# Yeast Fermentation for Production of Neutral Distilled Spirits
## 2 Overview of Neutral Distilled Spirits Production
### Substrates and Wort Preparation for Fermentation  
Regarding the starting carbohydrate materials for yeast fermentation, these can either be starch-based or sugar-based, and examples are provided in Table 2. Although this Table shows the typical spirit types produced from each starting material, it is possible to produce a neutral spirit from any substrate if the correct distillation processes are followed (see Section 2.5). Despite the resulting spirit being classified as neutral, different substrates can still introduce different qualities into the final spirit for sensory perceptions, such as mouthfeel and organoleptic characteristics. Figure 1: Summary of the production of diverse alcoholic beverages. The role of yeast, particularly _S. cerevisiae_, is pivotal in conversion of fermentable sugars to ethanol in all the beverages indicated. (Adapted from [9]). Processing of starch-based sources is more involved, as compared to sugar-based sources since the former necessitates pre-hydrolysis, whilst the latter does not. For example, molasses requires relatively straightforward pasteurisation, dilution, and nutrient supplementation prior to fermentation without polysaccharide pre-hydrolysis. Molasses, a by-product of sugar refining, has also been effectively fermented and distilled to produce rums and neutral spirits. Sugarcane molasses-based distilleries for the production of rectified ethanol are commonplace in many countries, notably Brazil and India (e.g., [11]). Similarly, fermentable sugars for fruit-based spirits (glucose, fructose, and sucrose) are simply extracted by physical means (e.g., pressing of grapes to yield wine musts). Being a polymer of glucose that cannot be fermented directly by yeast, starch requires hydrolysis (or amylolysis) for the conversion to fermentable sugars. Such processes firstly involve particle size reduction by milling, cutting, pressing, or grinding, depending on the starting raw material. This is then followed by mixing with water, unless the raw material is of a sufficiently high moisture content, and high-temperature hydration to gelatinise the starch. The temperature at which gelatinisation occurs, that is, the hydration, swelling, and bursting of the granules into which the starch is packed, is specific to each raw material. In the subsequent mashing stage, starch-degrading enzymes (amylases) then liberate the fermentable sugars into a liquid, known as _word_. Note, that for Scotch whisky production, exogenous enzymes (commercially available) are not allowed to be used due to legislation, and therefore, the saccharification of cereal starch is accomplished solely by the amylolytic enzymes present in barley malt. These are known as endogenous enzymes. The sugars fermented by yeast, predominantly glucose, maltose and maltotriose, are liberated from starch during the mashing stage by barley malt enzymes. However, for spirits to be used for gin and vodka production, namely grain neutral spirit (GNS), exogenous enzymes are allowed for conversion of starch (mainly from wheat or maize) into fermentable glucose. Starch is formed of two main structural components, amylose and amylopectin. Amylose is linear, whereas amylopectin is highly branched. Therefore, the enzymes applied must be able to hydrolyse both linear (\(\alpha\) 1-4) and branched (\(\alpha\) 1-6) glucoside linkages. Such starch-degrading enzymes are \(\alpha\)-amylase and amyloglucosidase (also known as glucoamylase),