Formation, Losses, Preservation and Recovery of Aroma Compounds in the Winemaking Process

A wine’s aroma profile is an important part of the criteria affecting wine acceptability by consumers. Its characterisation is complex because volatile molecules usually belong to different classes such as alcohols, esters, aldehydes, acids, terpenes, phenols and lactones with a wide range of polarity, concentrations and undesirable off-aromas. This review focused on mechanisms and conditions of the formation of individual aroma compounds in wine such as esters and higher alcohols by yeast during fermentation. Additionally, aroma losses during fermentation are currently the subject of many studies because they can lead to a reduction in wine quality. Principles of aroma losses, their prevention and recovery techniques are described in this review.

The Influence of Yeast Strain on Whisky New Make Spirit Aroma

Flavour in Scotch malt whisky is a key differentiating factor for consumers and producers alike. Yeast (commonly Saccharomyces cerevisiae) metabolites produce a significant amount of this flavour as part of distillery fermentations, as well as ethanol and carbon dioxide. Whilst yeast strains contribute flavour, there is limited information on the relationship between yeast strain and observed flavour profile. In this work, the impact of yeast strain on the aroma profile of new make spirit (freshly distilled, unmatured spirit) was investigated using 24 commercially available active dried yeast strains. The contribution of alcoholic, fruity, sulfury and sweet notes to new make spirit by yeast was confirmed. Generally, distilling strains could be distinguished from brewing and wine strains based on aroma and ester concentrations. However, no statistically significant differences between individual yeast strains could be perceived in the intensity of seven aroma categories typically associated with whisky. Overall, from the yeast strains assessed, it was found that new make spirit produced using yeast strains marketed as ‘brewing’ strains was preferred in terms of acceptability rating.

Effects of fermentation temperature and aeration on production of natural isoamyl acetate by Williopsis saturnus var. saturnus

Isoamyl acetate is a natural flavour ester, widely used as a source of banana flavour by the food industry. Williopsis saturnus var. saturnus is a yeast which can produce isoamyl acetate by esterification of amyl alcohols with acetyl coenzyme A via fermentation. The evaluation of this kind of production as an alternative way to obtain natural banana flavour could be possible, if the levels produced were high enough to make a commercial product. In this study, the effects of temperature (15°C and 25°C) and aeration (aerobic, semiaerobic, and anaerobic) on the production of isoamyl acetate by Williopsis saturnus var. saturnus from sugar beet molasses were examined. According to the results obtained, isoamyl acetate production rate and specific productivity were higher at 25°C than at 15°C and at semiaerobic condition than aerobic and anaerobic conditions. Williopsis saturnus var. saturnus showed a production rate of 0.703 mg L⁻¹ h⁻¹ and a specific productivity of 0.0297 mg L⁻¹ cell⁻¹ h⁻¹ isoamyl acetate with semiaerobic condition at 25°C. The maximum amount of isoamyl acetate reached with these conditions was 118 mg/L.

Effects of wort gravity and nitrogen level on fermentation performance of brewer's yeast and the formation of flavor volatiles

Normal gravity wort and high gravity wort with different nitrogen levels were used to examine their effects on the fermentation performance of brewer's yeast and the formation of flavor volatiles. Results showed that both the wort gravity and nitrogen level had significant impacts on the growth rate, viability, flocculation, and gene expression of brewer's yeast and the levels of flavor volatiles. The sugar (glucose, maltose, and maltotriose) consumption rates and net cell growth decreased when high gravity worts were used, while these increased with increasing nitrogen level. Moreover, high gravity resulted in lower expression levels of ATF1, BAP2, BAT1, HSP12, and TDH, whereas the higher nitrogen level caused higher expression levels for these genes. Furthermore, the lower nitrogen level resulted in increases in the levels of higher alcohols and esters at high wort gravity. All these results demonstrated that yeast physiology and flavor balance during beer brewing were significantly affected by the wort gravity and nitrogen level.

Modulating aroma compounds during wine fermentation by manipulating carnitine acetyltransferases in Saccharomyces cerevisiae

The wine yeast Saccharomyces cerevisiae is central in the production of aroma compounds during fermentation. Some of the most important yeast-derived aroma compounds produced are esters. The esters ethyl acetate and isoamyl acetate are formed from alcohols and acetyl-CoA in a reaction catalysed by alcohol acetyltransferases. The pool of acetyl-CoA available in yeast cells could play a key role in the development of ester aromas. Carnitine acetyltransferases catalyse the reversible reaction between carnitine and acetyl-CoA to form acetylcarnitine and free CoA. This reaction is important in transferring activated acetyl groups to the mitochondria and in regulating the acetyl-CoA/CoA pools within the cell. We investigated the effect of overexpressing CAT2, which encodes the major mitochondrial and peroxisomal carnitine acetyltransferase, on the formation of esters and other flavour compounds during fermentation. We also overexpressed a modified CAT2 that results in a protein that localizes to the cytosol. In general, the overexpression of both forms of CAT2 resulted in a reduction in ester concentrations, especially in ethyl acetate and isoamyl acetate. We hypothesize that overproduction of Cat2p favours the formation of acetylcarnitine and CoA and therefore limits the precursor for ester production. Carnitine acetyltransferase expression could potentially to be used successfully in order to modulate wine flavour.

The EstA esterase is responsible for the main capacity of Lactococcus lactis to synthesize short chain fatty acid esters in vitro

AIMS

Esters of short-chain fatty acids and alcohols participate significantly in the overall flavour of foods. The capacity of the lactic acid bacterium Lactococcus lactis to synthesize such esters is known even though the enzymes involved in the process are not well identified. The objective of our work is to determine whether the esterase is responsible for the whole capacity of L. lactis to synthesize esters in vitro.

METHODS AND RESULTS

A negative mutant for the esterase was constructed and its capacity to synthesize short chain fatty acid esters from different substrate couples was compared to that of the wild type. We observed that the esterase is responsible for the main ester synthesis activity of L. lactis in vitro. However, in the presence of some substrates, the esterase negative mutant still synthesizes low amounts of esters.

CONCLUSIONS

In favourable environmental conditions, the L. lactis esterase is responsible for the main ester synthesizing activity, even though another pathway for ester synthesis probably exists.

SIGNIFICANCE AND IMPACT OF THE STUDY

Since esters are potent aroma compounds, esterase is probably a key enzyme in the development of food flavour.