 | 2.24 ± 0.02 | 8.69 ± 0.94 |
Pyrazine compounds | 1.74 ± 0.08 | 3.92 ± 0.07 | 0.55 ± 0.01 | 0.77 ± 0.04 | 3.76 ± 0.35 |
Pyrazines (% of total GC-MS peak area) | 5.00 | 10.00 | 6.00 | 8.00 | 6.00 |
Among the pyrazine compounds generated, it should be noted that 2,5-dimethylpyrazine and 2,6-dimethylpyrazine were always eluted together at the selected chromatographic conditions and their peaks were linked together on the mass spectrometry. Thus, in our study, both were considered as 2,5(6)-dimethylpyrazine. In Table 3, although the peak area of pyrazines was relatively lower compared with other volatile compounds, their varieties were the greatest in all detected volatiles of native SSP model (Table S1†). The effect of heating conditions on substituted pyrazines and pyrazine was significant (p < 0.05). The increase in heating temperature could promote the formation of pyrazines. When the heating time was 150 min, 4 pyrazines were detected at 100 °C, while 9 pyrazines were detected at 140 °C in the native SSP model. Besides, the percentage of pyrazines in total GC-MS peak area increased significantly with heating temperature ranging from 100 to 140 °C, and the percentage of pyrazines were the highest at 140 °C (10%). Heating time could also affect the pyrazine formation. At the same temperature, the varieties of pyrazine formed increased with time. This may be because thermal degradation of the native SSP occurred, leading to generation of more amino compounds and promoting formation of pyrazines. The peak area of pyrazines showed an upward trend with the heating time increasing from 30 min to 120 min, whereas for a heating time of 150 min, the peak area of pyrazines decreased. This result indicated that the excessive heating time was not conducive to the formation of pyrazines.34
Substances | 30 min | 60 min | 90 min | 120 min | 150 min |
---|---|---|---|---|---|
a Numbers are represented as the means ± standard deviations. | |||||
The model was heated at 100 °C | |||||
Furan compounds | 1.96 ± 0.03 | 2.65 ± 0.08 | 6.22 ± 0.66 | 9.89 ± 0.34 | 10.12 ± 0.67 |
Aldehydes and ketones | 2.59 ± 0.11 | 3.03 ± 0.24 | 2.89 ± 0.07 | 3.94 ± 0.08 | 4.07 ± 0.44 |
Pyrazine compounds | 0 | 0.04 ± 0.003 | 0.18 ± 0.03 | 0.37 ± 0.01 | 0.54 ± 0.04 |
Pyrazines (% of total GC-MS peak area) | 0 | 0.52 | 1.94 | 2.61 | 3.67 |
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The model was heated at 110 °C | |||||
Furan compounds | 5.97 ± 0.27 | 10.89 ± 0.02 | 11.2 ± 0.44 | 12.45 ± 0.05 | 1.90 ± 0.13 |
Aldehydes and ketones | 4.02 ± 0.32 | 4.11 ± 0.05 | 3.74 ± 0.06 | 4.42 ± 0.07 | 0.44 ± 0.03 |
Pyrazine compounds | 0.09 ± 0.01 | 0.19 ± 0.01 | 0.25 ± 0.01 | 0.31 ± 0.07 | 0.09 ± 0.004 |
Pyrazines (% of total GC-MS peak area) | 0.89 | 1.25 | 1.65 | 1.80 | 3.70 |
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The model was heated at 120 °C | |||||
Furan compounds | 5.85 ± 0.09 | 9.34 ± 0.53 | 8.19 ± 0.16 | 10.62 ± 0.87 | 9.79 ± 0.34 |
Aldehydes and ketones | 3.53 ± 0.09 | 4.01 ± 0.03 | 3.47 ± 0.22 | 5.55 ± 0.55 | 5.01 ± 0.04 |
Pyrazine compounds | 0.08 ± 0.003 | 0.14 ± 0.01 | 0.25 ± 0.02 | 0.25 ± 0.12 | 0.32 ± 0.03 |
Pyrazines (% of total GC-MS peak area) | 0.85 | 1.04 | 2.10 | 1.52 | 2.12 |
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The model was heated at 130 °C | |||||
Furan compounds | 7.65 ± 0.07 | 9.33 ± 0.77 | 10.22 ± 0.03 | 12.97 ± 0.96 | 10.5 ± 0.10 |
Aldehydes and ketones | 2.57 ± 0.78 | 3.43 ± 0.45 | 5.55 ± 0.07 | 8.21 ± 0.34 | 7.10 ± 0.05 |
Pyrazine compounds | 0.34 ± 0.04 | 0.34 ± 0.02 | 0.64 ± 0.10 | 0.65 ± 0.03 | 0.71 ± 0.03 |
Pyrazines (% of total GC-MS peak area) | 3.22 | 2.60 | 3.90 | 2.98 | 3.88 |
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The model was heated at 140 °C | |||||
Furan compounds | 7.83 ± 0.05 | 7.1 ± 0.10 | 17.15 ± 0.28 | 37.28 ± 3.44 | 9.91 ± 0.44 |
Aldehydes and ketones | 1.64 ± 0.05 | 1.81 ± 0.03 | 8.49 ± 0.06 | 13.04 ± 1.12 | 5.36 ± 0.05 |
Pyrazine compounds | 0.27 ± 0.06 | 0.55 ± 0.06 | 5.60 ± 0.03 | 9.09 ± 0.06 | 1.72 ± 0.13 |
Pyrazines (% of total GC-MS peak area) | 2.77 | 5.81 | 17.93 | 15.3 | 10.12 |
In the native SSP & FAAs model, the varieties of detected pyrazines increased significantly with the increase of heating temperature. As shown in Table S2,† only two pyrazines were detected at 100 °C and three pyrazines at 110 °C, while ten pyrazines were detected at 140 °C. When the heating temperature was below 120 °C, the prolonged heating time has no significant effect on the formation of pyrazines (p > 0.05). However, when the temperature was higher than 120 °C, the influence of heating time on pyrazine formation was significant (p < 0.05). Five pyrazines were generated at 140 °C for 30 or 60 min, and eleven pyrazines were detected for 90 min or more time. These results may reflect that some pyrazines can only be generated at high heating temperature. For example, 2,3,5-trimethylpyrazine, 2-ethyl-3-methylpyrazine, 3-ethyl-2,5-dimethylpyrazine, 2-methyl-5-propylpyrazine and 3,5-diethyl-2-methylpyrazine were only detected at 140 °C for 90 min or more time in the native SSP & FAAs model, and these pyrazines were also detected at 140 °C for 150 min in native SSP model. These results were in agreement with the report by Misharina et al. who found that higher temperature could promote the formation of some specific pyrazines.34 Furthermore, the percentage of pyrazines in total GC-MS peak area increased with the increase of heating temperature and time in this model. The proportion of pyrazines in total volatiles increased from 2.77% to 17.93% at 140 °C with the heating time ranging from 30 to 90 min.
Substances | 30 min | 60 min | 90 min | 120 min | 150 min |
---|---|---|---|---|---|
a Numbers are represented as the means ± standard deviations. | |||||
The model was heated at 100 °C | |||||
Furan compounds | 10.43 ± 0.56 | 15.84 ± 0.09 | 17.88 ± 1.23 | 30.00 ± 2.67 | 28.11 ± 0.45 |
Aldehydes and ketones | 6.99 ± 0.32 | 7.60 ± 0.45 | 7.34 ± 0.88 | 5.5 ± 0.38 | 4.46 ± 0.12 |
Pyrazine compounds | 1.54 ± 0.03 | 3.84 ± 0.04 | 5.36 ± 0.34 | 7.31 ± 0.11 | 11.21 ± 0.54 |
Pyrazines (% of total GC-MS peak area) | 8.12 | 14.14 | 18.15 | 17.33 | 25.99 |
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The model was heated at 110 °C | |||||
Furan compounds | 5.32 ± 0.78 | 9.57 ± 0.45 | 11.26 ± 0.22 | 11.62 ± 0.05 | 11.97 ± 0.09 |
Aldehydes and ketones | 3.23 ± 0.21 | 2.29 ± 0.11 | 2.44 ± 0.31 | 2.15 ± 0.04 | 2.05 ± 0.02 |
Pyrazine compounds | 0.89 ± 0.02 | 2.52 ± 0.34 | 3.34 ± 0.22 | 2.91 ± 0.06 | 3.88 ± 0.01 |
Pyrazines (% of total GC-MS peak area) | 9.48 | 17.81 | 20.00 | 17.86 | 22.22 |
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The model was heated at 120 °C | |||||
Furan compounds | 22.83 ± 0.08 | 31.04 ± 1.66 | 35.83 ± 0.35 | 33.72 ± 0.98 | 62.65 ± 4.78 |
Aldehydes and ketones | 5.15 ± 0.05 | 6.08 ± 0.87 | 5.44 ± 0.44 | 4.78 ± 0.09 | 8.35 ± 0.22 |
Pyrazine compounds | 8.04 ± 0.02 | 11.01 ± 0.52 | 11.71 ± 0.21 | 14.24 ± 0.02 | 14.56 ± 0.99 |
Pyrazines (% of total GC-MS peak area) | 22.85 | 23.38 | 22.59 | 27.55 | 17.21 |
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The model was heated at 130 °C | |||||
Furan compounds | 9.45 ± 0.37 | 9.27 ± 0.03 | 6.93 ± 0.04 | 13.03 ± 0.07 | 14.3 ± 0.03 |
Aldehydes and ketones | 2.20 ± 0.02 | 2.44 ± 0.05 | 1.79 ± 0.02 | 3.05 ± 0.08 | 3.58 ± 0.06 |
Pyrazine compounds | 3.51 ± 0.07 | 2.51 ± 0.05 | 2.57 ± 0.01 | 3.38 ± 0.01 | 5.17 ± 0.09 |
Pyrazines (% of total GC-MS peak area) | 23.86 | 18.20 | 23.43 | 17.66 | 22.71 |
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The model was heated at 140 °C | |||||
Furan compounds | 25.19 ± 1.93 | 24.23 ± 0.05 | 16.84 ± 0.07 | 41.33 ± 0.34 | 40.36 ± 3.54 |
Aldehydes and ketones | 3.45 ± 0.05 | 6.26 ± 0.06 | 8.77 ± 0.08 | 14.43 ± 0.73 | 11.11 ± 0.93 |
Pyrazine compounds | 9.63 ± 0.06 | 12.05 ± 0.77 | 12.70 ± 0.44 | 11.26 ± 0.31 | 12.82 ± 0.05 |
Pyrazines (% of total GC-MS peak area) | 25.69 | 28.73 | 33.86 | 17.03 | 20.20 |
In Table S3,† 2-methylpyrazine and 2,5(6)-dimethylpyrazine were the main pyrazine compounds in hydrolyzed SSP model. In this model, the heating time and temperature have significant effect on pyrazine formation. When the heating temperature was 100 °C, two and six pyrazines were detected for 30 min and 150 min, respectively. As the heating temperature increased from 100 to 140 °C, more pyrazines were produced in this Maillard model, which was consistent with the results of other two models. It is worth noting that at 140 °C, when the heating time increased from 60 to 150 min, the varieties of pyrazines were similar and the changing of peak area of pyrazines was not significant (p > 0.05). This indicated that when the heating temperature was high enough, temperature exerted a major role in the kinetics of the Maillard reaction, and the heating time had little influence on the varieties of volatiles. Similar result was previously reported by Jousse et al.36 In addition, under the heating condition of 140 °C/90 min, eleven pyrazines were detected and the percentage of pyrazines in total volatiles was the highest: unsubstituted pyrazine (1.1%), 2-methylpyrazine (11.3%), 2,5(6)-dimethylpyrazine (8.0%), 2-ethylpyrazine (1.8%), 2-ethyl-5-dimethylpyrazine (1.9%), 2,3,5-trimethylpyrazine (1.7%), 2-ethyl-3-methylpyrazine (1.4%). Moreover, at the same heating temperature and time, the percentage of pyrazines was significantly higher in the hydrolyzed SSP model than that in the other tested model systems, especially at 140 °C/90 min, the percentage of pyrazines in the hydrolyzed SSP model was the most, accounted for 30% of total volatiles.
However, even if under longer heating time (150 min), only a few pyrazines were generated at 100 °C or 110 °C in all three Maillard model systems. This may be due to that the temperature was not high enough and restricted the production of pyrazine compounds. Compared with the other two model systems, the total percentage and varieties of pyrazines were higher in the hydrolyzed SSP model. This probably because the hydrolyzed SSP generated the short chain peptides fragments (Table 2) and the contribution of short chain peptides was more than that of FAAs for the pyrazines formation.
As shown in Fig. 1a–d and f–i, the varieties of pyrazines in the native SSP & FAAs model was close to pyrazines in native SSP model. This result indicated that the FAAs produced by hydrolysis of native SSP had no significant effect on formation of pyrazines (p > 0.05). Additionally, the most varieties and percentage of pyrazines were detected in the hydrolyzed SSP model at 140 °C/90 min and the content of pyrazines was higher than that of the other two Maillard models. Eleven pyrazines were detected and 2-methylpyrazine and 2,5(6)-dimethylpyrazine were the main pyrazine compounds in the hydrolyzed SSP model at 140 °C/90 min (12% and 8%, respectively) (Fig. 1b and c). Compared with other tested Maillard model systems, the hydrolyzed SSP model could facilitate the formation of pyrazines, which was consistent with the previous researches.7,37 A possible explanation is that after trypsin hydrolysis, native SSP was broken into short chain peptides, resulting in more pyrazines formation.19 Therefore, the formation of pyrazines in three Maillard models revealed that peptides, especially the short chain peptides, play an important role in the production of pyrazine compounds.
In addition, the general mechanism of pyrazine formation by peptides and FAAs has been reported.26,38,39 The pyrazine formation by FAAs involves decarboxylation, while peptides are lack of free carboxyl groups at the α-carbon; thus peptides could not follow the typical Strecker degradation reaction to produce α-aminoketones. The mechanism between the reaction of dipeptides and α-dicarbonyl compounds has been proposed, where a complex α-ketoamide is produced by the reaction of dipeptides and α-dicarbonyl compounds and further reacts with the α-aminoketone to form pyrazines.26
Fig. 2B presents the distribution of volatile compounds on two principal components. Heptanal was listed in the negative half axis of F1, while other volatile compounds were listed in the positive half axis of F1. This may be because heptanal was only detected in the native SSP & FAAs model and the detected peak area was low. Furan compounds, unsubstituted pyrazine, 2-methylpyrazine, 2-ethylpyrazine, 2,5(6)-dimethylpyrazine, 2,3-dimethylpyrazine and 2-ethyl-3-methylpyrazine, hexanal and 2-heptanone were distributed in the positive half axis of F2. However, heptanal, nonanal, 2-ethyl-5-methylpyrazine, 3,5-diethylpyrazine, 2,3,5-trimethylpyrazine, 2-methyl-5-propylpyrazine and 3-ethyl-2,5-dimethylpyrazine were distributed in the negative half axis of F2. This probably because furan compounds, hexanal and 2-heptanone unsubstituted pyrazine, 2-methylpyrazine, 2-ethylpyrazine, 2,5(6)-dimethylpyrazine, 2,3-dimethylpyrazine and 2-ethyl-3-methylpyrazine were detected in all tested Maillard models with relatively higher peak area. However, pyrazine compounds distributed in the negative axis of F2 were only detected in the hydrolyzed SSP model at 140 °C/90 min and their peak areas were relatively low. Thus, these volatile compounds were classified into three categories. Heptanal was divided into the first category. Furan compounds, unsubstituted pyrazine, 2-methylpyrazine, 2-ethylpyrazine, 2,5(6)-dimethylpyrazine, 2,3-dimethylpyrazine and 2-ethyl-3-methylpyrazine, hexanal and 2-heptanone were classified into the second category. Nonanal, 2-ethyl-5-methylpyrazine, 3,5-diethylpyrazine, 2,3,5-trimethylpyrazine, 2-methyl-5-propylpyrazine and 3-ethyl-2,5-dimethylpyrazine were classified into the third category.
Footnote |
† Electronic supplementary information (ESI) available: Tables S1–S3 were presented in the supplementary materials. See DOI: 10.1039/d1ra05140g |
This journal is © The Royal Society of Chemistry 2021 |