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.

Lager beer is the most consumed alcoholic beverage in the world. Its production process is marked by a fermentation conducted at low (8 to 15°C) temperatures and by the use of Saccharomyces pastorianus, an interspecific hybrid between Saccharomyces cerevisiae and the cold-tolerant Saccharomyces eubayanus. Recent whole-genome-sequencing efforts revealed that the currently available lager yeasts belong to one of only two archetypes, "Saaz" and "Frohberg." This limited genetic variation likely reflects that all lager yeasts descend from only two separate interspecific hybridization events, which may also explain the relatively limited aromatic diversity between the available lager beer yeasts compared to, for example, wine and ale beer yeasts. In this study, 31 novel interspecific yeast hybrids were developed, resulting from large-scale robot-assisted selection and breeding between carefully selected strains of S. cerevisiae (six strains) and S. eubayanus (two strains). Interestingly, many of the resulting hybrids showed a broader temperature tolerance than their parental strains and reference S. pastorianus yeasts. Moreover, they combined a high fermentation capacity with a desirable aroma profile in laboratory-scale lager beer fermentations, thereby successfully enriching the currently available lager yeast biodiversity. Pilot-scale trials further confirmed the industrial potential of these hybrids and identified one strain, hybrid H29, which combines a fast fermentation, high attenuation, and the production of a complex, desirable fruity aroma. With an annual production exceeding 1.97 billion hectoliters a year, beer is the most-produced fermented beverage in the world (1). The vast majority of currently produced beer is classified as either ale or lager beer, each type being produced by a unique fermentation process (2). Specifically, ale beer production uses the common brewer's yeast Saccharomyces cerevisiae and relatively high fermentation temperatures (typically 18 to 25°C) (2-5).In contrast, lager beer (with Pilsner beer as the most popular and commonly known type of lager beer) is fermented at lower temperatures (5 to 15°C), followed by a period of cold storage (lagering), which is a traditional practice vital for the beer's characteristically clean flavor and aroma. Lagers are not fermented by S. cerevisiae but by the closely related species Saccharomyces pastorianus (formerly known as Saccharomyces carlsbergensis), which combines the desirable fermentation characteristics of S. cerevisiae with the cold tolerance of its other parent, S. eubayanus (6). Lager beer currently accounts for more than 90% of the global beer market but has a much more recent origin than ales. The lager beer production process was developed in the 16th century in Bavaria (Germany), where brewing was only allowed during wintertime to minimize the microbial spoilage of beer. Later, in the 19th century, the advent of refrigeration enabled lager brewing throughout the whole year (2, 3, 7).Several recent studies have focused on analyzi...

Flavour changes of six Belgian pale lager beers were studied in order to estimate the importance of different parameters and reactions in relation to the ageing process. An attempt was made to link analytical data with sensory evaluation using multivariate statistical analysis. Partial least squares regression techniques (PLSR) were employed on the analytical and sensory data. As apparent from the PLSR model, significant indicators of lager beer ageing are aldehyde markers (especially total aldehydes, furfural, hexanal, 2-methylpropanal, 2-methylbutanal, and 3-methylbutanal), cold and permanent haze, and beer colour. Conversely, compounds or parameters that load negatively in the PLSR model for beer ageing are trans-isohumulones, cis-isohumulones, total bitterness, the T/C-ratio, polyphenolic markers (especially proanthocyanidins), the flavanoid content, and, to a lesser extent, the TB-index and reducing power (TRAP). The integrated analytical-sensorial methodology is proposed as a useful tool for evaluation of the flavour instability of pale lager beers.

Age-induced decomposition of iso-a-acids, the main bittering principles of beer, determines the consistency of the beer bitter taste. In this study, the profiles of iso-a-acids in selected highquality top-fermented and lager beers were monitored by quantitative high-performance liquid chromatography at various time intervals during ageing. Tlte degradation of the iso-a-acids as a function of time is represented by the ratio, in percentage, of the sum of the concentrations of trans-isocohumulone and trans-isohumulone to the sum of the concentrations of cis-isocohumulone and cis-isohumulone. Tfiis parameter is relevant with respect to the evaluation of bitterness deterioration in aged beers. Trans-iso-a-acids having a shelf half-life of less than one year proved to be significantly less stable than cis-iso-a-acids, but it appears feasible to counteract degradation if a stiitable beer matrix is available. Tlte fate of the trans-iso-a-acids in particular adversely affects beer bitterness consistency. In addition to using hop products containing low amounts of trans-iso-a-acids, brewers may profit of the remarkable stability of tetrahydroiso-aacids, even on prolonged storage, for the production of consistently bitter beers.KeyWords: Hops, iso-a-acids, tetrahydwiso-a-acid$, beer bitterness, beer ageing, HPLC analysis. INTRODUCTION associated with beer ageing, because such compounds, in particular /nms-2-nonenal, are considered responsibleOrganoleptic consistency of beer is of paramount for the so-called cardboard or staling offimportance to the modern brewing industry2. Raw navour3.6.io.2<>3:>.42.43. An array of enzymic and chemical materials are in general well characterised and reactions are supposed to be involved in the formation technological innovations continuously lead to of ageing carbonyls, including melanoidin-catalysed improvements in the brewing process, yet reproducible oxidation of higher alcohols1516, oxidative reactions of production of flavour-stable beers remains a challenging iso-a-acids4151617'2"-34, Strecker-type degradation of amino task for a brewer. In particular, lager beers are liable to acids61516 '28-11'32,auto-oxidation1516-27'32,photo-oxidationS7;w, undergo rapid taste and flavour changes that can readily and enzymic degradation7827^7 of unsaturated fatty acids, be tracked. Beer deterioration is an extremely complex aldol-type condensations of short-chain aldehydes1516, process, while, moreover, various beer types age in ancj secondary auto-oxidation of aIdehydes15l6--»2. different ways2. Importantly, the relative significance of these various pathways is not clear and even the nature of precursorsIn particular, the formation of volatile unsaturated for the carbonyls has not been unequivocally established43, carbonyl compounds with 6 to 10 carbon atoms has beenAmong the non-volatiles in beer, the iso-a-acids are *To whom correspondence should be addressed. pre-eminent as the main bitter principles. The iso-ct- Age-induced decomposition of iso-a-acids acids comprise 6 major components, ...

Over the past years, several insect species have gained increased attention as feedstock for food, feed, and industrial applications. One such species is Hermetia illucens, whose larvae can convert low-value organic waste into valuable fat-and protein-rich biomass. Previous research on extracting their lipids, proteins, and chitin has repeatedly focused on one life stage, while in practice different life stages coexist in the same rearing batch. In this study, the feasibility of the sequential extraction of said components from the larval, prepupal, and pupal stage of H. illucens was investigated. Additionally, the chemical composition of the life stages and their extracts was analysed. Following the lipid extraction with petroleum ether, insect proteins were extracted via solubilisation at pH 11.0 and precipitation at pH 4.0. This procedure delivered protein recoveries ranging between 27-57 % for the three life stages, with the extracts having high protein contents (85-98 %). After protein extraction, the residual impure chitin was treated sequentially with HCl and NaOH for further purification. No residual amino acids were detected by UPLC analysis of the purified chitin, which showed acetylation degrees of ± 90 %. Overall, it was concluded that the extraction procedure is indeed suitable for all investigated life stages of H. illucens, allowing for the extraction high-value biomolecules for use in industrial applications.

In this article, a detailed study on hop alpha-acid isomerization kinetics is presented. Because of the complex wort matrix and interfering interactions occurring during real wort boiling (i.e., trub formation and alpha-acids/iso-alpha-acids complexation), this investigation on alpha-acid isomerization kinetics was performed in aqueous buffer solution as a function of time (0-90 min) and heating temperature (80-100 degrees C). Rate constants and activation energies for the formation of individual iso-alpha-acids were determined. It was found that iso-alpha-acid formation follows first-order kinetics and Arrhenius behavior. Differences in activation energies for the formation of trans- and cis-isomers were noticed, the activation energy for the formation of trans-iso-alpha-acids being approximately 9 kJmol (-1) lower.

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BackgroundNon-conventional yeasts present a huge, yet barely exploited, resource of yeast biodiversity for industrial applications. This presents a great opportunity to explore alternative ethanol-fermenting yeasts that are more adapted to some of the stress factors present in the harsh environmental conditions in second-generation (2G) bioethanol fermentation. Extremely tolerant yeast species are interesting candidates to investigate the underlying tolerance mechanisms and to identify genes that when transferred to existing industrial strains could help to design more stress-tolerant cell factories. For this purpose, we performed a high-throughput phenotypic evaluation of a large collection of non-conventional yeast species to identify the tolerance limits of the different yeast species for desirable stress tolerance traits in 2G bioethanol production. Next, 12 multi-tolerant strains were selected and used in fermentations under different stressful conditions. Five strains out of which, showing desirable fermentation characteristics, were then evaluated in small-scale, semi-anaerobic fermentations with lignocellulose hydrolysates.ResultsOur results revealed the phenotypic landscape of many non-conventional yeast species which have not been previously characterized for tolerance to stress conditions relevant for bioethanol production. This has identified for each stress condition evaluated several extremely tolerant non-Saccharomyces yeasts. It also revealed multi-tolerance in several yeast species, which makes those species good candidates to investigate the molecular basis of a robust general stress tolerance. The results showed that some non-conventional yeast species have similar or even better fermentation efficiency compared to S. cerevisiae in the presence of certain stressful conditions.ConclusionPrior to this study, our knowledge on extreme stress-tolerant phenotypes in non-conventional yeasts was limited to only few species. Our work has now revealed in a systematic way the potential of non-Saccharomyces species to emerge either as alternative host species or as a source of valuable genetic information for construction of more robust industrial S. serevisiae bioethanol production yeasts. Striking examples include yeast species like Pichia kudriavzevii and Wickerhamomyces anomalus that show very high tolerance to diverse stress factors. This large-scale phenotypic analysis has yielded a detailed database useful as a resource for future studies to understand and benefit from the molecular mechanisms underlying the extreme phenotypes of non-conventional yeast species.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-017-0899-5) contains supplementary material, which is available to authorized users.