A New Model for the Regulation of Ester Synthesis by Alcohol Acetyltransferase in<i>Saccharomyces Cerevisiae</i>during Fermentation

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.

Section: (-7'977-32supporting

confidence: 91%

Section: -2863(9'8-32mentioning

confidence: 99%

Section: (-7'977-32mentioning

confidence: 99%

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

Moonjai

Verstrepen

Shen

et al. 2003

“…For example, higher wort aeration affects esters synthesis through reduced availability of acetyl-CoA (used for growth and lipid synthesis) and inhibition of AATases 26 . Although there is an overlap of the effects of different factors, the central role in beer ester formation seems to be the AATase activity and the regulation of its gene transcription 52,124 . In practice, to control the ester formation is rather difficult due to the many factors involved in the regulation of activity and the gene expression of AATases and regulation of substrate availability 103 .…”

Section: Estersmentioning

confidence: 99%

A Review of Flavour Formation in Continuous Beer Fermentations*

Brányik

Vicente

Dostálek

et al. 2008

The attractive prospect of a continuous beer fermentation system consists mostly of the accelerated transformation of wort into beer. Although continuous beer fermentation has been studied as a promising technology for several decades, the number of industrial applications is still limited. The major obstacle hindering the extensive industrial exploitation of this technology is the difficulty in achieving the correct balance of sensory compounds in the short time typical for continuous systems. This paper offers an integral view on the particularities of continuous systems, which may impart beer a sensorial character distinct from conventionally fermented counterparts. The main groups of flavour active compounds are discussed from the perspective of possible control strategies by means of process parameters and strain selection.

“…Esters are desirable components of beer when present in appropriate quantities and contribute to the fruity and flowery character of the beer. They are formed by the condensation reaction between acetyl/acyl-CoA and ethanol/higher alcohols catalysed by the alcohol acyltransferases of yeast 23 .…”

Section: Introductionmentioning

confidence: 99%

Characteristics of High Cell Density Fermentations with Different Lager Yeast Strains

Verbelen

Mulders

Saison

et al. 2008

To improve the productivity of the beer fermentation process, several strategies can be adopted. One of these promising strategies could be the increase of suspended yeast cells in the reactor. Therefore, the fermentation characteristics of 11 lager yeast strains were studied in normal pitched worts (20 × 10 6 cells/ mL) (LD) and in worts with a four-fold higher pitching rate (HD). The fermentation rate was 2-4 times increased when high initial cell levels were used. The net yeast growth was somewhat similar between the LD and the HD fermentations, although the FAN uptake level was about 35% higher in the HD fermentations compared with LD. High viabilities were observed throughout the fermentations with high cell loadings. HD fermentations resulted in higher concentrations of all the measured fusel alcohols and higher maxima and residual concentrations of total diacetyl were observed. In contrast, higher levels of most of the esters were found at the normal pitching rate, although the results of isoamyl acetate were not significant. With the help of "Principal Component Analysis", it became clear that the cell density had an important influence on the flavour profile, but that yeast specific preferences could not be overlooked as they determined the sensitivity of the yeast to the application of higher cell densities.