Moto

(Shubo / Yeast Starter) 

The purpose of making a moto ( also known as a shubo) is to grow up your yeast cell count and condition those yeast cells so they are healthy enough to withstand the osmotic pressure caused by a substantial sugar concentration in the moromi and the growing concentration of alcohol which is poisonous to the yeast cells (see: autolysis). In addition to fermentation benefits, a moto also has economical benefits, the most obvious is that you don't have to buy as much yeast. The moto is prepared before you start your main fermentation and can take 1-4 weeks to complete. Read more about Sake Yeast here

An alternative to making a moto is a procedure called Kobo-shikomi, in which you just build up a large enough yeast cell concentration to add directly into the Hatsuzoe (soe). By doing this, it will be much more costly to make the batch, but will cut at least 1 week off your production calendar. However, it should be noted that there are adverse effects, such as removing the benefits of building up cell walls, and forcing a lot of stress on the yeast. This can be overcome to some degree by “over-pitching” (using much more than needed) your yeast, but that has other effects such as speeding up the ferment and causing high-acidity in your final sake. Compensating for these can be difficult and should be considered before choosing this method. If you are making a home-brew size that is less than 25L, you probably don't need any moto at all, because the cell concentration you are using from the ampules or pouches contains enough cells already and will likely not require any build up to achieve a successful fermentation. 

How to think about yeast pitches: 

What does Yeast Pitch mean? “Yeast pitch” in the fermentation world just means “amount and concentration of yeast you put into your fermentation”. As many sources will confirm, the concentration of yeast cells you are looking for by the end of your moto is 10^8 cells / mL. This just means that every milliliter of moto will contain 100,000,000 cells. If you were to buy a small vial of yeast it might cost $8-20 for 10 mL of cells. Wyeast and Whitelabs are two popular sites for purchasing yeast in the United States. Wyeast boasts 100 Billion Cells in their 125gram package. However, as yeast cells go, chances are 50% will be gone by the second day, so it's already like 50 Billion, and if it goes through the mail and sits in warm areas… you guessed it… probably a lot less. Cell death will cause off flavors in your sake, so preventing this is very important. Try to use yeast that is as fresh and healthy as possible. Check out this yeast calculator to help with the heavy math.

How do I order yeast? Yeast Pitch is often referred to in ways like “what is your yeast pitch size?" or “what is the cell concentration of your yeast pitch”. In the beer world, because fermentations were/are measured in Barrels, like some form of arcane throw back to the good ol' days, rather than switching to … liters … [end rant], you might read something like “we use a 15 bbl pitch”. This is basically referring to a scale of your fermentation. If you had a tank that held 15 barrels of wort, then that's what you would order for a style type of beer recipe and the yeast would be calculated so that when you poured it in, the concentration of cells would be enough to sufficiently ferment the sugars available in that volume of beer without becoming overwhelmed by faults listed above, such as autolysis. We're going to show you equivalents for things, but because yeast suppliers typically supply commercial breweries in beer pitches, it's important to try to think in terms of liters and bbl ( "beer barrels" abbreviation ). 1 bbl is the same as 31 gallons / ~283.4 pints / ~117.3 liters. 

How a moto is prepared:

A moto is basically a small version of your fermentation. You provide it lots of oxygen through mixing it daily and that will allow the yeast to perform their aerobic metabolism (basically reproduction). This is different than the metabolic pathways that will create alcohol, but even if the majority of the yeast are performing aerobic fermentation, there are still others that are not, so alcohol will be produced regardless. This mostly has to do with how much free oxygen is suspended in the liquid. It's always important to remember that in any system, there are bound to be many different scenarios occurring at once, even if the document you read contains a more hyperbolized version of the events. 

Types of Moto (Shubo):

This section is HEAVILY informed by an episode of Sake Deep Dive by Jim Rion and Andrew Russell. I encourage you to listen to their podcast and also checkout Sake Today, and Aramasa's blog article on the subject.

There are three main types of yeast starters and one much less common these days. The most common today, Sokujo, is the most recently developed because it only existed since the discovery of microbiology, specifically the role that Lacto Bacillus plays in creating a favorable environment for growing yeast. All moto rely on having a low enough pH in the starter mash so that yeast have a safe environment to multiply. Here we'll take you through the iterations of this process in order of discovery (high-level descriptions):

• Type 1: Bodaimoto - Taking its name from the Bodaisen Temple in Nara, this is the earliest version of today's more common yeast starter, which is differentiated from the subsequent iterations mainly in that is uses only steamed rice and water. There is no koji used until the main yeast starter mash, meaning this one actually has 2 steps. After 3-8 days of letting this basically collect wild yeast and bacteria, the acidity will increase, mostly due to lactobacillus, and you'll notice CO2 bubbling up. At this point.. the mash is filtered to remove the rice. This liquid is called soyashi-mizu, to which more cooked rice as well as rice-koji are added. This sequential order of making a lactic solution and then pitching yeast along with your rice and koji differentiates it from the subsequent Kimoto System. During this next phase, temperatures are kept high for 6 more days, and then there should be enough yeast to begin the main fermentation mash.

• Type 2: Kimoto System (生酛系, kimoto-kei) - About 300 year later (first mentioned in 1687), someone figured out if you put steamed rice, water, and koji in it, keep the contents cold for the first few days, and then mix the contents with poles to pulverize the rice, the mixture seemed to have less off flavors. They would carry out this style in the winter because it is easier to keep cold and thus it was part of a style known as 寒酒, or “cold sake”. The first difference from previous methods is that the lactic fermentation happens all at the same time, in the same tank, rather than the two step method described above. The second is probably the more important, which is what is known as Utase (打瀬) or “knocking down", referring to the prevention of contamination. This is part of the yeast mash gathering  (酉元寄せ) process. In fact another name for Kimoto is Sodatemoto (育って酛) “raise the moto”, using a character synonymous with raising children.
  
  The Utase phase keeps temps cold for a few days to allow rice and koji to work their magic and generate sugar without the interference of ambient yeast. Lactobacillus will survive at these temperatures and will produce lots of lactic acid, lowing the pH to around 3.5-4. This acidic mash is called also called “nama moto” which could be where 生酛 (生 can be pronounced “nama” or “ki”) gets its name. Because bodaimoto didn't include this part, the moto was basically allowing anything to grow and establish itself. Even in modern times, many of the temples that make Bodaimoto in Japan still throw out batches because you just never know what's going to grow.
  
  After the low pH mash is created, pure yeast cultures are pitched in, the temp is slowly raised, and the yeast will start to multiply. After 2 more weeks the concentration on yeast cells can be as high as 200 million cells per gram.
  
  Kimoto-kei is an umbrella term for Yamahai Moto, Te-moto, and others. Basically, centering around this core utase principal. 
  
  Unlike Sokujo (described below) and Bodaimoto, which starts with higher temps to dissolve the rice quicker, the rice will take longer to break down at cooler temps. [describe Ikimeshi and the purpose]
  
  It should also be noted that there is no legal definition for kimoto, which further complicates this whole description, and will vary from brewer to brewer. Here is one example from kiku-masamune

• Type 3: Yamahai (山廃) - The full name “Yamaoroshi Haishi” means “omission of pole mashing”. Books Another 300 years pass before a Japan got super serious about science and Professor Kinichiro Kagi (嘉儀) discovered that you did not need to smash the contents with poles in order to melt the rice to provide sugar for the bacteria to grow. Instead, more water and higher temperatures could be used to achieve the same goal. They didn't even need to separate the moto into separate tanks. It could all be done in the same one. I'm sure there were plenty of kurabito that were rolling in there grave when at all the endless hours spend mixing for seemingly no reason, some brewers however, still continue the practice, and it is believed that there are other qualities that are developed through the Kimoto method that Yamahai doesn't offer. You'll have to try it for yourself. Yamahai is also about 4 weeks, you just don't need to smash rice while it happens.
   

• Type 4: Sokujo (速醸) - So, as one might imagine, as soon as it was realized that “pole-ramming” wasn't necessary to propagate yeast, it was also discovered that you could grow lactic acid, add a certain amount to the rice, water, and koji, and it would drop the pH low enough that you didn't even need to wait for lactic bacteria to do it for you. So 21-28 days became 5-10 days and the Sokujo moto was born. About 99% of all sake made uses this procedure and its not just because it is easier, it also has a drastic effect on flavor and reliability.

Role of pH in Moto

The point of a moto is to increase our yeast cells. We want to give them the best chance we can to replicate unimpeded by other environmental factors and free from resource competition. If other organisms exist in the same space, they will eat the yeast’s food and not only would that mean we'd have fewer, weaker yeast cells, but we'd also have bacteria that would produce off flavors and aromas. If you have ever tried to capture your own yeast, you might have tried some of the products that these organisms produce, for better or worse. Some bacteria can even create alcohol, acids, etc. These can be lovely in the right circumstances, but more often than not, they produce very unwanted smells and flavors.

By lowering the pH to around 3.5 or even 3.0 in the moto (or in the soe addition in the case of kobo-jikomi), we allow the sake yeast to perform its metabolism unimpeded, growing a dense cell culture and preparing for the main mash. This topic leads into the purpose of “San-Dan-Shikomi" or "three stage preparation", because it was calculated that by building the mash slowly, it gives time for the yeast to replicate, consuming all the nutrients in the mash before bacteria have a chance to do the same. If the moto was added to a full tank all at once it would be at risk of contamination because it would take several days for the yeast to consume the sugars, providing opportunities for other bacteria to feed. We could technically add more lactic acid during the moromi, but that would produce unwanted flavors as well. Rather, we just keep the pH low during the moto and the yeast will replicate by the time we get to the next addition. 

Calculations of moto size

The larger your moto ratio, the most acidic the fermentation will likely be. This is because you will typically drop the pH of the yeast starter using lactic acid and those volumes are based on the quantity of water in the moto. So, if you have a smaller size moto, it will be more diluted in the final sake. So while 7% is just fine, don’t be surprised if it comes out over 2.0 total acidity.

Koji Variations for Moto

Some moto are going to require more saccharification than others. This is completely a style decision, but you might encounter scenarios where koji is made differently in order to target a faster “melting” of the rice in order to promote yeast growth. For instance, ginjo temperatures will put a lot of stress on yeast because it forces them to live a long time in order to hit the required alcohol level and desired flavors. Koji at the beginning might be tailored to help generate lots of glucose and melt the rice, but in order to provide a consistently higher level of glucose throughout the ferment, the subsequent additions will actually use less spore and often reduce the time spent in the koji room during peak temp in order to cut down on the enzyme production, therefore slowing down the rice melt.

Other variations might be the spore type. For instance, one could actually skip using lactic acid if they are using a spore type that produces citric acid naturally, such as shochu koji spores. This is a odd use-case, but worth noting the diversity in koji moto decisions.