DIMETHYL SULPHIDE-A REVIEW

Metabolization of Free and Peptide-Bound Oxidized Methionine Derivatives by Saccharomyces cerevisiae in a Model System

In the brewing process, methionine is a decisive amino acid for (off-)flavor formation. A significant part of methionine is oxidized to methionine sulfoxide (MetSO) in malt. We hypothesized that MetSO and MetSO2 are metabolized to volatile compounds during yeast fermentation and examined whether the yeast Saccharomyces cerevisiae is able to catabolize l-MetSO and l-MetSO2 in free and dipeptide-bound forms. We also investigated the stability of l-methionine sulfoximine and S-methylmethionine. Cell viability in the presence of the test compounds was at least 90%. Both free and peptide-bound test substances were metabolized by Saccharomyces cerevisiae. l-MetSO was degraded most rapidly as the free amino acid, while l-MetSO2 was degraded most rapidly bound in dipeptides. We observed a different degradation behavior of the (R) and (S) diastereoisomers for l-MetSO and l-methionine sulfoximine. Furthermore, we detected methionol as the only metabolite of MetSO. Methionol sulfoxide was not formed. MetSO2 was not converted to methionol or methionol sulfone but to the respective α-hydroxy acid. We conclude that the reduction of MetSO to methionine proceeds faster than transamination. The occurrence of MetSO or MetSO2 in brewing malt will not lead to the formation of hitherto unknown volatile metabolites of the Ehrlich pathway.

Dimethyl Sulfide (DMS) in Amarone Wines: Influence of Aging, Withering, Grape Variety, and Geographical Origin

Occurrence of dimethyl sulfide (DMS), a potent aroma compound accumulating during aging, was investigated in commercial and experimental Amarone wines. In commercial Amarone, DMS was observed in concentrations ranging from 2.9 to 64.3 μg/L. Model aging studies on experimental wines indicated that DMS in Amarone is strongly associated with aging and that wines from different vineyards can vary significantly in their ability to accumulate DMS during aging. The capacity of certain vineyards to give wines with higher DMS-forming potential was consistent across three consecutive vintages, representing a true terroir factor to be expressed with aging. Wine content of primary amino acids (PAN), a commonly analyzed enological parameter of grape must, was shown to be positively correlated with DMS accumulation during aging. Grape withering also increased DMS-forming potential mostly due to increased PAN resulting from concentration due to water loss. Increased pH due to withering also contributed to a higher DMS content of withered wines, but to a lower extent. In certain vineyard sites, an influence of vintage conditions on DMS-forming potential was also observed.

Provocation: prolonged maturation of beer is of unproven benefit

Approaches to brewing are suffused with dogmatic insistence that certain techniques are unequivocally linked to the delivery of quality products. Amongst these belief sets is the perseverance with prolonged maturation (or ‘conditioning’) times post-fermentation. Historically the justification for these lagering techniques was to allow settling of solids, carbonation, flavour maturation and removal of chill haze entities. As science and technology have advanced it is unequivocally the case that solids and chill haze precursors can be dealt with in short order and without the need for lengthy treatments.  Equally it is perfectly possible to deliver specified levels of carbonation without the need for all the carbon dioxide to be introduced via yeast action. However, there remain many who feel that the nature of carbonation differs depending on which approach is taken. Herein lies one of the research areas that the author proposes. The perception of carbonation is not primarily due to bubble release on the palate, but rather is through the detection of carbonic acid. Is there a difference in the availability of this form of the gas depending on the mode of carbonation and to what extent does the adsorption of the carbonic acid on polypeptides in the beer have a role to play?  In terms of flavour, the advocates for lagering insist that there needs to be a handling of vicinal diketones, acetaldehyde, and hydrogen sulphide. However, all of these can be controlled through attention to primary fermentation. Then, the proponents for maturation insist that there is a desirable release of non-volatile materials into beer, which substances supposedly benefit the balance and mouthfeel of the lager. These include amino acids and nucleotides. It seems to this author however that the likeliest explanation for the greatly increased levels of these materials and of pH is autolysis of yeast. This, together with the disadvantageous impact of increased free amino nitrogen and higher pH on aspects such as biological stability, flavour stability and foam, should convince any brewer that there is a sound argument for avoiding the prolonged contact of beer with yeast. Indeed, a metabolomic approach to studying changes in non-volatile substances under conditions where there is little or no autolysis, revealed no detectable changes in any entity.  The author is open to being convinced that there are yet unidentified materials that are developed (whether through the action of viable yeast or by yeast autolysis) as beer is stored, substances which can be proven through sound organoleptic investigation to benefit the flavour of beer. Perhaps the Japanese term kokumi is what we are looking for here: ‘rich taste’. This is believed to be afforded by γ-glutamyl peptides and, inter alia, these are to be found in yeast extracts. Herein lies the second experimental approach that the author recommends for pursuit. 

Non-Conventional Yeast: Behavior under Pure Culture, Sequential and Aeration Conditions in Beer Fermentation

The use of wild yeasts, isolated from different environments, is becoming the most interesting option for the production of new beers. The objective of this study is to evaluate the potential of seven non-conventional yeast strains from five different species (Saccharomyces cerevisiae, Hanseniaspora guilliermondii, Metschnikowia pulcherrima, Torulaspora delbrueckii, and Zygosaccharomyces bailii) isolated from Madrid agriculture to produce type ale beer. Wild yeast strains were evaluated at laboratory and pilot plant scales under different fermentation conditions (pure, aerated, and sequential culture). Strain S. cerevisiae SafAle S-04 was used as a reference. Throughout the fermentation of beer, volatile compounds were determined by GC and residual sugars by HPLC, among other parameters. The yeast strains used for the fermentation in pure culture conditions were unable to ferment maltose and maltotriose (0.73-1.18% v/v of ethanol). The results of the study under aerated conditions showed varying levels of higher alcohol and ester concentrations. It should be noted that the strain CLI 1057 (S. cerevisiae) fermented maltose in the presence of oxygen (Kluyver effect). This strain also showed a high production of 4-vinyl guaiacol, making it suitable for producing beers with a phenolic profile. Finally, three strains (H. guilliermondii, Z. bailii, and T. delbrueckii) were evaluated in sequential culture together with commercial strain and found to improve the organoleptic characteristics of the brewed beer. These approaches offer the opportunity to add new product characteristics to the beers.

Repurposing brewery contaminant yeast as production strains for low-alcohol beer fermentation

A number of fungal isolates were recently obtained from a survey of the microbiota of multiple breweries and brewery products. Here, we sought to explore whether any of these brewery contaminants could be repurposed for beneficial use in beer fermentations, with particular focus on low-alcohol beer. There were 56 yeast strains first screened for the utilization of different carbon sources, ability to ferment brewer's wort, and formation of desirable aroma compounds. A number of strains appeared maltose-negative and produced desirable aromas without obvious off-flavours. These were selected for further scaled-up wort fermentations. The selected strains efficiently reduced wort aldehydes during fermentation, thus eliminating undesirable wort-like off-flavours, and produced a diverse volatile aroma profile. Two strains, Trigonopsis cantarellii and Candida sojae, together with a commercial Saccharomycodes ludwigii reference strain, were selected for 30-L-scale wort fermentations based on aroma profile and similarity to a commercial reference beer during sensory analysis using projective mapping. Both strains performed comparably to the commercial reference, and the T. cantarellii strain in particular, produced low amounts of off-flavours and a significantly higher amount of the desirable monoterpene alcohol trans-geraniol. The strain was also sensitive to common food preservatives and antifungal compounds and unable to grow at 37°C, suggesting it is relatively easily controllable in the brewery, and appears to have low risk of pathogenicity. This study shows how the natural brewery microbiota can be exploited as a source of non-conventional yeasts for low-alcohol beer production.

Simultaneous Optimization of Acetaldehyde and DMS Concentrations for Better Sensory Quality of Beer Fermented on an Industrial Scale

The levels of selected volatile components that affected the sensory properties of a lager beer were optimized under high-gravity brewing conditions (15.5 °P) in an industrial plant. The influence of different pitching rates (6–10 million cells/mL), aeration levels (8–12 mg/L), times (4.5–13.5 h) of filling CCTs (cylindroconical tanks, 3850 hl), and fermentation temperatures (8.5–11.5 °C) on the contents of acetaldehyde, diacetyl, acetone, 2,3-pentanedion, dimethyl sulfide (DMS), and on the sensory properties of beer were investigated. Response surface methodology (RSM, Box–Behnken design) was used to research the possibilities for optimizing the concentration of selected volatile components and sensory properties of bottom-fermented lager beers. Statistical analyses of the results showed that the experimental factors had a significant influence (R-squared for the original model with no significant lack-of-fit) on some of the volatile components. Based on the Multiple Response Optimization analysis, the values of independent factors that ensured the highest beer sensory quality were the following: a pitching rate of 10 million cells per mL; a fermentation temperature of 11.5 °C; an aeration level of 12 mg/L; and a CCT filling time of 4.5 h. These results proved that RSM modelling can be successfully applied to optimize fermentation and lagering processes in an industrial plant to manufacture lagers of enhanced sensory quality.

A new analytical method to measure S-methyl-l-methionine in grape juice reveals the influence of yeast on dimethyl sulfide production during fermentation

BACKGROUND

Dimethyl sulfide (DMS) is a small sulfur-containing impact odorant, imparting distinctive positive and / or negative characters to food and beverages. In white wine, the presence of DMS at perception threshold is considered to be a fault, contributing strong odors reminiscent of asparagus, cooked cabbage, and creamed corn. The source of DMS in wine has long been associated with S-methyl-l-methionine (SMM), a derivative of the amino acid methionine, which is thought to break down into DMS through chemical degradation, particularly during wine ageing.

RESULTS

We developed and validated a new liquid chromatography-tandem mass spectrometry (LC-MS/MS) method with a stable isotope dilution assay (SIDA) to measure SMM in grape juice and wine. The application of this new method for quantitating SMM, followed by the quantitation of DMS using headspace-solid phase micro-extraction coupled with gas chromatography-mass spectrometry (HS-SPME/GC-MS), confirmed that DMS can be produced in wine via the chemical breakdown of SMM to DMS, with greater degradation observed at 28 °C than at 14 °C. Further investigation into the role of grape juice and yeast strain on DMS formation revealed that the DMS produced from three different Sauvignon blanc grape juices, either from the SMM naturally present or SMM spiked at 50 mmol L^-1 , was modulated depending on each of the four strains of Saccharomyces cerevisiae wine yeast used for fermentation.

CONCLUSION

This study confirms the existence of a chemical pathway to the formation of DMS and reveals a yeast-mediated mechanism towards the formation of DMS from SMM during alcoholic fermentation. © 2019 Society of Chemical Industry.

Effect of Malt-Derived Potential Antioxidants on Dimethyl Sulfide Oxidation

During malt kilning, dimethyl sulfide (DMS) is partly oxidized to dimethyl sulfoxide (DMSO), which can be reduced by yeast to generate DMS during fermentation. The aim of this study was to test the effect of malt-derived potential antioxidants on DMS oxidation and to assess their applicability for DMSO minimization. In the presence of 18 μM copper, all tested antioxidants (250 μM) catalyzed DMS oxidation to deviating extents (sulfite > ascorbic acid (Asco) > gallic acid (GA) > L-cysteine (Cys) > L-glutathione (GSH)). Hydrogen peroxide was found as primary DMS oxidant for each substance except for sulfite. Electron spin resonance spectroscopy provided evidence for the formation of bisulfite radicals and peroxymonosulfate radicals, which are proposed as being capable of exhaustive DMS oxidation (∼100%) over a wide concentration. The data demonstrate that use of antioxidants per se cannot be suggested for the minimization of DMSO formation in malt and other foodstuffs. Potential shifts from pro- to antioxidative behavior of antioxidants and their implications on malt quality are discussed.

Enhancing the performance of brewing yeasts

Beer production is one of the oldest known traditional biotechnological processes, but is nowadays facing increasing demands not only for enhanced product quality, but also for improved production economics. Targeted genetic modification of a yeast strain is one way to increase beer quality and to improve the economics of beer production. In this review we will present current knowledge on traditional approaches for improving brewing strains and for rational metabolic engineering. These research efforts will, in the near future, lead to the development of a wider range of industrial strains that should increase the diversity of commercial beers.

Physiology, ecology and industrial applications of aroma formation in yeast

Yeast cells are often employed in industrial fermentation processes for their ability to efficiently convert relatively high concentrations of sugars into ethanol and carbon dioxide. Additionally, fermenting yeast cells produce a wide range of other compounds, including various higher alcohols, carbonyl compounds, phenolic compounds, fatty acid derivatives and sulfur compounds. Interestingly, many of these secondary metabolites are volatile and have pungent aromas that are often vital for product quality. In this review, we summarize the different biochemical pathways underlying aroma production in yeast as well as the relevance of these compounds for industrial applications and the factors that influence their production during fermentation. Additionally, we discuss the different physiological and ecological roles of aroma-active metabolites, including recent findings that point at their role as signaling molecules and attractants for insect vectors.

Effect of l-Cysteine and Transition Metal Ions on Dimethyl Sulfide Oxidation

During malt kilning, significant amounts of dimethyl sulfide (DMS) oxidize leading to the formation of dimethyl sulfoxide (DMSO), a precursor of DMS during fermentation. Yet, knowledge regarding reaction mechanisms of DMSO formation during malt production is limited. The role of thiols in sulfide oxidation is unclear as they possess sulfoxide reducing ability as well as pro- and antioxidative properties. This study investigated the effects of the thiol l-cysteine (Cys), molecular oxygen, transition metal ions, and EDTA on DMS oxidation in aqueous model solutions. Highest oxidative DMS consumption was observed when Cys was combined with iron(II) (∼12%) and copper(II) (∼40%). Response surface modeling (RSM) revealed that Cys together with copper(II) had a strictly prooxidative effect and no antioxidative behavior was found. Hydrogen peroxide, as generated via autoxidation of Cys-Cu(I)-Cys complexes, was supposed to be the primary DMS oxidant in this work. Based on redox kinetics, potential reaction mechanisms, and their impact on oxidative processes in thermal food processing, such as malt and beer production, are discussed.

Study of the effect of H2S, MeSH and DMS on the sensory profile of wine model solutions by Rate-All-That-Apply (RATA)

The effect of hydrogen sulfide (H₂S), methanethiol (MeSH) and dimethyl sulfide (DMS) on the odor properties of three wine models-WM- (young white, young red and oaked red wines) was studied. Wine models were built by mixing a pool of common wine volatile and non-volatile compounds and further spiked with eight different combinations of the three sulfur compounds present at two levels (level 0: 0μgL^-1 and level 1: 40μgL^-1 of H₂S, 12μgL^-1 of MeSH; 55μgL^-1 of DMS). For each wine matrix eight WMs were produced and further submitted to sensory description by Rate-All-That-Apply (RATA) method. Hydrogen sulfide and methanethiol were clearly involved in the formation of reductive aromas and shared the ability to act as strong suppressors of fruity and floral attributes. Specifically, hydrogen sulfide generated aromas of rotten eggs, while methanethiol generated significant increases in camembert and decreases in citrus, smoky/roasted and oxidation aromas. The simultaneous presence of hydrogen sulfide and methanethiol enhanced the intensity of the unspecific term reduction, while the specific nuances individually imparted by each of the two compounds could not be further identified. DMS did not exert any outstanding effect on the reductive character of wines and its sensory effect was matrix-dependent. It was involved in the formation of fruity notes such as cooked/candied and red/black fruits in young wines, and vegetal notes (canned vegetables) in oaked red WMs.

Dimethyl sulfide as a source of the seaweed-like aroma in cooked soybeans and correlation with its precursor, S-methylmethionine (vitamin U)

Among the soybean germplasm in Japan, two varieties, Nishiyamahitashi 98-5 (NH) and Shinanokurakake (SKK), have an intense seaweed-like flavor after cooking. Gas-liquid chromatography with mass spectrometry (GC-MS) indicated that a significant amount (11.5 ± 3.46 μg g(-1) for NH and 6.66 ± 0.91 μg g(-1) for SKK) of dimethyl sulfide (DMS) was formed after heat treatment. DMS is formed from S-methylmethionine (SMM, vitamin U). SMM was detected in all soybean varieties examined here, but its concentration in NH and SKK seeds was >100-fold higher than in the other varieties and ranged from 75 to 290 μg g(-1). The SMM content and the ability to form DMS upon heat treatment correlated among them. The plumes and radicles contained SMM exclusively. This is the first report of soybean varieties containing SMM at a level equivalent to or higher than that in vegetables known to contain high levels of SMM, for example, turnip, cabbage, and celery.

Quantitative analysis of free and bonded forms of volatile sulfur compouds in wine. Basic methodologies and evidences showing the existence of reversible cation-complexed forms

This paper examines first some basic aspects critical to the analysis of Volatile Sulfur Compounds (VSCs), such as the analytical characteristics of the GC-pFPD system and the stability of the different standard solutions required for a proper calibration. Following, a direct static headspace analytical method for the determination of exclusively free forms of VSCs has been developed. Method repeatability is better than 4%, detection limits for main analytes are below 0.5μgL(-1), and the method dynamic linear range (r(2)>0.99) is expanded by controlling the split ratio in the chromatographic inlet to cover the natural range of occurrence of these compounds in wines. The method gives reliable estimates of headspace concentrations but, as expected, suffers from strong matrix effects with recoveries ranging from 0 to 100% or from 60 to 100 in the cases of H2S and the other mercaptans, respectively. This demonstrates the existence of strong interactions of these compounds with different matrix components. The complexing ability of Cu(2+) and to a lower extent Fe(2+) and Zn(2+) has been experimentally checked. A previously developed method in which the wine is strongly diluted with brine and the volatiles are preconcentrated by HS-SPME, was found to give a reliable estimation of the total amount (free+complexed) of mercaptans, demonstrating that metal-mercaptan complexes are reversible. The comparative analysis of different wines by the two procedures reveals that in normal wines H2S and methanethiol can be complexed at levels above 99%, with averages around 97% for H2S and 75% for methanethiol, while thioethers such as dimethyl sulfide (DMS) are not complexed. Overall, the proposed strategy may be generalized to understand problems caused by VSCs in different matrices.

Production of a saccharifying rice malt for brewing using different rice varieties and malting parameters

This study was conducted to produce rice malt suitable for beer brewing. An all-rice beer would be particularly appealing to individuals with celiac disease because rice does not contain gluten proteins. Furthermore, rice malt could also contribute to new beer flavors and brands. A screening of 10 rice varieties was conducted. The varieties Balilla and Centauro were found to be suitable for the production of an all-rice malt beer without the need of exogenous enzymes. They were characterized by a low diastatic power but nevertheless they saccharified well, likely due to other endogenous amylolytic enzymes such as limit dextrinase and α-glucosidase. The addition of CaCl2 and lactic acid during mashing lowered the pH value and increased saccharification. However, the Balilla variety saccharified without the need of these additives. We also show that the soluble nitrogen and free amino nitrogen content of rice malt wort can be increased by the incorporation of the acrospires and rootlets during mashing.

The microbiology of malting and brewing

Brewing beer involves microbial activity at every stage, from raw material production and malting to stability in the package. Most of these activities are desirable, as beer is the result of a traditional food fermentation, but others represent threats to the quality of the final product and must be controlled actively through careful management, the daily task of maltsters and brewers globally. This review collates current knowledge relevant to the biology of brewing yeast, fermentation management, and the microbial ecology of beer and brewing.