Release of deuterated nonenal during beer aging from labeled precursors synthesized in the boiling kettle

Effect of Addition of Different Phenolic-Rich Extracts on Beer Flavour Stability

Flavour stability is a key factor in the beer production process. The stabilizing effect of six commercial phenolic-rich extracts was studied. The extracts were added to beer before bottling. Quality parameters (colour, turbidity, foam and dissolved oxygen content), antioxidant activity by 2,2'-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), ferric reducing antioxidant power (FRAP), 1,1-diphenyl-2-picrylhydrazyl radical (DPPH), total polyphenols content, and sensorial analysis by a trained panel were performed over the course of storage. The beers were analyzed every 3 months for a total period of 6 months. Results indicated that all studied phenolic-rich extracts positively affected the beer flavour stability. In particular, the condensed tannins showed a significant protective effect. The condensed green tea tannins resulted as the most promising source of natural antioxidant able to prolong beer shelf-life and bring interesting organoleptic characteristics to beer. Also, grape seed tannins appeared suitable to boost flavour stability and improve organoleptic properties of beer.

Insights into chemical and sensorial aspects to understand and manage beer aging using chemometrics

Beer chemical instability remains, at present, the main challenge in maintaining beer quality. Although not fully understood, after decades of research, significant progress has been made in identifying "aging compounds," their origin, and formation pathways. However, as the nature of aging relies on beer manufacturing aspects such as raw materials, process variables, and storage conditions, the chemical profile differs among beers. Current research points to the impact of nonoxidative reactions on beer quality. The effect of Maillard and Maillard intermediates on the final beer quality has become the focus of beer aging research, as prevention of oxidation can only sustain beer quality to some extent. On the other hand, few studies have focused on tracing a profile of whose compound is sensory relevant to specific types of beer. In this matter, the incorporation of "chemometrics," a class of multivariate statistic procedures, has helped brewing scientists achieve specific correlations between the sensory profile and chemical data. The use of chemometrics as exploratory data analysis, discrimination techniques, and multivariate calibration techniques has made the qualitatively and quantitatively translation of sensory perception of aging into manageable chemical and analytical parameters. However, despite their vast potential, these techniques are rarely employed in beer aging studies. This review discusses the chemical and sensorial bases of beer aging. It focuses on how chemometrics can be used to their full potential, with future perspectives and research to be incorporated in the field, enabling a deeper and more specific understanding of the beer aging picture.

Why Craft Brewers Should Be Advised to Use Bottle Refermentation to Improve Late-Hopped Beer Stability

The aromatic complexity of craft beers, together with some particular practices (use of small vessels, dry hopping, etc.), can cause more oxidation associated with pre-maturated colloidal instability, Madeira off-flavors, bitterness decrease, and aroma loss. As bottle refermentation is widely used in Belgian craft beers, the aim of the present work is to assess how this practice might impact their flavor. In fresh beers, key flavors were evidenced by four complementary techniques: short-chain fatty acids determination, esters analysis, XAD-2 extract olfactometry, and overall sensory analysis. In almost all of the fresh beers, isovaleric acid was the sole fatty acid found above its sensory threshold. Selected samples were further analyzed through natural aging at 20 °C. The presence of yeast in the bottle minimized the trans-2-nonenal released from Schiff bases and proved less deleterious than suggested by previous studies with regard to fatty acid release and ester decrease through aging. Furthermore, according to the yeast species selected, some interesting terpenols and phenols were produced from glucosides during storage.

Forced into aging: Analytical prediction of the flavor-stability of lager beer. A review

Despite years of research, sensory deterioration during beer aging remains a challenge to brewing chemists. Therefore, sensorial and analytical tools to investigate aging flavors are required. This review aims to summarize the available analytical methods and to highlight the problems associated with addressing the flavor-stability of beer. Carbonyls are the major contributors to the aroma of aged pale lager beer, which is especially susceptible to deterioration. They are formed via known pathways during storage, but, as recent research indicates, are mainly released from the bound-state during aging. However, most published studies are based on model systems, and thus the formation and breakdown parameters of these adducts are poorly understood. This concept has not been previously considered in previous forced-aging analysis. Only weak parallels can be drawn between forced and natural aging. This is likely due to the different activation energies of the chemical processes responsible for aging, but may also be due to heat-promoted release of bound aldehydes. Thus, precursors and their binding parameters must be investigated to make appropriate technological adjustments to forced-aging experiments. In combination with sophisticated data analysis, the investigation of volatile indicators and non-volatile precursors can lead to more reliable predictions of flavor stability.

How sotolon can impart a Madeira off-flavor to aged beers

4,5-Dimethyl-3-hydroxy-2(5H)-furanone or sotolon is known to impart powerful Madeira-oxidized-curry-walnut notes to various alcoholic beverages. It has been much studied in oxidized Jura flor-sherry wines, aged Roussillon sweet wines, and old Port wines, in which it contributes to the characteristic "Madeira-oxidized" aroma of these beverages. No scientific paper describes how sotolon might be involved in the Madeira off-flavor found in aged beers. The specific extraction procedure applied here allowed us to quantify this lactone in 7 special beers, at levels sometimes well above its threshold (from 5 to 42 μg/L after 6, 12, 18, and 24 months of natural aging, while unquantifiable in fresh beer). Investigation of spiked beers led us to highlight the key role of pro-oxidants and acetaldehyde. Addition of ascorbic acid without sulfites should be avoided by brewers, as the former would intensify sotolon synthesis. Acetoin, a beer fermentation byproduct, also emerged as possible precursor in beer when combined with serine.

On the origin of free and bound staling aldehydes in beer

The chemistry of beer flavor instability remains shrouded in mystery, despite decades of extensive research. It is, however, certain that aldehydes play a crucial role because their concentration increase coincides with the appearance and intensity of "aged flavors". Several pathways give rise to a variety of key flavor-active aldehydes during beer production, but it remains unclear as to what extent they develop after bottling. There are indications that aldehydes, formed during beer production, are bound to other compounds, obscuring them from instrumental and sensory detection. Because freshly bottled beer is not in chemical equilibrium, these bound aldehydes might be released over time, causing stale flavor. This review discusses beer aging and the role of aldehydes, focusing on both sensory and chemical aspects. Several aldehyde formation pathways are taken into account, as well as aldehyde binding in and release from imine and bisulfite adducts.

Influence of endogenous ferulic acid in whole wheat flour on bread crust aroma

The influence of wheat flour type (refined (RWF)/whole (WWF)) on bread crust aroma was investigated. Differences were characterized by aroma extract dilution analysis and quantified utilizing stable isotope surrogate standards. For RWF breads, five aroma compounds were higher in concentration, 2-acetyl-1-pyrroline, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, 2-phenylethanol, 2-acetyl-2-thiazoline, and 2,4-dihyroxy-2,5-dimethyl-3(2H)-furanone, by 4.0-, 3.0-, 2.1-, 1.7-, and 1.5-fold, respectively, whereas three compounds were lower, 2-ethyl-3,5-dimethylpyrazine, (E,E)-2,4-decadienal, and (E)-2-nonenal by 6.1-, 2.1-, and 1.8-fold, respectively. A trained sensory panel reported the perceived aroma intensity of characteristic fresh refined bread crust aroma was significantly higher in RWF compared to WWF crust samples. Addition of 2-acetyl-1-pyrroline, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, 2-phenylethanol, 2-acetyl-2-thiazoline, and 2,4-dihyroxy-2,5-dimethyl-3(2H)-furanone to the WWF crust (at concentrations equivalent to those in the RWF crust) increased the intensity of the fresh refined bread crust aroma attribute; no significant difference was reported when compared to RWF crust. The liberation of ferulic acid from WWF during baking was related to the observed reduction in these five aroma compounds and provides novel insight into the mechanisms of flavor development in WWF bread.

Influence of malt roasting on the oxidative stability of sweet wort

Influence of malt roasting on the oxidative stability of sweet wort was evaluated based on radical intensity, volatile profile, content of transition metals (Fe and Cu) and thiols. Malt roasting had a large influence on the oxidative stability of sweet wort. Light sweet worts were more stable with low radical intensity, low Fe content, and ability to retain volatile compounds when heated. At mild roasting, the Fe content in the wort increased but remained close to constant with further roasting. Dark sweet worts were less stable with high radical intensities, high Fe content, and a decreased ability to retain volatiles. Results suggested that the Maillard reaction compounds formed during the roasting of malt are prooxidants in sweet wort. A thiol-removing capacity was observed in sweet wort, and it was gradually inhibited by malt roasting. It is possibly caused by thiol oxidizing enzymes present in the fresh malt.

Potentiality of red sorghum for producing stilbenoid-enriched beers with high antioxidant activity

trans-Piceid and trans-resveratrol were authenticated for the first time by high-resoution mass spectrometry in red sorghum grains. A 0.4-1 mg/kg amount of trans-piceid and up to 0.2 mg/kg trans-resveratrol were quantified by reversed phase high-performance liquid chromatography-atmospheric pressure chemical ionization(+)-tandem mass spectrometry. The white sorghum samples contained only traces of trans-piceid (up to 0.1 mg/kg), and trans-resveratrol was absent. In much lower amounts than procyanidins, stilbenoids are not able to contribute significantly to the exceptional antioxidant activity of red sorghum (ORAC, 83-147 μmol TE/g; AAPH, 0.61-1.79 min/mg kg(-1)). More than 10 mg/kg of total stilbenoids have been reported in some hop varieties. Yet, as hop is a minor wort ingredient as compared to cereals, red sorghum could be the main source of trans-piceid in beer. Hop remains, however, the single source of cis-piceid.

Release and evaporation of volatiles during boiling of unhopped wort

The release and evaporation of volatile compounds was studied during boiling of wort. The observed parameters were boiling time, boiling intensity, wort pH, and wort density. The effect of every parameter was discussed and approached chemically, with an eye on beer-aging processes. The results indicated that pH highly influenced the release of flavor compounds and that the formation of Strecker aldehydes was linear with boiling time. However, because of evaporation of volatiles, information about the applied thermal load on wort is lost when using a volatile heat load indicator. The thiobarbituric acid (TBA) method, which includes the nonvolatile precursors of volatile aging compounds, proved to be a more reliable method to determine all kinds of heat load on wort. Finally, it was discussed how the obtained insights could help to understand the mechanism of beer aging.

Influence of lipids in the generation of phenylacetaldehyde in wort-related model systems

The effect of lipids on the formation of the Strecker aldehyde phenylacetaldehyde during wort boiling was studied to determine the role that small changes in the lipid content of the wort have in the production of significant flavor compounds in beer. Wort was treated with 0-2.77 mmol per liter of glucose, linoleic acid, or 2,4-decadienal and heated at 60-98 degrees C for 1 h. After this time, the amount of the Strecker aldehyde phenylacetaldehyde increased in the samples treated with linoleic acid or decadienal but not in the samples treated with glucose. Thus, the amount of phenylacetaldehyde produced in the presence of linoleic acid was 1.1-2.5 times the amount of the Strecker aldehyde produced in the control wort, and this amount increased to 3.6-4.6 times when decadienal was employed. The higher reactivity of decadienal than linoleic acid for this reaction decreased with temperature and was related to the oxidation of linoleic acid that occurred to a higher extent at higher temperatures. The above results suggest that lipids can contribute to the formation of Strecker aldehydes during wort boiling and that changes in the lipid content of the wort will produce significant changes in the formation of Strecker aldehydes in addition to other well-known consequences in beer quality and yeast metabolism. On the other hand, because of the high glucose content in wort, small changes in its content are not expected to affect the amount of Strecker aldehydes produced.

Identification of a stale-beer-like odorant in extracts of naturally aged beer

For a long time, beer staling has been a prime concern in brewery research. Yet, to improve flavor stability, better knowledge of all chemicals involved is still needed. From our aroma extract dilu-tion analyses (AEDA) applied to naturally aged lager beers emerged an old-beer-like odorant at RICP-SIL 5 CB = 1532 and RIFFAP = 2809, with a FD value close to that of trans-2-nonenal (the well-known cardboard off-flavor found in aged beers). Specific phenol extraction, GC cold trapping, and mass spectrometry (electron impact and chemical ionization) enabled us to identify it as 4-vinylsyringol. Although already mentioned in some fresh beers, this compound had never been highlighted as involved in the aging process of lager beers.

Further insights into the role of methional and phenylacetaldehyde in lager beer flavor stability

This work attempts to measure the importance of methional and phenylacetaldehyde on the flavor stability of beer, as direct participants or as indicators of aroma deterioration. A trained sensory panel identified the most important descriptors related to the typical aroma of aged beer: "malty", "honey-like", "cooked potato", and "metallic". By GC-olfactometry analysis, six aromatic zones related to the selected descriptors were highlighted, and by using GC-MS techniques it was possible to identify methional and phenylacetaldehyde as being responsible for two odor zones. The quantification of these molecules in samples submitted to forced aging treatments showed that the levels of methional and phenylacetaldehyde are dependent on the temperature of storage. Normal aged beers were also analyzed, and it was observed that these compounds accumulate with time of storage. Furthermore, these molecules were negatively correlated with the aroma quality of beer as evaluated by a sensorial panel. To validate the sensory impact of these substances, a fresh beer was spiked with these molecules and also with trans-2-nonenal, singly and in combination, and the similarity value between samples and the aged beer was then determined. The highest value from the similarity tests was 72% when the three compounds were added simultaneously. The combination of the two Strecker aldehydes increases by 54% the degree of similarity, indicating the key role played by these molecules in the aroma deterioration of beer. Finally, the kinetic parameters, Ea and k, were calculated, and it was observed that the Arrhenius equation described well the temperature dependence of the reaction rate constant. Measuring the concentration of methional and phenylacetaldehyde may provide information about the key steps along the process that most affect the flavor stability of beer, which may be useful in establishing the best storage conditions.

Furfuryl ethyl ether: important aging flavor and a new marker for the storage conditions of beer

Recently, it was reported that furfuryl ethyl ether is an important flavor compound indicative of beer storage and aging conditions. A study of the reaction mechanism indicates that furfuryl ethyl ether is most likely formed by protonation of furfuryl alcohol or furfuryl acetate followed by S(N)2-substitution of the leaving group by the nucleophilic ethanol. For the reaction in beer, a pseudo-first-order reaction kinetics was derived. A close correlation was found between the values predicted by the kinetic model and the actual furfuryl ethyl ether concentration evolution during storage of beer. Furthermore, 10 commercial beers of different types, aged during 4 years in natural conditions, were analyzed, and it was found that the furfuryl ethyl ether flavor threshold was largely exceeded in each type of beer. In these natural aging conditions, lower pH, darker color, and higher alcohol content were factors that enhanced furfuryl ethyl ether formation. On the other hand, sulfite clearly reduced furfuryl ethyl ether formation. All results show that the furfuryl ethyl ether concentration is an excellent time-temperature integrator for beer storage.

Evolution of chemical and sensory properties during aging of top-fermented beer

The aging and consequent changes in flavor molecules of a top-fermented beer were studied. Different aging conditions were imposed on freshly bottled beer. After 6 months of aging, the concentration changes were recorded for acetate esters, ethyl esters, carbonyls, Maillard compounds, dioxolanes, and furanic ethers. For some flavor compounds, the changes with time of storage were monitored at different temperatures, either with CO(2) or with air in the headspace of the bottles. For some molecules a relationship was determined between concentration changes and sensory evaluation results. A decrease in volatile esters was responsible for a reduced fruity flavor during aging. On the contrary, various carbonyl compounds, some ethyl esters, Maillard compounds, dioxolanes, and furanic ethers showed a marked increase, due to oxidative and nonoxidative reactions. A very high increase was found for furfural, 2-furanmethanol, and especially the furanic ether, 2-furfuryl ethyl ether (FEE). For FEE a flavor threshold in beer of 6 mug/L was determined. In the aged top-fermented beer, FEE concentrations multiple times the flavor threshold were observed. This was associated with the appearance of a typical solvent-like flavor. As the FEE concentration increased with time at an almost constant rate, with or without air in the headspace, FEE (and probably other furanic ethers) is proposed as a good candidate to evaluate the thermal stress imposed on beer.

Release of deuterated (E)-2-nonenal during beer aging from labeled precursors synthesized before boiling

Although lipid autoxidation in the boiling kettle is a key determinant of the cardboard flavor of aged beers, recent results show that mashing is another significant source of wort nonenal potential, the well-known indicator of how a beer will release (E)-2-nonenal during storage. Although unstable, deuterated (E)-2-nonenal nitrogen adducts created during mashing can in some cases partially persist in the pitching wort, to release deuterated (E)-2-nonenal during beer aging. In the experiment described here, the relative contributions of mashing and boiling were estimated at 30 and 70%, respectively. The presence of oxygen during mashing and, to a lesser extent, high lipoxygenase activity can intensify the stale cardboard flavor.

How low pH can intensify beta-damascenone and dimethyl trisulfide production through beer aging

Flavor quality is of major importance to the consumer, but the flavor characteristics of beer appear to deteriorate greatly with time, at a rate depending on the composition of the beer and its storage conditions (notably pH). Prior to identifying the influence of pH on the development of the most intense staling flavors found in aged lager beers, the corresponding key flavor compounds were determined by aroma extract dilution analysis. In addition to trans-2-nonenal, beta-damascenone seems at least as important in the flavor of aged beer. Ethyl butyrate, dimethyl trisulfide, 2-acetylpyrazine, 3-(methylthio)propionaldehyde, 2-methoxypyrazine, maltol, gamma-nonalactone, and ethyl cinnamate are also relevant to the sensory profile of aged beer. Upon aging, a beer having a higher pH produces less beta-damascenone, because acid-catalyzed glycoside hydrolysis is decreased. On the other hand, it produces more 3-(methylthio)propionaldehyde, owing to Strecker degradation of methionine. Raising the beer pH additionally causes the release of 3-(methylthio)propionaldehyde from sulfitic adducts. These adducts, more stable at a lower pH, protect the aldehyde against premature oxidation to 3-(methylthio)propionic acid, thus making it available for dimethyl trisulfide formation during aging.

Investigation of the beta-damascenone level in fresh and aged commercial beers

This study investigated the increase of beta-damascenone content during aging in a variety of commercial Belgian beers. Quantities detected in fresh beers were generally low (from 6 ng/g to 25 ng/g). After 5 days at 40 degrees C, the level increased (to as much as 210 ng/g) in most of the beers studied, according to the type of beer. Further experiments showed that wort initially contains large quantities of beta-damascenone (450 ng/g), but that degradation of the compound during fermentation accounts for the low concentrations observed in fresh beers. Production during beer aging can be partially explained by acid hydrolysis of glycosides.

3-methylthiopropionaldehyde as precursor of dimethyl trisulfide in aged beers

Hop S-methylcysteine sulfoxide has previously been postulated as the precursor of dimethyl trisulfide (DMTS) in beers. The present data point to 3-methylthiopropionaldehyde, the Strecker aldehyde issued from methionine, as another potential precursor in aged beers. Spiking either fresh beer or wort before boiling leads in all cases to higher levels of DMTS after storage. Moreover, special malts with a high level of 3-methylthiopropionaldehyde also favor polysulfide synthesis. A higher pH should increase this onion-like off-flavor, whereas a low pH is unfortunately known to enhance the cardboard flavor of aged beers. 3-methylthiopropanol, issued from yeast reducing activity, can be considered as an additional DMTS source during aging.

Measuring antioxidant efficiency of wort, malt, and hops against the 2,2'-azobis(2-amidinopropane) dihydrochloride-induced oxidation of an aqueous dispersion of linoleic acid

This paper presents a simple, convenient method for determining the efficiency of antioxidants in aqueous systems. Production of conjugated diene hydroperoxide by oxidation of linoleic acid in an aqueous dispersion is monitored at 234 nm. 2, 2'-Azobis(2-amidinopropane) dihydrochloride is used as a free radical initiator. Among 12 antioxidants tested, phenolic compounds proved to be the most efficient, both kinetically and in terms of the inhibition time (T(inh)). Applied to wort, malt, and hops, the method confirmed a significant antioxidant activity in such products, especially hops. This assay can be used to follow oxidative changes throughout the brewing process and to understand the contribution of each raw material.