Wort Aeration Research Articles

Influence of the yeast aeration method on the quality characteristics of beer

Aeration of wort in brewing, as a traditional way to reduce the need for yeast oxygen, has several disadvantages associated with a change in the quality of the finished beverage. Another approach to solving this problem is the pre-fermentation preparation of brewer’s yeast, which consists in short-term aeration (30 min) of the inoculum followed by exposure without air for 2 hours. The effect of various methods of supplying the yeast culture with atmospheric oxygen on the formation of flavoring substances and change in physicochemical characteristics of beer. Objects of research are bottom fermentation yeast Saccharomyces cerevisiae, young and finished beer. It was revealed that in young beer fermented with yeast after aeration, the content of sensory significant components (higher alcohols, acetaldehyde, diacetyl) is on average 27% lower than in the variant with wort aeration. A possible reason for this is the high activity in the prepared inoculum in comparison with the initial culture of enzymes (1.7-1.8 times increased) involved in the conversion of by-products. Stimulation of the metabolic processes of yeast with oxygen, especially in the sample with aeration of the wort, creates the conditions for reducing (from 7 to 23%) the main mutating components. However, pre-aeration of the inoculum allows you to get a ready drink with the best taste and aromatic characteristics, which is the advantage of this method in comparison with aeration of the wort.

Elucidation of the Formation Mechanism of 2-Mercapto-3-Methyl-1-Butanol in Beer. Part I: Identification of the Key Factors during Fermentation

2-Mercapto-3-methyl-1-butanol (2M3MB) imparts an onion-like off-flavor to beer and is believed to result from the hot aeration of brewer's wort. However, little has been reported regarding the mechanism of formation of 2M3MB. In this study, the origin of a 2M3MB precursor (2,3-epoxy-3-methylbutanal, EMB) and the mechanism of formation of 2M3MB from EMB were investigated. EMB was generated by oxidation of the side chain of iso-alpha acids from the hops. It was revealed that EMB reacts with hydrogen sulfide to give 2-mercapto-3-hydroxy-3-methylbutanal, which is then converted into 2M3MB by yeast. When the concentrations of these compounds were measured during beer fermentation using gas chromatography/mass spectrometry, it became clear that only a portion of EMB was converted into 2M3MB, since the majority of EMB was reduced to 2,3-epoxy-3-methylbutanol without conversion to 2M3MB.

Elucidation of the Formation Mechanism of 2-Mercapto-3-Methyl-1-Butanol in Beer. Part II: Identification of the Key Enzymes in Yeast

2-Mercapto-3-methyl-1-butanol (2M3MB) imparts an onion-like off-flavor to beer and is known to be formed from 2,3-epoxy-3-methylbutanal (EMB), which is generated in the reaction of iso-alpha acids with oxygen during the hot aeration of brewer's wort. Our previous results revealed that 2-mercapto-3-hydroxy-3-methylbutanal (2M3H3MB), formed from EMB by a nonenzymatic reaction with hydrogen sulfide, serves as the intermediate to 2M3MB. However, little is known regarding the 2M3H3MB to 2M3MB conversion process. In the present study, preparative column chromatography coupled with an enzyme assay revealed that two enzymes of Saccharomyces pastorianus, Old Yellow Enzyme 2 (Oye2p) and alcohol dehydrogenase 6 (Adh6p), are involved in the conversion process. An in vitro reaction containing these enzymes purified from recombinant Escherichia coli cells converted 2M3H3MB to 2M3MB. Furthermore, a yeast mutant with deletion of the genes encoding these two enzymes dramatically decreased 2M3MB formation. Our series of studies has therefore revealed the overall mechanism of 2M3MB formation starting from EMB through 2M3MB.

Control of Fermentation By-Products and Aroma Features of Beer Produced with Scottish Ale Yeast by Variation of Fermentation Temperature and Wort Aeration Rate

ABSTRACTFermentation and pitching conditions provide numerous opportunities for the control of the beer's final sensory characteristics. The fermentation temperature and wort aeration rate relate significantly to the formation of many flavor-relevant substances, in particular esters, acids, and higher alcohols. A commercially available ale yeast was selected for fermentation trials investigating fermentation temperature and dissolved oxygen effects. Both parameters were varied in accordance with a central composite design. All obtained data were used for the development of a predictive model for the by-product control in dependence of pitching and fermentation conditions, using regression analysis, principal component analysis (PCA), and hierarchical cluster analysis (HCA). It was shown that the selection of certain technological parameters, in particular fermentation temperatures, enhanced specific flavor characteristics such as fruitiness and acidity. Higher fermentation temperatures promoted the formation of many aroma relevant substances, which were clustered in relation to technological parameters and the final flavor perception. Samples fermented at 20°C achieved the highest sensory preference. In addition, specific substances (e.g., acetaldehyde and alcohols) in the final beer correlated with integrals of the cell density curves in dependence of the fermentation time. Fermentation phases in which substances varied corresponding to yeast cell state were detected.

Search for compounds contributing to onion-like off-flavor in beer and investigation of the cause of the flavor

Onion-like off-flavor is a highly undesirable property in beer. Although several compounds that impart onion-like odors have been identified, the individual contribution of these compounds to the onion-like off-flavor in beer is not clear. In the present study, we searched for compounds that impart an onion-like odor by gas chromatography (GC)-olfactometry. The analysis of several types of beer revealed that 2-mercapto-3-methyl-1-butanol (2M3MB) and 3-mercapto-3-methyl-1-butanol (3M3MB) were possible causative compounds. Based on the difference threshold values in beer (0.13ng/mL for 2M3MB and 17.5ng/mL for 3M3MB) and the quantification values of these compounds in beer samples, only 2M3MB was considered to contribute to the onion-like off-flavor in beer. A further formation factor analysis of 2M3MB revealed that 2M3MB was formed in hopped wort after fermentation, and that the concentration of 2M3MB increased following the hot aeration treatment of wort. These results suggest that preventing the hot aeration of wort is a key factor for reducing 2M3MB levels in beer. In a previous report, 3-methyl-2-buten-1-ol (3MBol) was speculated to be the precursor of 2M3MB and 3M3MB; however, the results of the present quantification analysis and wort addition tests indicate that 3MBol did not contribute to the formation of 2M3MB in the brewing process and that unknown precursors of 2M3MB originated in wort. Identifying the precursor of 2M3MB may facilitate elucidation of the mechanism of 2M3MB formation.

The effect of wort aeration on fermentation, maturation and volatile components of beer produced on an industrial scale

The aim of the study was to determine the effect of the initial beer wort aeration on the process of fermentation, maturation, content of the volatile components of beer and abundance and vitality of yeast biomass. The experiments were performed on an industrial scale, with fermentation and maturation performed in fermentation tanks with a capacity of 3800 hL. The wort was aerated with sterile air in quantities as follows: 7, 10 and 12 mg/L. During fermentation and maturation, the changes in the content of the extract, yeast growth and vitality and more importantly volatile components were investigated. The experiments showed that differentiated aeration has a significant impact on the course of fermentation and metabolic changes. With the increase in wort aeration, the content of acetaldehyde decreased and the concentration of higher alcohols increased. On the other hand, the contents of esters and vicinal diketones did not change. The level of aeration did not affect the final quality of beer. Copyright © 2017 The Institute of Brewing & Distilling

Strategies for Reducing Succinic Acid Concentrations in Beer

Recent publications highlight the effect of succinic acid from alcoholic fermentation products on stomach acidity, which affects drinkability. To date, scientific information on succinic acid production during the brewing process is scarce. In order to make advances on this problem, a screening of technological fermentation parameters was carried out and the data compared with analyses of samples from several beers on the market. Based on the information in the literature, we tested process parameters such as temperature, substrate composition (available nitrogen), original gravity, CO2 concentration, pH value, pitching rate, wort aeration, and yeast propagation for factors that influence succinic acid production during the brewing process. It was found that the selection of yeast strain and pitching rate have a strong effect. Additionally, the nitrogen source of the substrate and oxygen availability during fermentation show an impact. In a last step, we compared the resulting succinic acid concentrations to those of samples taken from a variety of commercially available beers. The research results indicate that it is possible to produce a beer with a reduced level of succinic acid.

Analysis of aging in lager brewing yeast during serial repitching

Serial repitching of brewing yeast inoculates is an important economic factor in the brewing industry, as their propagation is time and resource intensive. Here, we investigated whether replicative aging and/or the population distribution status changed during serial repitching in three different breweries with the same brewing yeast strain but different abiotic backgrounds and repitching regimes with varying numbers of reuses.Next to bud scar numbers the DNA content of the Saccharomyces pastorianus HEBRU cells was analyzed. Gene expression patterns were investigated using low-density microarrays with genes for aging, stress, storage compound metabolism and cell cycle.Two breweries showed a stable rejuvenation rate during serial repitching. In a third brewery the fraction of virgin cells varied, which could be explained with differing wort aeration rates. Furthermore, the number of bud scars per cell and cell size correlated in all 3 breweries throughout all runs. Transcriptome analyses revealed that from the 6th run on, mainly for the cells positive gene expression could be seen, for example up-regulation of trehalose and glycogen metabolism genes. Additionally, the cells’ settling in the cone was dependent on cell size, with the lowest and the uppermost cone layers showing the highest amount of dead cells.In general, cells do not progressively age during extended serial repitching.

Stability of high cell density brewery fermentations during serial repitching

The volumetric productivity of the beer fermentation process can be increased by using a higher pitching rate (i.e. higher inoculum size). However, the decreased yeast net growth observed in these high cell density brewery fermentations can adversely affect the physiological stability throughout subsequent yeast generations. Therefore, different O(2) conditions (wort aeration and yeast preoxygenation) were applied to high cell density fermentation and eight generations of fermentations were evaluated together with conventional fermentations. Freshly propagated high cell density populations adapted faster to the fermentative conditions than normal cell density populations. Preoxygenating the yeast was essential for the yeast physiological and beer flavor compound stability of high cell density fermentations during serial repitching. In contrast, the use of non-preoxygenated yeast resulted in inadequate growth which caused (1) insufficient yield of biomass to repitch all eight generations, (2) a 10% decrease in viability, (3) a moderate increase of yeast age, (4) and a dramatic increase of the unwanted flavor compounds acetaldehyde and total diacetyl during the sequence of fermentations. Therefore, to achieve sustainable high cell density fermentations throughout the economical valuable process of serial repitching, adequate yeast growth is essential.

The role of oxygen in yeast metabolism during high cell density brewery fermentations

The volumetric productivity of the beer fermentation process can be increased by using a higher pitching rate (i.e., higher inoculum size). However, the decreased yeast net growth observed in these high cell density fermentations can have a negative impact on the physiological stability throughout subsequent yeast generations. The use of different oxygen conditions (wort aeration, wort oxygenation, yeast preoxygenation) was investigated to improve the growth yield during high cell density fermentations and yeast metabolic and physiological parameters were assessed systematically. Together with a higher extent of growth (dependent on the applied oxygen conditions), the fermentation power and the formation of unsaturated fatty acids were also affected. Wort oxygenation had a significant decreasing effect on the formation of esters, which was caused by a decreased expression of the alcohol acetyl transferase gene ATF1, compared with the other conditions. Lower glycogen and trehalose levels at the end of fermentation were observed in case of the high cell density fermentations with oxygenated wort and the reference fermentation. The expression levels of BAP2 (encoding the branched chain amino acid permease), ERG1 (encoding squalene epoxidase), and the stress responsive gene HSP12 were predominantly influenced by the high cell concentrations, while OLE1 (encoding the fatty acid desaturase) and the oxidative stress responsive genes SOD1 and CTT1 were mainly affected by the oxygen availability per cell. These results demonstrate that optimisation of high cell density fermentations could be achieved by improving the oxygen conditions, without drastically affecting the physiological condition of the yeast and beer quality.

The influence of wort aeration and yeast preoxygenation on beer staling processes

The influence of oxygen on the beer staling process is considered to be of major importance. Therefore, the impact of wort aeration, which is thought to cause wort oxidation processes, on beer ageing, has been examined. Pilsner and ale beers were produced with the classical wort aeration technique or by the use of the yeast preoxygenation process, in which yeast cells are exposed to oxygen before fermentation. The staling of these beers was studied using P&T GC–MS and sensory analysis. GC–MS analyses of the natural and forced aged beers showed no significant differences between the two treatments. Sensory evaluation of natural and forced beers confirmed these results. Thus, normal wort aeration (8 mg/l) does not appear to determine flavour stability in a direct manner. This is probably due to the short contact of the wort with oxygen at low temperatures before the onset of fermentation.

Flavour Control in Small-Scale Beer Fermentations

Fermentation productivity can be improved by the use of high temperatures, high yeast pitching rates and high wort specific gravities. However these approaches can lead to altered beer flavour and detrimental effects on yeast viability and vitality. Our investigations have sought to help us understand how process variables affect beer flavour profiles with the aim of developing intensified processes which produce beers with normal flavour characteristics. In order to assess the interactive effects of process variables on beer flavour production, statistically designed fermentation trials were performed using a range of process conditions. Empirical correlations were derived for predicting the average yields of some flavour volatiles and for predicting fermentation time. Higher temperatures and higher yeast pitching rates yielded faster fermentations. Ester yield was reduced under conditions of high wort aeration, lower temperature, lower wort gravity and higher pitching rates. The yield of higher alcohol was generally reduced under conditions of lower temperature, lower initial aeration levels and higher gravity. The effect of pitching rate on the yield of higher alcohol was strongly dependent on the wort gravity used. Comparison of the predicted flavour volatile yield and process time ‘maps’ indicate operating zones where process intensification occurs, without changes to flavour volatile concentration.

Flavor-Active Esters: Adding Fruitiness to Beer

As they are responsible for the fruity character of fermented beverages, volatile esters constitute an important group of aromatic compounds in beer. In modern high-gravity fermentations, which are performed in tall cylindroconical vessels, the beer ester balance is often sub-optimal, resulting in a clear decrease in beer quality. Despite the intensive research aimed at unravelling the precise mechanism and regulation of ester synthesis, our current knowledge remains far from complete. However, a number of factors that influence flavor-active ester production have already been described, including wort composition, wort aeration and fermentor design. A thoughtful adaptation of these parameters allows brewers to steer ester concentrations and thus to control the fruity character of their beers. This paper reviews the current knowledge of the biochemistry behind yeast ester synthesis and discusses the different factors that allow ester formation to be controlled during brewery fermentation.

Linoleic Acid Supplementation of a Cropped Brewing Lager Strain: Effects on Subsequent Fermentation Performance with Serial Repitching

Most breweries collect yeast from a previous fermentation cycle for further use in a subsequent cycle. However, the cropped cells are deficient in membrane sterols and unsaturated fatty acids (UFAs) which are required for good fermentation performance in the next cycle. Consequently, the cellular levels of these compounds must be restored to obtain an optimal fermentation performance. There are currently three possibilities to satisfy this requirement. The common practice is aeration of the wort before pitching, thus providing oxygen needed for lipid synthesis during the first stages of fermentation. Oxygenation (aeration) of cropped yeast slurries is a second alternative. Finally, the addition of the required lipids to wort is sometimes suggested as an alternative to aeration. We examined a fourth possibility, namely the supplementation with UFA of cropped cells. Previously, we reported that the supplementation of stationary phase cropped brewer's yeast with linoleic acid is a good alternative to wort aeration. This conclusion resulted from results obtained with a well-defined stirred synthetic fermentation medium. We also showed that cells cropped from non-stirred tall-tube fermented malt wort incorporated linoleic acid into different cellular lipid fractions, when suspended and supplemented in fermented wort. Now, we report that such yeast, pitched in malt wort in non-stirred tall-tubes, showed growth and attenuation profiles comparable to unsupplemented yeast in pre-aerated wort. Moreover, the synthesis of acetate esters, which is known to be affected when UFAs are added directly to the wort, was not significantly affected. We hypothesize that the active uptake of linoleic acid during fermentation and its activation by coenzyme A (CoA) and phospholipid synthesis are responsible for the effects on ester synthesis, through repression of the alcohol acetyltransferase-encoding gene, ATF1. In supplemented cropped yeast, these reactions occur prior to fermentation, thus avoiding interferences with acetate ester synthesis. In serial repitching experiments with repeated linoleic acid supplementation of the cropped yeast, the fermentation performances of the yeast remained comparable to those of non-supplemented yeast in pre-aerated wort. However, due to a progressive increase of cellular UFA, negative effects on acetate ester synthesis appeared. Nevertheless, the supplementation of cropped yeast with UFAs can be considered as an interesting alternative to wort oxygenation to restore optimal membrane functions.

Technological Approach to Improve Beer Flavor Stability: Adjustments of Wort Aeration in Modern Fermentation Systems Using the Electron Spin Resonance Method

From the point of view of improving beer flavor stability, wort aeration methods used in modern fermentation technology, such as high-gravity brewing and large cylindroconical tank systems, were investigated in detail using a novel electron spin resonance method, which could determine the endogenous antioxidant activity (EA) value. The results showed that the optimization of wort aeration methods in the multifilling fermentation systems (“Drauflassen”) was essential for controlling the EA value of the finished beer as well as fermentation performance. The key point in the optimization of wort aeration methods was to consider the yeast growth phase at the time of wort aeration and the quantitative ratio of additional aeration to the first aeration depending on the fermentation conditions. Based on the extensive series of studies on improving the oxidative flavor stability of beer, the relationships between the EA value and sulfite level in beer are considered. A strategy for improving oxidative flavor stability of beer is also proposed.

ビール醸造におけるエステル生成制御法としての麦汁通気

ビール醸造におけるエステル生成制御法としての麦汁通気

Influence of the Aeration of Wort during Continuous Beer Fermentation

(1965). Influence of the Aeration of Wort during Continuous Beer Fermentation. Proceedings. Annual meeting - American Society of Brewing Chemists: Vol. 23, Proceedings of the Annual Meeting 1965, pp. 112-116.

Reflections on Wort Aeration, Yeast Growth, and Fermentation

(1962). Reflections on Wort Aeration, Yeast Growth, and Fermentation. Proceedings. Annual meeting - American Society of Brewing Chemists: Vol. 20, Proceedings of the Annual Meeting 1962, pp. 119-126.

Utilization of aspartic acid and asparagine by yeast.

MUCH work has been carried out upon the mechanisms whereby yeast is able to utilize nitrogen from amino-acids for its nutrition, and the mechanisms worked out by Ehrlich and Stickland illustrating nitrogen uptake are well known1,2. These and other authors (for example, Thorne3) have been concerned with the value of amino-acids to yeasts simply as sources of nitrogen. Working with aspartic acid and asparagine, however, we have for some time observed that yeast growth has been stimulated under some conditions to a remarkable extent if either aspartic acid or asparagine has been present in nutrient solutions containing sugars and inorganic salts and nutrilites under various conditions of wort aeration. Under several conditions of experimental procedure, always with good aeration, it has been found that aspartic acid and asparagine added in relatively large amounts have always had the effect of increasing the yield of yeast obtained from a standard amount of fermentable sugar. Thus, under one set of conditions, it has been found that the addition of 1 gm. of asparagine would yield an additional 2.7 gm. of yeast (at 27 per cent dry matter) or 0.729 gm. of yeast dry matter. Also, while we have never been able to obtain yields of yeast (at 27 per cent dry matter) greater than 210 per cent of the hexose sugar supplied, this figure has, under some conditions, been greatly exceeded when either aspartic acid or asparagine has been present in the wort in sufficient amount.