New Sake Brewer - Oxygen Questions

[Tim Hoke]

Hey All.  New sake brewer here.  I owned a craft brewery in Virginia a few years ago, lost the business, and ended up moving to the West Coast.  I've been scouring every book and forum to get myself as educated on the science of sake brewing as I can.  I'm one of those over the top brewers that likes to control as many factors as I can.  Wanted to throw a few questions out that I've had a hard time finding answers to. I have ordered a full kit from Will at HomeBrewSake (premade koji, rice, yeast, brewing salts).  Wanted to be as ready as I can when the materials arrive. If anybody has used his kits, love to hear any feedback.

1. I have my old brewing equipment and was curious if anyone uses a stir plate to begin the yeast starter versus mixing the night before and letting it wit.  It was the number one practice that improved my fermentations with beer, but couldn't find any references to agitation prior to pitching for the shubo.

2. Every reference I see to the shubo and main fermentation steps have it in open containers, moving to a typical fermenter in secondary.  I'm used to having my fermentations sealed and a blow off tube for CO2. Is there a reason the mash needs to be exposed to air the whole time?  With stirring every 12 hours, I am assuming a continuous exposure to additional oxygen through the stirring process.  Does the koji conversion process contribute sufficient sterols to offset the oxygen consumption of the yeast?  What are the oxygen requirements? Can I oxygenate my brewing liquor prior to adding it in each of my 3 additions to increase yeast function?

So glad to find other brewers to bounce ideas off of.  We have a handful of breweries here in CA, and I'm planning on reaching out to them as well for advice too. You all do the same? 

Cheers, Tim

[Timothy Jacoby]

Hey there Tim,

I am citing this ONE paper:

Global Gene Expression Analysis of Yeast Cells during Sake Brewing▿ †

Hong Wu,1 Xiaohong Zheng,1 Yoshio Araki,1 Hiroshi Sahara,2 Hiroshi Takagi,3 and Hitoshi Shimoi1,*

Appl Environ Microbiol. 2006 Nov; 72(11): 7353–7358.    

Which states, among other things:

"Ergosterol is an important constituent of the plasma membrane, and ergosterol content is important for ethanol tolerance (8). Because its biosynthesis requires molecular oxygen (9, 18), yeast cells under anaerobic fermentation conditions require oxygen during the cell growth phase. In fact, aeration to supply oxygen is frequently performed in wine making (22). Sake brewing does not require the addition of oxygen because ergosterol is provided by koji, which is cultured in aerobic conditions and contains ergosterol. As shown in Fig. ​Fig.3B,3B, however, expression levels of many genes related to ergosterol biosynthesis increased during the cell growth phase, and their signal intensities were relatively high throughout the sake brewing process (data not shown). It has been reported that expression of some ergosterol synthesis genes are induced in anaerobic conditions (19). In microarray analysis of wine fermentation, sufficient ergosterol was added to facilitate yeast cell growth, and increased expression of the ergosterol synthesis genes was not observed (22). Taken together, these data seem to show that ergosterol supply is not sufficient during the cell growth phase in sake mash and that expression of ergosterol biosynthetic genes is therefore induced. "

My take is: I may be wrong, but if you are growing yeast cells in rice and koji, then you have enough O2 (sterols).

If you grow your cells in barley wort, then you do not have enough O2.

But, then they say that "some ergosterol biosynthesis genes were turned on throughout the sake brewing process". They do not show this data!

Then, " these data seem to show that ergosterol supply is not sufficient during the cell growth phase in sake mash and that expression of ergosterol biosynthetic genes is therefore induced"

Your sake is exposed to air because you need to get in there and stir to agitate and help break down rice solids/ release trapped CO2,

thus it is not an O2 thing?

And it's the right way because that's the way it has been perfected over the years?

SO... Sake brewing does not require the addition of oxygen, because koji supplies ergosterols. Then the yeast turn on the genes that are involved in ergosterol synthesis, because ergosterol supply from koji is not sufficient during growth phase. But during the anaerobic phase, the related ergosterol gene expressions remain at a relatively high level. So the yeast might be making more ergosterol as alcohol concentration increases? Maybe, but they don't say that in this paper.

Clear as mud!

Tim