Although it’s a mouthful, yamahai-moto is a shortened version of the formal yamaoroshi haishi moto.Yamaoroshi refers to the practice of using a wooden pole to pound steamed rice and koji.Haishi and hai mean “stop”. So yamahai-moto literally means “the starter (moto) which eliminates the need for yamaoroshi. On the label, most sake shorten this further and only say yamahai.
The National Institute of Brewing Research (Nihonjyozoukyoukai) trialed use of “water-koji” (a mix of koji and water) to replace the labor-intensive pounding procedure. In doing so, they discovered that the enzymes can produce saccharification without pounding, simply by “punching down ” the water, koji and steamed rice to mix them thoroughly.
However, this works best at a slightly higher temperature than used for ki-moto.Therefore, the differences between ki-moto and yamahai methods are:
1. whether or not the koji-rice-water mix is pounded (ki-moto) or dissolved by the water koji enzyme (yamahai); and
2. temperature being 6-8℃ (ki-moto) or 9-10℃ (yamahai)
However even though yamahai-moto is a more scientific approach than ki-moto, some producers such as Kikumasamune in Hyōgo prefecture and Daishichi in Fukushima prefecture only use the ki-moto method for sake production.
Generally speaking,Yamahai-moto has less umami than ki-moto, but provides a pleasant complexity and authenticity of flavor.