ϊ{κ Atmospheric and Vacuum Distillation
Authentic shochu is typically distilled under normal pressure or reduced pressure.
In recent years, there are also products that are distilled under a slight vacuum, which falls between these two methods.
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Atmospheric Distillation (left diagram)
Atmospheric distillation is a traditional method carried out at the same pressure as the atmosphere. Since atmospheric pressure does not affect the boiling point, the mash is boiled at around 85-95 to extract alcohol. The high temperature promotes chemical reactions, leading to the formation of chemical compounds through heating. Additionally, non-alcoholic components are more likely to vaporise. As a result, shochu distilled under atmospheric pressure tends to have a rich flavour.
Vacuum Distillation (right diagram)
Vacuum distillation is a method developed by a sake brewery in Fukuoka Prefecture in 1973, where distillation is conducted under lower pressure than atmospheric pressure. As the pressure decreases, the boiling point of compounds also decreases, allowing alcohol to be extracted at a low temperature of 45-55. Due to the low temperature, there is minimal decomposition of floral aromas and evaporation of flavour components, resulting in a light and floral shochu.
The above information is an excerpt from the Japan Sake and Shochu Makers Association's website, "Understanding Authentic Shochu and Awamori."