# Yeast Fermentation for Production of Neutral Distilled Spirits
## 2 Overview of Neutral Distilled Spirits Production
### Properties of Yeast for Neutral Spirit Production  
The general desirable characteristics of distilling strains of yeast exploited for production of different distilled spirits are outlined in Table 5. When considering yeasts used for the production of flavoursome distilled spirits (e.g., malt whisky), rather than neutral alcohol destined for gin and vodka, the desired yeast attributes have been focused on congener flavour development, rather than ethanol yield [21]. Conversely, for the production of neutral spirits, the desired attributes of yeasts include rapid fermentation with high alcohol but low congener yield and tolerance to environmental stresses. The physiological \begin{table}
\begin{tabular}{c c c} \hline
**Alcohol Type** & **Yeast Involved** & **Comments** \\ \hline \hline  & & Scotch whisky producers currently use selected distilling strains of _S. cerevisiae_ in three main formats: cream yeast, pressed (cake), and dried yeast. Malt whisky distilleries traditionally use pressed yeast, but larger grain distillers have now adopted cream yeast. Dried yeasts are not as prevalent as pressed and cream formats in whisky fermentation; however, they are becoming more popular with smaller scale producers due to their shelf-life stability at room temperature. \\ \hline \hline  & & _S. cerevisiae_ strains in run fermentation are developed as starter cultures and provide faster fermentation with higher alcohol levels and more fatty acids, but fewer esters, resulting in lighter styles of rum. \\ Rum & _S. cerevisiae_ and _Schiz. pombe_ & _Schiz. pombe_ in rum fermentation provides slower fermentation for lower alcohol levels and fewer fatty acids but more esters, resulting in heavier styles of rum with stronger aromas. Growth of _Schiz. pombe_ is favoured by low pH with higher sugar concentrations. \\ \hline \hline  & & Various yeasts have been isolated from such processes: _S. cerevisiae_, _Kluyveromyces marxianus_, _Pichia spp._, _Bretanomyces spp._, _Rhodototumal_ spp., etc. [20]. \\ \hline \hline  & & For brandies, cognac, etc., the base wine is produced by pure starter cultures of _S. cerevisiae_. For gin, vodka, etc., selected distilling strains of _S. cerevisiae_ are used. \\ \hline \hline  & & Lactose-fermenting yeast produce ethanol destined for gin, vodka, and cream libueurs. \\ \hline \end{tabular}
\end{table} 
Table 4: Yeasts employed for production of distilled spirits. (Adapted from [19]). and biochemical characteristics of a distilling yeast strain for neutral spirits are strongly influenced by the chosen fermentatable substrate, which could either be sugar-based (e.g., molasses) or starch-based (e.g., wheat). The following salient features of yeasts to be employed specifically for neutral spirit fermentation should be considered:  
* Fast utilisation of all available fermentable sugars
* Production of high alcohol yields at the end of fermentation
* Good thermotolerance (e.g., ability to ferment at temperatures as high as 37 \({}^{\circ}\)C)
* Non-flocculent behaviour (aggregation of yeast cells into sedimentary clumps)
* Tolerance to osmotic stress (e.g., high-sugar molasses and high-gravity cereal word)
* Low production of flavour compounds (congeners)  
Regarding the latter attribute, several commercially produced yeasts are available that inherently synthesise low levels of congeners, such as higher alcohols, aldehydes, and esters. Although higher alcohols account for the majority of congeners present, the esters are the most influential congener group formed during fermentation from a flavour perspective. Figure 3 compares the ester profiles of a cereal word fermented with one such neutral spirit yeast, to a whisky yeast. Although the overall concentration of esters present remained lower when using a neutral yeast, this difference was reduced at higher temperatures.