# Yeast Fermentation for Production of Neutral Distilled Spirits
## 2 Overview of Neutral Distilled Spirits Production
### Secondary Metabolic Products of Yeast
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Metabolite Class** & **General Flavour Characteristics** & **Examples of Compounds** & **Taste/Aroma** \\ \hline \multirow{6}{*}{Higher alcohols} & Within certain concentration & \(n\)-Propanol & Alcoholic \\  & limits, higher alcohols impart & Isobutanol & Pharmacy \\  & desirable flavour and aromas to & Isoamyl alcohol & Fusel, alcoholic, fruity, banana \\  & fermented beverages, notably in & (3-MethylButan-1-ol) & Rose petals, perfume \\  & distilled spirits. & 2-Phenylethanol & \\ \hline \multirow{6}{*}{Esters} & Ethly acetate & Solvent, acetone \\  & & Ethly butyrate & Pineapple, banana, mango \\  & These compounds impart fruity & Ethly caprate & Apple, aniseed \\  & and floral flavours and aromas to & Ethly caprylate & Apple \\  & fermented beverages and & Ethly hexaonate & Pineapple, unripe banana \\  & distilled spirits. & Ethly lactate & Butter, cream \\  & & Ethly octanoate & Sour apple, apricot \\  & & Isoamyl acetate & Banana, pear, fruity \\ \hline \multirow{6}{*}{Carbonyl compounds} & Above its flavour threshold, this & \multirow{2}{*}{Acetaldehyde} & \multirow{2}{*}{Green apple} \\  & compound can impart a “grassy” & & \\  & or “green apple” flavour. & & \\ \hline \multirow{6}{*}{Organic acids} & These compounds contribute in a & \multirow{2}{*}{Crosmic acid} & \multirow{2}{*}{Sour, acidic notes} \\  & beneficial way to the astringency, & & \\  & or “sharpness”, of fermented & & \\ \cline{1-1}  & beverages. The presence of some & & \\ \cline{1-1}  & acids, notably lactic acid, may & & \\ \cline{1-1}  & indicate undesirable & & \\ \cline{1-1}  & bacterial spoilage. & & \\ \hline \multirow{6}{*}{Polyols} & Glycerol is produced during & \multirow{6}{*}{Crosmic acid} & \multirow{6}{*}{Crosmic acid} \\  & normal yeast metabolism or when & & \\ \cline{1-1}  & yeasts are confronted with & & \\ \cline{1-1}  & osmotic stress. Glycerol & & \\ \cline{1-1}  & contributes desirable viscosity to & & \\ \cline{1-1}  & fermented beverages, & & \\ \cline{1-1}  & notably wines. & & \\ \hline \multirow{6}{*}{Vicinal diketones} & Diacetyl in some beers is & \multirow{6}{*}{Diacetyl} & \multirow{2}{*}{Diacetyl} has a rancid-butter, \\ \cline{1-1}  & undesirable, imparting a & & \\ \cline{1-1}  & rancid-butter or “buttersocotch” & & \\ \cline{1-1}  & flavour, but levels can be reduced & & \\ \cline{1-1}  & during beer conditioning. & & \\ \cline{1-1}  & Diacetyl is also found in & & \\ \cline{1-1}  & distilled spirits. & & \\ \hline \multirow{6}{*}{Sulphur compounds} & These are important flavour and & Hydrogen sulphide & \multirow{2}{*}{Rotten eggs, struck match, burnt} \\ \cline{1-1}  & aroma compounds but, at higher & Di-/tri-methyl sulphide & rubber. \\ \cline{1-1}  & concentration, are generally & & \\ \cline{1-1}  & regarded as undesirable. & & \\ \cline{1-1}  & & & \\ \hline \multirow{6}{*}{Phenolic compounds} & & 4-Vinyl guaiacol & \multirow{2}{*}{Spicy, smoly, clove-like, \\ \cline{1-1}  & & 4 Ethly phenol & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Important yeast-derived flavour congeners in distilled spirits. (Adapted from [10]). by condensation reactions between alcohols and acyl CoAs. Ethyl acetate is the predominant ester produced, with isoamyl acetate formed at lower concentrations to impart a fruity (banana) aroma to spirits. However, certain esters, notably ethyl lactate, are linked to lactic acid bacterial metabolism. Another group of flavour congeners derived from yeast metabolism that are important in distilled spirits are higher alcohols. Their production by yeast is related to the concentrations of amino acids in a fermentation medium. For example, phenylalanine stimulates phenylethanol production, giving rise to a rose-like aroma. In many distilled spirits, yeast-derived congeners are beneficial contributors to the flavour characteristics of distillates, if controlled within certain limits.