Simply referred to as yamahai, yamahai-jikomi (yamahai-shikomi) is a method of brewing sake (Japanese rice wine) that belongs to the kimoto-kei (traditional moto, or yeast starter, developing method).
The official name for yamahai-jikomi is 'yamaoroshi-haishi-moto,' and the sake made from the moto is generally called 'yamahai' (yamahai-jikomi).
Simply put, yamaoroshi is a process in which steamed rice, koji (mold rice) and water are mixed and rammed until they turn into a porridge-like texture.
Before the Meiji period when sake brewing became modernized, yamaoroshi was extremely hard work: rice milling was conducted powered by a water wheel; then, steamed rice and water were put into a flat large tub and were patiently rammed with a mash paddle. Since the Meiji period, a new wave of industrial revolution had been gradually affecting the brewing industry in Japan. Rice milling was automated to some extent. As a result of this automation, rice was able to absorb enzymes from koji, and the yamaoroshi process of smashing steamed rice was not necessarily required.
Brewery workers tell each other about the essence of yamaoroshi in the following way, 'Break down steamed rice with koji rather than ramming with a mash paddle!'
The yamaoroshi-haishi-moto was developed in 1909 in the National Research Institute of Brewing established through the initiation of the Meiji government. It is the moto (yeast starter) representing the moto of the kimoto-kei. To develop the yamaoroshi-haishi-moto takes approximately two to four times longer than that of the sokujo-kei (faster moto developing method), normally requiring up to thirty days. Additionally, the yamaoroshi-haishi-moto is difficult to use since the utase (a part of the brewing process) can not be performed without maintaining the environmental temperature below 5 degrees Celsius.
The utase is a part of the brewing process from the aragai (stirring with a mash paddle) to the hatsudaki (the first warming process). In the case of the yamaoroshi-haishi-mot, the utase means the following processes: lactobacillus and nitrate reducing bacteria that improve the taste of sake are slowly developed to produce lactic acid at about pH 3.5 and to convert starch to sugar while the environmental temperature is kept low to prevent harmful wild yeast and bacteria that ruin sake from becoming active and growing.
Quality of sake
With these complicated processes, the sake made from the yamaoroshi-haishi-moto has a rich taste thanks to the high amino acid composition of the moto itself, delivering a solid taste and a deep, pleasant flavor. Therefore, the sake like premium whisky tastes the same even when mixed with water. Additionally, the yamaoroshi-haishi-moto has the advantage of being able to keep the effect of the moto intact even the karashi, the period from the completion of the moto development to its use in the brewing process, is prolonged.
However, yamahai-jikomi which requires a master brewer to have years of experience and talent as a producer carries a significant risk of going bad along the way. Yamahai-jikomi tends to be avoided nowadays because it requires a certain level of care; moreover, rationalization such as kobo-jikomi (brewing using cultured moto), kon-toka-moto (high heat saccharification moto), and chuon-sokujo-moto (medium heat saccharification moto) has caused many sake breweries to stop using the yamahai method.