Diacetyl Production during Brewing and Its Management through Process Optimization and Molecular Evolution of Yeast

Abstract

Diacetyl is butter-tasting off-flavor compound produced as by-product of yeast valine metabolism during brewery fermentation. Yeasts produce diacetyl during primary fermentation and then reabsorb it in secondary fermentation. This causes a non-productive lengthy maturation period, which is costly. Several strategies have been proposed to manage diacetyl and improve the productivity of brewery industries. This review aimed to assess diacetyl production and proposed strategies to manage diacetyl production during brewing. Diacetyl production and its amount in the green beer are influenced by brewing condition and type of strain used. Green beer conditioning and brewing process optimization are regarded as simple and feasible approaches. However, these have their own inherent drawbacks. On the other hand, a plethora of researches declared that genetic manipulation of yeasts is an effective strategy in reducing diacetyl amount and ultimately to shorten the maturation period and thereby maximize profitability of brewery industries. But the applicability genetic engineering limited, due to firm regulation of utilization of genetically modified organisms in food processing industries. Therefore, though extensive research was done on identifying and understanding factors which influence yeast diacetyl formation and reduction, diacetyl management is persisting as a challenge in brewing systems.

1. Introduction

The art of brewing accounts for a longer period of time and seems to be a well-established process. However, the brewery industry and researchers are still striving for optimization of fermentation conditions. The development of high-performing yeasts and, in general, affirming consumers’ claims are crucial to the sustained profitability of the brewery industry. Indeed, nowadays, brewery industries are facing increasing product demand along with enhanced product quality and optimal profitability [1, 2]. One of the bottlenecks to ensuring the increased availability of beer and brewery economics is the longer beer maturation period, which is in turn associated with diacetyl formation during brewery fermentation [1].

Diacetyl is one of the vicinal diketones produced by yeast and imparts a distinct buttery/butterscotch flavor to fermented foods and beverages [3]. In fermented beverages, diacetyl content may be perceived positively or negatively depending on the product type, style, and concentration. In lager beers, diacetyl is considered a serious off-flavor compared to ale beers [3, 4]. The reported acceptable diacetyl threshold values in a final beer are around 0.1–0.2 mg/l in lager-style beers and 0.1–0.4 mg/l in ale-style beers [5], although threshold values as low as 0.017 mg/l [6] and 0.014–0.016 mg/l [7] have also been reported. The variation in flavor threshold value is due to variations in a taster’s geographical background, ethnicity, diet, and smoking practices [8].

As result, brewers always attempt various strategies to manage the diacetyl level below the threshold [2, 4]. Beer stabilization, or keeping the green beer for an extended maturation period, is the most commonly used method to reduce the diacetyl amount in the final beer. In lager beer, the green beer has to be stored for 2 to 3 weeks at a temperature close to the freezing point until the diacetyl concentration declines below its taste threshold value. However, this prolonged maturation demands cost for additional storage facilities and ultimately makes the process economically unfeasible [9].

Consequently, considering the off-flavor property of diacetyl and the prolonged maturation period as a result of it, a number of alternative strategies are being used and have been proposed. This includes selection of yeast strain, optimization of fermentation conditions such as increased fermentation temperature [10], normal wort specific gravity and pitching rate [11], wort valine content [12], decreased wort pH value [13] and optimal wort aeration and oxygen availability [14], addition of exogenous enzyme [15], passing the beer through a column of immobilized yeast [16], and genetic modification of yeast [17, 18]. However, most strategies have inherent drawbacks and lack of social acceptability in the case of genetic modification [3, 4, 19].

Though the brewing process is a highly complex series of chemical reactions, identifying, understanding, and optimizing factors that influence the formation and reabsorption of diacetyl is critical to managing the diacetyl content in the final beer. This in turn shortens the maturation time and consequently increases the productivity, profitability, and competitiveness of the brewery industry in the global market. Therefore, the purpose of this review was to perform a critical analysis of diacetyl production and its management strategies during the brewing process.

2. Production of diacetyl in brewery fermentation

It is a fact that understanding the diacetyl biosynthesis pathway, identifying the enzymes and coding genes involved, and other influencing brewing conditions play a tremendous role in reducing the diacetyl amount in final beer. Diacetyl is produced by the brewer’s yeast during fermentation. It is formed extracellularly through spontaneous nonenzymatic oxidative decarboxylation of α-acetolactate, which is an intermediate metabolite in the valine biosynthesis pathway, as indicated in Figure 1 [4].

Figure 1.

Graphic outline of diacetyl formation and reduction process in valine biosynthesis pathways of Saccharomyces spp.

The conversion of α-acetolactate to 2,3-dihydroxy-isovalerate is rate-limiting; hence, during fermentation, excessively produced α-acetolactate is secreted out through the cell’s plasma membrane into the wort, where it is spontaneously decarboxylated to diacetyl. Likely, the spontaneous decarboxylation of α-acetolactate to diacetyl is also rate-limiting. Therefore, during fermentation, the concentrations of free diacetyl in the wort are usually low, whereas α-acetolactate exists at higher concentrations. As time goes on, it decarboxylates into diacetyl [20]. Hence, the total diacetyl concentration in beer is expressed as the sum of free diacetyl and α-acetolactate (potential diacetyl) [4, 20].

Where: AHA synthase (α-acetolactate synthase), AHARI (acetohydroxyacid reductoisomerase), DHAD (dihydroxyacid dehydratase), BCAA transaminase (branched-chain amino acid transaminase), and DR (diacetyl reductase).

2.1 Green beer conditioning and brewing process optimization

2.1.1 Optimizing fermentation process

Fermentation is the central and most important step in alcoholic beverage production. The progression rate of fermentation is affected by the viability and vitality of brewery yeast strains, which in turn are affected by fermentation conditions, such as temperature [4], wort pH [21], dissolved oxygen content [22], and osmotic pressure (associated with wort specific gravity and pitching rate) [11].

Indeed, the flavor profile of the final beer is greatly affected by the fermentation conditions. Diacetyl is a normal metabolic byproduct of yeast during brewery fermentation. But its amount, rate of formation, and subsequent removal rate from the green beer (immature beer) are influenced by the above-mentioned factors [8]. For instance, brewing conditions, such as lower fermentation temperature, deviation from the normal amount (in the case of wort gravity and pitching rate), higher wort pH, and excessive wort aeration or oxygenation, have a considerable positive effect on the diacetyl content of the final beer. In contrast, increased fermentation temperature, normal wort gravity and pitching rate, lower wort pH, and optimal wort oxygenation have a recognizable effect on reducing diacetyl content in beer and then minimizing the maturation period without altering other beer parameters [4, 11, 21, 22, 23].

The steps in modern brewing procedures are almost the same as in ancient craftsmanship, except for the technological development and the implementation of modern industrial equipment [2, 4, 24, 25]. But several reports complain that the technical advancement of the brewing process did not lead to an improvement in the quality of the final product in a manner that satisfied consumers’ demands. This is due to the complexity of the brewing process, which in turn induces a lack of understanding of much of the fermentation phenomena taking place. This makes it challenging to exactly define the effect of process alterations on the processing condition and product composition [24, 25].

However, the growing global population, along with the continual increase in consumers, has resulted in an ever-increasing demand for beer products. Thus, to be competitive in the global market, it is imperative that brewers operate their production processes effectively and maximize their productivity. Thus, identification, understanding, and optimization of factors that influence the formation and reabsorption of diacetyl without affecting other beer quality parameters are required to be studied. This in turn shortens the maturation time and consequently increases the productivity, profitability, and competitiveness of the brewery industry in the global market.

2.1.2 Lowering preferred amino acids and improving wort valine content

Studies reported that wort amino acids are divided into four groups (Groups A, B, C, and D) depending on their preference and uptake rate by yeast, as indicated in Table 1. Groups A, B, C, and D [12].

Category of amino acids	Degree of absorption/preference	Amino acids
Group A	Highly preferred and absorbed	Glutamic acid, aspartic acid, asparagine, glutamine, serine, threonine, lysine, and arginine
Group B	Moderately preferred and absorbed	Valine, methionine, leucine, isoleucine, and histidine
Group C	Poorly preferred and absorbed	Glycine, phenylalanine, tyrosine, tryptophan, and alanine
Group D	No/Least preferred and absorbed	Proline

Table 1.

The classification of wort amino acids based on their uptake rate in S. cerevisiae.

Since diacetyl is an intermediate metabolite in the biosynthesis of valine, its amount produced during fermentation is dependent on the wort valine content. The level of diacetyl content is reduced when the amount of valine in the wort is increased and the amount of other preferred amino acids is lowered. The study of Krogerus and Gibson [12] showed that the increment of valine supplementation to the wort from 100 to 300 mg/L resulted in 37 and 33% diacetyl concentration reductions during primary fermentation and at the end of fermentation, respectively, without affecting the quality of the beer. Moreover, lowering the amount of highly preferred amino acids (those taken up faster than valine) will create an opportunity for increased uptake of valine because of less competition for amino acid transporter proteins (permease) interactions. This implies a reduced total diacetyl amount in green beer [12, 26, 27, 28].

On the other hand, malt type and its processing conditions (such as steeping, germination, and kilning) greatly affect the amino acid profile of wort. The study by Nie et al. [29] found that different varieties of barley have different amino acid profiles. The same study also showed that different malting conditions resulted in different wort amino acid profiles. Likewise, the study by Samaras et al. [30] revealed that high malt kilning temperatures can decrease the concentration of amino acids in the malt, whereas malts kilned at lower temperatures contained a higher concentration of both valine and the total amount of amino acids. Thus, it is possible to conclude that diacetyl can be managed by adjusting the amino acid content of wort, selecting the best barley variety, and optimizing malting conditions.

2.1.3 Green beer conditioning

2.1.3.1 Addition of exogenous enzyme to the green beer

The brewing process is based on a myriad of endogenous and exogenous enzymatic activities. The use of commercial exogenous enzymes is a common practice in modern breweries. These enzymes help breweries be competitive by increasing process efficiency, enhancing product quality, extending product ranges, and resolving intermittent process problems, such as incomplete fermentation, turbidity/haze, and increased diacetyl content. Endogenous enzymes are needed to convert starch into sugars, complex proteins into simple amino acids, and breakdown and solubilize plant cell wall materials for yeast nutrition. In addition, brewery industries add exogenous enzymes such as α-acetolactate decarboxylase and β-1,4-glucanase to hasten the fermentation process and avoid incomplete fermentation, respectively [15, 31].

Αlpha-Acetolactate decarboxylase (ALDC) is an enzyme that directly converts α-acetolactate to acetoin by bypassing diacetyl formation. This avoids the slow oxidative decarboxylation of α-acetolactate into diacetyl and the subsequent yeast diacetyl reabsorption stage and/or maturation period [15]. However, ALDC could not be produced by the pitching yeast. But this enzyme is produced by different species of bacteria, such as Bacillus subtilis, Klebsiella ternigena, Enterobacter aerogenes, Lactobacillus casei, Bacillus brevis, and Streptococcus diacetylactis. Hence, safe production of these enzymes from the aforementioned producer organisms and the addition of ALDC to the fermentation accelerate the maturation of beer, resulting in quick and improved vessel utilization and thus increasing the brewing capacity of brewery industries [32, 33, 34].

The study of Choi et al. [15] aimed to assess the efficacy of ALDC in reducing diacetyl content in two different types of beer (Jinyang and Dahyang) and showed a remarkable diacetyl reduction. The diacetyl content of Jinyang decreased from 5.17 to 1.31 ppm, while the content of Dahyang decreased from 9.75 to 1.54 ppm. These values showed that the diacetyl content was reduced by 25% in Jinyang and 15% in Dahyang compared to the controls.

2.1.3.2 Treatment of green beer with immobilized yeast cell

Nowadays, a variety of immobilized cell technologies are available and have been used in bioprocess industries such as the brewery industry. As compared with traditional fermentation with freely suspended cells and operated in batch mode, immobilized cell systems offer many advantages, such as a faster fermentation rate, increased volumetric productivity, and the possibility of continuous operation. Hence, immobilized cell technology has been attracting the attention of fermentation industries and is used for different stages in the beer fermentation process [16].

Immobilized cell technology is being used in secondary fermentation to remove diacetyl and then shorten the maturation period [16, 35]. In this process, immobilized yeast cells absorb diacetyl from the green beer and are then converted into acetoin, which in turn is converted into 2,3-butanediol. Diacetyl removal from green beer involves rapid heating of the beer after the primary fermentation to facilitate the conversion of α-acetolactate into diacetyl. Then the heat-treated green beer will be passed through a column of immobilized yeast, in which the diacetyl reduction will take place (Figure 2) [36].

Figure 2.

Schematic presentation suggested optimization in the brewing process for lowering diacetyl production.

2.2 Molecular evolution of yeasts

It is worth mentioning that the brewer’s yeasts played a major role in diacetyl production and removal from green beer. The molecular evolution of yeast could be the right and effective diacetyl management strategy. However, blind accusations and frustration with genetically modified yeast are common phenomena in breweries. The applicability of genetically modified brewery yeasts is still not allowed and is continuing to be under strict control due to illogical perceptions and ethical and legislative barriers. Moreover, brewers are hesitant to use engineered yeast because they suspect that it might alter the branding characteristics of their beer. Hence, though genetically modified microbes are used in medical aspects and foods originated from genetically modified crops are consumed, the use of genetically modified brewer’s yeast in the brewery industry is lagging behind [17].

Genetic engineering is a recent, faster, more accurate, and very effective technique, and considerable outcomes are obtained in the improvement of microbes, crops, and livestock [17, 18, 37]. Brewer’s yeast strains that show improved ethanol tolerance, fermentation speed, and attenuation produce very specific flavors, less diacetyl, ferment starch, and/or dextrin are developed through genetic engineering. Therefore, breweries are starting to appreciate the applications of genetically modified yeasts to increase the productivity of breweries and afford the increasing consumer demand [17, 18, 37].

Moreover, different reports revealed that genetically modified brewer’s yeast strains produce beer with less undesirable flavor compounds, such as diacetyl and 2,3-pentanedione, which increase the productivity of breweries and satisfy consumer demand. For instance, a yeast strain that can degrade some off-flavor molecules was patented in the USA [38]. Genetic modification approaches involving manipulation of the genes associated with diacetyl production and reduction are stated as effective, though they have limited applicability [1, 18, 37].

2.2.1 Altering genes involved in diacetyl synthesis pathway

Reports showed that manipulating genes involved in the diacetyl production pathway can effectively manage diacetyl content to the threshold limit. S. cerevisiae has four genes (Sc-ILV2, ILV3, ILV5, ILV6, and BDH2) that are associated with diacetyl production. Sc-ILV2 and ILV6 encode AHA synthase and its regulatory subunit. Whereas ILV3, ILV5, and BDH2 code for dihydroxyacid dehydratase, acetohydroxyacid reductoisomerase, and diacetyl reductase, respectively, as indicated in Figure 1. Extensive research conducted so far has declared that Sc-ILV2 and ILV6 are directly associated with diacetyl production. But ILV3, ILV5, and BDH2 are inversely related to diacetyl production and diacetyl content in the final beer. Previous studies suggested that reduction in the gene copy numbers of Sc-ILV2 and ILV6 and overexpression of ILV3, ILV5, and BDH2 play a tremendous role in minimizing diacetyl production and then managing its amount below the threshold level in the final beer [18, 37, 39, 40].

2.2.2 Deletion of Sc-ILV2 and ILV6 genes

The results of different studies claimed that Sc-ILV2 and ILV6 gene copy numbers and expression levels are positively associated with diacetyl content in the final beer. Multiple copy numbers and overexpression of these genes lead to higher diacetyl production during beer production. The Sc-ILV2 gene encodes the enzyme AHA synthase, which facilitates the conversion of pyruvate to α-acetolactate and leads to a higher diacetyl content in the final beer. The increased expression of this gene results in increased AHA synthase production and activity, which in turn increase the production of α-acetolactate. The increased production of α-acetolactate results in an increased diacetyl level in the final beer [37, 39].

Reports showed that deletion of one more of this gene reduced diacetyl production and then shortened the nonproductive, lengthy maturation period [39, 41]. The study of Zhang et al. [39] showed that disruption of one allele of the Sc-ILV2 gene decreased AHAS activity by 59 and 41% in T1 and Q9 recombinant yeasts, respectively, as compared with their normal parent. Moreover, the acetoin amount at the end of fermentation is decreased by 35 and 40% in T1 and Q9 than that of their parent, as a result of limited diacetyl production (precursor metabolite of acetoin) reduction [39]. Furthermore, the study by Gibson et al. [42] indicated that cells treated with a sublethal amount of chlorsulfuron (specifically, that inhibits AHA synthase production) produce beer with a 60% lower diacetyl content as compared with nontreated cells of the same species.

The other gene that is responsible for diacetyl production is Sc-ILV6, which encodes a subunit of the AHAS multisubunit enzyme. The study of Duong et al. [41] found that lowered expression of Sc-ILV6 during fermentation in a lager yeast strain produced beer with a low diacetyl level as compared with other yeast strains, in which Sc-ILV6 was overexpressed. The same study also observed lower diacetyl production by a lager yeast strain with a disrupted Sc-ILV6 gene compared with the unmodified strain, suggesting that the Sc-ILV6 encodes an AHA synthase enzyme subunit. In this study, a Sc-ILV6 double deletion mutant brewery yeast resulted in a 65% reduction in the final diacetyl concentration [41].

2.2.3 Overexpression of Sc-ILV5, ILV-3, and BDH2 genes

Contrary to the Sc-ILV2 and ILV6 genes, the higher gene copy number and expression level of Sc-ILV5, ILV3, and BDH2 are inversely related to diacetyl production and total diacetyl concentration at the end of fermentation. ILV3 and ILV5 genes encode for dihydroxyacid dehydratase and acetohydroxyacid reductoisomerase, which catalyze the second and third reactions in the valine biosynthesis pathway, respectively. Whereas BDH2 encodes diacetyl reductase that catalyzes the reduction of diacetyl into acetoin within the yeast cell after it is absorbed [18, 37, 40].

Overexpression of these genes increases the activity of the respective encoded enzymes. The increased activity of dihydroxyacid dehydratase and acetohydroxyacid reductoisomerase leads to an increased anabolic flux of α-acetoacetate to the intermediate metabolites of valine. This situation consequently decreases the amount of α-acetolactate leaking out of the yeast cells during fermentation. On the other hand, the greater the activity of diacetyl reductase, the faster the conversion of leaked diacetyl to acetoin, a compound with a high threshold value, ultimately resulting in a low total diacetyl concentration at the end of fermentation [18, 37, 40].

The study of Qin and Park [37] reported that recombinant brewery yeast strains that have taken up the ILV5 gene showed 3.7 times higher diacetyl reduction as compared with the not-transformed parent strain. The utilization of those transformed yeast strains for wine fermentation using Campbell Early and Muscat Baily A grapes brought approximately 35 to 39% diacetyl content reduction as compared with nontransformed control yeasts. Moreover, the same study also assessed the activity of cell-free acetohydroxyacid reductoisomerase. Acetohydroxyacid reductoisomerase extracted from it showed approximately 4 to 5 times higher activity than the control yeasts.

On the other hand, studies that involved deletion of Sc-ILV2 and overexpression of Sc-ILV5 and BDH2 played a remarkable role in diacetyl reduction. Three strains, WY1, WY1-2, and WY1-12, transformed with BDH1, BDH2, and both BDH1 and BDH2, produced beer with diacetyl concentrations that decreased by 39.81, 33.42, and 46.71%, respectively, compared with the control strain [43]. The study of Kusunoki and Ogata [40], involving construction of self-cloning lager yeast via insertion of Sc-ILV5 DNA upstream of ILV2 in bottom-fermenting yeast to increase the copy number of Sc-ILV5, achieved an approximately 60% reduction in the diacetyl amount at the end of fermentation and shortened the maturation period during which diacetyl was reduced to within the threshold value, with no impact on beer quality. Similarly, the report of [18] claimed a 55.65% diacetyl reduction in the recombinant strain, where BDH2 was an overexpressed gene and one Sc-ILV2 allelic gene was deleted. Moreover, a strain in which the Sc-ILV5 gene was overexpressed and one Sc-ILV2 allelic gene knockout showed a 69.13% diacetyl reduction.

There are also studies that indicate the role of co-expression of those genes in reducing the diacetyl amount to the threshold level at the end of fermentation. The study of [44] reported that the co-expression of Sc-ILV3 and Sc-ILV5 in brewer’s yeast resulted in a 60% diacetyl reduction in beer. The same study revealed that overexpression of ILV3 and ILV5 in the S1 yeast strain resulted in a 40 and 70% diacetyl reduction, respectively, when compared to that of the normal parent strain.

2.2.4 Introduction of α-acetolactate decarboxylase enzyme gene to brewer’s yeast

Alpha-acetolactate decarboxylase (ALDC) is an enzyme that catalyzes the nonoxidative decarboxylation of α-acetolactate into acetoin directly without the formation of diacetyl [15]. However, the ALDC gene is not found in brewery yeast strains, and they did not produce ALDC. In contrast, ALDC is produced by different species of bacteria, such as Bacillus subtilis, Klebsiella ternigena, Enterobacter aerogenes, Lactobacillus casei, Bacillus brevis, and Streptococcus diacetylactis. The results of different studies confirmed that the integration of ALDC genes originated from different bacterial species into brewery yeasts plays a crucial role in the reduction of total diacetyl content [33, 34].

The study by Blomqvist et al. [32] on the integration of Klebsiella ternigena and Enterobacter aerogenes genes encoding α-acetolactate decarboxylase into brewery yeasts revealed that the diacetyl amount was reduced even below the threshold value at the end of primary fermentation. Similarly, Saccharomyces cerevisiae with a truncated active ALDC from Acetobacter aceti ssp. xylinum attached to the cell wall using the C-terminal anchoring domain of α-agglutinin decreased the diacetyl concentration by 30% as compared with control yeasts displaying only the anchoring domain [45].

2.2.5 Overexpression of BAP2 and BAP3 genes

It is mentioned that the amount of diacetyl produced in the green beer is influenced by the wort amino acid composition and yeast uptake rate [12]. Apart from this, the uptake rate of amino acids via the yeast cell is determined by the expression level of amino acid transporter protein (permease) encoding genes [4, 12, 46]. In yeast, various amino acid permeases are involved in the transport of amino acids across the plasma membrane with different affinities, specificities, capacities, and regulations. Most of the branched-chain amino acids are transported by Bap2p, Bap3p, and Tat1p. These amino acid permease membrane proteins indirectly play a critical role in the flavor profile of the final beer. For instance, Bap2p and Bap3p are amino acid permeases involved in the uptake of leucine, isoleucine, and valine (branched-chain amino acids) [12, 46]. The overexpression of these amino acid permease genes increases the uptake level of valine and isoleucine. When cells have sufficient valine uptake, diacetyl levels are reduced due to feedback inhibition in the valine biosynthesis pathway. Similarly, the increased uptake rate of isoleucine resulted in a remarkable 2,3-pentanedione reduction [12, 46]. This implies that inducing the transcription of those genes increases the uptake of branched-chain amino acids such as valine, which then reduces the diacetyl amount through feedback inhibition (Figure 3).

Figure 3.

Schematic presentation of expression of genes involved in valine biosynthesis in brewer’s yeast targeting lowering diacetyl amount in the final beer.

The study of Kodama [27] found that the constitutive expression of BAP2 in a brewing yeast strain accelerated the assimilation rate of branched-chain amino acids, while the disruption of BAP2 did not affect assimilation rates for these amino acids during the brewing process. These could be Bap3p, Tat1p, and/or other branched-chain amino acid permease homologs, which exist in lager brewing yeasts. The transcription of the branched-chain amino acid permease genes, particularly BAP2 and BAP3, is induced by some amino acids, such as leucine and phenylalanine [47], in the medium, and this induction requires Ssy1p as a sensor for external amino acids [48].

3. Conclusion

Brewing is the oldest and most well-established process, and beer is the most economical and widely produced beverage all over the world. Nowadays, as a result of the increasing global population and the advancement of human life, the brewery industry is facing increasing product demand and consumer’s quality claims, along with sustaining their profitability. One of the bottlenecks to ensuring increased availability and avoiding quality claims is associated with a longer maturation period. This in turn allied with diacetyl production, a buttery-tasting off-flavor compound produced by yeasts during brewery fermentation as byproduct of valine metabolism.

Though yeast diacetyl reduction is not well understood, the diacetyl amount in green beer is affected by the brewing conditions and genetic constituents of the yeasts used. As a result, green beer conditioning, brewing process optimization, and genetic modification of yeasts have been used and proposed to manage the diacetyl amount below the detectable or threshold amount. However, these approaches have an inherent problem that affects other beer quality parameters and increases the production cost.

Whereas genetic modification of yeast strains, particularly manipulation of genes responsible for diacetyl production and removal, is believed to be an efficient and economically feasible approach. But it has been inapplicable due to the stringent regulation of utilizing genetically modified organisms in the food processing industry. Considerable research has been conducted and huge publications are available, but diacetyl management is still a logjam to brewery productivity. Therefore, there must be an amalgamated pressure against the illogical frustration of breweries and societies with new and/or engineered yeasts.

Acknowledgments

The current study has not received external funding.

Conflict of interest

The authors declare that they have no any competing interests.

Compliance with ethical standards

Ethical statement

This manuscript is in compliance with Ethical Standards. Since it does not involve any human or animals participant.