It was another seven years before the apple yeast could actually be used in sake production. Initially, apple yeast was added and the sake was brewed in the traditional manner. But by the time the sake was pressed the apple yeast had completely disappeared and the resulting sake had none of its unique and favorable qualities. This is because during the fermentation process, the apple yeast was overwhelmed by stronger yeasts naturally occurring in the brewery.
To overcome this issue, Kiyoma embarked on the development of a new brewing method. In 1947, he perfected his method for producing the starter mash (shubo) and called it the high-temperature starter mash saccharification method.
Steamed rice, koji rice mold and water are mixed to produce the moromi mashin the sake production process. If the yeast is added to this large moromi mash, it takes a very long time to multiply and permeate, and there is greater potential for wild yeasts and other microorganisms to infiltrate the mash. To prevent this, and ensure the cultivation of a pure mixture abundant in the desired yeast, a starter mash known as shubo is first produced. It is approximately 1/12 the size of the required moromi mash, and the Japanese characters for shubo refer to “sake” and “mother”, much like the “mother” used for bread-making.
During the production of the shubo starter mash, the yeast multiplies approximately 2000 times. Subsequent to this step, the yeast only multiplies about 12 times. This means that as long as the mix contains the desired yeast in pure form at the starter mash stage of the process, the impact of other yeasts subsequently entering the mixture will be negligent.
However, in the traditional production method for the starter mash, the mixture is heated to just 20°C(68°F), making it very easy for potent yeasts present in the brewery, in the koji or on various implements, to multiply and change the nature of the mixture. When the mixture gets overrun by the vast numbers of brewery yeasts at the starter mash stage, the desired yeast loses out. It is extremely difficult to avoid this issue, however, because the koji used in sake production is prepared by hand using many cloth and fabric implements.
In contrast, in the first step when the steamed rice, koji and water are mixed during the high-temperature starter mash saccharification method developed by Kiyoma, a temperature of 55°C (131°F) is maintained for 8 hours. These conditions allow for the creation of a starter mash free of microorganisms, making it possible to produce a starter mash containing no brewery yeasts.
In addition to the sterilization effect, maintaining 55°C (131°F) for 8 hours means that the enzymes in the rice mold, which do not stand up well to heat and become inactive at approximately 60℃(140℉), are not destroyed, and the rice mold can be completely and effectively converted into sugar. Thus it is clear that when creating the starter mash, the temperature can neither be too high or too low.
At the end of the 8-hour period, the mixture is cooled to 20°C (68°F) after which the apple yeast is added. The yeast is allowed to multiply in the mixture for approximately 10 days resulting in a purely apple yeast starter mash. Apparently close to half of all Japanese breweries now employ the high-temperature starter mash saccharification method when producing sake for submission to national competitions. The influence of brewery yeasts is clearly a negative factor during judging of new sake varieties from around the country.
In recognition of the major contribution made to the sake industry, the high-temperature starter mash saccharification method was awarded the Inaugural Prize for Services to the Craft of Sake-Making by the Japan Sake and Friends Association in 1955, eight years after its discovery.