- Wild yeast
Yeast other than that deliberately inoculated for brewing, or of strains other than the main variety responsible for a healthy fermentation, is known as wild yeast. The great majority of wild yeast is thought to derive from sources such as mashing water (shikomi-sui), rice and kome-koji, and can grow well even in an environment rendered acidic by lactic acid. Wild yeast must be eliminated in order to raise pure shubo, and, in the case of kimoto-school shubo, the early growth of wild yeast is contained by the action of nitrous acid and lactic acid, causing it to die off gradually.
In the case of ko-on-toka shubo (high temperature saccharification shubo), it dies because of the high temperature (55-58°C) maintained. This is not the case with regular sokujo shubo (quick-fermenting shubo). Some wild yeasts have the capacity to consume the small quantities of nutrients in the medium at high speed, where others secrete substances with an inhibitory (“killer”) effect. These can suppress or kill the desired yeast, leading to contaminated moromi. For this reason, when making sokujo shubo, contamination is prevented by adding large amounts of pure yeast cultures at an early stage of mashing to prevent wild yeast growth.