Yeast : the soul of beer’s aroma—a review of flavour-active esters and higher alcohols produced by the brewing yeast

Among the most important factors influencing beer quality is the presence of well-adjusted amounts of higher alcohols and esters. Thus, a heavy body of literature focuses on these substances and on the parameters influencing their production by the brewing yeast. Additionally, the complex metabolic pathways involved in their synthesis require special attention. More than a century of data, mainly in genetic and proteomic fields, has built up enough information to describe in detail each step in the pathway for the synthesis of higher alcohols and their esters, but there is still place for more. Higher alcohols are formed either by anabolism or catabolism (Ehrlich pathway) of amino acids. Esters are formed by enzymatic condensation of organic acids and alcohols. The current paper reviews the up-to-date knowledge in the pathways involving the synthesis of higher alcohols and esters by brewing yeasts. Fermentation parameters affecting yeast response during biosynthesis of these aromatic substances are also fully reviewed.Eduardo Pires gratefully acknowledges the Fundacao para a Ciencia e a Tecnologia (FCT, Portugal) for the PhD fellowship support (SFRH/BD/61777/2009). The financial contributions of the EU FP7 project Ecoefficient Biodegradable Composite Advanced Packaging (EcoBioCAP, grant agreement no. 265669) as well as of the Grant Agency of the Czech Republic (project GACR P503/12/1424) are also gratefully acknowledged. The authors thank the Ministry of Education, Youth and Sports of the Czech Republic (MSM 6046137305) for their financial support

Ca isotope fingerprints of early crust-mantle evolution

Among the most important factors influencing beer quality is the presence of well-adjusted amounts of higher alcohols and esters; as well as the successful reduction of undesirable by-products such as diacetyl. While higher alcohols and esters contribute rather positively to the beer aroma, diacetyl is mostly unwelcome for beer types with lighter taste. Thus, the complex metabolic pathways in yeast responsible for the synthesis of both pleasant and unpleasant by-products of fermentation were given special attention in this last chapter

How to improve the enzymatic worty flavour reduction in a cold contact fermentation

Although much more efficient at 28 degrees C than at low temperature, enzymatic removal of Strecker aldehydes by brewer's yeast, Saccharomyces cerevisiae, is always limited to 60-85% of the: initial concentration, whatever the fermentation conditions. This asymptotic reduction pattern leads to residual concentrations imparting the well-known unpleasant worty taste to alcohol-free beers. Low-energy binding to flavanoids is shown to hinder more complete enzymatic reduction in the cold contact fermentation process. (C) 2000 Elsevier Science Ltd. All rights reserved

Fate of the worty flavours in a cold contact fermentation

3-Methylbutanal, 2-methylbutanal and 3-methylthiopropionaldehyde are described in the literature as preponderantly responsible for the worty taste of alcohol-free beers. Even in a cold contact process, such aldehydes are reduced through fermentation. The chemical removal of aldehydes by amino acids or proteins does not exceed 20% of the initial concentration, although such mechanisms appear much more effective at 28 degrees C. The role of Saccharomyces cerevisiae brewer's yeast, in the reduction of wort aldehydes, is confirmed here. Only viable yeasts allow a significant decrease in carbonyl level. Unfortunately, residual concentrations are higher for Strecker aldehydes among which 3-methylthiopropionaldehyde is characterized by a very low flavour threshold. Effects of yeast strain, pitching rate and inhibitors have been assessed. (C) 1999 Elsevier Science Ltd. All rights reserved

Yeast ADHI disruption: A way to promote carbonyl compounds reduction in alcohol-free beer production

Alcohol-free beers are usually characterized by worty off-flavors and the lack of the pleasant fruity or ester aroma found in regular beers. Enhancing yeast reduction of 3-methylthiopropionaldehyde, 3-methylbutanal, and 2-methylbutanal appears to be a good means of improving the organoleptic quality of alcohol-free beers. Upon screening of various Saccharomyces cerevisiae yeasts for in vitro reductase activity, a haploid adh0 strain emerged as the most efficient yeast for nicotinamide adenine dinucleotide phosphate (reduced form) (NADPH)-dependent Strecker aldehyde reduction. Tetrad analysis of diploids (adh0 x 15D wild-type) demonstrated the predominant role of adh1 mutation to enhance aldehyde reductase. Consequent to such alcohol dehydrogenase gene disruption, acetaldehyde accumulates, although partly oxidized into acetic acid by the NADP cofactor. We propose that regeneration of this cofactor in adh1 strains can be improved by promoting NADPH-dependent aldehyde reductase activity

Varietal discrimination of hop pellets by essential oil analysis I. Comparison of fresh samples

The aim of this study was to differentiate hop pellets by essential oil analysis. Volatile compounds of five aromatic cultivars (Styrie, Saaz, Lublin, Mount Hood, and Hallertau) and seven bitter cultivars (Northern Brewer, Nugget, Pride of Ringwood, Northdown, Galena, Target, and Challenger) were extracted with a Likens-Nickerson simultaneous solvent extractor. The extracts had a strong hop aroma that varied according to the type of hop. Approximately 100 compounds were separated by gas chromatography (GC) and identified by GC-mass spectrometry. An identification flowchart including seven terpenic compounds, four esters, and one methyl ketone was established to discriminate between fresh samples of the 12 investigated cultivars. High amounts of bergamotene and farnesene were found only in Saaz, Lublin, and Styrie samples. Quantification of 4-decenoic acid methyl ester and 3-methyl butyl isobutyrate proved a quick means of distinguishing non-European and European bitter hops from aromatic cultivars

Uptake of amino acids during beer production: The concept of a critical time value

According to M. Jones and J. Pierce in 1964, amino acid uptake occurs sequentially when Saccharomyces cerevisiae grows in a complex medium. whatever the individual concentration. The objective of the current work was to determine to what extent, under different conditions, the four previously defined amino acid families can still be distinguished. For this purpose. amino acid uptake was monitored during fermentation in the presence of one ale and two lager yeasts conducted at various scales (1-L tall EBC tubes and 1-L conical flasks) and temperatures (23 and 28degreesC). Industrial production (1-hL rectangular vessel) conducted with a co-culture of three ale yeasts was also investigated. A critical time (Tc), defined as the time it takes the amino acids of group A' (named A' in the Current work) to be totally consumed, emerged from all our experiments. This group coincides with the A class defined by M. Jones (aspartate, threonine. serine. glutamate. lysine, and arginine) plus methionine and minus arginine. Tc also corresponds with the beginning of consumption of an amino acid group (named C' in the current work) that includes only glycine and alanine. All other amino acids. defining the B' group, (named B' in the current work) are slowly and gradually taken up without any lag phase

Amplified fragment-length polymorphism, a new method for the analysis of brewer's yeast DNA polymorphism

This study sought to confirm the utility of amplified fragment-length polymorphism for identifying brewery yeast strains. Our results were promising, since single primer pairs can be used to distinguish most yeast strains. The use of several primer pairs, however, should still increase the method's reproducibility, Furthermore, this technique yields quantitative data on genetic polymorphism. Among the 26 strains studied, we calculated a 55% average of shared fragments. This similarity indicator was higher for bottom-fermenting strains (72%) than for top-fermentation yeasts (45%)