Molecules 2015, 20, 19002-19013; doi:10.3390/molecules201019002 
 
molecules 
ISSN 1420-3049 
www.mdpi.com/journal/molecules 
Article 
The Occurrence of Propyl Lactate in Chinese Baijius  
(Chinese Liquors) Detected by Direct Injection Coupled with 
Gas Chromatography-Mass Spectrometry 
Jihong Wu 1,2, Yang Zheng 1,2, Baoguo Sun 1,2,3, Xiaotao Sun 1,2, Jiyuan Sun 1,2,  
Fuping Zheng 1,2 and Mingquan Huang 1,2,3,* 
1 School of Food and Chemical Engineering, Beijing Technology and Business University,  
Beijing 100048, China; E-Mails: wujihong12@126.com (J.W.); an2zhengyang@126.com (Y.Z.); 
sunbg@btbu.edu.cn (B.S.); sunxiaotao@th.btbu.edu.cn (X.S.); sunjinyuan@th.btbu.edu.cn (J.S.); 
zhengfp@th.btbu.edu.cn (F.Z.) 
2 Beijing Key Laboratory of Flavor Chemistry, Beijing Technology and Business University,  
Beijing 100048, China 
3 Beijing Innovation Centre of Food Nutrition and Human Health, Beijing 100048, China 
* Author to whom correspondence should be addressed; E-Mail: huangmq@th.btbu.edu.cn;  
Tel./Fax: +86-10-6898-5382. 
Academic Editor: Luca Forti 
Received: 1 September 2015 / Accepted: 13 October 2015 / Published: 19 October 2015 
 
Abstract: As one of the oldest distillates in the world, flavor compounds of Chinese Baijiu 
(Chinese liquor) were extremely complex. Propyl lactate was ﬁrstly detected by direct injection 
and gas chromatography-mass spectrometry (GC-MS) in 72 Chinese Baijius. The objectives 
were to detect the contents of propyl lactate and evaluate its contribution to the aroma of 
Chinese Baijiu based on odor activity values (OAVs). The levels of propyl lactate in these 
distillates were determined by internal standard method and selective ion monitoring (SIM), 
which ranged from 0.050 to 1.900 mg·L−1 under investigation. Its detection threshold was 
determined by Three-Alternative Forced-Choice (3-AFC) and curve fitting (CF), which was 
0.740 mg·L−1 in 38% ethanol solution. The contribution of propyl lactate on the aroma of 
these distillate drinks was evaluated by their odor activity values (OAVs), which varied  
from 0.066 to 4.440. The OAVs of propyl lactate were found to exceed 1 in 13 Chinese 
Baijius, including 50° Jingzhi Guniang 5 years (4.440), 52° Jingzhi Guniang 10 years (3.024), 
Jingyanggang (2.568), Xianghe Ronghe Shaofang (2.313), and 1956 Laolang (1.431), which 
indicated that propyl lactate was one of odor-active components in these Chinese Baijius. 
OPEN ACCESS
Molecules, 2015, 20 
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Keywords: Chinese Baijiu; propyl lactate; gas chromatography-mass spectrometry (GC-MS); 
threshold; odor activity values (OAVs) 
 
1. Introduction 
Chinese Baijiu (Chinese liquor) is one of the oldest distillates in the world, and it is the most popular 
spirits in China with the annual production of about 4 million metric tons. The general process for the 
production of Chinese Baijiu is as follows. At first, the grain raw materials, such as wheat, sorghum, corn, 
rice or glutinous rice, are cooked with steam. And then some saccharification and fermentation agents 
(“DaQu” or “XiaoQu”) are added into the cooked grain matrix. Finally, the liquors are distilled out with 
steam from the fermentation products after several months or years of fermentation [1]. The fresh distillates 
need to be aged for a long time in order to balance the flavors. The final commercialized products are 
blended with aged distillate drinks, fresh distillate drinks and water based on certain ratios according to 
different formulations of spirit drinks [2]. 
Flavor compounds of Chinese Baijiu are extremely complex due to different raw materials, various 
microorganisms and diverse procedures in different production regions, and a great number of compounds 
have been studied extensively [3–6], such as esters, alcohols, ketones, acids, and so on. Lactates have been 
reported to occur in Chinese liquors widely, such as methyl lactate [4], ethyl lactate [7], butyl lactate [8], 
hexyl lactate [9], isopropyl lactate [10], isobutyl lactate [4] and isoamyl lactate [4]. Lactates are recognized 
to be important flavor compounds in Chinese Baijiu [11]. Ethyl lactate and butyl lactate were detected 
in “Gujing” Baijiu during investigation carried out in our lab. In the meantime, lactic acid and propanol 
were also found in this distillate, which had been reported in other Chinese Baijius [12,13]. It is a little 
strange that propyl lactate has not been reported in Chinese Baijiu up to now. The same are other alcoholic 
beverages, except distilled Calvados [14] and Chinese rice wine [15]. Meanwhile, the odor properties of 
propyl lactate and its contribution to aroma of these spirits are also not known. We checked the newest 
edition (2011 edition) of NIST (National Institute of Standards and Technology) library and found that 
the mass spectrogram of propyl lactate was not included in this library. The occurrence of propyl lactate 
in Chinese Baijiu might be overlooked since the identification of volatile components was usually 
performed by searching NIST library. The objective of this work were (i) the detection of the occurrence 
of propyl lactate in 72 Chinese Baijius and (ii) the evaluation of its contribution to the aroma of Chinese 
Baijius based on odor activity values (OAVs). 
2. Results and Discussion 
2.1. Identification of Propyl Lactate in Distillate Samples 
The mass spectra of an unknown compound shown at 17.4 min in sample 33 and propyl lactate standard 
were presented as Figure 1, and the differential spectrum of them was located at the bottom, which indicated 
the unknown compound shown at 17.4 min being matched with propyl lactate. The two magnified TICs 
of the unknown compound in sample 33 (a TIC) and propyl lactate standard (b TIC) at 17.4 min were 
shown in Figure 2. These two peaks were completely overlapped. 
Molecules, 2015, 20 
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Figure 1. TIC of propyl lactate in GC-MS. The mass spectra of an unknown compound shown 
at 17.4 min in sample 33 and propyl lactate standard are presented. The differential spectrum 
of them is located at the bottom. 
 
Figure 2. The magnifying TIC of sample 33 and propyl lactate at 17.4 min. The two magnified 
TICs are shown, one is the unknown compound in sample 33 (a TIC), and the other is propyl 
lactate standard (b TIC) at 17.4 min. 
2.2. Quantitative Analysis of Propyl Lactate in Distillate Samples 
The concentrations of propyl lactate were determined in these spirit drinks by the internal standard 
method with the selected ions monitoring mode of GC-MS. The LOD and the LOQ of the method were 
0.025 mg·L−1 and 0.050 mg·L−1, respectively. The internal standard curve equations included three 
equations in different concentration ranges, including y = 4.4587x + 0.0112 (0.050 < y < 0.60),  
y = 1.9987x + 0.2673 (0.60 < y < 1.00) and y = 2.1972x + 0.0569 (1.00 < y < 4.50). The corresponding 
correlation coefficients (R2) were 0.9994, 0.9917 and 0.9993, respectively. The quantitative results of 
propyl lactate were shown in Table 1. The content of propyl lactate in other 10 Chinese Baijiu, numbered 
73 to 82, were also included in Table 1, which our lab had reported before [16].
Molecules, 2015, 20 
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Table 1. The concentrations of propyl lactate in 82 Chinese Baijiu samples (Detection threshold 0.740 mg·L−1). 
No. 
Sample Name  
(Alcohol % by Volume) 
Manufacturer 
Concentrations of Propyl 
Lactate (mg·L−1) 
OAV of Propyl 
Lactate 
1 
Guizhou Yuanjiang Chenniang 18 Years (52°) 
Kweichow Moutai Co., Ltd. 
0.064 ± 0.003 
0.086 
2 
1956 Laolang (53°) 
Sichuan Langjiu Group Co., Ltd. 
1.059 ± 0.008 
1.431 
3 
Jiabin Lang (50°) 
Sichuan Langjiu Group Co., Ltd. 
0.060 ± 0.002 
0.081 
4 
Xiaohutuxian (52°) 
Guizhou Xingyi Yunfeng Co., Ltd. 
a tr 
- b 
5 
Tianzhilan (42°) 
Jiangsu Yanghe Distillery Co., Ltd 
tr 
- b 
6 
Xiangquan (54°) 
Jiugui Liquor Co., Ltd. 
0.068 ± 0.001 
0.092 
7 
Jinliufu Sixing (38°) 
Wuliangye Group 
0.064 ± 0.001 
0.086 
8 
Jinliufu Hongsejingdian (38°) 
Wuliangye Group 
tr 
- b 
9 
Shuanggou Daqu (38°) 
Shuanggou Distillery 
0.091 ± 0.002 
0.123 
10 
Niulanshan Bainian (38°) 
Shunxin Agriculture Ture 
tr 
- b 
11 
Jianzhuang Chenjiu (52°) 
Wuliangye Group 
tr 
- b 
12 
Laishigang Chenniang 3 Years (53°) 
Laishigang Group 
0.062 ± 0.001 
0.084 
13 
Zhijiang Zhixin 5 Years (52°) 
Zhijiang Group 
0.081 ± 0.002 
0.109 
14 
Jiannanchun (52°) 
Sichuan Jiannanchun Jituan Co., Ltd. 
tr 
- b 
15 
Liulingzui 3 (52°) 
Liulingzui Group 
0.100 ± 0.002 
0.135 
16 
Zhonghua Dukang K3 (50°) 
Luoyang Dukang Holdings Limited Official Website 
0.052 ± 0.001 
0.070 
17 
Yangshao Caitaofang Jiuliang Miaopin (52°) 
Yangshao Co., Ltd. 
0.553 ± 0.006 
0.747 
18 
Mianrou Dukang (50°) 
Luoyang Dukang Holdings Limited Official Website 
0.065 ± 0.001 
0.088 
19 
Shamochun Shengshi (42°) 
Neimenggu Dahekou Co., Ltd. 
0.064 ± 0.001 
0.086 
20 
Luzhou LaojiaoTouqu (52°) 
Luzhou Laojiao Co., Ltd. 
tr 
- b 
21 
Luzhou Laojiao Chentouqu 8 Years (52°) 
Luzhou Laojiao Co., Ltd. 
tr 
- b 
22 
Hetao Laojiao Jinzun (42°) 
Hetao Liquor 
tr 
- b 
23 
Yingjia K6 (38°) 
Yingjia Gongjiu Co., Ltd. 
0.075 ± 0.003 
0.101 
24 
Neimenggu Sorghum Blue Era (38°) 
Neimenggu Dahekou Co., Ltd. 
0.068 ± 0.001 
0.092 
25 
Rouhe Shuanggou (42°) 
Shuanggou Distillery 
0.059 ± 0.001 
0.080 
26 
Xinghuacun Baishun (45°) 
Fenjiu Group 
0.094 ± 0.001 
0.127 
27 
Jingjiu Jixing (36°) 
Wuliangye Group 
0.065 ± 0.002 
0.088 
28 
Guizhou Yuanjiang Zhenpin 9 Years (38°) 
Guizhou Maotai Distillery Group Technology Development Company 
0.068 ± 0.002 
0.092 
Molecules, 2015, 20 
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Table 1. Cont. 
No. 
Sample Name  
(Alcohol % by Volume) 
Manufacturer 
Concentrations of Propyl 
Lactate (mg·L−1) 
OAV of Propyl 
Lactate 
29 
Guizhou DongcangYuanjiu 30 Years (38°) 
Zhenpin Jiuye 
0.302 ± 0.010 
0.408 
30 
Xifeng Yucang (38°) 
Xifeng Co., Ltd. 
0.056 ± 0.001 
0.076 
31 
Furuiwang (38°) 
Furui Co., Ltd. 
0.079 ± 0.001c 
0.107 
32 
Laobaifen Fengtan 15 Years (38°) 
Fenjiu Group 
0.112 ± 0.001 
0.151 
33 
Honghuaci Erguotou (56°) 
Sanhe Fucheng Co., Ltd. 
0.224 ± 0.042 
0.303 
34 
Jingdu Yujiu (42°) 
Shuangqinghe Co., Ltd. 
tr 
- b 
35 
Zhoufuji Erguotou (42°) 
Zhoufuji Co., Ltd. 
tr 
- b 
36 
Mendaolv (62°) 
Ningheyuan Co., Ltd. 
tr 
- b 
37 
Weirenmin Fuwu (53°) 
Guizhou Maotai Distillery Group 
0.130 ± 0.002 
0.176 
38 
Beijing Erguotou Qinghuaci (52°) 
Jiuzhongjiu Co., Ltd. 
0.064 ± 0.002 
0.086 
39 
Zhougong Baisui (35°) 
Huangjia Jingdu Co., Ltd. 
0.052 ± 0.003 
0.070 
40 
Tianshan Laobing (38°) 
Tianshan Co., Ltd. 
tr 
- b 
41 
Beijing Erguotou I (56°) 
Tongquanyong Co., Ltd. 
tr 
- b 
42 
Xianghe Ronghe Shaofang (53°) 
Kweichow Moutai Co., Ltd. 
1.712 ± 0.023 
2.313 
43 
Dajinjiu (42°) 
Dajin Co., Ltd. 
tr 
- b 
44 
Jingdu Heitan (42°) 
Huangjia Jingdu Co., Ltd. 
0.066 ± 0.003 
0.089 
45 
Wuliang Yuanjiu (50°) 
Yuqiao Co., Ltd. 
0.300 ± 0.054 
0.405 
46 
Guocuijiu (52°) 
Luzhou Guocui Co., Ltd. 
0.052 ± 0.001 
0.070 
47 
Hongdu Tezhen 25 Years (50°) 
Hongdu Co., Ltd. 
tr 
- b 
48 
Beijing Erguotou II (56°) 
Shuangqinghe Co., Ltd. 
0.075 ± 0.006 
0.101 
49 
Beijing Fangzhuang Erguotou (52°) 
Fangzhuang Co., Ltd. 
tr 
- b 
50 
Laobeijing Erguotou (41°) 
Duxing Co., Ltd. 
tr 
- b 
51 
Sichuan Sorghumjiu I (40°) 
Yaoquan Laojiao Co., Ltd. 
tr 
- b 
52 
Guantoushan (40°) 
Guantoushan Co., Ltd. 
0.093 ± 0.001 
0.126 
53 
Mengguwang (44°) 
Mengguwang Co., Ltd. 
0.090 ± 0.002 
0.122 
54 
Caoyuan Andaqing (62.8°) 
Andaqing Co., Ltd. 
0.100 ± 0.004 
0.135 
55 
Luchun (52°) 
Luzhou Laojiao Co., Ltd. 
tr 
- b 
56 
Yujingfang Shaojiu (38°) 
Yujingfang Shaojiu Group 
0.058 ± 0.002 
0.078 
Molecules, 2015, 20 
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Table 1. Cont. 
No. 
Sample Name  
(Alcohol % by Volume) 
Manufacturer 
Concentrations of Propyl 
Lactate (mg·L−1) 
OAV of Propyl 
Lactate 
57 
Sichuan Sorghum II (42°) 
Culiangfang Co., Ltd. 
tr 
- b 
58 
Xianli Jianguo 60 Years (50°) 
MaotaiJiucheng Co., Ltd. 
tr 
- b 
59 
Longfeng (38°) 
Longfeng Co., Ltd. 
tr 
- b 
60 
Caoyuan Liema (62°) 
Menggudao Co., Ltd. 
tr 
- b 
61 
Shuijing Kongdong (52°) 
Liuhuchun Co., Ltd. 
0.066 ± 0.001 
0.089 
62 
Gubei Chunliang (42°) 
Gubeichun Co., Ltd. 
0.050 ± 0.001 
0.066 
63 
Banmasuo Chenniao 3 Years (65°) 
Muniu Co., Ltd. 
0.074 ± 0.001 
0.100 
64 
Heitudi (38°) 
Hecheng Co., Ltd. 
tr 
- b 
65 
Hengshui Laobaigan (39°) 
Hengshui Laobaigan Co., Ltd. 
0.149 ± 0.005 
0.201 
66 
Bancheng Laojiu (42°) 
Qianlongzui Co., Ltd. 
0.075 ± 0.001 
0.101 
67 
Jingyanggang (38°) 
Jingyanggang Co., Ltd. 
1.900 ± 0.002 
2.568 
68 
65667 Troops Tegong T99B (38°) 
Beimao Co., Ltd. 
0.250 ± 0.002 
0.338 
69 
Huanghelong Laoliangfang (52°) 
Huanghelong Group 
0.100 ± 0.001 
0.135 
70 
Guojiao 1573 (52°) 
Luzhou Laojiao Co., Ltd. 
tr 
- b 
71 
Mengzhilan M6 (40.8°) 
Jiangsu Yanghe Distillery Co., Ltd 
0.176 ± 0.006 
0.238 
72 
Kouzijiao Zhencang 20 Years (41°) 
Kouzi Yjiuye 
0.052 ± 0.002 
0.070 
73 
Moutai (53°) c 
Kweichow Moutai Co., Ltd. 
0.851 ± 0.001 c 
1.150 
74 
Xifeng (55°) c 
Xifeng Co., Ltd. 
0.818 ± 0.011 c 
1.105 
75 
Guojing 1# (65°) c 
Shandong Bandaojing Co., Ltd. 
1.008 ± 0.018 c 
1.362 
76 
Gujing Yuanjiang (65°) c 
Anhui Gujing Group Co., Ltd. 
2.237 ± 0.022 c 
3.023 
77 
Jinshiyuan Yuanjiang (59°) c 
Jiangsu King’s Luck Brewery Joint-Stock Co., Ltd. 
0.932 ± 0.024 c 
1.259 
78 
Jinshiyuan (53°) c 
Jiangsu King’s Luck Brewery Joint-Stock Co., Ltd. 
0.810 ± 0.017 c 
1.095 
79 
Jingzhi Guniang10 Years (52°) c 
Shandong Jingzhi Liquor Co., Ltd. 
3.024 ± 0.025 c 
4.086 
80 
Jingzhi Guniang 5 Years (50°) c 
Shandong Jingzhi Liquor Co., Ltd. 
3.286 ± 0.060 c 
4.440 
81 
Wuyue Duzun (52°) c 
Taishan Liuor Group Co., Ltd. 
0.788 ± 0.006 c 
1.065 
82 
Jiuchao Chenxiang (42°) c 
Shandong Lanling Meijiu Co., Ltd. 
0.910 ± 0.014 c 
1.230 
a tr: The concentrations of propyl lactate were between 0.025 mg·L−1 and 0.050 mg·L−1; b No OAVs because of the concentrations of propyl lactate was much less than LOQ; 
c the concentrations of propyl lactate were from the reported article [16]. 
Molecules, 2015, 20 
19008 
 
As shown in Table 1, propyl lactate did occur in all Chinese Baijius under investigation, though the 
concentrations in some distillate drinks were between 0.050 mg·L−1 (LOQ) and 0.025 mg·L−1 (LOD). 
The top 5 distillate samples were Jingzhi Guniang 5 years (50°) (3.286 mg·L−1), Jingzhi Guniang 10 years 
(52°) (3.024 mg·L−1), Gujing Yuanjiang (65°) (2.237 mg·L−1), “Jingyanggang” (38°) (1.900 mg·L−1), 
and “Xianghe Ronghe Shaofang” (53°) (1.712 mg·L−1). 
2.3. Detection Threshold of Propyl Lactate and OAV Analysis 
The determination of detection threshold of propyl lactate was conducted by the statistical analyses 
and the Curve Fitting, and the result was shown in Figure 3. 
 
Figure 3. Scatter diagram by CF for determination of propyl lactate detection threshold. The 
X-axis represents the concentration of propyl lactate to the base 10 logarithm (X = LogA) and 
Y-axis is the correct recognition ratio, which is the ratio of the correct recognition numbers 
in total recognition numbers ሺY ൌ
୒ሺୡ୭୰୰ୣୡ୲ሻ
୒ሺ୲୭୲ୟ୪୪ሻ ሻ. 
The calibration curve equation was y = 0.1623x + 0.6877, and the correlation coefficient R2 = 0.9663. 
The threshold was the corresponding X value (0.740 mg·L−1) when Y = 66.67% [17]. 
2.4. Discussion 
The unknown compound shown at 17.4 min in sample 33 was identified to be propyl lactate based on 
the comparison of mass spectra and retention time with the standard. Propyl lactate was also discovered in 
all other samples under investigation by the same method. 
Propyl lactate was possibly formed through the esterification of lactic acid with propanol, both of 
which have been reported during the fermentation. There were a great number of lactobacilli during the 
fermentation process of Chinese spirit drinks, which could convert sugars to lactic acid [18,19]. Higher 
alcohols could be formed during the fermentation under aerobic condition from sugar or under anaerobic 
conditions from amino acids [20] since the raw materials, sorghum, rice, sticky rice, wheat and corn, were 
rich sources of amino acids. Propanol can be produced from threonine by yeast via the Ehrlich metabolic 
pathway [21]. Small amounts of propanol could also be formed by yeast through reduction of propanal. 
The esterification of lactic acid with propanol could be taken place directly or catalyzed by esterases during 
the fermentation and aging process of Chinese liquor production [20]. The esterases might be from yeasts, 
Molecules, 2015, 20 
19009 
 
 
molds, or bacteria, which existed in “Daqu” or “Xiaoqu” [21]. Fan [22] also reported that the “Daqu” had 
high esterase activities. The main formation process of propyl lactate was as Figure 4. 
 
Figure 4. The pathway of propyl lactate formation by esterase catalyzation. 
The odor activity values (OAV) equaled to the ratio of the concentration of propyl lactate and its 
detection threshold value. If a compound has an OAV > 1.0, then it would contribute to the flavor of a 
product [23]. The OAVs of propyl lactate in 82 Chinese Baijius were listed in Table 1. There were 13 
distillate samples with the OAVs of propyl lactate higher than 1, including 50° Jingzhi Guniang  
5 years (4.440), 52° Jingzhi Guniang 10 years (3.024), 38° Jingyanggang (2.568), 53° Xianghe Ronghe 
Shaofang (2.313), and so on, whereas other 69 liquor samples with the OAVs lower than 1. Propyl lactate 
had a grape-like fruity, milk and ester odor [17]. The results indicated that propyl lactate was one of 
odor-active components of these 13 Chinese Baijius. Whether propyl lactate was a key odor compound 
or not based on more experiments and proofs, which we would be to study next. 
3. Experimental Section 
3.1. Chemicals 
Chemicals and standards were GC grade with a high purity (>99.0%).The water was boiling for at least 
0.5 h and redistilled twice before use. Methyl lactate (PubChem CID: 11040), used as internal standard 
(IS), and propyl lactate (PubChem CID: 92821) were obtained from Tokyo Chemical Industry CO., Ltd. 
(Shanghai, China). Absolute ethanol (PubChem CID: 702) was obtained from Merck (Darmstadt, Germany). 
3.2. Spirit Drink Samples 
A total of 72 spirit drinks, shown as Table 1, were obtained from different factories in China, or 
supermarkets, such as Wal-Mart and Carrefour in Beijing, China. 
3.3. Qualitative and Quantitative Analysis by GC-MS 
1.0 µL of Chinese spirit drinks was injected and analyzed by GC-MS with the full scan mode, and 
the occurrence of propyl lactate was confirmed by comparing its retention time and mass spectrum with 
the standards. 
Starches
amylase
sugars
lactic acid
lactobacilli
H3C
H
C
COOH
OH
H3C
H2
C
CH2OH
threonine
yeast
Ehrlic metabolic pathway
CH
H
C
COOH
H3C
HO
NH2
Propanol
H3C
H
C
COOCH2C2H5
OH
esterases
Propyl lactate
Molecules, 2015, 20 
19010 
 
 
The concentrations of propyl lactate in these distillates were determined by the internal standard method 
with the selected ions monitoring mode of GC-MS. At first, a series of the standard solutions, such as 
5.000 mg·L−1, 2.500 mg·L−1, 1.250 mg·L−1, 0.625 mg·L−1, 0.313 mg·L−1, 0.156 mg·L−1, 0.078 mg·L−1, 
0.039 mg·L−1 and 0.020 mg·L−1, were prepared with absolute ethanol and analyzed by GC-MS. Then 
1.0 mL of each distillate drink sample with 10.0 µL of methyl lactate solution (100.000 mg·L−1) were 
placed in 72 tightly closed sample vials, numbered 1 to 72, for GC-MS analysis. Finally, the concentrations 
of propyl lactate were calculated by the software of GC-MS. The conditions of GC-MS (6890A-5975C, 
Agilent technologies Co., Ltd., Beijing, China) were as follows. 
GC conditions: DB-FFAP capillary column (30 m × 0.25 mm, 0.25 μm film thickness, Santa Clara, CA, 
USA); carrier gas, helium, 99.9995%; flow rate, 1.0 mL·min−1; The oven temperature was programmed at 
50 °C for 2 min, then raised to 100 °C at 6 °C·min−1, then raised to 170 °C at 3 °C·min−1 for 2 min, and 
then raised to 200 °C at 10 °C·min−1 for 2 min, and finally raised to 230 °C at 15 °C·min−1 for 5 min; 
inlet temperature, 250 °C; transfer line temperature, 250 °C; injection volume, 1 µL; split ratio, 20:1. 
MS conditions: electron ionization source, 70 eV; ion source and quadruple temperatures, 230 °C and 
150 °C, respectively; The monitored ions and other parameters of selected-ion-monitoring (SIM) mode 
were listed in Table 2. 
Table 2. The monitored ions and other parameters. 
Compound 
Mode 
Mass List or Range 
Methyl lactate 
Full Scan 
50–500 
SIM 
45, 75, 89,105 
Propyl lactate 
Full Scan 
50–500 
SIM 
45, 75, 117 
3.4. Determination of the Detection Threshold of Propyl Lactate 
3-AFC test was recommended as the national standard method for determination of Chinese Baijiu 
flavors thresholds by GB/T 22366-2008 [16], which is a general guidance for measuring odor, flavor 
and taste detection threshold, because of its higher efficiency and accuracy. Meanwhile, CF method was 
adopted as threshold calculation method, which was recommended by ASTM E1432-2004 Standard 
Practice. So 3-AFC and CF were selected to determine the detection threshold of propyl lactate in Chinese 
spirit drinks. 
Most concentrations of alcohol in the distillate samples under investigation were 38 vol% or nearby, 
so 38% ethanol solution was used as the benchmark. A series of propyl lactate solutions were prepared for 
sensory evaluation with 38% ethanol solution, such as 0.137 mg·L−1 (A1), 0.412 mg·L−1 (A2), 1.235 mg·L−1 
(A3), 3.704 mg·L−1 (A4), 11.111 mg·L−1 (A5) and 33.333 mg·L−1 (A6). 
There were eighteen samples for sensory evaluation, which were equally divided into six groups. All 
these samples were placed in 10 mL tulip-like glass wine cups. Two samples of each group were the control 
samples and the remaining one had different concentration of propyl lactate in it. Each sample was marked 
in a random four-digit number. A group of 30 untrained and normal olfaction assessors were invited to 
determine propyl lactate detection threshold. All samples were assessed at room temperature. For each 
test, the assessors needed to pick out the one which was “very different from the references”, and wrote 
down the number. Each test was replicated 3 times, so that 90 responses were obtained for each testing 
Molecules, 2015, 20 
19011 
 
 
concentration. Then the Curve Fitting (CF) was conducted by the statistical analyses, and the curve was 
drawn. When Y = 66.7%, the corresponding X value was the detection threshold value of propyl lactate. 
4. Conclusions 
In summary, this work reported the occurrence of propyl lactate in 72 Chinese Baijius for the first 
time. The concentration of propyl lactate ranged from 0.050 to 1.900 mg·L−1 in 72 Chinese Baijius. The 
detection threshold of propyl lactate in 38% ethanol solution was 0.740 mg·L−1. Based on OAV analysis 
in this research, propyl lactate had much contribution to the aroma of 13 Chinese Baijius , including 50° 
Jingzhi Guniang 5 years (4.440), 52° Jingzhi Guniang 10 years (3.024), Jingyanggang (2.568), Xianghe 
Ronghe Shaofang (2.313), and 1956 Laolang (1.431). 
Acknowledgements 
The financial supports from National Natural Science Foundation of China (31471665 and 31301466) 
and National Natural Science Foundation of Beijing (KZ201410011015) are gratefully acknowledged. 
Author Contributions 
Conceived and designed the experiments: B.S., F.Z. and M.H. Performed the experiments: J.W. 
(mostly), Y.Z., X.S. and J.S. Analyzed the data: J.W. Wrote the paper: J.W. and M.H. 
Conflicts of Interest 
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study, 
in the collection, analyses, or interpretation of data, in the writing of the manuscript, and in the decision to 
publish the results. 
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© 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article 
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The Occurrence of Propyl Lactate in Chinese Baijius (Chinese Liquors) Detected by Direct Injection Coupled with Gas Chromatography-Mass Spectrometry.
Wu, Jihong,Zheng, Yang,Sun, Baoguo,Sun, Xiaotao,Sun, Jiyuan,Zheng, Fuping,Huang, Mingquan