Current Updates on Lactic Acid Production and Control during Baijiu Brewing

Abstract

Lactic acid is closely linked to the safety and quality of baijiu, the traditional Chinese fermented alcoholic beverage. Produced by lactic acid bacteria during fermentation, it creates an acidic environment that inhibits the growth of spoilage organisms and harmful microbes, thereby enhancing the safety and stability of the final product. Additionally, lactic acid is a key contributor to baijiu’s flavor profile, providing a smooth and rounded taste. Its levels can significantly impact consumer experience. An excess of lactic acid can result in a sour, undesirable flavor, while insufficient levels may lead to a flat and less appealing taste. Maintaining balanced lactic acid levels is crucial for ensuring that baijiu is both safe and enjoyable to drink, ultimately contributing to the product’s success and marketability. This paper reviews the mechanisms of lactic acid production in baijiu, examines its effects on flavor and the potential causes of imbalances, explores regulatory measures for controlling lactic acid during brewing, and discusses the impact of these measures on baijiu’s quality, taste, and yield, along with practical applications by various distilleries. The goal of this paper is to provide a reference for regulating lactic acid in the baijiu production processes.

1. Introduction

As China’s national spirit, baijiu is made from grains that are converted into mash and distilled after solid-state fermentation [1,2]. Many acidic substances that affect baijiu’s aroma, taste, and functionality are produced during fermentation [3,4]. Lactic acid is an essential structural acid in baijiu. In the mash, it regulates the growth of other brewing microorganisms [5]; in the spirit, it imparts a sour fragrance to baijiu, eliminates the rough, spicy, and bitter tastes, and enhances its richness [6]. However, too high a concentration of lactic acid in the spirit can lead to increased acidity in baijiu [7], while too low a concentration can cause a low degree of alcohol esterification [8]. Therefore, strictly controlling the lactic acid content in baijiu is beneficial for ensuring the quality of the spirit. Currently, both the domestic and international controls on lactic acid in baijiu primarily focus on the fermentation process, mainly involving the use of high-yielding lactic acid bacteria to increase the lactic acid content in the spirit [9], and the addition of lactic acid-degrading bacteria to reduce the lactic acid content [10]. This review systematically summarizes the microbial fermentation pathways of lactic acid in baijiu, proposes an overall approach to controlling lactic acid during the brewing process, and discusses the impact of different control techniques on the quality of baijiu and their practical application in some distilleries, providing a theoretical reference for enhancing the quality of baijiu.

2. Fermentation Pathways of Lactic Acid Production in Baijiu

In the brewing of baijiu, lactic acid is primarily generated through fermentation by various lactic acid bacteria, with many molds such as Mucor and Rhizopus also producing lactic acid. The raw materials undergo pre-fermentation processing to produce glucose, and mixed fermentation produces monosaccharides, disaccharides, malic acid, and other carbon-containing substances [6,11], which are transported into bacterial cells by the lactic acid bacteria transport system [12,13], as shown in Figure 1. Fructose enters the cell and is converted into fructose-1-phosphate, and sucrose is converted into sucrose-6-phosphate [14]; sugar substances in the cell generate pyruvate, which is then converted into lactic acid by the action of lactate dehydrogenase (LDH) [14]. Lactic acid is transported across the membrane to the outside of the cell [15]. Some of the extracellular lactic acid is utilized by lactic acid-utilizing bacteria [10], some is combined with ethanol to form lactate ethyl ester [16], some enters the spirit via distillation, and the remaining stays in the mash [17]. Due to differences in bacterial strains, metabolic pathways, and the products formed, lactic acid fermentation can be divided into homolactic fermentation, heterolactic fermentation, and bifid fermentation.

2.1. Homolactic Fermentation

Homolactic fermentation refers to the process where lactic acid bacteria convert glucose into pyruvate via the glycolytic pathway (Embden-Meyerhof-Parnas, or EMP, pathway), and pyruvate is then reduced to lactic acid by lactate dehydrogenase [18]. One molecule of glucose produces two molecules of lactic acid, with a conversion rate of 100%. Initially, glucose is converted into fructose-6-phosphate under the action of phosphofructokinase and glucose phosphate isomerase. This is then converted into fructose-1,6-bisphosphate by phosphofructokinase. Subsequently, it is broken down into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate under the action of aldolase and triose phosphate isomerase. Dihydroxyacetone phosphate can also be directly converted into glyceraldehyde-3-phosphate. Glyceraldehyde-3-phosphate is then transformed into pyruvate through a series of reactions involving glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, phosphoglyceromutase, enolase, and pyruvate kinase. Finally, pyruvate is converted into lactic acid by lactate dehydrogenase (LDH). This type of fermentation is characterized by the direct production of lactic acid from glucose, resulting in a rapid fermentation process and high efficiency in lactic acid production.

2.2. Heterolactic Fermentation

Heterolactic fermentation involves lactic acid bacteria converting glucose into ribulose-5-phosphate via the hexose monophosphate pathway (HMP), then to glyceraldehyde-3-phosphate and pyruvate, and finally reduced to lactic acid [18]. One molecule of glucose produces one molecule of lactic acid and one molecule of ethanol, with a conversion rate of 50%. Glucose enters the bacterial cell through the phosphotransferase system and is then converted by glucokinase into glucose-6-phosphate. It is further broken down by glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase into ribulose-5-phosphate. Ribulose-5-phosphate is then converted by phosphopentose isomerase into xylulose-5-phosphate, which subsequently undergoes a cleavage reaction catalyzed by phosphoketolase to produce acetyl phosphate and glyceraldehyde-3-phosphate. The conversion of glyceraldehyde-3-phosphate into pyruvate follows the latter part of the EMP pathway.

In this pathway, the bacteria do not only produce lactic acid; other byproducts, such as ethanol and carbon dioxide, are also formed, depending on the specific enzymes present in the different bacterial strains. This metabolic pathway allows for the production of lactic acid alongside other compounds, contributing to the complex flavor profile of products like some types of sourdough bread and fermented beverages.

2.3. Bifid Fermentation

Bifid fermentation is the process where Bifidobacteria convert glucose into lactic acid via the hexose monophosphate shunt pathway [18]. One molecule of glucose yields one molecule of lactic acid and one and a half molecules of acetic acid, with a conversion rate of 50%. Glucose is first transformed into fructose-6-phosphate by the action of phosphofructokinase and glucose phosphate isomerase. Fructose-6-phosphate is then split into erythrose-4-phosphate and xylulose-5-phosphate. Erythrose-4-phosphate can also be converted into xylulose-5-phosphate, which is subsequently cleaved by xylulose-5-phosphate phosphoketolase into glyceraldehyde-3-phosphate and acetyl phosphate. The conversion of glyceraldehyde-3-phosphate to pyruvate follows the latter part of the EMP pathway.

This metabolic route is distinctive to Bifidobacteria and characterizes their unique ability to ferment glucose, resulting in a mix of lactic acid and acetic acid as end products rather than only lactic acid. This combination contributes to a distinct flavor profile and various health benefits attributed to Bifidobacteria in fermented foods.

3. Lactic Acid in Baijiu Brewing

3.1. The Role of Lactic Acid in Baijiu

The concentration of lactic acid in baijiu typically ranges from 210 to 623 mg/L, with the highest contents found in sauce-aroma, sesame-aroma, and special-aroma types of baijiu [19]. Lactic acid in finished baijiu mainly plays two roles:

First, it enhances the sensation of richness. Due to the weak acidity of lactic acid and its easily dissociable hydroxyl and alcohol groups, which facilitate hydrogen bond formation by providing a proton (as proton donor), lactic acid strengthens hydrogen bond associations in the ethanol–water system [20,21]. By forming a stable ternary association structure that binds alcohol molecules, it effectively enhances the association degree in baijiu, stabilizing the liquor. The hydrogen bonding involving lactic acid reduces the harshness of ethanol on the palate, producing a smooth taste.

Second, as a flavoring and aroma-contributing substance, it affects the quality of various types of baijiu. Lactic acid, because it is a shorter carbon chain organic acid, ionizes H⁺ and produces a sour taste [22]. Lactic acid, which contains only one hydroxyl group, releases H⁺ with weak acidity. Additionally, because of the negative groups on the lactic acid carbon chain, it produces a variety of sour tastes, gives the spirit an astringent quality, and also moderates its acidic sting, making its sour taste mild [22,23].

The aromatic contribution of lactic acid is reflected in the spirit’s subtle aroma. An appropriate amount of lactic acid stabilizes the main aroma components of baijiu (such as lactate ethyl ester, hexanoate ethyl ester, and acetate ethyl ester) while also imparting a lingering sweet taste [4,23]. Excessive lactic acid content can cause off-flavors, sourness, and astringency in the liquor. The content of lactic acid in baijiu and the ratio of lactic acid to other organic acids are important indicators affecting the quality of baijiu and also significantly influence the aroma, taste, and style of different aroma types of baijiu [24,25]. Currently, there are no national standards regulating the content of lactic acid in finished baijiu, but the industry has general standards for evaluating the lactic acid content of different aroma types. For example, in strong-aroma baijiu, the ratio of hexanoic acid to lactic acid usually ranges from 1: (0.5–1) [26]; in light-aroma baijiu, this ratio typically ranges from 1: (0.6–0.8) [22].

3.2. The Causes of Abnormally High or Low Lactic Acid Levels during Baijiu Brewing

The baijiu brewing process begins with the crushing of the raw grains, which are then mixed in a specific ratio and blended with water. The mixture is then evenly spread in a brewing pot, heated, and cooked thoroughly. After cooking, the mash is removed from the pot, sprayed with cold water, and cooled using fans. Once cooled, the distiller’s Daqu is evenly spread over the cooled mash. Next, the mixture is transferred to fermentation tanks, where it ferments. Finally, the fermented mash is evenly spread into a distillation pot, covered, and heated to begin distillation. The basic steps in the production of baijiu and the use of “Qu” as a fermentative agent in the process are illustrated in the flow diagram (Figure 2):

In the baijiu brewing process, high levels of lactic acid can primarily be attributed to the following factors:

High starch content in the raw materials: if the raw materials have high starch content, lactic acid bacteria can utilize this starch for growth and metabolism, producing lactic acid during fermentation [11,27].

Presence of lactic acid bacteria in the distiller’s Daqu: high-temperature Daqu and stacked Daqu often contain a significant amount of lactic acid bacteria, which can enter the mash and produce lactic acid during fermentation [28].

Sealing of the fermentation pits with pit mud: pit mud sealing is prone to cracking, which can allow air-borne lactic acid bacteria to infiltrate the mash and produce lactic acid [29].

Extended fermentation time and high temperature: long fermentation times and high temperatures can lead to a significant proliferation of lactic acid bacteria (as lactic acid bacteria are thermotolerant), thereby increasing lactic acid production [7,30].

Conversely, low lactic acid levels are mainly due to the following:

Short fermentation time and low temperature: short fermentation periods and low temperatures (container fermentation or production during winter) can result in fewer lactic acid bacteria in the mash, leading to lower lactic acid production [31,32].

Mechanized production processes: the use of mechanized production processes reduces the chances of environmental lactic acid bacteria entering the mash, thus decreasing the amount of lactic acid produced [33].

We have summarized some of the known causes of abnormally high or low lactic acid levels in baijiu in

Table 1. The causes of abnormally high/low lactic acid levels during baijiu brewing.

Abnormally High/Low Lactic Acid Levels	The Causes	The Type of Baijiu	Reference
High	High starch content in raw materials, rapid temperature rise in the pit, high residual sugar, large proliferation of lactic acid bacteria	Strong aroma	[11,27]
	Lactic acid bacteria introduced into the fermentation mash from starter culture and pit mud	Strong aroma	[28]
	Cracks in the pit mud after sealing the pit, allowing lactic acid bacteria to invade the fermentation mash	Strong aroma	[29]
	High ambient temperature during summer production, lactic acid bacteria resistant to high temperatures and widely distributed in the fermentation mash	Light aroma	[7,30]
	Use of high-temperature starter culture and heap fermentation, rapid temperature rise in the pit, large proliferation of lactic acid bacteria	Strong aroma	[7,30]
Low	Container fermentation (lower temperature inside the container), because lactic acid bacteria prefer higher-temperature environments	Strong aroma	[31,32]
	Lower ground and air temperatures during winter production, leading to lower lactic acid bacteria content in the fermentation mash	Strong aroma	[31,32]
	Application of mechanized and clean production processes, reducing the chance of lactic acid bacteria entering the fermentation mash from the environment	Light aroma	[33]

. Understanding these factors can provide theoretical support for discussing regulatory measures of lactic acid in baijiu brewing. By managing these elements effectively, distilleries can maintain an optimal level of lactic acid, which is crucial for the quality and characteristic flavor profile of baijiu.

4. Lactic Acid Production Control in Baijiu Brewing

The flavor and post-consumption comfort are core criteria for evaluating the quality of baijiu, significantly affecting consumer acceptance and industrial application [4,34]. Therefore, controlling lactic acid not only requires the consideration of the specific demands and practical situations of the different distilleries but also affects the quality of baijiu. In the baijiu brewing process, possible approaches to controlling lactic acid content include: first, controlling the growth and reproduction of lactic acid bacteria to increase or decrease their numbers; second, the biochemical regulation of lactic acid production, either adding or reducing certain substances in the biochemical pathway that forms lactic acid to promote or hinder its normal progression; third, microbial decomposition, using microorganisms that utilize lactic acid as a carbon source to degrade lactic acid; and fourth, adjusting baijiu production process parameters, such as distillation or steaming conditions, to affect the extraction of lactic acid. When considering which control method to adopt, the specific requirements and actual situations of different distilleries, as well as the impact on baijiu quality, should also be taken into account. Specific measures for controlling lactic acid are as follows.

4.1. Impact of Brewing Ingredients on Lactic Acid in Baijiu

Lactic acid, a non-volatile acid, tends to accumulate during the multi-cycle fermentation process of sauce-flavor baijiu, leading to higher lactic acid content [35]. Different studies have shown that using glutinous sorghum in the brewing of sauce-flavor baijiu can reduce lactic acid levels. For instance, Li Tonghuan et al. [36] conducted production trials at Shandong Qinchi Distillery and found that baijiu brewed with glutinous sorghum had lower lactic acid content (439.57 mg/L) compared to that brewed with Japonica sorghum (453.14 mg/L) and that the glutinous sorghum batch also had a higher yield and a smoother and cleaner taste. Similarly, Wang Guijun et al. [37] found at Hunan Wuling Distillery that sauce-flavor baijiu brewed with glutinous sorghum had lower lactic acid content (480 mg/L) than that brewed with Japonica sorghum (497 mg/L), with a higher yield (49.3%) and a higher quality rate (18.1%). The reason is that glutinous sorghum has a higher ratio of branched starch, which decomposes slowly during the gelatinization process in cooking, matching the production characteristics of sauce-flavor baijiu.

The impact of different brewing ingredients on lactic acid levels varies. For example, Li Kefa et al. [38] found at Si Te Distillery that baijiu brewed with premium rice had higher lactic acid content (153.33 mg/L) than baijiu brewed with ordinary rice (150 mg/L). Han Jing and Xiao Dongguang [39] found that a rice medium produced more lactic acid (298.56 g/L) than a corn medium (267.56 g/L) in a simulated baijiu fermentation experiment. The reason might be related to the material differences in the ingredients, relevant when starch is converted into sugars and organic compounds during saccharification, providing a substrate and energy source for lactic acid bacteria [39]. Ingredients with higher starch content favor an increase of lactic acid in the spirit, and those with lower starch content favor a decrease of lactic acid [39]. The structure of starch divides into amylose and amylopectin, with amylose being more conducive to lactic acid production [36,37]. Tannins inhibit the growth of lactic acid bacteria [40], so ingredients high in tannins can reduce lactic acid levels in baijiu. Additionally, different levels of protein, fat, and moisture in ingredients can also affect lactic acid levels during baijiu fermentation [41].

Selecting the right ingredients is essential to baijiu production. Rice is the main ingredient in special-aroma, rice-aroma, and bean-aroma baijiu [42], with rice generally having higher starch content than sorghum and lacking tannins [43], allowing for the adjustment of lactic acid levels in these aroma types by choosing different rice varieties. Sauce-, strong-, and light-aroma baijiu primarily use sorghum [42], which is available in stem and glutinous varieties and contains varying levels of tannins [43]. Selecting different sorghum varieties can help control lactic acid levels in these baijiu types. Current research shows that glutinous sorghum is more suitable for sauce-flavor baijiu [36,37]. However, starting control measures from ingredient selection involves considerations of production costs (ingredient prices, logistics, transportation time, labor costs), and the research and application cycle is lengthy, which may not be feasible for small and medium distilleries but could be suitable for large distilleries aiming to innovate their spirit profiles and develop new products.

4.2. The Impact of Distiller’s Daqu on Lactic Acid in Baijiu

Distiller’s Daqu, serving as a saccharification and fermentation agent, is added to the mash in a proportion of 10% to 20% [44]. This addition introduces lactic acid bacteria into the fermenting mash. Li Lianhui [44] found that eight dominant lactic acid bacteria strains in the fermenting mash of Luzhou Laojiao, namely Lactobacillus brevis, Lactobacillus plantarum, Lactobacillus curvatus, Lactobacillus pentosus, Lactococcus lactis, Leuconostoc citreum, Weissella confusa, and Weissella cibaria, originated from Daqu. Numerous studies have shown that the lactic acid bacteria in Daqu are one of the primary sources of lactic acid bacteria in the mash [45,46]. Therefore, increasing the concentration of lactic acid bacteria in the distiller’s Daqu can enhance the production of lactic acid during fermentation, thereby increasing the lactic acid content in baijiu.

Yang Fengying [47] added Geotrichum candidum to Daqu during fermentation. The study showed that the lactic acid bacteria content in the experimental group’s Daqu was 6.02 times that of traditional Daqu. This increase was attributed to the synergistic saccharification effect of Geotrichum candidum and Rhizopus, which promoted the growth of lactic acid bacteria in Daqu. Wu Zhengkun et al. [48] observed at Hubei Baiyunbian Distillery that the number of lactic acid bacteria in high-temperature Daqu initially increased and then decreased over storage time, reaching a maximum at 120 days (7.6 × 10⁵ CFU/g). This fluctuation may be due to the interactions among microbial communities affecting the quantity of lactic acid bacteria in Daqu.

Therefore, adding Geotrichum candidum to the distiller’s Daqu and controlling the storage time of the Daqu can regulate the amount of lactic acid bacteria in the Daqu. Further research could explore the impact of various factors on lactic acid bacteria in the distiller’s Daqu and their effect on lactic acid production in baijiu. Given that this method has not yet been applied in actual production and requires a long research period, it is more suitable for large distilleries that need to increase the lactic acid content in their baijiu. This approach allows distilleries to fine-tune the flavor profile and quality of the spirit by managing the microbial dynamics during fermentation.

4.3. Control of Lactic Acid During Fermentation

4.3.1. Application of High-Yielding Lactic Acid Bacteria in Fermentation

Utilizing high-yielding lactic acid bacteria (LAB) isolated from the brewing environment can enhance the lactic acid content in baijiu. For instance, Ding Haimei [49] inoculated Lactobacillus casei isolated from Hengshui Laobaigan Daqu into the fermentation mash of this baijiu, resulting in a 98.02% increase in lactic acid content. Li Xiaoxiao [50] inoculated Lactobacillus plantarum, selected from the brewing environment, into sauce-flavor baijiu fermentation mash. At an inoculation rate of 3%, the highest lactic acid production was achieved (2.56 g/L). Zhang Long and others [51] inoculated two high-yielding LAB strains isolated from Guangxi Tianlongquan Distillery into a culture medium for fermentation, showing an increase in lactic acid content by 629.32 mg/L and 717.22 mg/L compared to the control group.

Extensive research has been conducted on the isolation and identification of high-yielding LAB across different aroma types of baijiu, providing a theoretical basis for its practical application in baijiu production. Zhao Xinyi et al. [52] isolated two high-yielding LAB strains (Lactobacillus rod-shaped and Lactobacillus lactis) from light-aroma baijiu mash, laying a theoretical foundation for improving the flavor and taste of light-aroma baijiu. Zhang Yan and others [53] isolated two high-yielding LAB strains (Lactobacillus fermentum and Lactobacillus brevis) from sauce-aroma baijiu mash, finding that they could interact with Daqu and promote the formation of acids, alcohols, and esters, which are crucial for the characteristic flavors in baijiu. Zhao Wei and others [54] isolated a high-yielding LAB strain (Lactobacillus plantarum) from light-aroma baijiu mash, providing an experimental basis for the industrial production of high-quality light-aroma baijiu.

While extensive experimental research on increasing baijiu’s lactic acid content using high-yielding LAB has been conducted, its specific application in actual production at distilleries has yet to be documented. Therefore, future research could continue to explore the application of high-yielding LAB in the actual production of different aroma types of baijiu across various distilleries. This method is especially suitable for well-funded large distilleries that need to significantly increase the lactic acid content in their baijiu. Such an approach could greatly improve the control over the fermentation process and enhance the quality and distinctiveness of the final product.

4.3.2. Application of Lactic Acid-Degrading Bacteria in Fermentation

Lactic acid-degrading bacteria, typically sourced from the brewing environment, can degrade lactic acid in various aroma types of baijiu. Yang Wangjun et al. [55] isolated two strains of lactic acid-degrading bacteria from the light-aroma baijiu production process. During solid-state simulation fermentation, the degradation efficiencies of Bacillus amyloliquefaciens and Bacillus subtilis were 25.9% and 38.5%, respectively. Wu Shengwen and others [56,57] isolated a strain of Propionibacterium from the old pit mud of Si Te Distillery, which was applied in the production trials of special-aroma baijiu, reducing the lactic acid content by 3.67 g/L as Propionibacterium converted lactic acid into propionic acid. When applied in actual production at the distillery, it enhanced baijiu quality and consumer satisfaction, leading to an increase in annual sales revenue of 47.326 million Chinese yuan (RMB) [58].

Lactic acid-degrading bacteria are microbes that use lactic acid as an electron acceptor or carbon source [56], primarily producing alcohols, acids, and esters. They are mainly found in Daqu, pit mud, and spent grains and are often facultative anaerobes [10]. The lactic acid-degrading bacteria isolated during different baijiu production processes vary. Those isolated from strong-aroma baijiu production are primarily Propionibacterium, which mainly convert lactic acid into propionic acid and a small amount of acetic acid [10]. Those isolated from light-aroma baijiu production are mainly Bacillus species, such as Bacillus amyloliquefaciens and Bacillus subtilis [59,60]. The fermentation products of Bacillus strains from different sources vary; for example, Bacillus isolated from huangshui fluid only produce ethanol, whereas Bacillus from Daqu produce ethanol, propionic acid, hexanoic acid, isobutanol, and isoamyl alcohol, according to Zhen Da and others [61].

The impact of different strains of lactic acid-degrading bacteria on baijiu quality and their utilization of lactic acid varies, so the screening and application cycle for these bacteria can be lengthy. This method is suitable for well-funded large distilleries that need to significantly reduce lactic acid content and enhance baijiu quality. Deploying these bacteria can help refine the flavor profile of baijiu by adjusting the balance of acids and esters, thereby producing a product that meets specific consumer preferences and market demands.

4.3.3. Impact of Fermentation Time on Lactic Acid

The concentration of lactic acid in baijiu mash tends to increase over the course of fermentation. Zhang Baoyu et al. [62] conducted tests on baijiu mash samples from different periods at the Huanghelou Distillery and found that lactic acid content increased from 1.55–1.66% at the time of potting to 3.20–3.37% after 80 days, showing a gradual upward trend. Similarly, Yang Fan [63] observed that in sauce-flavor baijiu mash, the lactic acid content (3 g/kg to 12 g/kg) increased with fermentation time (from 0 to 30 days). Thus, controlling the fermentation time can regulate the lactic acid content in the fermenting mash and, consequently, in the baijiu itself. Shortening the fermentation time can help reduce the lactic acid content in the spirit, while extending the fermentation time can increase it. It should be noted, however, that the fermentation times for different types of baijiu can vary greatly, and the effects of shortening or extending fermentation times will be similarly dependent on the type of baijiu [30,64].

Future studies could delve deeper into controlling fermentation time to regulate lactic acid in actual distillery production. However, it is important to consider the potential impact of this method on the flavor and mouthfeel of baijiu. This approach is more suitable for medium to large distilleries, where adjustments to fermentation processes can be managed without disproportionately affecting production schedules and where there is sufficient capacity to experiment with different fermentation durations to optimize the balance of flavors and acids in baijiu.

4.3.4. Using Microbial Interactions to Regulate Lactic Acid in Baijiu

The interactions among microorganisms can effectively increase or decrease lactic acid during the baijiu brewing process. To explore the impact of functional microbes in fermentation pit mud on baijiu flavor, Zhou Xiaojing et al. [65] conducted a simulated fermentation using Clostridium butyricum isolated from the pit mud of Jiannanchun Distillery. The results showed that Clostridium butyricum promoted the formation of lactic acid during fermentation. To reduce lactic acid in sauce-flavor baijiu brewing, Luo Han and others [66] added an enhanced microbial agent prepared from Pichia membranifaciens, isolated from Guizhou Maotai Distillery to the sauce-flavor baijiu mash. Compared to the control group, the lactic acid content in the experimental group’s baijiu was reduced by 6.06 g/kg due to the inhibitory effect of Pichia membranifaciens on lactic acid bacteria in the mash. Additionally, Nan D and others [67] found that Kazachstania exigua, a strain of Pichia, inhibited the growth of lactic acid bacteria and the production of lactic acid during baijiu fermentation.

Baijiu is brewed through a complex solid-state fermentation process involving multiple microorganisms, where these interactions are key to maintaining a symbiotic microbial community structure. These interactions affect microbial metabolism during fermentation and, consequently, influence the flavor and quality of baijiu [24]. However, the fermentation process involves a variety of microbial species, and the mechanisms of interaction between different microbes vary. Utilizing microbial interactions to regulate lactic acid in baijiu may cause instability in other microbial species in the mash, difficulties in controlling microbial quantities, and imbalances in microbial ratios.

Further research is needed to deeply and comprehensively understand the effects of microbial interactions on the lactic acid content in baijiu. Utilizing microbial interactions is most suitable for well-funded large distilleries that need to increase or decrease the lactic acid content. This approach allows distilleries to tailor the fermentation environment to their specific product goals, potentially creating unique flavor profiles and improving product quality through enhanced microbial management.

4.4. Distillation Process Impact on Lactic Acid in Baijiu

Lactic acid, being water-soluble and having a high boiling point, can be challenging to manage during the distillation process. As distillation progresses, the alcohol content decreases and temperature increases, facilitating the diffusion of lactic acid from the mash into the distillate, particularly during the later stages of distillation.

Huang Ting and colleagues [68] conducted a study at the Xifeng Distillery and found that the lactic acid content in the distillate increased from an initial 34.31 mg/L to a peak of 1417.59 mg/L after 30 min of distillation, showing a gradual increase over time. This finding indicates that lactic acid tends to remain in the aqueous phase and becomes more concentrated as more volatile compounds, like ethanol, are distilled off.

Collecting the distillate early in the distillation process can help reduce the lactic acid content in baijiu, as the concentration of lactic acid tends to increase later in the distillation. Conversely, collecting the distillate later can increase the lactic acid content in baijiu. This approach might be used to enhance the flavor profile of certain types of baijiu that benefit from a higher acidity. By collecting and storing different fractions of the distillate separately, distilleries can more precisely control the lactic acid content in the final product. This method allows for blending different fractions in varying proportions to achieve the desired flavor and acidity in the baijiu.

This method of controlling lactic acid content through strategic distillation and collection is suitable for any distillery that needs to adjust the lactic acid levels in its products. It provides flexibility in flavor profiling and can be particularly useful in crafting premium baijiu where specific taste characteristics are highly valued.

Overall, understanding the behavior of lactic acid during distillation allows distillers to fine-tune their processes and produce baijiu with desired flavor profiles, meeting both traditional standards and consumer preferences. This control is crucial for maintaining quality and consistency across batches.

4.5. Strictly Controlling Environmental Hygiene to Reduce Lactic Acid in Baijiu

The process of baijiu brewing involves capturing beneficial microorganisms while controlling harmful ones, which requires strict environmental hygiene standards. If hygiene standards are not met, this can lead to significant infiltration of lactic acid bacteria into the baijiu fermentation process, thereby affecting the lactic acid content in the final product. The prevention and control of contamination by non-target microorganisms can be approached from four main aspects [44,69]:

Maintain cleanliness of production tools: ensure that the production tools, the equipment, and the various containers are cleaned and sanitized before and after use. Steam or boiling water is often used for this purpose to eliminate any residual microorganisms that could influence the fermentation process.

Clean production areas thoroughly: it is essential to clean and tidy the production area both before and after production. This includes ensuring that the floors and the areas around the distillation pots are dry, with no standing water or leftover grains or mash. Such residues can harbor unwanted bacteria that may disrupt the fermentation dynamics.

Manage waste water promptly: it is crucial to timely clean out the water collected at the bottom of the pots to prevent it from being reused in distillation. This “pot-bottom water” can contain concentrations of microorganisms that, if reintroduced to the distillation process, could alter the microbial balance desired for optimal fermentation.

Personal hygiene of workers: workers should pay close attention to their personal hygiene. This includes regularly washing their work clothes and shoes to prevent any potential contamination of the production environment by microorganisms that could be carried on clothing or footwear.

By adhering to these rigorous hygiene practices, distilleries can significantly mitigate the risk of unwanted lactic acid production, thereby maintaining the desired quality and flavor profile of baijiu. This aspect of production is crucial, especially in high-scale operations where the impact of minor contaminations can be amplified, affecting entire batches of product. Proper sanitation and hygiene not only ensure the consistency and safety of baijiu but also enhance its marketability by fulfilling stringent quality standards.

4.6. Summary of Lactic Acid Production Control in Baijiu Brewing

Baijiu producers can assess the appropriate levels of lactic acid in different types of baijiu as discussed in Section 3.1. If the lactic acid content is too high, various strategies can be employed to reduce it, including the following:

• Selecting raw materials with lower starch content: using ingredients with lower starch levels can naturally decrease the amount of lactic acid produced during fermentation [36,37].
• Shortening fermentation time: reducing the duration of fermentation limits the time available for lactic acid bacteria to produce lactic acid [30,64].
• Using lactic acid-degrading bacteria: introducing microorganisms that break down lactic acid can help lower its concentration in the mash [56,57,58].
• Leveraging microbial interactions: manipulating the interactions among different microorganisms can influence lactic acid levels by enhancing the activity of bacteria that reduce lactic acid [66,67].
• Early distillate collection: collecting distillate early in the distillation process when lactic acid concentration is lower can reduce its presence in the final product [68].

Conversely, if the lactic acid content is too low, producers can increase it by the following:

• Selecting raw materials with higher starch content: ingredients with higher starch content provide more substrate for lactic acid bacteria, thereby boosting lactic acid production [38,39].
• Increasing lactic acid bacteria in the Daqu: enriching the distiller’s Daqu with lactic acid-producing bacteria can elevate lactic acid levels during fermentation [47].
• Extending fermentation time: allowing a longer fermentation period gives lactic acid bacteria more time to produce lactic acid [62,63].
• Using high-yielding lactic acid bacteria: introducing strains known for their high lactic acid production can directly increase lactic acid levels [49,50,51,52,53,54].
• Microbial interactions: utilizing the synergistic effects of microorganisms can also increase lactic acid production [65].
• Late distillate collection: collecting distillate later in the process, when lactic acid concentrations are higher, can increase its presence in the final product [68].

The appropriate method of regulation should be chosen based on the specific circumstances of each baijiu producer, including scale, financial conditions, and specific requirements such as the degree of lactic acid regulation needed, the yield, and the quality rate. Each of these factors will influence the decision on how best to control lactic acid levels to achieve the desired quality and flavor profile of baijiu.

5. Conclusions and Future Prospects

This article reviewed the mechanisms of lactic acid formation in the production of baijiu, summarized research progress on lactic acid control techniques during the raw material selection, yeast starter preparation, fermentation process, and distillation stages, and analyzed how controlling lactic acid content in different distilleries can enhance baijiu quality, improve taste, increase production yields, and boost the economic benefits for some distilleries.

To better leverage lactic acid control, with the continuous development of biotechnology, genetic engineering, and metabolic engineering, future efforts should focus on flavor and health-oriented approaches. These efforts should aim to integrate the corporate culture and development philosophies of different baijiu enterprises to optimize, explore, and establish more efficient and environmentally friendly lactic acid control technologies tailored for their practical application in baijiu production. This approach is expected to provide new strategies and pathways for regulating lactic acid in baijiu, contributing significantly to the industry’s advancement and the global appreciation of this unique spirit. By enhancing the precision and efficacy of these control measures, the baijiu industry can ensure the production of high-quality spirits that meet both traditional standards and contemporary consumer preferences.