Lactic Acid Dynamics in Baijiu Brewing: Microorganisms, Roles, and Control Strategies

Abstract

This manuscript examines the critical role of lactic acid in baijiu brewing, focusing on the microorganisms involved in its production, the importance of lactic acid in the brewing process, and the methods used to control its levels. The study is, to our knowledge, the first review of lactic acid in baijiu, explicitly focusing on the multiple roles of lactic acid in various stages of the baijiu brewing process, including its regulatory function during fermentation, maintaining acidity, participating in microbial metabolism, and shaping the flavor of the liquor. The review compiles and organizes data that are scattered in the literature on aspects including lactic-acid-producing microbial communities, their distribution in different aroma types of baijiu, and relevant control strategies supported by recent research. By providing a comprehensive overview of these aspects, this manuscript aims to improve the understanding of lactic acid dynamics in baijiu brewing and offers insights into improving production efficiency and product quality. It also identifies current knowledge gaps and suggests future directions, including the use of molecular tools to investigate lactic acid metabolic pathways in complex fermentation systems.

1. Introduction

Baijiu is a traditional Chinese distilled spirit with strong cultural and economic significance [1]. As one of the most consumed spirits globally, it typically contains 40–60% (v/v) alcohol and known for its diverse aroma types and complex flavor profiles [1].

Baijiu brewing is a multistep process that combines traditional fermentation, distillation, and time-honored aging, resulting in baijiu’s complex aroma, depth, and characteristic profile [2].

Lactic acid is primarily a metabolic product generated by various lactic-acid-producing microorganisms through homolactic or heterolactic fermentation of sugars found in baijiu brewing materials [3]. On one hand, lactic acid in the mash interacts with the microbial community within the mash [4]; on the other hand, it subsequently enters the pit mud (the microbial-rich clay lining of the fermentation pit) through huangshui (a liquid byproduct from fermentation), interacting with both the pit mud chemical components and the microbial community presented [4,5]. Some lactic acid and lactic acid esters in the mash enter the base liquor during solid-state distillation through the distillate vapor [1]. The lactic acid esters in the base liquor decompose into lactic acid under the influence of acid–ester equilibrium [1,6]. Because of the presence of hydroxyl groups, lactic acid can effectively enhance the association of hydrogen bounds between alcohol molecules and water molecules, thereby stabilizing the base liquor and the final product while reducing the sharpness of taste and resulting in a smoother mouthfeel [7]. Additionally, lactic acid in huangshui can form aromatic and flavorful substances, such as ester in low-quality baijiu when processed huangshui is blended with it.

This illustrates that lactic acid plays a crucial role in various stages of baijiu brewing; it is not only involved in the fermentation process and microbial interactions but has significant impacts on the quality of the liquor [8]. This encompasses multiple aspects, including brewing microbiology and flavor chemistry [8], the effect from environmental microbiota, and potential impact of climate change on microbiota changes. Therefore, an in-depth study of lactic acid in the baijiu brewing process is of great significance, as it can clarify the role of lactic acid at different stages of production and further improve the quality of baijiu by controlling the lactic acid content. The objective of this review is to systematically summarize the current understanding of lactic acid’s microbial origins, biochemical functions, and roles throughout the baijiu brewing process; highlight the existing research gaps; and propose future research directions to support quality improvement and process optimisation.

2. Microorganisms That Produce Lactic Acid in Baijiu Brewing

In baijiu production, the microorganisms involved in lactic acid production mainly include three categories: bacteria, mold, and yeast [2,9,10].

2.1. Bacteria

In baijiu production, the majority of the lactic acid is produced by lactic acid bacteria, which utilize fermentable sugars to generate lactic acid [11,12]. This group includes genera such as Lactobacillus, Enterococcus, Pediococcus, Leuconostoc, Carnobacterium, Tetragenococcus, Vagococcus, and Weissella. Lactic acid bacteria produce organic acids such as lactic acid and acetic acid, as well as antagonistic substances such as bacteriocins, and they compete with other microorganisms for substrates to stimulate the growth of other microbes [11]. Currently, several types of lactic acid bacteria have been identified in baijiu production, as shown in

Table 1. Lactobacillus.

Strain	Region *	Microbial Source	Aroma Type	Reference
Lactobacillus rhamnosus	Sichuan	Pit mud	Unknown	[13]
	Shandong	Mash	Sesame aroma	[14]
	Hubei	Pit mud	Strong aroma	[15]
Lactobacillus delbrueckii	Shandong	Mash	Sesame aroma	[14]
Lactobacillus coryniformis	Shanxi	Mash	Light aroma	[16]
	Hubei	Mash	Light aroma	[17]
	Shandong	Mash	Sesame aroma	[14]
Lactobacillus casei	Guangdong	Starter ball, starter cake, fermented liquid	Chi flavor	[18]
	Hubei	Mash, pit mud	Light aroma, strong aroma	[15,17,19,20]
	Sichuan	Pit mud	Unknown	[13]
	Guizhou	Mash	Sauce aroma	[21]
	Shanxi	Starter cake	Light aroma	[22]
Lactobacillus paracasei	Jiangsu	Mash	Sesame aroma	[23]
	Hubei	Mash, pit mud	Light aroma, strong aroma	[15,24,25]
	Shandong	Mash	Sesame aroma	[14]
	Guizhou	Mash	Sauce aroma	[21,26]
	Shanxi	Mash	Light aroma	[27]
	Sichuan	Pit mud	Strong aroma	[28]
	Inner Mongolia	Mash	Unknown	[29]
Lactobacillus sakei	Hubei	Mash, pit mud	Light aroma, strong aroma	[15,17]
	Guizhou	Mash	Sauce aroma	[26]
	Shanxi	Daqu (fermentation starter)	Light aroma	[30]
Lactobacillus acidophilus	Jiangsu	Mash	Sesame aroma	[23]
	Sichuan	Mash	Strong aroma	[31]
	Shandong	Daqu	Strong aroma	[32]
Lactobacillus brevis	Shanxi	Mash	Light aroma	[21,33,34,35]
	Hubei	Mash, xiaoqu (rice-based fermentation starter)	Light aroma, unknown	[17,19,20,24,36,37]
	Shandong	Mash	Sesame aroma	[14]
	Guizhou	Mash	Sauce aroma	[38]
	Sichuan	Pit mud, mash	Unknown, strong aroma	[13,39]
	Anhui	Daqu	Strong aroma	[40]
	Henan	Medial-temperature daqu	Strong aroma	[41]
	Inner Mongolia	Mash, pit mud	Unknown, strong aroma	[29,42]
	Jiangsu	Mash	Unknown	[43]
	Beijing	Daqu	Light aroma	[44]
	Shaanxi	Mash	Xifeng-style aroma	[45]
Lactobacillus buchneri	Jiangsu	Mash	Sesame aroma	[23]
	Sichuan	Mash	Strong aroma	[46]
	Shanxi	Mash	Light aroma	[21,33,34,35]
	Guizhou	Mash	Sauce aroma	[21,38,47,48]
	Shandong	Mash	Sesame aroma	[14]
	Hubei	Mash	Light aroma, strong and sauce aroma blended	[20,49,50]
	Anhui	Daqu	Strong aroma	[40]
	Inner Mongolia	Mash	Unknown	[29]
	Shaanxi	Mash	Xifeng-style aroma	[45]
Lactobacillus fermentum	Jiangsu	Mash	Sesame aroma	[23]
	Guangdong	Starter ball, starter cake, qu, fermented liquid	Chi flavor	[18,51]
	Guizhou	Mash	Sauce aroma	[38]
	Hebei	Pit mud, daqu	Strong aroma	[52,53]
	Shandong	Pit mud, mash	Strong aroma, sesame aroma	[14,52,54]
	Sichuan	Pit mud, mash, daqu	Strong aroma	[39,52,55]
	Heilongjiang	Pit mud	Strong aroma	[52]
	Shanxi	Mash	Light aroma	[27]
	Anhui	Daqu	Strong aroma	[40]
	Inner Mongolia	Mash, pit mud	Unknown, strong aroma	[29,42]
	Hubei	Mash, xiaoqu	Light aroma	[20,56]
Lactobacillus johnsonii	Guizhou	Mash	Sauce aroma	[26]
	Shanxi	Mash	Light aroma	[33]
Lactobacillus plantarum	Shanxi	Mash	Light aroma	[16,21,34,35,57,58,59]
	Guangdong	Starter ball, starter cake	Chi flavor	[18]
	Guizhou	Mash	Sauce aroma	[38,47]
	Shandong	Mash	Sesame aroma	[14,54]
	Jiangsu	Mash	Strong aroma	[60]
	Tianjin	Mash	Sauce aroma	[61,62]
	Inner Mongolia	Mash	Unknown	[29]
	Sichuan	Pit mud	Strong aroma	[63]
	Hubei	Pit mud, xiaoqu, mash	Strong aroma, unknown, blended strong and sauce aroma, light aroma	[20,36,49,50,64,65]
	Anhui	High-temperature daqu, pit mud	Strong aroma	[40,66]
	Tibet	Qu	Strong aroma	[67]
	Hebei	Daqu	Strong aroma	[53]
Lactobacillus helveticus	Hubei	Mash	Light aroma	[19]
	Guizhou	Mash	Sauce aroma	[26]
	Shanxi	Mash	Light aroma	[27]
	Guangdong	Fermented liquid	Chi flavor	[51]
Lactobacillus pentosus	Shanxi	Mash	Light aroma	[33,34,35]
	Jiangsu	Mash	Strong aroma	[60]
	Shandong	Mash	Sesame aroma	[14]
	Sichuan	Mash	Strong aroma	[39]
	Heilongjiang	Pit mud	Strong aroma	[68]
	Tianjin	Mash	Sauce aroma	[62]
	Tibet	Qu	Strong aroma	[67]
	Shaanxi	Mash	Xifeng-style aroma	[45]
Lactobacillus panis	Guangdong	Starter ball, starter cake, fermented liquid	Chi flavor	[18]
	Guizhou	Mash	Sauce aroma	[38,47,69]
	Shandong	Mash	Sesame aroma	[14]
	Jiangsu	Mash	unknown	[43]
	Hubei	Xiaoqu, mash	Light aroma	[56,70]
	Shaanxi	Mash	Xifeng-style aroma	[45]
Lactobacillus curvatus	Shandong	Mash	Sesame aroma	[14]
	Sichuan	Mash	Strong aroma	[39]
	Tianjin	Mash	Sauce aroma	[61]
Lactobacillus fructivorans	Sichuan	Mash	Strong aroma	[71,72]
Lactobacillus dextrinicus	Guizhou	Mash	Sauce aroma	[38]
	Shandong	Mash	Sesame aroma	[14]
Lactobacillus pontis	Guangdong	Fermented liquid	Chi flavor	[18,51]
	Guizhou	Mash	Sauce aroma	[26,38,47]
	Shandong	Mash	Sesame aroma	[14]
	Anhui	Daqu	Strong aroma	[40]
Lactobacillus zeae	Jiangsu	Mash	Sesame aroma	[23]
	Heilongjiang	Pit mud	Strong aroma	[68]
	Hubei	Pit mud	Strong aroma	[15]
	Beijing	Daqu	Light aroma	[44]
Lactobacillus acetotolerans	Shanxi	Mash, daqu	Light aroma	[16,59,73]
	Shandong	Mash, pit mud	Sesame aroma, strong aroma	[14,54]
	Guizhou	Mash	Sauce aroma	[26,47]
	Heilongjiang	Pit mud	Strong aroma	[52]
	Jiangsu	Mash, pit mud	Unknown, strong aroma	[43,63]
	Hubei	Pit mud	Strong aroma	[15,65]
	Anhui	Pit mud	Strong aroma	[63]
	Shaanxi	Mash	Xifeng-style aroma	[45]
	Sichuan	Huangshui, mash, pit mud, qu	Strong aroma	[46,52,63,74,75,76,77,78,79]
Lactobacillus hilgardii	Hubei	Mash	Light aroma	[19,20,24]
	Guizhou	Mash	Sauce aroma	[26]
	Shanxi	Mash	Light aroma	[59,80]
Lactobacillus diolivorans	Shanxi	Mash	Light aroma	[16]
	Inner Mongolia	Mash	Unknown	[29]
Lactobacillus crispatus	Guangdong	Fermented liquid	Chi flavor	[18]
	Sichuan	Pit mud	Strong aroma	[81]
Lactobacillus reuteri	Guangdong	Fermented liquid	Chi flavor	[18]
	Shandong	Mash	Sesame aroma	[14]
	Anhui	Daqu	Strong aroma	[40]
	Beijing	Daqu	Light aroma	[44]
Lactobacillus gastricus	Guangdong	Fermented liquid	Chi flavor	[18]
Lactobacillus jinshani	Hubei	Mash	Blended strong and sauce aroma	[82]
Lactobacillus kefiri	Hubei	Mash	Light aroma	[17]
Lactobacillus manihotivorans	Hubei	Mash, pit mud	Light aroma, strong aroma	[15,17]
	Guizhou	Mash	Sauce aroma	[26]
	Jiangsu	Mash	unknown	[43]
Lactobacillus suebicus	Shandong	Mash	Sesame aroma	[14]
Lactobacillus paralimentarius	Shanxi	Mash, daqu	Light aroma	[16,30]
	Hubei	Mash	Light aroma	[17]
	Shandong	Mash	Sesame aroma	[14]
	Anhui	Daqu	Strong aroma	[40]
	Beijing	Daqu	Light aroma	[44]
Lactobacillus concavus	Shandong	Mash	Sesame aroma	[14]
Lactobacillus sanfranciscensis	Shanxi	Mash	Light aroma	[16]
	Shandong	Mash	Sesame aroma	[14]
	Inner Mongolia	Daqu	Strong aroma	[83]
Lactobacillus acidipiscis	Shandong	Mash	Sesame aroma	[14]
	Guizhou	Mash	Sauce aroma	[26]
	Inner Mongolia	Mash	unknown	[29]
	Sichuan	Mash, pit mud	Strong aroma	[39,63]
Lactobacillus xiangfangensis	Shandong	Mash	Sesame aroma	[14]
Lactobacillus intestinalis	Shandong	Mash	Sesame aroma	[14]
Lactobacillus vaccinostercus	Shandong	Mash	Sesame aroma	[14]
Lactobacillus taiwanensis	Shandong	Mash	Sesame aroma	[14]
Lactobacillus agilis	Shandong	Mash	Sesame aroma	[14]
Lactobacillus pantheris	Shandong	Mash	Sesame aroma	[14]
	Hubei	Mash	Light aroma	[20]
Lactobacillus oryzae	Shandong	Mash	Sesame aroma	[14]
Lactobacillus mucosae	Shandong	Mash	Sesame aroma	[14]
Lactobacillus rossiae	Shandong	Mash	Sesame aroma	[14]
	Anhui	Daqu	Strong aroma	[40]
Lactobacillus pobuzihii	Shandong	Mash	Sesame aroma	[14]
Lactobacillus amylovorus	Shandong	Mash	Sesame aroma	[14]
Lactobacillus kimchicus	Guizhou	Mash	Sauce aroma	[26]
Lactobacillus namurensis	Guizhou	Mash	Sauce aroma	[26]
Lactobacillus rumini	Guizhou	Mash	Sauce aroma	[26]
Lactobacillus secaliphilus	Guizhou	Mash	Sauce aroma	[26]
Lactobacillus odoratitofui	Hebei	Pit mud	Strong aroma	[52]
	Shandong	Pit mud	Strong aroma	[52]
	Sichuan	Pit mud	Strong aroma	[52]
	Shaanxi	Mash	Xifeng-style aroma	[45]
Lactobacillus ingluviei	Guizhou	Mash	Sauce aroma	[26]
Lactobacillus parabuchneri	Hubei	Mash	Light aroma	[20]
	Inner Mongolia	Mash, pit mud	Unknown, strong aroma	[29,42]
Lactobacillus nagelii	Hubei	Mash	Light aroma	[20]
Lactobacillus mali	Hubei	Mash	Light aroma	[20]
	Inner Mongolia	Pit mud	Strong aroma	[42]
Lactobacillus spicheri	Hubei	Mash	Light aroma	[20]
Lactobacillus paracollinoides	Sichuan	Mash	Strong aroma	[39]
Lactobacillus crustorum	Sichuan	Mash	Strong aroma	[39]
	Hubei	Mash, xiaoqu	Light aroma	[56,70]
Lactobacillus Citreum	Sichuan	Mash	Strong aroma	[39]
Lactobacillus mindensis	Shanxi	Daqu	Light aroma	[30]
Lactobacillus Lactis	Henan	Medial-temperature daqu	Strong aroma	[41]
	Inner Mongolia	Mash	Unknown	[29]
Lactobacillus nasuensis	Inner Mongolia	Pit mud	Strong aroma	[42,83]
Lactobacillus versmoldensis	Inner Mongolia	Pit mud	Strong aroma	[42,83]
Lactobacillus harbinensis	Inner Mongolia	Mash	Unknown	[29]
Lactobacillus sunkii	Inner Mongolia	Mash	Unknown	[29]
Lactobacillus paraplantarum	Inner Mongolia	Mash	Unknown	[29]
	Guangdong	Qu	Chi flavor	[51]
Lactobacillus fuchuensis	Jiangsu	Mash	Unknown	[43]
	Shanxi	Mash	Light aroma	[21]
Lactobacillus kitasatonis	Hubei	Pit mud	Strong aroma	[15]
Lactobacillus pasteurii	Hubei	Pit mud	Strong aroma	[15]
Lactobacillus gigeriorum	Hubei	Pit mud	Strong aroma	[15]
Lactobacillus camelliae	Hubei	pit mud	Strong aroma	[15]
Lactobacillus porcinae	Hubei	Pit mud	Strong aroma	[15]
	Shaanxi	Mash	Xifeng-style aroma	[45]
Lactobacillus brantae	Hubei	Pit mud	Strong aroma	[15]
Lactobacillus saniviri	Hubei	Pit mud	Strong aroma	[15]
Lactobacillus bifermentans	Sichuan	Pit mud	Strong aroma	[63]
Lactobacillus farciminis	Sichuan	Pit mud	Strong aroma	[63]
Lactobacillus animalis	Sichuan	Pit mud	Strong aroma	[63]
Lactobacillus mash	Guangxi	Fermented liquid	Rice aroma	[84]
Lactobacillus homohiochii	Guizhou	Mash	Sauce aroma	[21,48,69]

* “Region” here refers to a province in China.

.

Common bacteria found in baijiu production also include Bacillus species, which are generally not known for producing lactic acid as their primary fermentation product [85]. However, studies have reported that some species within the Bacillus genus can produce various organic acids, including lactic acid, as shown in

Table 2. Bacillus.

Strain	Region *	Microbial Source	Aroma Type	Reference
Bacillus coagulans	Shandong	Mash	Sesame aroma	[54]
Bacillus subtilis	Hubei	Mash, xiaoqu	Light aroma, strong aroma	[19,24,56,86]
	Guizhou	Mash	Sauce aroma	[38]
	Henan	Pit mud	Strong aroma	[87]
	Sichuan	Mash, daqu, qu	Strong aroma	[55,71,72,78]
	Shaanxi	Mash	Xifeng-style aroma	[88]
	Inner Mongolia	Daqu	Strong aroma	[83]
	Jiangsu	Mash	Unknown	[43]
	Shandong	Medial-temperature daqu	Strong aroma	[32]
	Shanxi	Daqu	Light aroma	[73]
Bacillus methylotrophicus	Sichuan	Mash	Strong aroma	[71,72,76]
	Guizhou	Mash	Sauce aroma	[38,69]
Bacillus licheniformis	Hubei	Mash, xiaoqu	Light aroma	[24,56]
	Guizhou	Mash	Sauce aroma	[38]
	Henan	Pit mud	Strong aroma	[87]
	Beijing	Daqu	Light aroma	[89]
	Sichuan	Mash, daqu	Strong aroma, unknown	[55,71,72,90]
	Shaanxi	Mash	Xifeng-style aroma	[88]
Bacillus paralicheniformis	Hubei	Mash	Light aroma	[19]
Bacillus amyloliquefaciens	Hubei	Mash	Light aroma	[24]
	Guizhou	Mash	Sauce aroma	[38,47]
	Jiangsu	Mash	Strong aroma, unknown	[43,60]
	Beijing	Daqu	Light aroma	[89]
	Sichuan	Mash	Strong aroma	[71,72,76]
	Shaanxi	Mash	Xifeng-style aroma	[88]
Bacillus cereus	Hubei	Mash	Light aroma, strong aroma	[24,86]
	Jiangsu	Mash	Strong aroma	[60]
	Shaanxi	Mash	Xifeng-style aroma	[88]
	Inner Mongolia	Pit mud	Strong aroma	[42]
	Shandong	Medium-to-high-temperature daqu	Strong aroma	[32]
	Shanxi	Daqu, mash	Light aroma	[73,80]
Bacillus pumilus	Hubei	Mash	Light aroma	[24]
	Henan	Pit mud	Strong aroma	[87]
	Shaanxi	Mash	Xifeng-style aroma	[88]
Bacillus Mojavensis	Beijing	Daqu	Light aroma	[89]
Bacillus aerius	Beijing	Daqu	Light aroma	[89]
Bacillus megaterium	Sichuan	Mash	Strong aroma	[71,72]
	Shaanxi	Mash	Xifeng-style aroma	[88]
	Hubei	Mash, xiaoqu	Strong aroma, light aroma	[56,86]
	Shanxi	Mash	Light aroma	[80]
Bacillus safensis	Sichuan	Mash	Strong aroma	[71,72]
Bacillus siamensis	Sichuan	Mash	Strong aroma	[71,72]
	Hubei	Mash	Light aroma	[91]
Bacillus australimaris	Hubei	Mash	Light aroma	[91]
Bacillus vallismortis	Hubei	Mash	Light aroma	[91]
Bacillus proteolyticus	Hubei	Mash	Light aroma	[91]
Bacillus kokeshiiformis	Guizhou	Mash	Sauce aroma	[47]
Bacillus wiedmannii	Shandong	Daqu	Unknown	[90]
Bacillus tequilensis	Shandong	Daqu	Unknown	[90]
	Guizhou	Mash	Sauce aroma	[69]
Bacillus stearothermophilus	Shandong	Medium-to-high-temperature daqu	Strong aroma	[32]
Bacillus cohnii	Shanxi	Daqu	Light aroma	[73]
Oceanobacillus	Jiangxi	Daqu	Si’te distillery special aroma	[92]
Bacillus velezensis	Sichuan	Mash	Strong aroma	[76]

* “Region” here refers to a province in China.

. Other lactic acid bacteria genera found in baijiu production are Enterococcus (

Table 3. Enterococcus.

Strain	Region *	Microbial Source	Aroma Type	Reference
Enterococcus faecalis	Hubei	Mash, high-temperature daqu	Light aroma, blended strong and sauce aroma	[24,49,50,64]
	Tianjin	Mash	Sauce aroma	[62]
	Liaoning	Daqu	Unknown	[90]
	Hebei	Daqu	Strong aroma	[53]
Enterococcus casseliflavus	Sichuan	Pit mud	Unknown	[13]
Enterococcus lactis	Hubei	Pit mud	Strong aroma	[65]
	Shandong	Daqu	Unknown	[90]
Enterococcus faecium	Inner Mongolia	Mash	Unknown	[29]
	Hubei	Pit mud	Strong aroma	[65]
Enterococcus xiangfangensis	Inner Mongolia	Mash	Unknown	[29]
Enterococcus thailandicus	Inner Mongolia	Mash	Unknown	[29]
Enterococcus durans	Shanxi	Daqu	Light aroma	[73]

* “Region” here refers to a province in China.

), Pediococcus (

Table 4. Pediococcus.

Strain	Region *	Microbial Source	Aroma Type	Reference
Pediococcus parvulus	Shanxi	Mash	Light aroma	[16]
	Hubei	Mash	Light aroma	[17]
	Sichuan	Pit mud	Strong aroma	[81]
	Inner Mongolia	Mash	Unknown	[29]
	Guizhou	Mash	Sauce aroma	[21]
Pediococcus acidilactici	Jiangsu	Mash	Sesame aroma, strong aroma	[23,60]
	Hubei	Mash, high-temperature daqu, xiaoqu	Light aroma, blended strong and sauce aroma, unknown	[24,36,37,49,50,64]
	Shanxi	Mash	Light aroma	[16]
	Guangdong	Starter ball, starter cake, fermented liquid	Chi flavor	[18]
	Guizhou	Mash	Sauce aroma	[38,47,93]
	Shandong	Mash	Sesame aroma	[14]
	Heilongjiang	Pit mud	Strong aroma	[68]
	Henan	Medial-temperature daqu	Strong aroma	[41]
	Tianjin	Mash	Sauce aroma	[62]
	Sichuan	Mash	Strong aroma	[76]
Pediococcus pentosaceus	Jiangsu	Mash	Sesame aroma, strong aroma	[23,60]
	Guizhou	Mash	Sauce aroma	[26,38]
	Shandong	Mash	Sesame aroma	[14,54]
	Beijing	Daqu	Light aroma	[89]
	Sichuan	Mash	Strong aroma	[39,71,72]
	Hubei	Mash, xiaoqu	Light aroma, blended strong and sauce aroma, unknown	[17,20,37,49,50]
	Henan	Medial-temperature daqu	Strong aroma	[41]
	Tianjin	Mash	Sauce aroma	[62]
	Inner Mongolia	Mash, pit mud	Unknown, strong aroma	[29,42]
	Guangdong	Qu, fermented liquid	Chi flavor	[51]
	Shanxi	Starter cake	Light aroma	[22]
	Hebei	Daqu	Strong aroma	[53]
Pediococcus lolii	Inner Mongolia	Mash	Unknown	[29]

* “Region” here refers to a province in China.

), Leuconostoc (

Table 5. Leuconostoc.

Strain	Region *	Microbial Source	Aroma Type	Reference
Leuconostoc lactis	Shandong	Mash	Sesame aroma	[14]
	Shanxi	Mash	Light aroma	[58]
Leuconostoc pseudomesenteroides	Shandong	Mash	Sesame aroma	[14,54]
	Guizhou	Mash	Sauce aroma	[26]
Leuconostoc mesenteroides	Shandong	Mash	Sesame aroma	[14]
	Hubei	Mash	Light aroma	[20]
	Shanxi	Mash	Light aroma	[27,59]
	Sichuan	Daqu	Strong aroma	[74]
	Tibet	Qu	Strong aroma	[67]
Leuconostoc citreum	Shanxi	Mash, daqu	Light aroma	[16,30]
	Shandong	Mash, pit mud	Sesame aroma, strong aroma	[14,52]
	Sichuan	Pit mud, daqu, mash	Strong aroma	[52,74]
	Heilongjiang	Pit mud	Strong aroma	[52]
	Guizhou	Mash	Sauce aroma	[26]

* “Region” here refers to a province in China.

), Weissella (

Table 6. Weissella.

Strain	Region *	Microbial Source	Aroma Type	Reference
Weissella paramesenteroides	Shanxi	Mash	Light aroma	[16]
	Guizhou	Mash	Sauce aroma	[26,38,47]
	Shandong	Mash	Sesame aroma	[14,54]
	Sichuan	Pit mud, huangshui	Strong aroma	[77,81]
Weissella cibaria	Jiangsu	Mash	Strong aroma	[60]
	Shandong	Mash	Sesame aroma	[14]
	Guizhou	Mash	Sauce aroma	[26]
	Sichuan	Mash, daqu	Strong aroma	[39,74]
	Shanxi	Daqu	Light aroma	[30]
	Inner Mongolia	Daqu	Strong aroma	[83]
	Hubei	Xiaoqu, mash	Light aroma	[56,70]
	Hebei	Daqu, mash	Laobaigan aroma	[94]
Weissella confuse	Shandong	Mash, pit mud	Sesame aroma, strong aroma	[14,52]
	Sichuan	Pit mud, mash, daqu	Strong aroma	[52,71,72,74]
	Hubei	Mash, high-temperature Daqu	Light aroma, blended strong and sauce aroma	[20,64]
Weissella hellenica	Shandong	Mash	Sesame aroma	[14]
	Sichuan	Daqu	Strong aroma	[74]
Weissella soli	Shanxi	Mash	Light aroma	[16]
Weissella viridescens	Hubei	Mash	Light aroma	[17]
	Tianjin	Mash	Sauce aroma	[61]
Weissella confusa	Tianjin	Mash	Sauce aroma	[62]
	Hubei	Mash, pit mud	Xin aroma	[95]

* “Region” here refers to a province in China.

), Lactococcus (

Table 7. Lactococcus.

Strain	Region *	Microbial Source	Aroma Type	Reference
Lactococcus lactis	Guangdong	Starter ball	Chi flavor	[18]
	Sichuan	Mash	Strong aroma	[39]
Lactococcus garvieae	Jiangsu	Mash	Strong aroma	[60]
	Shandong	Mash	Sesame aroma	[14]
	Sichuan	Mash	Strong aroma	[39]
	Hubei	High-temperature daqu	Blended strong and sauce aroma	[64]
Lactococcus taiwanensis	Shandong	Mash	Sesame aroma	[14]
	Hubei	High-temperature daqu	Blended strong and sauce aroma	[64]

* “Region” here refers to a province in China.

), Streptoccoccus (

Table 8. Streptoccoccus.

Strain	Region *	Microbial Source	Aroma Type	Reference
Streptococcus lutetiensis	Sichuan	Xiaoqu	Unknown	[96]
Streptococcus lactis	Henan	Medial-temperature daqu	Strong aroma	[41]

* “Region” here refers to a province in China.

), Streptochaeta (

Table 9. Streptochaeta.

Strain	Region *	Microbial Source	Aroma Type	Reference
Streptochaeta angustifolia	Sichuan	Mash	Strong aroma	[72]

* “Region” here refers to a province in China.

), and Micrococcus (

Table 10. Micrococcus.

Strain	Region *	Microbial Source	Aroma Type	Reference
Micrococcus luteus	Henan	Medial-temperature daqu	Strong aroma	[41]
	Shandong	Medium-to-high-temperature daqu	Strong aroma	[32]
Micrococcus colpogenes	Henan	Medial-temperature daqu	Strong aroma	[41]
Micrococcus varians	Henan	Medial-temperature daqu	Strong aroma	[41]

* “Region” here refers to a province in China.

).

2.2. Mold

The primary function of molds in the baijiu brewing process is to produce various active enzymes (such as saccharifying enzymes, amylases, proteases, etc.) that convert large molecules such as starch and proteins in the brewing materials into fermentable sugars and amino acids [9,10]. This provides the essential substances for the growth and metabolism of other microorganisms, such as yeasts and bacteria [9,10]. In baijiu production, the primary lactic-acid-producing mold is Rhizopus [9]. Rhizopus secretes a large amount of amylase during its growth process, converting starch into fermentable sugars, making it an important saccharifying microorganism in baijiu production. Its metabolic products include lactic acid and other organic acids [9]. Currently, five types of lactic-acid-producing Rhizopus have been identified in baijiu production, as shown in

Table 11. Lactic-acid-producing Rhizopus during baijiu brewing.

Strain	Region *	Microbial Source	Aroma Type	Reference
Rhizopus oryzae	Hubei	Mash	Light aroma	[19]
	Tibet	Qu	Strong aroma	[97]
	Hubei	Daqu	blended strong and sauce aroma	[98]
Rhizopus stolonifer	Hubei	Daqu	Blended strong and sauce aroma	[98]
Rhizopus nigricans	Tibet	Qu	Strong aroma	[97]
Rhizopus chinensis	Tibet	Qu	Strong aroma	[97]
Rhizopus microsporus	Tibet	Qu	Strong aroma	[97]

* “Region” here refers to a province in China.

.

Mucor may also be related to lactic acid in baijiu brewing [99]. Mucor is highly adaptable to environmental conditions, grows rapidly, and reproduces quickly. It can saccharify starch and generate small amounts of ethanol, producing proteases and secreting various carbohydrate metabolizing enzymes, including amylases and lipases. Many species of Mucor can produce lactic acid [99]. Recent studies have shown that a significant amount of Mucor is present during the fermentation and brewing process of sauce-flavor baijiu, indicating it is a dominant strain in the brewing process [100]. However, there have been no reports related to lactic-acid-producing Mucor in baijiu production to date.

2.3. Yeast

Yeasts are important functional microorganisms in the baijiu brewing process [101]. The main role of yeast is to convert sugars into ethanol in baijiu production. Some lactic acid is also produced under anaerobic conditions. The brewing yeast Saccharomyces cerevisiae has strong fermentation capabilities, with the primary function of alcoholic fermentation, converting sugars into ethanol and carbon dioxide. Additionally, it can produce small amounts of organic acids (including lactic acid) and esters, contributing to flavor [102]. Nonbrewing yeasts, non-Saccharomyces, have lower fermentation abilities but a strong enzyme production capacity. Their main function is to produce esters, converting precursor substances in the brewing materials into organic acids (including lactic acid) and esters through biochemical reactions [4]. Currently, three types of lactic-acid-producing yeasts have been identified in baijiu production, as shown in

Table 12. Lactic-acid-producing yeast during baijiu brewing.

Strain	Region *	Microbial Source	Aroma Type	Reference
Saccharomyces cerevisiae	Hubei	Mash	Light aroma	[19]
	Sichuan	Mash	Strong aroma	[46]
	Guizhou	Mash	Sauce aroma	[103]
	Guangdong	Qu	Chi aroma	[104]
	Tibet	Qu	Strong aroma	[97]
Candida tropicalis	Sichuan	Daqu	Strong aroma	[105]
Pichia kudriavzevii	Shandong	Mash	Sesame aroma	[23]
	Sichuan	Mash	Strong aroma	[46]
	Guizhou	Mash	Sauce aroma	[106]

* “Region” here refers to a province in China.

.

Although numerous species of lactic acid producing microorganisms have been identified in baijiu fermentation systems, the specific metabolic pathways and regulatory mechanisms governing lactic acid production by individual microorganisms remain largely unclear. Because of differences in bacterial stains, enzyme systems, and metabolic intermediates, lactic acid fermentation can be categorized into homolactic fermentation, heterolactic fermentation, and bifid fermentation [107]. However, under the complex, mixed-culture, solid-state fermentation conditions unique to baijiu, how these pathways function and interact remains poorly understood. Therefore, further mechanistic investigations are needed to elucidate the regulation and coordination of lactic acid biosynthesis in this intricate microbial ecosystem.

3. The Role of Lactic Acid in the Baijiu Brewing Process

Lactic acid influences baijiu brewing by regulating the growth and activity of the microorganism involved and interacting with the flavor compounds it contains.

3.1. Lactic Acid in Qu (Fermentation Starter)

Microorganisms in the qu (fermentation starter) environment are enriched and cultivated from the surrounding environment and the raw materials used to make the qu, allowing these microorganisms to grow and reproduce on the qu dough while secreting various enzymes [2]. During the qu-making process, starch is degraded into sugars, and with the action of enzymes in the qu, microorganisms further decompose these sugars, catalyzing their conversion into lactic acid, ethanol, and other compounds [2]. Therefore, the qu is rich in various microorganisms such as molds, yeasts, and bacteria, as well as a variety of enzymes (including amylases, saccharifying enzymes, and proteases) and microbial metabolites (such as lactic acid) [2]. The lactic acid in the qu primarily serves the following three functions.

3.1.1. Participation in Microbial Metabolism During Fermentation

Lactic acid serves as a source of energy and carbon for microorganisms and is involved in their metabolic processes. In addition, as one of the end products of microbial metabolism, lactic acid helps to regulate the acidity of the qu, which in turn affects the species and population of microorganisms present and their metabolic activities [108].

3.1.2. Influencing the pH of Qu

Lactic acid bacteria are thermotolerant, anaerobic, or facultative anaerobic bacteria that grow and multiply in qu by utilizing sugars and proteins. They produce lactic acid as a metabolic product, which influences the pH value of the qu. During the qu production process, the lactic acid bacteria multiply and produce lactic acid, which is beneficial for flavor development and production control [3,109]. Both high and low levels of lactic acid in the qu can affect its quality by influencing the pH, thereby impacting the yield and quality of the liquor [109]. For example, the optimal pH for the growth of molds in the qu and for starch saccharification is between pH 4 and 6 [110,111], while the optimal pH for yeast growth is between pH 5 and 6 [112]. Microorganisms can only achieve good growth and reproduction under suitable acidic conditions. For instance, in the preparation of the starter culture, lactic acid bacteria are inoculated to produce lactic acid, acidifying the saccharified mash to a pH of 3.7–4.2. The lactic acid bacteria are then inactivated by heating before the yeast is inoculated to promote its growth [113]. Wei Jiayi and others used three types of high-temperature qu (high-, medium-to-high-, and medium-temperature qu) in solid-state fermentation experiments to analyze the differences in enzyme activity among them and the flavor composition of the baijiu produced [114]. The experimental results showed that the high-temperature qu had the highest acidity, with significant differences compared with the medium-to-high- and medium-temperature qu. At the same time, the saccharification and fermentation capabilities of the high-temperature qu were significantly lower than those of the medium-to-high- and medium-temperature qu. The results indicated that under high-temperature conditions, the proliferation of thermotolerant microorganisms such as lactic acid bacteria is robust, leading to increased acidity in the qu. This has a negative effect on the growth and metabolic activities of yeast and other microorganisms and leads to lower saccharification and fermentation capabilities, which in turn lead to lower yields, higher costs, and lower economic benefits [115].

3.1.3. Flavor Contribution

Lactic acid undergoes esterification with alcohol, forming lactic acid ester products. Both lactic acid and its esterified product, ethyl lactate, are components that contribute to the unique aroma of the qu [116,117]. During the qu production process, microbial metabolites and the degradation products of the raw materials together form the flavor compounds and flavor precursors in the qu. Because of differences in raw materials, qu production processes, and microbial communities, the content and type of flavor compounds and precursors in qu vary [118]. For example, the free lactic acid content in sauce-flavor qu is 507 µg/g (dry weight), while in strong-flavor qu, it is 419.1 µg/g (dry weight), and in light-flavor qu, it is 475.3 µg/g (dry weight) [119]. The specific content of lactic acid in the qu is determined by the variety and style of the liquor. However, there is currently a lack of in-depth research on flavor substances in the qu, especially concerning the aroma activity of these flavor substances, and it remains unclear what significant contributions lactic acid makes to the aroma of the qu [120].

3.2. Lactic Acid in the Mash

Lactic acid is the main acid in the mash, accounting for most of the total acid and having a significant impact on the yield and quality of the final liquor. For example, a study by the Inner Mongolia Light Industry Institute found that the lactic acid content in the mash of liquor fermented for 21 days was 2.01%, accounting for 86% of the total acidity [113]. Liu Jian and others analyzed the organic acids in fenjiu mash and found that lactic acid had the highest concentration at 0.5 g/100 g of mash [121]. The lactic acid in the mash mainly fulfills the following four functions.

3.2.1. Impact on Mash pH

Lactic acid is a nonvolatile organic acid, so after distillation, the majority of the lactic acid remains in the zaopei (distilled grains used in baijiu production), with only a small amount entering the base liquor [5]. In the traditional baijiu brewing process, zaopei is mixed with qu and fermented raw materials in a certain ratio (depending on the type of baijiu) to make the mash, using the abundant lactic acid in the zaopei to regulate the acidity of the fermentation process, thereby maintaining an optimal pH. For example, Zhang Baoyu and others analyzed the acidic components of Huanghelou-brand baijiu mash and found that the lactic acid content at the time of entering the cellar was 1.55–1.66%, while the lactic acid content after 80 days of fermentation was 3.20–3.37% [122].

3.2.2. Participation in Mash Microbial Metabolism

Lactic acid serves as a carbon source for caproic-acid-producing bacteria. Wang Huilin and others discovered a dominant new strain of caproic acid bacteria, Caproicibaterium sp. JNU-WLY1368, in the old pit mud of Wuliangye, with an average content of 11% [123]. This strain can utilize both glucose and lactic acid and shows fermentation potential with various substrates (sugar, lactic acid). Its main substrates include glucose, lactic acid, maltose, and starch, with the most important fermentation product being caproic acid. Cultivation with starch and glucose as carbon sources produces lactic acid, butyric acid, and caproic acid, with caproic acid yields reaching 5 g/L. When lactic acid and glucose are used, growth is worse. While butyric acid and caproic acid are produced, the ATP yield from lactic acid utilization is only 1/5 of that from glucose, resulting in higher caproic acid production, reaching 7.5 g/L. Utilizing glucose ensures biomass accumulation, while using lactic acid gradually raises the pH of the surrounding microenvironment, preventing cell death due to excessive acidification. Thus, when glucose exceeds lactic acid, fermentation lowers pH; when lactic acid exceeds glucose, fermentation raises pH [124].

3.2.3. Flavor Enhancement

Lactic acid serves as a precursor to ester compounds such as lactic acid ethyl ester and lactic acid propyl ester, both of which are significant flavor compounds. During fermentation, lactic acid reacts with alcohols such as ethanol and propanol in the presence of esterifying enzymes to form lactic acid esters, which subsequently enter the liquor after distillation. For example, lactic acid ethyl ester is an important flavor component in baijiu, and among all alcoholic beverages, Chinese baijiu has the highest content of lactic acid ethyl ester, with the main aroma of light-flavor baijiu being attributed to this compound [119]. Additionally, lactic acid propyl ester has a rich aroma and has been found in 72 types of baijiu, including various aroma types, ranging from 0.05 to 3.346 mg/L [125,126].

3.2.4. Involvement of Interaction Between Mash and Pit Mud

The accumulation of lactic acid salts in the pit mud can lead to hardening and aging of the pit, a common issue in the production of strong-flavor baijiu [127]. This is because the fermentation process occurs in the pit, where the mash and the microbial community in the pit mud undergo various symbiotic and cometabolic interactions across solid, liquid, and gas phases. In this process, changes in microbial metabolism affect the chemical ecology of the mash, and fermentation products seep into the pit mud, providing nutrients for the microbial community there. The chemical ecological changes in the mash feed back into the microbial community of the pit mud. The microbes in the pit mud utilize these nutrients to grow and multiply, and their metabolic products then re-enter the mash. The constant interaction between the mash and pit mud regulates physical and chemical changes and the succession of microbial species, which affect the quality and yield of the liquor. Therefore, when the mash produces large amounts of lactic acid and accumulates it over time, the lactic acid can easily form salts with metal ions such as calcium and iron in the pit mud, such as calcium lactate and ferrous lactate. These salts can precipitate out of the pit mud, forming white, powdery substances or needle-like crystals, leading to salinization of the pit mud surface and reduced moisture content and permeability, causing the pit to harden [128,129]. Studies have shown that when the content of calcium lactate and ferrous lactate reaches 0.1%, it can inhibit the growth of caproic acid bacteria, reducing the population of functional bacteria in the pit mud and causing aging. Thus, the accumulation of calcium lactate and ferrous lactate is one of the main reasons affecting the quality of pit mud and causing its aging. As the pit mud ages, the exchanges with the mash are hindered, disrupting the normal exchange of material and energy and creating a localized microecological environment that suppresses the growth and reproduction of functional bacteria, while lactic acid bacteria and acetic acid bacteria from the mash actively multiply, resulting in high lactic acid and low huangshui. This localized environment has a negative effect on the saccharification ability of molds and the fermentation ability of yeast, slowing down the conversion of starch into alcohol and reducing the yield. In addition, because of insufficient huangshui, the metabolic products of functional bacteria in the pit mud cannot be effectively transported into the mash or migrate, resulting insufficient enrichment of flavor components and deterioration of liquor quality.

3.3. Lactic Acid in Pit Mud

Lactic acid is the main organic acid in pit mud, with normal concentrations ranging from 10 to 80 g/L [130,131,132]. The functions of lactic acid in pit mud are summarized as follows.

3.3.1. Impact on Pit Mud pH

Lactic acid has the greatest influence on the pH of pit mud and plays a crucial role in maintaining the acidic environment of the pit. The literature indicates that the pH of both new and old pit mud typically falls between 3 and 6 [133,134,135]. Lactic acid maintains suitably acidic conditions so that molds and yeast can maximize their saccharification and fermentation capabilities.

3.3.2. Influence on Microbial Community in Pit Mud

Lactic acid demonstrates a strong correlation with dominant bacterial genera such as Caproiciproducens and Hydrogenispora in the pit mud [130]. Lei Xuejun and others studied the changes in microbial community structure in strong-flavor baijiu pit mud under lactic acid stress by using lactic acid as a standalone physicochemical factor to regulate pH [136]. The results indicated that lactic acid has an inhibitory effect on Proteobacteria. In the absence of lactic acid stress (pH = 6.8) or with mild lactic acid stress (pH = 5.0), the dominant bacterial groups in the pit mud were Firmicutes and Proteobacteria. Under greater lactic acid stress (pH = 4.0 and pH = 5.0), only Firmicutes remained dominant. The experimental results also showed that lactic acid inhibited Thermacetogerium and Sedimentibacter. Thermacetogenium uses ethanol, sugars, and amino acids as substrates to convert acetic acid into methane [137]. Sedimentibacter can degrade amino acids, with the decomposition products providing ammonium nitrogen as a nitrogen source for pit mud microbes [138]. Zhang Huimin and others [131] compared and analyzed the physicochemical factors and bacterial community structure differences between new and old pit mud. In new pit mud, the composition of the microbial community is relatively simple, with lactic acid bacteria being the dominant group and lactic acid content being high. As the pit mud matures, the microbiome community composition becomes richer, with a greater number and type of microorganisms. Different bacteria work together to break down lactic acid, leading to an increase in the pH of the pit mud, which in turn further influences the microbial diversity and improves the metabolic functions of the microbial community.

However, excess lactic acid in the pit mud can be caused by the strong proliferation of lactic acid bacteria. In addition to the lactic acid produced during fermentation, lactic acid bacteria also produce other acidic substances, such as acetic acid and butyric acid [139]. These strong acids can corrode the surface of the pit, causing cracks and further accelerating the corrosion of the pit.

3.4. Lactic Acid in the Huangshui

Huangshui is a viscous, yellow liquid that accumulates at the bottom of the fermentation pit, resulting from the seepage of extract water from the fermented mash and leachate from the pit mud. During its formation, huangshui transports and migrates metabolic products between the pit mud and the mash, enriching all substances produced during the fermentation process. It is rich in organic substances, including acids, esters, alcohols, and aldehydes, with lactic acid being the primary organic acid [140]. The functions of lactic acid in huangshui are summarized as follows.

3.4.1. Removal of Iron Ions from Pit Mud

Iron is abundant in pit mud, second only to silicon and aluminium. Under the specific conditions of sealed, anaerobic fermentation in the pit, iron compounds in the pit mud are reduced from their oxidized state to ferrous ions, leading to a rapid increase in iron ion concentration. For example, in aging mud from the Hunan Jiugui Distillery, the active iron content in old mud was 884.80 mg/100 g of dry soil, over 200 times that in mature pit mud [141]. When lactic-acid-rich leachate from the mash penetrates the pit mud, it reacts chemically with the ferrous ions to form ferrous lactate. Ferrous lactate is relatively soluble, with a solubility of about 1% in water, and is carried away by the dissolving action of the leachate, resulting in a decrease in the total iron content in the pit mud. Under normal conditions, when the moisture content in the pit is sufficient, the leaching rate of ferrous lactate balances with its formation, maintaining the optimal concentration of active iron as a necessary trace element to promote microbial growth and metabolism in the pit mud. However, if the moisture in the pit is insufficient, so that the leaching rate of ferrous lactate is lower than its formation rate, ferrous lactate accumulates in the mud and crystallizes, which has toxic effects on the microbial community and leads to aging of the pit mud [129,141].

3.4.2. Effects on Flavoring and Aromatic Substances

In some distilleries, huangshui is collected daily and combined with food-grade alcohol and clarified before being directly used to blend low-grade baijiu. By effectively utilizing the lactic acid and ethyl lactate flavor compounds in huangshui, the flavor of the liquor can be improved, enhancing its natural characteristics while also reducing production costs [142].

3.4.3. Providing Precursor Substances for Esterification

The Qingdao Langyatai Group treated huangshui through esterification by adding food-grade alcohol and last distillate before further fermentation. The concentration of ethyl lactate in the huangshui increased from 2.933 g/L to 4.08 g/L after esterification. The resulting esterified huangshui could be used to improve the ester content of baijiu or to blend with new types of baijiu, thus enhancing the quality, natural flavor, and richness of baijiu [142].

3.4.4. Maintenance of Pit Mud

Utilizing huangshui for nurturing and maintaining pit mud is a common practice in the baijiu industry. The principle is to use various microorganisms that have adapted to the long-term high-acid, anaerobic environment in the huangshui and the organic components such as acids, esters, alcohols, and aldehydes that contribute to the flavor of baijiu. The lactic acid in the nurturing huangshui provides a substrate for various lactic-acid-utilizing bacteria in the pit mud, promoting their growth and reproduction, and preventing the aging of the pit mud [142].

3.5. Lactic Acid in the Base Liquor

The lactic acid in the base liquor is introduced into the liquor body through the distillation of the fermented mash, whereby part of the alcohol and the accompanying aromatic components evaporate and condense. As lactic acid is a high-boiling, nonvolatile organic acid, its content in the base liquor is relatively low. Nonetheless, it is an important organic acid that contributes to the flavor of baijiu, giving it a slightly acidic, mildly sweet, and slightly astringent taste with a hint of richness. The amount of lactic acid varies with the different aroma types of baijiu. Typically, the lactic acid content in the base liquor ranges from 366 to 1581 mg/L, being highest in sauce-flavor, sesame-flavor, and special-flavor baijiu, exceeding 1000 mg/L. In contrast, the lactic acid content in strong-flavor baijiu is only about half that of these three flavor types, or even lower [119]. As an important flavor compound, lactic acid plays an important role in the base liquor.

3.5.1. Flavoring Effect

An appropriate amount of lactic acid can stabilize the aroma of the main flavor components of baijiu (such as ethyl lactate, caproic acid ethyl ester, and ethyl acetate), mask the stimulating taste of alcohol, and reduce the bitterness of the liquor, resulting in a smooth and sweet taste [124]. Conversely, too high a lactic acid content can lead to off flavors, sourness, and astringency in the liquor.

3.5.2. Promotion of Maturation

The lactic acid in base liquor primarily arises from the hydrolysis of ethyl lactate during storage. In new liquor, the lactic acid content is low while ethyl lactate content is high. Under the acid–ester equilibrium, the decomposition of ethyl lactate generates lactic acid and ethanol, gradually increasing the lactic acid content until a new balance is reached. Higher-alcohol liquors have higher ethanol content than lower-alcohol liquors, resulting in a slower hydrolysis of ethyl lactate during storage. This means that high-alcohol liquor’s lactic acid content increases more slowly than that of lower-alcohol liquor, necessitating a longer storage period to achieve equilibrium. Furthermore, because lactic acid contains weakly acidic, easily dissociable hydroxyl and alcohol hydroxyl groups, it readily forms hydrogen bonds. By dissociating protons (as proton donors), it enhances hydrogen bond association in the ethanol–water system [143]. The formation of stable ternary association structures contributes to the stability of the liquor body and mitigates the stimulating effect of ethanol. The hydrolysis and esterification of acids and esters, along with the hydrogen bond association involving lactic acid, collectively promote the maturation of the liquor [107].

3.5.3. Flavoring and Aromatic Components for Blending Low-Grade Liquor

For example, research from the Inner Mongolia Light Industry Institute found that the lactic acid content in the last distillate was very high, accounting for 94.2% of the total distillate [113]. A practical method to address the flavor quality of liquid fermented baijiu involves the reasonable use of high-quality base liquor last distillate blended with food-grade alcohol, effectively utilizing aromatic components such as lactic acid and ethyl lactate from the tails [113].

3.6. Lactic Acid in Finished Baijiu Product

Lactic acid is commonly present in finished baijiu, with considerable variations in content across different aroma types and brands. The highest concentrations are found in sauce-flavor, sesame-flavor, and special-flavor baijiu. For example, the lactic acid content in finished products was found as follows: Langjiu (sauce aroma): 623 mg/L, Jian Nan Chun (strong aroma): 210 mg/L, Luzhou Laojiao Tequ (strong aroma): 600.7 mg/L, Gu Jing Gong Jiu (strong aroma): 580 mg/L, Yanghe Daqu (strong aroma): 482.6 mg/L. These differences highlight how production processes and fermentation conditions can influence the lactic acid content in baijiu [119].

As discussed in preceding sections, lactic acid performs multiple functional roles throughout the baijiu brewing process. In addition to regulating pH and influencing microbial ecology, lactic acid contributes directly to the sensory characteristics of the final product. It acts as a precursor to various lactic acid esters, such as ethyl lactate, which can help soften the sharpness of strong alcohols, and enhance the complexity and richness of baijiu’s overall flavor profile. Moreover, lactic acid participates in acid–ester equilibrium reactions, affecting both the formation and hydrolysis of esters during aging. Its influence on pH further alters enzyme activity and microbial metabolism, indirectly impacting the levels of other volatile compounds such as aldehydes and organic acids. Taken together, these functions underscore lactic acid’s central role not only in microbial and biochemical processes, but as a key flavor precursor and modulator in baijiu production.

4. Macro Control and Comprehensive Application of Lactic Acid in Baijiu Brewing

4.1. Macro Control of Lactic Acid in Baijiu Brewing

From the formation process of lactic acid and its role in the brewing process, it can be concluded that controlling lactic acid levels is essential in baijiu production. Over time, the issue in baijiu production has not been a shortage of lactic acid, but rather an excess. This is particularly true for strong-aroma baijiu, where increasing ethyl caproate and reducing lactic acid is a key technique to ensure the quality of the baijiu. There are four potential approaches to controlling lactic acid levels [107]. Detailed case studies and experimental data supporting these control strategies were discussed in our previous publication (“Current Updates on Lactic Acid Production and Control during Baijiu Brewing”, 2024), which is cited in this review.

4.1.1. Controlling the Growth and Reproduction of Microorganisms

By regulating the growth and multiplication of microorganisms, lactic acid content can be controlled. Rhizopus oryzae, for exmaple, produces significant amounts of lactic acid, especially L-lactic acid, which accounts for about 70% of the total acidity and greatly impacts the quality of baijiu. Therefore, in the production of qu, it is essential to control the growth and metabolic activity of Rhizopus oryzae. This requires ensuring its presence in the qu while limiting its lactic acid production through temperature control. Since the optimal growth temperature for Rhizopus oryzae is 37–41 °C, and the optimal fermentation temperature for qu is 30–35 °C, many baijiu producers maintain the temperature of daqu below 40 °C in the early stages of cultivation and standardize the production process and quantity of Rhizopus oryzae to control its reproduction and lactic acid production [144].

4.1.2. Biochemical Regulation of Lactic Acid Production

By adding or reducing certain substances in the biochemical pathway of lactic acid production, the production of lactici acid can either be promoted or inhibited, thereby regulating the amount of lactic acid [84]. For example, Gong Shixuan found that addition of fumaric acid (0.6–1 g/kg of mash) to the fermentation pit of Xifengjiu could inhibit the growth of lactic acid bacteria, thereby controlling lactic acid and its ester products [145]. However, this method may affect the metabolism of other microorganisms, potentially affecting the quality of the baijiu.

4.1.3. Microbial Degradation of Lactic Acid

Using microorganisms that utilize lactic acid as a carbon source to degrade it can reduce its levels. For instance, Propionibacteria can convert lactic acid into propionic acid and acetic acid, reducing the amounts of lactic acid and its ester compounds in the baijiu. This helps to maintain a balanced ratio between ethyl caproate and ethyl lactate. In addition, under the action of esterases produced by other microorganisms, propionic and pentanoic acid esters are formed, which enhance the distinctive flavor of baijiu. This is one of the main methods to increase ethyl caproate and reduce lactic acid in strong-aroma baijiu and to increase propionic acid and reduce lactic acid in Te-aroma baijiu [146].

4.1.4. Adjusting Production Parameters

Altering baijiu production parameters, such as adjusting distillation or fermentation conditions, can affect the extraction of lactic acid [147]. Additionally, changing the production season can influence lactic acid production [148]. For example, Wuliangye Group found that the lactic acid and ethyl lactate are retained in the residues in distillation pot during vacuum distillation and fractional condensation of base liquor. Under different vacuum distillation conditions, the concentration and ratio of flavor compounds in each fraction of the base liquor vary. The lactic acid content in baijiu can be specifically controlled by adjusting vacuum distillation conditions. In addition, lactic acid bacteria thrive in the summer months with high temperature and high humidity, resulting in higher lactic acid production, which increases the acidity of the fermention grains and has negative impacts on the yield and quality of the baijiu. As a result, most distilleries suspend production for maintenance during the summer.

When considering which control method to adopt, the specific requirements and practical conditions of different distilleries, as well as the potential impact on baijiu quality, must also be taken into account.

4.2. Comprehensive Application of Lactic Acid in Baijiu Brewing

The byproducts generated during baijiu brewing, such as huangshui and spent grains, contain large amounts of lactic acid. Comprehensive utilization of the lactic acid from these baijiu byproducts is of great significance for maximizing resource utilization and promoting sustainable development in the baijiu industry [149]. For instance, as mentioned in Section 3.4, the flavor components in huangshui, including lactic acid and ethyl lactate, can be used to formulate baijiu flavoring solutions for blending low-grade baijiu. Alternatively, spent grains can be processed into animal feed, where the lactic acid in the grains inhibits the growth of spoilage bacteria, serving as preservative.

5. Conclusions and Perspective

Lactic acid plays a central role in the baijiu brewing process and influences the microbial dynamics, the efficiency of fermentation, and the quality of the final product. This review summarizes the current understanding of lactic acid dynamics in baijiu brewing and highlights the multiple functions of lactic acid, including its regulatory roles during fermentation, its interactions with the microbial communities in both the mash and pit mud, and its influence on the flavor and mouthfeel of the liquor. By contributing to the formation of aromatic compounds and stabilizing the base liquor, lactic acid significantly enhances the quality of baijiu.

In light of the important role of lactic acid in baijiu brewing, four potential control strategies have been proposed. To better support their implementation, a gradual integration of modern industrial approaches into traditional brewing practices may be beneficial. By doing this, better control and management of lactic acid levels, as well as other vital parameters throughout the brewing process, can be achieved with greater efficiency. In addition, modern industrial brewing practices would be more resilient to potential impact on the flavor and quality of baijiu production resulting from changes in the microbiota in the environment due to climate change. A better understanding of the composition of the microbial community and its changes during baijiu brewing and its interaction with the surrounding microbial ecosystem will be important for transformation. Future research should aim to elucidate the metabolic networks of lactobacilli involved in baijiu fermentation using advanced molecular biology techniques, such as genome sequencing, transcriptomics and proteomics. These approaches can provide deeper insights into the regulation of lactic acid biosynthesis, strain-specific metoblic traits, and interactions with other microbial populations. A better understanding of these networks will support the development of targeted strategies for microbial management and flavor optimization in baijiu production.