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Review

Regulation and Mechanisms of L-Lactic Acid and D-Lactic Acid Production in Baijiu Brewing: Insights for Flavor Optimization and Industrial Application

by
Yabin Zhou
1,2,3,* and
Jin Hua
1,4
1
School of Biological Engineering, Sichuan University of Science and Engineering (SUSE), Yibin 644000, China
2
Brewing Science and Technology Key Laboratory of Sichuan Province, Sichuan University of Science and Engineering (SUSE), Yibin 644000, China
3
College of Science and Engineering, Flinders University, Adelaide 5064, Australia
4
College of Public Health and Medicine, Flinders University, Adelaide 5064, Australia
*
Author to whom correspondence should be addressed.
Fermentation 2025, 11(4), 213; https://doi.org/10.3390/fermentation11040213
Submission received: 30 December 2024 / Revised: 4 April 2025 / Accepted: 7 April 2025 / Published: 12 April 2025
(This article belongs to the Special Issue Feature Review Papers in Fermentation for Food and Beverages 2024)
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Versions Notes

Abstract

:
L-lactic acid and D-lactic acid are chiral forms of lactic acid that significantly influence the flavor and health-related properties of Baijiu. Their production during brewing is primarily driven by lactic acid bacteria (LAB), with L-lactic acid being favored at higher fermentation temperatures and by specific high-producing strains, while D-lactic acid predominates at lower temperatures and with limited microbial utilization. Various factors, including fermentation mash composition, microbial communities, and brewing conditions, affect the balance between these isomers. This review synthesizes recent research on regulating L- and D-lactic acid production in Baijiu brewing, highlighting advancements in raw material selection, fermentation starter composition, temperature control, LAB strain selection, and distillation techniques. It critically evaluates strategies aimed at increasing L-lactic acid content while minimizing D-lactic acid levels to optimize flavor and promote health benefits. This review aims to provide theoretical insights and practical guidance for controlling these chiral isomers in Baijiu production. By consolidating the latest findings, it serves as a resource for industrial applications, offering strategies to enhance lactic acid ratios, improve Baijiu flavor, and support sustainable development in the industry.

1. Introduction

Baijiu, China’s national liquor, is made primarily from grains as the main raw material, with Qu (fermentation starter) and yeast serving as saccharification and fermentation agents [1]. The production process involves steaming, saccharification, fermentation, and distillation, as shown in Figure 1. The process begins by steaming grains such as sorghum or rice to prepare them for fermentation. Qu, made from compacted grains, is inoculated with molds, yeasts, and bacteria and mixed with the steamed grains. This mixture is then placed in fermentation pits lined with pit mud, which houses lactic acid bacteria and other microorganisms essential for flavor development. In the pits, the mixture undergoes an extended anaerobic fermentation, where microbes produce ethanol and flavor compounds such as various alcohols, organic acids, aldehydes, and esters. The fermented mash is then distilled through steam, collecting base liquor in stages with unique flavor profiles. After distillation, the base liquor is transferred to ceramic vessels for storage and aging, where flavors continue to develop and integrate over months, even years. Different aged batches are then blended to create a balanced, consistent finished product—Baijiu. By-products of Baijiu brewing include huangshui (a nutrient-rich yellow water from fermentation) and zaopei (distilled grains), both of which are used in further production or agriculture.
During the fermentation of Baijiu, up to 157 acidic compounds are produced, which contribute to its aroma, taste, and functional properties [2]. Among these, lactic acid is a significant non-volatile organic acid. In Baijiu, lactic acid plays a crucial role as a flavor and aroma compound and serves as a precursor for ethyl lactate, an essential flavor substance in Baijiu [3]. According to studies, the lactic acid content in Baijiu can reach up to 218 mg/100 mL [3,4]. Due to the presence of an asymmetric carbon atom in the lactic acid molecule, it exists in two chiral (optical) isomeric forms: L-lactic acid and D-lactic acid [4].
Baijiu is categorized into several aroma types, with sauce aroma and strong aroma being the most well-known and commercially popular. These classifications arise from variations in raw materials, fermentation processes, and microbial communities. In sauce-aroma Baijiu, both D-lactic acid and L-lactic acid are present in higher concentrations compared to strong-aroma Baijiu. The average concentration of D-lactic acid in commercial sauce-aroma Baijiu is 130.02 mg/100 mL, significantly higher than the 49.58 mg/100 mL found in commercial strong-aroma Baijiu. Similarly, the average concentration of L-lactic acid in commercial sauce-aroma Baijiu is 39.75 mg/100 mL, significantly higher than the 15.14 mg/100 mL found in commercial strong-aroma Baijiu.
The two chiral forms of lactic acid have different effects on Baijiu fermentation mash and the finished product. In the fermentation mash, L-lactic acid is more readily esterified, producing a higher quantity of esterification products such as L-methyl lactate, L-ethyl lactate, and so on. In contrast, D-lactic acid is less prone to esterification, which can lead to an increased acidity in the mash, thereby disrupting normal saccharification and fermentation processes [5]. In the finished product, L-lactic acid contributes a certain flavor profile with a balance of acidity and a slight sweetness, while D-lactic acid imparts a sharp spiciness that may negatively affect the sensory quality of the liquor [3,6]. Additionally, L-lactate dehydrogenase is the primary lactic acid metabolism enzyme in the human body, allowing for the metabolism of L-lactic acid, whereas D-lactic acid is metabolized inefficiently [7].
Currently, research and application of the regulation of the two chiral lactic acids in Baijiu are limited, focusing primarily on breeding high-L-lactic acid-producing strains and using them in Baijiu brewing to increase L-lactic acid content. Therefore, controlling the levels of these two chiral lactic acids is of great importance for improving Baijiu quality and supporting healthy and comfortable human consumption. This paper reviews the mechanisms of production of the two chiral lactic acids, describes briefly the differences in their content and ratios in Baijiu, and summarizes their metabolic roles in the human body. It also explores control measures for influencing L-lactic acid and D-lactic acid through factors such as raw materials, fermentation starters, fermentation mashes, fermentation processes, temperature, high-L-lactic acid-producing strains, distillation techniques, and pit mud. Lastly, it discusses the impact of these two chiral lactic acids on Baijiu quality, aiming to provide a theoretical reference for improving Baijiu quality.

2. The Production Mechanism of the Two Chiral Forms of Lactic Acid in Baijiu

In Baijiu production, lactic acid bacteria (LAB) from Qu, pit mud, fermentation liquid (Huang Shui), and the environment migrate into the fermentation mash to participate in the fermentation process [8]. The brewing grains (such as sorghum, rice, glutinous rice, wheat, and corn) undergo pre-fermentation treatments (including grain input, crushing, soaking, steaming, and saccharification) to produce sugars, which are utilized by the lactic acid bacteria and are metabolized into pyruvate [9]. Lactate dehydrogenase (LDH) is a key enzyme in the glycolysis pathway and plays a critical role in LAB fermentation [10,11]. Using reduced nicotinamide adenine dinucleotide (NADH) as a coenzyme, LDH catalyzes the biochemical conversion of pyruvate into lactic acid [10,12], as shown in Figure 2. During Baijiu brewing, two types of LDH enzymes within LAB—L-lactate dehydrogenase (L-LDH) and D-lactate dehydrogenase (D-LDH)—produce the two chiral forms of lactic acid: L-lactic acid and D-lactic acid [13]. Some LAB contain only a single type of LDH and thus produce only one chiral form of lactic acid. For example, Lactobacillus rhamnosus produces L-lactic acid due to the presence of L-LDH [14], while Lactobacillus delbrueckii produces D-lactic acid via D-LDH [15]. However, most LAB contain both L-LDH and D-LDH, enabling them to produce both chiral forms. For instance, Lactobacillus helveticus produces both L-lactic acid and D-lactic acid [16]. In 2020, the genus Lactobacillus was taxonomically revised [17]. In the present review, we retained the original nomenclature for consistency with previously published literature and for better readability.
The activity of LDH is influenced by temperature. For example, Lactobacillus coryniformis primarily produces D-lactic acid at 30 °C, shifting to L-lactic acid production when the fermentation temperature exceeds 40 °C [18]. Lactobacillus plantarum exhibits optimal activity for L-LDH at 40 °C and for D-LDH at 30 °C [19]. Additionally, Bacillus coagulans produces only L-lactic acid at 50 °C [20]. In most Baijiu brewing facilities, fermentation temperatures range from 20 °C to 40 °C [21,22]. For example, strong-flavor Baijiu often ferments within the range of 20 °C to 35 °C [22,23], while sauce-flavor Baijiu is usually fermented at slightly higher temperatures, often between 36 °C and 42 °C [24]. Sesame-flavor Baijiu, on the other hand, is fermented at a relatively lower range, generally around 22 °C to 36 °C [25]. Consequently, LAB in the fermentation mash produce both chiral forms of lactic acid, with their relative content and ratio varying depending on the LAB strains and brewing processes used by different Baijiu producers.
Lactate racemase (Lar) is a nickel-dependent enzyme capable of catalyzing the stereoisomeric conversion of lactic acid, allowing organisms to utilize both configurations of lactic acid simultaneously [26]. LAB (lactic acid bacteria) can produce Lar under the induction of lactic acid as a substrate, enabling the interconversion of lactic acid isomers [27]. The reported LAB capable of producing Lar include Lactobacillus plantarum [27], Lactobacillus sakei [28], Lactobacillus curvatus [29], Lactobacillus helveticus [30], Lactococcus lactis [31], and Lactobacillus bulgaricus [32]. The expression and activity of Lar are influenced by the growth cycle and environmental factors such as pit mud and temperature. For instance, Iino T et al. reported that Lactobacillus sakei expresses Lar during the late logarithmic growth phase to convert L-lactic acid into D-lactic acid [33]. Similarly, Dai Jinfeng et al. found that Lar was undetectable in aged pit mud, hypothesizing that certain common physicochemical properties of aged pit mud may lead to the degradation of Lar messenger RNA (mRNA) [34]. Additionally, Desguin B. et al. discovered that Lactobacillus plantarum Lar partially converts D-lactic acid into L-lactic acid at temperatures ranging from 30 °C to 50 °C, although the exact proportion remains unclear [35].
The two chiral forms of lactic acid produced by LAB metabolism are transported into the fermentation mash via transmembrane transport proteins in the LAB. In the mash, part of the lactic acid is metabolized by lactic acid-utilizing bacteria (such as Propionibacterium, Bacillus, and Clostridium), producing compounds like acids (e.g., propionic acid and acetic acid), alcohols (e.g., ethanol and isobutanol), and esters (e.g., butyl propionate and ethyl butyrate). Another portion reacts with ethanol to synthesize ethyl lactate, while the remaining lactic acid is partially carried into the base liquor during distillation [36].

3. The Impact of the Two Chiral Forms of Lactic Acid on the Sensory Attributes of Baijiu

Sensory evaluation studies conducted following the National Standard of the P. R. China, Guidelines for Determination of Liquor Flavor Substances Threshold (GB/T 33406-2016, Baijiu flavor evaluation guidelines), have shown that L-lactic acid has a lower recognition threshold (9.819 mg/100 mL) than D-lactic acid (19.418 mg/100 mL) in a 46% ethanol solution [3]. This means that L-lactic acid imparts a smoother, rounder acidity, while D-lactic acid contributes a sharper, more lingering acidity, affecting the balance of Baijiu’s flavor. It can be concluded that L-lactic acid helps balance acidity with a subtle sweetness, while excessive D-lactic acid can introduce a sharp spiciness, potentially impacting the sensory quality of the liquor [3,6].

4. The Metabolic Roles of the Two Chiral Forms of Lactic Acid in the Human Body

Recent research has revealed that dietary intake of lactic acid has a regulatory effect on the immune system, particularly in suppressing localized intestinal inflammation [37,38,39,40,41]. This has shifted attention to lactic acid as a functional substance in food [42].
The metabolism of lactic acid is crucial in the body. As a high-energy molecule, lactic acid supplies energy by being converted back to pyruvate via lactate dehydrogenase (LDH). Pyruvate then enters the mitochondria for oxidation, producing carbon dioxide, water, and energy [43]. Additionally, lactic acid metabolism promotes the body’s utilization and storage of glucose. This occurs through gluconeogenesis in the liver and skeletal muscle cells, where lactic acid is converted into glucose or glycogen to replenish blood sugar levels and glycogen stores [44]. Lactic acid also stimulates the secretion of critical hormones such as growth hormone and adrenaline, which play significant roles in metabolism and energy regulation [7].
Research on the health effects of lactic acid is currently focused mainly on lactic acid-based products and the relationship between lactic acid bacteria, gut microbiota, and metabolic disorders [45,46]. The human body contains mainly L-lactate dehydrogenase for the efficient metabolism of L-lactic acid but not D-lactic acid [7].
Cai Hao found that dietary supplementation with L-lactic acid significantly enhanced energy metabolism in high-fat-diet (HFD) mice. The mechanism involves L-lactic acid suppressing ileal FXR signaling, promoting hepatic bile acid synthase CYP7A1 expression, and activating adipose tissue TGR5 signaling, thereby boosting energy metabolism [47]. Pohanka M reported that D-lactic acid from food may accumulate in the body, potentially causing D-lactic acidosis, elevated uric acid levels, and increased glycogen levels in the liver [48]. Yan Y et al. demonstrated that the two chiral forms of lactic acid have different effects on inflammation and glucose–lipid metabolism [49]. Since the liver is the primary organ for glucose and lipid metabolism, dietary intake of different chiral combinations of lactic acid may influence liver function and glucose–lipid metabolism. Additionally, their impact on local immune systems could further affect liver function [50,51].
During Baijiu consumption, alcohol metabolism occurs mainly in the liver. As the D-lactic acid content in Baijiu is generally higher than that of L-lactic acid, the D-lactic acid in Baijiu may affect alcohol metabolism in the liver and contribute to drinking discomfort [52,53]. Therefore, exploring control measures for the two chiral forms of lactic acid during Baijiu brewing could be beneficial for healthy and comfortable consumption.

5. The Content and Ratio Difference of the Two Chiral Forms of Lactic Acid in Baijiu

5.1. The D-Lactic Acid Content in Baijiu Is Generally Higher than That of L-Lactic Acid

Currently, there are no national or industry-specific regulations regarding the content and ratio of L-lactic acid and D-lactic acid in Baijiu. Limited studies to date have observed that the D-lactic acid content in Baijiu is generally higher than L-lactic acid [3,4].
Jiang Feng et al. used chiral column HPLC to measure the content of L-lactic acid and D-lactic acid in Baijiu [4]. The results showed that in most Baijiu, the L-lactic acid to D-lactic acid ratio was below 0.5, with the lowest reaching 0.083. Only a small number of Baijiu sample exhibited an L/D-lactic acid ratio of approximately 1. Hao Xu et al. analyzed lactic acid in various strong-flavor, sauce-flavor, and light-flavor Baijiu types, finding L/D-lactic acid ratios ranging from 0.053 to 3.167 [3]. Most Baijiu samples had an L/D-lactic acid ratio around 0.35, with only two samples having a ratio greater than 1. Preliminary results also indicated that the two chiral forms of lactic acid contribute differently to Baijiu’s flavor.

5.2. The Content of the Two Chiral Forms of Lactic Acid and the L:D Ratio Vary Significantly Among Different Aroma Types of Baijiu

In sauce-aroma Baijiu, both the content of both chiral forms of lactic acid and the L:D ratio are relatively high, as shown in Figure 3 and Figure 4. For example, in Qinghualang, the L-lactic acid and D-lactic acid contents are 755.38 mg/L and 1567.41 mg/L, respectively, with an L:D ratio of 0.294. In Langjiu, the L-lactic acid and D-lactic acid contents are 755.38 mg/L and 1803.99 mg/L, respectively, with an L:D ratio of 0.435 [3].
In contrast, strong-aroma Baijiu generally has lower contents of both chiral forms. Luzhou Laojiao, however, exhibits a higher L:D ratio. For instance, in Luzhou Laojiao Touqu, the L-lactic acid and D-lactic acid contents are 130.08 mg/L and 483.60 mg/L, respectively, with an L:D ratio of 0.263. In Luzhou Laojiao Tequ, the L-lactic acid and D-lactic acid contents are 143.04 mg/L and 402.67 mg/L, respectively, with an L:D ratio of 0.357. Conversely, Wuliangye has a lower L:D ratio, with L-lactic acid and D-lactic acid contents of 35.02 mg/L and 407.70 mg/L, respectively, and an L:D ratio of 0.087 [3].
As mentioned earlier, L-lactic acid contributes positively to fermentation, Baijiu flavor, and healthy and comfortable consumption. However, most Baijiu contains lower levels of L-lactic acid compared to D-lactic acid, highlighting the need to improve the L:D ratio. At present, there is no established ideal range for the L:D ratio in Baijiu due to the limited research available on this topic. Future studies are necessary to investigate and determine the optimal L:D ratio to better understand its role in Baijiu production and flavor.

6. Control Measures to Balance the Two Chiral Forms of Lactic Acid Production in Baijiu Brewing

The content of the two chiral forms of lactic acid in Baijiu is closely related to the fermentation process. Therefore, their levels can be regulated by controlling the brewing raw materials, Qu (fermentation starters), fermentation mash, fermentation process, fermentation temperature, breeding high-L-lactic acid-producing strains, distillation techniques, and pit mud.

6.1. The Impact of Brewing Raw Materials on the Production of the Two Chiral Forms of Lactic Acid in Baijiu

6.1.1. High-Quality Rice Increases the L-Lactic Acid Content in Baijiu

Different brewing raw materials have varying effects on L-lactic acid production. Li Kefan et al. conducted a study at the Sitijiu distillery and found that the L-lactic acid content in Baijiu brewed with high-quality rice (153.33 mg/L) was slightly higher than that in Baijiu brewed with regular rice (150 mg/L) [54]. Sensory evaluations of Baijiu samples by professionals also showed that the ones brewed from high-quality rice scored slightly higher than the ones brewed from regular rice.
Currently, national standards classify brewing raw materials (e.g., rice, sorghum, and corn) into different quality grades based on differences in factors such as broken grain percentage, moisture content, impurity levels, and amylose content [55]. Brewing with high-quality raw materials may increase the L-lactic acid content in Baijiu, possibly because high-quality materials contain higher levels of starch and protein, providing lactic acid bacteria with more nutrients required for growth and fermentation [55]. Future research could further explore how to enhance the L-lactic acid content in Baijiu by using premium brewing raw materials.

6.1.2. Corn Reduces the D-Lactic Acid Content in Baijiu

To address the issue of flavor deficiency in Hengshui Laobaigan, Han Jing et al. studied the correlation between different culture media and the production of chiral lactic acid. They identified seven strains of lactic acid bacteria, Bacillus coagulans, Lactobacillus casei, Lactobacillus brevis, Pediococcus pentosaceus, Lactobacillus plantarum, Lactobacillus bulgaricus, and Pediococcus acidilactici, and inoculated these seven strains into fermentation media based on two different raw materials, rice and corn, and fermented them for five days. These results demonstrated that D-lactic acid production was consistently lower in the corn-based culture medium (0.35–14.24 g/L) compared to the rice-based culture medium (3.24–19.82 g/L) across all strains. Among them, Bacillus coagulans exhibited the most significant reduction, with its D-lactic acid content decreasing by approximately 96.9% when using corn instead of rice. Lactobacillus bulgaricus also showed a notable reduction of nearly 50%. Other strains, including Lactobacillus casei, Lactobacillus brevis, Lactobacillus plantarum, and Pediococcus acidilactici, also displayed varying degrees of reduction, though to a lesser extent. The smallest difference was observed in Pediococcus pentosaceus, where D-lactic acid production remained nearly the same between the two media. In contrast, the L-lactic acid content showed little variation between the two culture media. These findings suggest that certain nutrients present in corn may influence the gene expression or enzymatic activity of D-LDH (D-lactate dehydrogenase), leading to reduced D-lactic acid synthesis [5].
In summary, this study provides an important reference for using corn as a brewing raw material to modulate the D-lactic acid content in Baijiu. However, it is important to note that this study was conducted under controlled fermentation conditions rather than full-scale Baijiu production. The practical application of these findings in commercial brewing requires further validation due to the complexity of Baijiu fermentation and the lengthy research and implementation cycles associated with such adjustments. Moreover, regulating raw materials requires a holistic approach that considers factors such as moisture, acidity, starch content, temperature, and microbial activity in the fermentation pit, as well as key metrics like yield, flavor compounds, and sensory scores. Additionally, production costs such as raw material prices, logistics, and labor must be taken into account. The research and application cycles for such adjustments are lengthy, making this approach more suitable for large-scale distilleries.

6.2. The Impact of Lactic Acid Bacteria in Qu on the Production of the Two Chiral Forms of Lactic Acid in Baijiu

Fermentation starters (Qu) are a critical ingredient in Baijiu production, containing a diverse range of lactic acid bacteria. Through high-throughput sequencing analysis, Li Lianhui identified 19, 49, and 3 species of lactic acid bacteria in Daqu (a type of Qu), fermentation mash, and pit mud, respectively. Through comparative analysis, Li Lianhui found that 16 species of lactic acid bacteria were present in both the Daqu and fermentation mash. Among these, eight were dominant (relative abundance > 0.1%) lactic acid bacteria in the fermentation mash of Luzhou Laojiao originated from Daqu. These dominant strains were Lactobacillus brevis, Lactobacillus crustorum, Lactobacillus curvatus, Lactobacillus pentosus, Lactococcus lactis, Leuconostoc citreum, Weissella cibaria, and Weissella confusa. Additionally, three lactic acid bacteria species were detected exclusively in Daqu, with two of them (Leuconostoc mesenteroides and Weissella hellenica) being dominant (relative abundance > 0.1%) but absent in the fermentation mash [56]. Among these, Lactobacillus curvatus and Lactococcus lactis contain lactate racemase (Lar), while Pediococcus pentosaceus is a high L-lactic acid-producing strain, which may explain why the L:D ratio of chiral lactic acids in some Luzhou Laojiao Baijiu approaches 1 [57].
As a saccharification and fermentation agent, Qu is added to the fermentation mash at a proportion of 10–20%, introducing specific lactic acid bacteria into the mash [57]. Therefore, by adjusting the composition of lactic acid bacteria in Qu, it is possible to regulate the levels of the two chiral forms of lactic acid in Baijiu.
However, exploring the effects of different raw material ratios and processing conditions for high L-lactic acid-producing bacteria in Qu requires careful consideration of several factors: interactions between lactic acid bacteria and other microorganisms in Qu; whether high L-lactic acid-producing bacteria can dominate in the fermentation mash; and raw material costs, labor expenses, and research timelines. These factors are critical for achieving the effective control of chiral lactic acid production in Baijiu through modifications to Qu.

6.3. The Impact of Lactic Acid Bacteria in Fermentation Mash on the Production of the Two Chiral Forms of Lactic Acid in Baijiu

The production of lactic acid is closely tied to the lactic acid bacteria in the fermentation mash. During Baijiu brewing, only facultative anaerobic, acid-tolerant, and ethanol-tolerant lactic acid bacteria can become dominant strains. Studies show that the lactic acid bacterial community in fermented grain undergoes significant changes during the first week of fermentation before stabilizing. Li Lianhui found that during the fermentation process of Luzhou Laojiao, the relative abundance of lactic acid bacteria was 51.7% at the start of fermentation. However, after one day, the relative abundance dropped sharply to 11.2%, indicating that many lactic acid bacteria introduced into the mash could not adapt to the new environment and perished [57]. In the study, on day 0, Weissella confusa was the dominant lactic acid bacterium, accounting for 88% of the lactic acid bacterial community. However, its abundance declined rapidly after fermentation began, and it became undetectable by the end of fermentation. On day 1, lactic acid bacteria such as Lactobacillus curvatus, Lactococcus lactis, Lactobacillus brevis, Lactobacillus pentosus, Pediococcus pentosaceus, Lactobacillus acetotolerans, Lactobacillus panis, Lactococcus garvieae, Leuconostoc citreum, Weissella cibaria, Lactobacillus acidipiscis, and Lactobacillus paracollinoides rapidly proliferated, with their relative abundance reaching 65%. In contrast, Lactobacillus acetotolerans started with a relative abundance of less than 1% but proliferated rapidly, becoming the dominant lactic acid bacterium in the fermented grains by day 2, with a relative abundance exceeding 70%. After day 3, its relative abundance exceeded 90% and remained consistently above this level, reaching a peak of 98% by the end of the 40-day fermentation period [56].
After day 7, the growth of lactic acid bacteria, although slowing down, continued until the fermentation ended on day 40. The dominant bacteria, Lactobacillus curvatus and Lactococcus lactis, contain lactate racemase (Lar) that is capable of conversion of the two chiral forms of lactic acid [57]. Additionally, dominant strains such as Lactobacillus brevis, Lactobacillus pentosus, and Pediococcus pentosaceus are high L-lactic acid producers. This may explain why some Luzhou Laojiao Baijiu have a relatively high L-lactic acid content.

6.4. The Impact of the Fermentation Process on the Production of the Two Chiral Forms of Lactic Acid

The fermentation process of the mash affects the production of the two chiral forms of lactic acid. As fermentation progresses, the L-lactic acid content shows a pattern of initial increase followed by a decrease, while the D-lactic acid content gradually rises [58]. Li Xiaoxiao found that the D-lactic acid content in sauce-aroma Baijiu increases with the number of brewing cycles [59]. Liang Zhen et al. [58] observed that the L-lactic acid content in soy-sauce-aroma Baijiu fermentation initially increases and then slowly decreases. This is likely because L-lactic acid undergoes esterification and is utilized by other microorganisms, while D-lactic acid is less readily utilized. Therefore, controlling the fermentation process is beneficial for regulating the levels of the two chiral forms of lactic acid in Baijiu.

6.5. The Impact of Fermentation Temperature on the Production of the Two Chiral Forms of Lactic Acid

Lactic acid bacteria have different optimal environmental temperatures for producing L-lactic acid and D-lactic acid. Most lactic acid bacteria have a higher optimal temperature for producing L-lactic acid (35–50 °C) compared to a lower temperature for D-lactic acid (20–35 °C). Thus, increasing the fermentation temperature can promote a higher L-lactic acid content in Baijiu.
For instance, the high L-lactic acid content (755.38 mg/L) and the high L:D ratio (1.0:2.3) in Langjiu may be associated with its “Four Highs and Two Longs” production process, where high-temperature conditions are more conducive to the growth and metabolism of lactic acid bacteria. Additionally, high-temperature distillation (37–45 °C) allows for the maximum collection of L-lactic acid in the liquor [60,61]. In contrast, most Baijiu producers maintain fermentation temperatures between 20 and 40 °C. For example, Luzhou Laojiao ferments at 26–28 °C [22]. These temperatures favor the production of D-lactic acid, which may explain the higher D-lactic acid content in most Baijiu.
Since most microbial communities in the mash (including lactic acid bacteria and yeasts) are mesophilic, increasing fermentation temperatures could affect the growth and metabolism of certain microbial groups, thereby influencing the production of other flavor compounds such as acids, alcohols, and esters. Therefore, further research is needed to determine the optimal fermentation temperatures for regulating the production of the two chiral forms of lactic acid.

6.6. The Impact of High L-Lactic Acid-Producing Strain Selection and Application on the Production of the Two Chiral Forms of Lactic Acid

Selecting high L-lactic acid-producing strains from the brewing environment and applying them in production can potentially increase the L-lactic acid content in Baijiu. Ding Haimei isolated Lactobacillus casei from Daqu and applied it to the production of Laobaigan Baijiu, resulting in a 98.02% increase in L-lactic acid content [62]. Similarly, Li Xiaoxiao screened Lactobacillus plantarum GT-L-22 from the brewing environment and applied it to the production of sauce-aroma Baijiu. When inoculated at 3%, the strain achieved the highest lactic acid yield (2.56 g/L) [59].
Currently, extensive research on the isolation and identification of high L-lactic acid-producing strains provides LAB strain material for practical application in Baijiu production. For instance, Lactobacillus plantarum GT-L-22 and Pediococcus acidilactici GT-L-56, isolated from sauce-aroma Baijiu production, have been applied in sauce-aroma fermentation mash to promote ester production and enrich Baijiu flavor [59].
However, research on high D-lactic acid-producing strains remains limited. To date, no studies have reported the isolation and identification of high D-lactic acid-producing strains for Baijiu fermentation. Further research is needed to explore the potential applications of D-lactic acid-producing strains and their impact on the overall fermentation process.

6.7. The Impact of Distillation on the Content of the Two Chiral Forms of Lactic Acid

Lactic acid, being a water-soluble acid, is less likely to diffuse from the fermentation mash during the early stages of distillation when the alcohol content is high and the temperature is relatively low. As distillation progresses and the alcohol content decreases while temperature increases, lactic acid, which dissolves in water, is more easily distilled out of the mash [63]. Additionally, L-lactic acid (boiling point: 125 °C) has a higher boiling point than D-lactic acid (boiling point: 103 °C), making it more likely to distill out during later stages when the alcohol content is low and the temperature is high.
Huang Ting found that the lactic acid content increased from 34.31 mg/L at the start of distillation to 1417.59 mg/L after 30 min, showing a gradual increase [63]. He Fei measured the L-lactic acid content at different distillation stages and detected it only in the tail fractions of each layer of fermentation mash and in the middle fraction of the fourth layer (excluding the head and tail fractions). In the distillation process, Tequ (the fraction collected 5 min after the head fraction) has a high alcohol content. Touqu (collected after tequ) has a slightly lower alcohol content. Erqu (collected after touqu) has the lowest alcohol content. As distillation progresses, the alcohol content decreases, the temperature increases, and the total lactic acid and L-lactic acid content tend to rise. This aligns with findings where it was found that the average lactic acid content in Touqu (613.68 mg/L) is higher than in Tequ (545.7 mg/L), and the L-lactic acid content in Erqu (235.50 mg/L) is higher than in Tequ (143.04 mg/L) and Touqu (130.08 mg/L). However, the L-lactic acid content in Tequ and Touqu showed little difference, possibly due to close distillation times and limited sample data [64].
These studies suggest that the L-lactic acid content is lower in the early distillation stages and higher in the later stages. Categorizing and storing Baijiu from different distillation stages could help regulate L-lactic acid content during the liquor blending process.

6.8. The Impact of Pit Mud on the Production of the Two Chiral Forms of Lactic Acid

The microbial community in pit mud plays a crucial role in Baijiu fermentation by influencing the production of lactic acid enantiomers. High L-lactic acid-producing bacteria in pit mud, such as Weissella confuse and Lactobacillus plantarum, can migrate into the fermentation mash and contribute to the fermentation process. Lactobacillus acetotolerans, the dominant strain found in Luzhou Laojiao mash, has been identified as originating from the pit mud, further supporting the role of pit microbiota in shaping the fermentation environment [57].
Interestingly, microbial distribution within the fermentation pit is not uniform, which may explain the variations in liquor quality across different mash layers. Zhang Hong et al. [65] discovered that the lower layers of Luzhou Laojiao mash produced higher-quality liquor compared to the middle and upper layers, and the mash near the edges of the pit outperformed the central mash. This variation in liquor quality correlates with differences in microbial abundance and activity, suggesting that certain lactic acid bacteria from the pit mud establish more favorable fermentation conditions in specific regions of the mash.
Moreover, the microbial composition of pit mud has been found to influence the enantiomeric balance of lactic acid in Baijiu. In Luzhou Laojiao’s pit mud, the presence of high L-lactic acid-producing strains, such as Weissella confusa and Lactobacillus plantarum, may explain the relatively higher L-lactic acid content observed in some of its Baijiu [65]. Given that L-lactic acid contributes positively to Baijiu fermentation and sensory characteristics, optimizing pit mud microbiota by promoting the growth of high L-lactic acid-producing strains could be a potential strategy to enhance Baijiu quality.
In summary, the interaction between the pit mud microbiota and the fermentation mash significantly influences the production of the two chiral forms of lactic acid. These findings highlight the importance of microbial community structure in pit mud and its potential influence on Baijiu fermentation outcomes.
Currently, research on methods to regulate lactic acid in Baijiu is limited, focusing mainly on isolating and applying high L-lactic acid-producing strains in production. This approach is feasible because such strains originate from the brewing environment and have minimal impact on other fermentation microorganisms. However, other regulatory methods involve challenges such as long research cycles and high costs, making their application in practical production difficult.
It is also important to note that there is a notable gap in the current literature regarding the adaptability and genetic stability of high L-lactic acid-producing strains under different brewing environments. To date, no studies have systematically investigated how variations in conditions—such as temperature, pH, and nutrient availability—might affect the performance and consistency of these strains over multiple fermentation cycles. Future studies should therefore involve large Baijiu enterprises and university key laboratories to systematically explore these issues. By addressing these challenges, further research will provide practical guidance and valuable insights for regulating the two chiral forms of lactic acid and for enhancing sustainable practices in the Baijiu industry.

6.9. Environmental Implications and Sustainability Considerations

Although environmental and sustainability metrics have not traditionally been the focus of research in the Baijiu brewing industry, their integration is increasingly recognized as vital for long-term industrial advancement. Currently, comprehensive data regarding the environmental impacts of Baijiu production—including resource utilization, energy consumption, and waste generation—is scarce. This limitation is largely due to the historical emphasis on flavor optimization and production efficiency over sustainability assessments. Recognizing this gap, we emphasize the need for future studies to incorporate sustainability metrics—such as carbon footprint analysis and waste management efficiency—to better understand and address the environmental impact of Baijiu production.
By highlighting this research gap, we aim to draw attention to the need for systematic evaluations of environmental performance in Baijiu production. Integrating such assessments into fermentation technology could offer dual benefits: optimizing product quality while promoting greener production practices. We encourage further investigations to develop reliable methodologies for measuring sustainability parameters in this traditional industry, thereby paving the way for environmentally friendly innovations in Baijiu brewing.

7. Conclusions and Future Prospects

The development trend in Chinese food focuses on both flavor and health [66,67]. Increasing L-lactic acid and reducing D-lactic acid in Baijiu without compromising flavor can improve Baijiu’s taste and support healthier and more comfortable consumption, contributing to the sustainable growth of the Baijiu industry. Currently, there are no national or industry-specific regulations governing the content or ratio of L-lactic acid and D-lactic acid in Baijiu, meaning that adjustments to these components are driven primarily by market preferences including consumer sensory expectations and industry demands. Therefore, any strategy to alter the L/D-lactic acid content and ratio must be carefully balanced to avoid negative impacts on Baijiu’s flavor profile and traditional taste, as L-lactic acid has a lower sensory threshold than D-lactic acid, making its sour taste more noticeable at lower concentrations.
This paper reviewed the mechanisms behind the formation of the two chiral forms of lactic acid in Baijiu brewing and summarized the research progress on regulating L-lactic acid and D-lactic acid in areas such as brewing raw materials, fermentation starters, mash, fermentation processes, fermentation temperature, high L-lactic acid-producing strain selection, distillation techniques, and pit mud. These findings provide important data and form the basis for controlling the two chiral forms of lactic acid in Baijiu production.
Future research should delve deeper into the production mechanisms of the two chiral forms of lactic acid in Baijiu to better understand their changes during the brewing process. Additionally, as brewing techniques and microbial technologies continue to advance, it will be important to develop innovative, environmentally friendly methods to enrich L-lactic acid and reduce D-lactic acid without affecting Baijiu flavor. Strengthening collaboration among industry, academia, and research institutions is crucial for accelerating the transformation and application of scientific achievements, as well as for promoting the practical adoption of regulation methods in production. These efforts will contribute to the sustainable development of China’s Baijiu industry with its dual focus on flavor and health.

Author Contributions

Conceptualization, Y.Z.; writing—original draft preparation, Y.Z.; writing—review and editing, Y.Z. and J.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Brewing Science and Technology Key Laboratory of Sichuan Province, grant number NJ2023-06, and the Wuliangye Industry University research cooperation project, grant number CXY2022ZR009.

Data Availability Statement

No new data were created or analyzed in this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Production process of Baijiu brewing.
Figure 1. Production process of Baijiu brewing.
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Figure 2. Production mechanism of the two chiral forms of lactic acid in LAB.
Figure 2. Production mechanism of the two chiral forms of lactic acid in LAB.
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Figure 3. Average concentration of two chiral forms of lactic acid in different aromatic Baijiu samples. This figure is based on the data from Refs. [3,4].
Figure 3. Average concentration of two chiral forms of lactic acid in different aromatic Baijiu samples. This figure is based on the data from Refs. [3,4].
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Figure 4. Proportion of two chiral forms of lactic acid (L:D ratio) in different aromatic Baijiu samples. This figure is based on the data from Refs. [3,4].
Figure 4. Proportion of two chiral forms of lactic acid (L:D ratio) in different aromatic Baijiu samples. This figure is based on the data from Refs. [3,4].
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Zhou, Y.; Hua, J. Regulation and Mechanisms of L-Lactic Acid and D-Lactic Acid Production in Baijiu Brewing: Insights for Flavor Optimization and Industrial Application. Fermentation 2025, 11, 213. https://doi.org/10.3390/fermentation11040213

AMA Style

Zhou Y, Hua J. Regulation and Mechanisms of L-Lactic Acid and D-Lactic Acid Production in Baijiu Brewing: Insights for Flavor Optimization and Industrial Application. Fermentation. 2025; 11(4):213. https://doi.org/10.3390/fermentation11040213

Chicago/Turabian Style

Zhou, Yabin, and Jin Hua. 2025. "Regulation and Mechanisms of L-Lactic Acid and D-Lactic Acid Production in Baijiu Brewing: Insights for Flavor Optimization and Industrial Application" Fermentation 11, no. 4: 213. https://doi.org/10.3390/fermentation11040213

APA Style

Zhou, Y., & Hua, J. (2025). Regulation and Mechanisms of L-Lactic Acid and D-Lactic Acid Production in Baijiu Brewing: Insights for Flavor Optimization and Industrial Application. Fermentation, 11(4), 213. https://doi.org/10.3390/fermentation11040213

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