Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This review is interesting. Apparently, the chinese baijiu is a kind of most important alcoholic beverage in china. The lactic acid is the accompaniment of baijiu fermentation, and it directly affect the flavor and quality of baijiu. This paper reviewed the current research updates about lactic acid during the production of baijiu, so this paper provide a new information about lactic acid in baijiu. This paper have flaws that need improvement. The detailes are listed below.

1. The serial number is repeat. line 46, 116 is repeated, line 65, 82, 101, should be 2.1, 2.2, 2.3, please change the serial number here after.

2. figures and tables is the key elements of a review, this paper only have one figue, I suggest the author to summarize the content to figures and tables, that will greatly improve the expression of this review.

3. the lactic acid affect the flavor and sensation of baijiu , as the line 117.  how to control the lactic acid in proper level in baijiu? decline and increase?

Author Response

Comments 1: The serial number is repeat. line 46, 116 is repeated, line 65, 82, 101, should be 2.1, 2.2, 2.3, please change the serial number here after.

Response 1: Thank you for pointing this out. We have made the correction.

Comments 2: Figures and tables is the key elements of a review, this paper only have one figure, I suggest the author to summarize the content to figures and tables, that will greatly improve the expression of this review.

Response 2: Thank you for pointing this out. This is a similar suggestion to comment 1 reviewer 2, and we have added a flow diagram to illustrate the production process of Baijiu (refer to figure 2). We have also included a table summarising the causes of abnormally high or low lactic acid levels during Baijiu brewing (refer to table 1).

Comments 3: The lactic acid affect the flavor and sensation of Baijiu, as the line 117. How to control the lactic acid in proper level of baijiu? Decline and increase?

Response 3: There is not a universal standard or level of lactic acid in Baijiu. Instead, each type of Baijiu has different level of lactic acid that contribute to their distinct flavor. So, each brand and type of Baijiu will control the amount of lactic acid present to their own standard (refer to lines 141-5). The different methods of controlling the level of lactic acid in Baijiu have been discussed in detail in section 4, with the key points summarised in sub-section 4.6.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript “Current Updates on Lactic Acid Production and Control During Baijiu Brewing” presents an important review proposal on the interference of lactic acid in the traditional Chinese fermented alcoholic drink Baijiu. Below are my considerations:

- I suggest that the authors mention the production process of the drink in a diagram, illustrating Distiller's Daqu.

- Line 240: “found that eight dominant lactic acid bacteria” mention which are the main species involved.

- Line 308: “primarily producing alcohols, acids, and esters”. Mention the main volatile compounds present in the drink and the identification techniques used. This item was missing from the review.

-Line 337: “Future studies could delve deeper into controlling fermentation time to regulate”. What is the average fermentation time of the drink? What would be the negative sensory impacts of excessively reducing fermentation time to reduce lactic acid production?

- Line 346: Replace the term microbes.

-Line 422-424: How the introduction of environmental contaminants and the prevalence of starter bacteria are controlled in practice.

- Line 471: How can biotechnology, genetic engineering, and metabolic engineering be applied efficiently in the production of Baijiu?

- What would you say are the main difficulties faced by small and medium-sized distilleries in implementing strict control methods for the lactic acid content, compared to large distilleries?

- Lactic acid stabilizes the main aromatic components of Baijiu. What are the safe limits so that excess lactic acid doesn't cause unwanted changes in taste?

- Describe premium baijius and how the lactic acid content in the distillate fractions is controlled. If lactic acid tends to concentrate in the final stages of distillation, are there ways to optimize the collection of distillation fractions to maintain the balance between acidity and other desirable compounds in Baijiu?

 

Author Response

Comment 1: I suggest that the authors mention the production process of the drink in a diagram, illustrating Distiller's Daqu.

Response 1: Thank you for the suggestion. we have added a flow diagram illustrating the baijiu production process (refer to figure 2).

Comment 2:  Line 240: “found that eight dominant lactic acid bacteria” mention which are the main species involved.

Response 2: We have modified the manuscript to include the requested information (see lines 254-256):Lactobacillus brevis, Lactobacillus plantarum, Lactobacillus curvatus, Lactobacillus pentosus, Lactococcus lactis, Leuconostoc citreum, Weissella confusa, Weissella cibaria.

Comment 3:  Line 308: “primarily producing alcohols, acids, and esters”. Mention the main volatile compounds present in the drink and the identification techniques used. This item was missing from the review.

Response 3: It is referring to the metabolic by-products of lactic acid-degrading bacteria which can contribute to the final flavour profile of the baijiu but are not the main determinants.

The aromatic and flavor compounds in baijiu number in the thousands, and they collectively constitute the flavor of Baijiu. However, in the industry, the detection of these flavor compounds in Baijiu is mainly carried out in the final product rather than during the fermentation process.

Although it will be an interesting exploration, a discussion of the volatiles and their identification techniques fall outside the scope of the current review, which is focussed on lactic acid production and the control of it during Baijiu brewing. We look forward to investigating this subject in a future review.

Comment 4:  Line 337: “Future studies could delve deeper into controlling fermentation time to regulate”. What is the average fermentation time of the drink? What would be the negative sensory impacts of excessively reducing fermentation time to reduce lactic acid production?

Response 4: It would not be possible to give an average fermentation time for baijiu fermentation as it can vary from days to months, depending on the type of baijiu being produced. For example, light-flavor baijiu fermentation time is 14 days (Tang et al. 2022), while strong aroma and sauce aroma baijiu require several months (Wang et al. 2022). We have modified the text to include a statement indicating this high variability in fermentation times depending on the type of baijiu (refer to line 351-353).

A reduction in lactic acid due to an excessively shortened fermentation time will obviously have a negative impact the final flavour profile of the Baijiu. However, the effects are highly dependent on the type of Baijiu the production processes specific to the Baijiu distillery.

Comment 5:   Line 346: Replace the term microbes.

Response 5: We have replaced the term microbes with microorganisms.

Comment 6:  Line 422-424: How the introduction of environmental contaminants and the prevalence of starter bacteria are controlled in practice.

Response 6: The effects of environmental contaminants and their control in practice are discussed in detail in section 4.5

Comment 7:  Line 471: How can biotechnology, genetic engineering, and metabolic engineering be applied efficiently in the production of Baijiu?

Response 7:  These technologies can be applied in the optimization of the production of desired metabolic products, which in the specific case of our present review, is lactic acid. This can be achieved by for example, investigating the specific metabolic pathways involved in the production of lactic acid in the bacterial species involved in Baijiu production, optimizing the necessary metabolic substrates, and by genetic manipulation to enhance or modify these metabolic processes to produced desired changes. A detailed discussion of these will fall outside of the scope of the current review. We look forward to investigating this subject in a future review.

Comment 8:   What would you say are the main difficulties faced by small and medium-sized distilleries in implementing strict control methods for the lactic acid content, compared to large distilleries?

Response 8:  we have briefly alluded to this point in section 4, lines 245-249, 273-277, 302-309,334-340,355-360, and 384-388. But in general, the difficulties face by smaller distilleries are related to limitation of their resources, which restrict their ability to access more advanced control techniques.

Comment 9:   Lactic acid stabilizes the main aromatic components of Baijiu. What are the safe limits so that excess lactic acid doesn't cause unwanted changes in taste?

Response 9:  There is no universal "safe limit" for lactic acid. As mentioned in our response to comment 4 and to comment 10, lactic acid levels vary greatly across different types and brands of Baijiu. Suitable levels of lactic acid that will produced the desired flavour profile are determined independently by each Baijiu producer.

Comment 10:   Describe premium baijius and how the lactic acid content in the distillate fractions is controlled. If lactic acid tends to concentrate in the final stages of distillation, are there ways to optimize the collection of distillation fractions to maintain the balance between acidity and other desirable compounds in Baijiu?

Response 10:  There are no strict definitions of what makes a premium baijiu. Premium baijiu can have greatly varying levels of lactic acid depending on their brand. For example, Langjiu and Wuliangye are considered to be two of the premium Baijius. However lactic acid of Langjiu is around 3 times of that of Wuliangye. Baijiu producers achieve their distinct combination of flavours and acidity by blending early and late distillate fractions in differing proportions.