Technical Yearbook 2023

floral) and propyl acetate (pear). Common fatty acid ethyl esters produced in this pathway include ethyl decanoate (grape/brandy aroma) and ethyl butyrate (fruity pineapple). Off-odours on their own might be off-putting, but in small amounts and/or in interaction with other aroma compounds, they contribute to aromatic complexity and can greatly enhance the bouquet of the wine. Yeast species and strain selection for aroma production The production of aromatic compounds from amino acids and ammonium can vary greatly depending on the metabolic flux of the species and the strain of yeast in the fermentation. Some yeasts may be more efficient at utilising different amino acids as nitrogen sources. Similarly, some might channel more amino acids into fermentative aromas, while others will mostly produce these compounds from sugars. This partly explains why so many active dry yeast products are available on the market. These yeasts have been specifically selected for their aroma compound Winemaking conditions, such as temperature, oxygen levels and nutrient availability, can also impact the production of aroma compounds. Higher temperatures result in increased evaporation rates and loss of sensitive aroma compounds. This includes the esters, which are especially sensitive to high temperatures. Indeed, at 24°C, most esters produced during the fermentation will be lost due to evaporation. The presence of oxygen limits the production of acetate esters from higher alcohols. Higher levels of oxygen lead to an accumulation of higher alcohols. Fermentations that have been oxygenated, such as red wine fermentations, also result in lower levels of fatty acids and ethyl esters. The levels of lipids in the grape must also affect the production of esters. Higher levels of lipids have been correlated with lower concentrations of acetate esters as the enzymes converting higher alcohols to acetate esters are repressed. On the other hand, lipid deficiency can also result in slow or stuck fermentations. Besides the source of nitrogen, the levels of yeast assimilable nitrogen (YAN) also play a large role in producing aromatic compounds. Grape musts with higher levels of YAN tend to produce higher levels of fruity esters and lower levels of undesirable higher alcohols and hydrogen sulphide (H 2 S) during fermentation. If the YAN is too low, yeasts must produce their own amino acids, including the sulphur containing amino acids methionine and cysteine. To obtain the sulphur necessary to produce the latter two amino acids, the yeasts take up sulphate from grape juice. During the reduction of sulphate to sulphur, they produce SO 2 and H 2 S. Excess accumulation of these intermediates results in their liberation into the fermenting grape juice, with the latter generating the well-known rotten egg off-odour in the wine. production to suit any requirement. Impact of winemaking conditions

Differences in SO 2 and H 2 S production between yeast strains remain poorly understood, but are typically connected to their differing nitrogen requirements. YAN supplementation in nitrogen-deficient musts will ensure that (1) yeasts produce sufficient biomass and avoid stuck fermentation and (2) produce sought-after aroma compounds. It is now well understood that a single addition in grape juice of a large amount of nitrogen (especially ammonium) is likely to induce very high biomass production coupled with a very high, but short-lived, fermentation rate accompanied by excessive temperature increase. A sluggish second phase of fermentation that may even become stuck is common. A better solution is to split the addition as follows: before fermentation in the grape juice (a source of organic nitrogen) and the second addition when yeasts stop growing after around one-third of the sugar has been depleted (organic and/or inorganic nitrogen). Indeed, adding an organic source of YAN at the beginning of growth will ensure that sufficient biomass is produced, while not increasing the fermentation rate excessively. YAN supplementation during the yeast growth phase during fermentation is also observed to increase fatty acid ethyl esters, acetate esters, as well as ethyl acetate, but decrease levels of higher alcohols. The supplementation of YAN at the beginning of the stationary phase will “boost” fermentation kinetics and the production of aroma compounds. Nevertheless, significant differences are observed in the production of specific higher alcohols. This is mainly because, as mentioned above, some of the compounds mostly originate from sugar metabolism, while others mostly originate from amino acid metabolism in a yeast species/strain-dependent manner. Finally, the timing of addition seems to play a role, but once again, the impact differs depending on the aroma compound. Indeed, recent results showed that the addition of nitrogen during the stationary phase increases the production of sought-after acetate esters by inducing transcription of the genes coding for the enzyme that converts higher alcohols to acetate esters. It is, however, advised not to add excessive amounts too late, because the fermenting yeasts will not be able to take them up fully, and the remaining amounts can be used by spoilage organisms after the completion of fermentation. YAN addition to grape must It is permissible in most wine regions to raise the YAN of grape juice with the addition of ammonium salts in various forms. This includes the regularly utilised diammonium phosphate (DAP) and ammonium sulphate, which should be used when confronted with severe YAN deficiencies. However, whereas the addition of specific amino acids to grape must to “flavour” the final wine by influencing the production of specific aromatic compounds is not permitted, that of inactive yeasts (containing ammonium, amino

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TECHNICAL YEARBOOK 2023