To foam or not to foam, that is a question of the yeast?

Sake brewers have long used the appearance of the foam to tell the stages of the moromi’s progress (fermentation progress). The named stages are as follows:

Pictures for each of these stages as seen through the foam can be seen at the Daishichi’s site.

In 1959, Mr. Takao Nihei of the Honolulu brewing company began brewing with a partially or mostly non-foaming yeast strain. He isolated the strain which had a higher percentage of non-foaming to foaming cells than are normally present. Later, a pure stain of yeast #701 was isolated from the then standard strain being used at the Honolulu brewing company. The way this was done was to take a fermenting mash and slowly bubble CO₂ up through the mash. The foaming strain would attach to the rising CO₂ and bubble over leaving the non-foaming yeast. After multiple prolonged fermentation sessions a virtually pure non-foaming stain exists that can be cultured to produce a pure strain.

Looking at these two strains under a microscope while in the presence of CO₂ bubbles is revealing. A CO₂ bubble with foaming yeast present literally has a shell formed by the yeast cells as they hug the surface of the bubble. CO₂ bubbles in the presence of non-foaming yeast are simply ignored by the yeast cells. What is the difference and why do non-foaming yeast cells ignore CO₂ bubbles while foaming strains don’t?

As it turns out, foaming strains of yeast have a protein on the surface of the cell wall that is hydrophobic (eschews water) and so adhere to gas bubbles. This protein has been named awa1p; awa (泡) being foam and p for protein. This protein is not present on or in the cell walls of non-foaming yeast strains. Going deeper, researchers have tracked the production of the awa1p protein back to a gene (awa1) in chromosome XV. Non-foaming yeast have the gene replaced with, in at least one case for yeast #7, the left subtelomeric region of chromosome IX. Without the awa1 gene the yeast become hydrophilic (embraces water) and readily mix with water while ignoring gas bubbles.

While the presence of the awa1 gene determines whether the yeast will be foam producing or not, it has very little if any effect on all the other characteristics related to sake brewing. So in choosing between a foaming and non-foaming version of a particular yeast strain your desire for foam is the primary if not all encompassing factor.