Terpenes are compounds that include monoterpenes, sesquiterpenes, diterpenes, and terpenes, where isoprene is the basic structural unit of multiple oxygenated hydrocarbons. Terpenes are often found in plants, especially in some Zingiberaceae or tulips, and they have been obtained by direct extraction, chemical synthesis, and microorganisms. They exhibit anti-bacterial, anti-viral, anti-oxidant, and analgesic activity, aid digestion, and have other biological activities. Terpenes are widely used as factors in food health and as anti-cancer treatments. They are used especially as active substances in fermented Chinese baijiu products and gives Chinese baijiu its fruity aroma. To a certain extent, terpenes affect the quality of baijiu. In pharmaceutical products, terpenes, especially limonene and elemene, have strong biological activity that reduces the mitosis of tumor cells, induces tumor cell apoptosis, and inhibits tumor cell growth. However, the low yield of terpenes limits its application. Therefore, we review the sources of terpenes, focus on the biosynthesis pathway of sesquiterpenes, and explore the latest research progress on terpenes that play a biologically active and functional role in fermented products and pharmaceutical products, especially in the development and utilization of the Chinese baijiu industry. We provide new research ideas for the improvement and optimization of terpenes in various industries.

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.