Pasteurization

The Purpose of Pasteurization is to “denature” yeast and enzymes, therefore chemically deforming their shapes or actually causing death, in the case of yeast, so that they are unable to further change the flavor of sake. If there are no active enzymes, nothing can cleave sugars, proteins, or fats into smaller chains or individual molecules. This will prevent increases in acidity or sugar levels. If there is no active yeast, the sugar level will not decrease and the alcohol level will not increase.

There are many ways to achieve this goal, but the important information for those coming from other forms of fermentation is that sake enzymes don’t require quite the same amount of time and temp to denature. Competition sake is heated to between 62 and 65°C and immediately cooled. For some other products that require more stability, this might be extended to seconds or even minutes at the peak temp of 65°C, but Japanese sake doesn’t use the same Pasteurization Units (PUs) that you may be familiar with.

In the Tamonin Nikki (Diary of Tamonin, 1570), a method of pasteurization is described in which sake is heated to 63-65°C to kill harmful bacteria and inactivate remaining enzymes. It is well known that this was 300 years before the discovery of Pasteurization by Louis Pasteur of France. [Reference: 変わり行く日本酒]

Strategies

Depending on the intended flavor and aroma of the sake, decisions need to be made on how to pasteurize or even to pasteurize at all. Typically these are categorized into the following:

  • Do Not Pasteurize (Namazake - 生酒)

  • Pasteurizing before storage, but not before shipping. (NamaZume - 生詰め)

  • Store unpasteurized, but pasteurized before shipping. (NamaChozo - 生貯蔵)

  • Pasteurize before storage and before shipping (Twice Pasteurized - 二回火入れ)

These strategies represent high-level ideas of how much sake is allowed to change. For instance, by pasteurizing before letting it sit in storage for 3-6 months, the sake will change much slower and when it tastes ready, it will be most likely bottled, then pasteurized, then shipped immediately. Other strategies might want the sake to change much more, so it will remain fresh and the enzymes and residual yeast will be allowed to alter it further.

Flavor and Aroma Effects of Pasteurization

Aroma 

Increase: aldehyde, wood, spices, Caramel-like (honey, dried fruit, molasses, soy sauce), Sulfide-like (polysulfide)

Decrease: Ginjo scent, fruit-like, esters, alcohol (ethanol)

Flavor 

Increase: bitter taste, Pungent taste (smooth)

Decrease: astringency, stimulating taste (coarseness, roughness)