# Yeast Fermentation for Production of Neutral Distilled Spirits
## 2 Overview of Neutral Distilled Spirits Production
### Properties of Yeast for Neutral Spirit Production
\begin{table}
\begin{tabular}{c c c c c} \hline
**Yeast Attributes** & \begin{tabular}{c} **Malt Whisky** \\ **Fermentation** \\ \end{tabular} & \begin{tabular}{c} **Grain Whisky** \\ **Fermentation** \\ \end{tabular} & \
\begin{tabular}{c} **Neutral Spirit** \\ **Fermentation** \\ \end{tabular} &
\begin{tabular}{c} **Fruit-Based** \\ **Fermentation** \\ \end{tabular} \\ \hline Rapid fermentation & 2 & 4 & 4 & 4 \\ (e.g., 1–2 days) & 1 & 2 & 2 & 2 \\ \hline Continuous fermentation & 1 & 2 & 4 & 4 \\ \hline Alcohol yield & 2 & 4 & 4 & 4 \\ \hline Acceptable spirit character & 5 & 4 & 2 & 3 \\ (congener profile) & 4 & 4 & 2 & 1 \\ \hline Maltotriose utilisation & 4 & 4 & 2 & 1 \\ \hline Less yeast biomass & 2 & 3 & 3 & 2 \\ \hline Stress tolerance: & 2 & 4 & 4 & 2 \\ High gravity & 3 & 4 & 4 & 4 \\ Temperature & 3 & 4 & 4 & 4 \\ Ethanol & 2 & 3 & 4 & 3 \\ \hline Competitive (vs. bacteria) & 3 & 5 & 5 & 5 \\ \hline Fermentation consistency & 3 & 5 & 5 & 5 \\ Yield & 5 & 3 & 2 & 4 \\ Flavour & 5 & 5 & 5 & 5 \\ Yeast vitality & 3 & 5 & 5 & 3 \\ \hline Availability as: & 3 & 5 & 5 & 3 \\ Cream/liquid yeast & 3 & 1 & 2 & 3 \\ Cake yeast & 4 & 1 & 2 & 2 \\ Dry yeast & 1 & 4 & 4 & 2 \\ Freshly propagated & 1 & 1 & 3 & 1 \\ Recycled & 1 & 1 & 3 & 1 \\ \hline \end{tabular}  
* Qualitative descriptors: 1 = undesired (or not applicable); 2 = less desired; 3 = moderately desired; 4 = highly desired; 5 = very highly desired (essential).  
\end{table} 
Table 5: Distilling yeast desired attributes: qualitative criteria (adapted from [22]). Distilling yeast strains can be cultured _in house_ or supplied on a commercial basis by yeast supply companies [1]. Some small-scale and artisanal distilleries have adopted spontaneous fermentation that rely on indigenous microbiological flora, such as wild yeasts and bacteria. These microbes are found in the raw materials and are part of the fabric of distillery environments. Spontaneous fermentation have typically been exploited in small distilleries in Mexico (for Tequila and Mezcal production) and in Brazil (for Cachaca production), and their products are typically rich in congeners. Therefore, this is not an ideal practice for the production of a neutral spirit. In addition to _S. cerevisiae_, several non-_Saccharomyces_ yeasts can be used as pure starter cultures or can occur naturally in beverage fermentation. For example, _Schiz. pombe_ was found in molasses fermentation for rum production, and _K. marxianus_ strains can ferment lactose in cheese-whey to produce white spirits, such as vodka and gin. Several wild yeast species, such as non-distilling strains of _S. cerevisiae_, _Pichia membranefaciens_, _Toruspora delbrueckii_, and _Candidia_ species, can also be found in some distillery fermentations [9]. Although the levels of such yeasts can be deleterious in affecting fermentation progress, they are usually controlled at low cell numbers due to the predominance of _S. cerevisiae_ as the main yeast strain if used at high starting cell densities during production. However, another wild yeast, _Dekera bruxellensis_ (anamorph _Brettanomyces bruxellensis_), was found to dominate and out-compete _S. cerevisiae_ in wheat-based distillery fermentation [23].