Comparative Analysis of Volatile Flavor Compounds in Strongly Flavored Baijiu under Two Different Pit Cap Sealing Processes

Abstract

The solid-state fermentation process of strongly flavored Baijiu is complicated by the co-fermentation of many different microorganisms in the fermentation pools. The traditional fermentation pools of strong flavor Baijiu are sealed with mud, and this sealed-pit mud is not easy to maintain; therefore, the pit cap is prone to cracks and to caving in. The destruction of the sealed-pit mud may lead to instability in the composition and an abundance of microorganisms in the fermentation process that results in fluctuations of product quality. Thus, the production method of replacing the mud cap with a new steel cap is gradually attracting the attention of scientific and technical workers in the industry. However, so far, there have been relatively few reports on the use of steel lids for sealing pits for fermentation and brewing. In this study, the volatile flavor components of 270 Baijiu samples from mud-sealing and steel-sealing pits of a Chinese Baijiu distillery were studied qualitatively and quantitatively using Gas Chromatography-Mass Spectrometry (Abbreviated as GC-MS). Our statistical methods included Hierarchical Cluster Analysis (Abbreviated as HCA), Principal Component Analysis (Abbreviated as PCA), and Discriminant Analysis (Abbreviated as DA). A statistical analysis was carried out on the yield of strongly flavored Baijiu, and we made a comprehensive evaluation of the Baijiu produced under the two pit-sealing modes with regard to flavor and economic efficiency. The yield of strong flavored Baijiu was 6.7% higher with steel-sealing pits compared with mud-sealing pits. Cluster analysis categorized the strongly flavored Baijiu samples into two categories initially: (1) samples produced using mud-sealing pits and (2) samples using steel-sealing pits. Our analysis also indicated that the 28 compounds used for quantification were selected correctly. Surprising to the experimental staff, the overall score for the steel-sealing pits was greater than that of the mud-sealing pits based on PCA. Using DA, the prediction results were 100% accurate. In summary, through a comparative analysis of the flavor and yield, which are the two main factors that affect the quality of Baijiu in a distillery, and systematic combination at both experimental and theoretical levels, the differences between the Baijiu production by steel-sealing and the traditional mud-sealing were clear. Regardless of the impact of age, the detectable flavor components of Baijiu from the mud-steeling pits were very consistent with those of the steel-sealing pits in terms of richness or concentration. However, steel-sealing pits were significantly superior to mud-sealing pits with respect to output, consistency in quality, and cost (human and economic) savings.

Keywords: Gas Chromatography-Mass Spectrometry; flavor compounds; mud-sealing pits; pit cap; steel-sealing pits; strong flavor Baijiu.

Figure 1

Profile of traditional fermentation device (pit cap: mud) for producing strongly flavored Baijiu in China. (1—mud-sealing cap; 2—Grains of Baijiu; 3—Brewing pit; 4—Sealing the pit opening; 5—Included angle of prism clamp, Modified from Figure 1 in Lu et al., 2012 [14]).

Figure 2

Profile of pit cap improved fermentation device A(pit cap: steel) for producing strongly flavored Baijiu in China. (1—steel-sealing cap; 2—Lifting ring; 3—Fixing brackets; 4—Pit mud board; 5—Cover top; 6—Lower box; 7—Pit mud; 8—Grains of Baijiu; 9—Brewing pit, Modified from Figure 2 in Zhang, 2015 [11]).

Figure 3

Profile of pit cap improved fermentation device B(pit cap: steel) for producing strongly flavored Baijiu in China. (1—Lower box; 2—Upper cover body; 3—Lifting ring; 4—Lower box side reinforcements; 5—Pressure Gauges; 6—Material handling ports; 7—Outer side wall of the lower box; 8-Brewing pit; 9—Grains of Baijiu, Modified from Figure 3 in Zhang and Zhao et al., 2015 [17]).

Figure 4

Variation in the yield of strongly flavored Baijiu from the same pit using mud sealing or steel sealing under five sampling batches. ((a)—Pit No. 1; (b)—Pit No. 2; (c)—Pit No. 3).

Figure 5

Yield of strongly flavored Baijiu from two different pit sealing methods. ((a)—1 segment of strong flavor Baijiu; (b)—2 segment of strong flavor Baijiu; (c)—3 segment of strong flavor Baijiu).

Figure 6

Amount of volatile flavor compounds in the different layers of fermented grains in the process of making strongly flavored Baijiu in China.

Figure 7

Clustering analysis of strongly flavored Baijiu with different pit sealing methods. ((a)—1 segment strong flavor Baijiu; (b)—2 segment strong flavor Baijiu; (c)—3 segment strong flavor Baijiu; The names of the letters in the above picture mean: initial “M” for mud-sealing pit, initial “S” for steel-sealing pit).

Figure 8

Scatter plot of the main components of the flavor substances of Baijiu. ((a)—three-dimensional scatter plot of PC1, PC2, and PC3; (b)—plan view of PC1 and PC2).

Figure 9

Scatter diagram of a typical discriminant analysis of the two sealing methods for strongly flavored Baijiu. ((a)—upper layer of grain; (b)—middle layer of grain; (c)—lower layer of grain; The names of the letters in the above picture mean: MSP for mud-sealing pits, SSP for steel-sealing pits; 1—1 segment strong flavor Baijiu; 2—2 segment strong flavor Baijiu; 3—3 segment strong flavor Baijiu).