Moromi is a general term for a liquid of starter with steamed rice, water and koji. In moromi, like the starter, starch is dissolved into glucose, which is converted into alcohol. This process is called“Multiple parallel fermentation” because saccharification and the alcohol producing process are carried out parallel. Only a few types of alcohol beverage, such as Shaoxingjiu (Shaoxing rice wine) from China and Korean makgeolli use this technique.During this process, it is very important to achieve a certain level of alcohol to create flavor while protecting from contamination.
When the starter is finished, yeast dominates the starter and alcohol by volume (ABV) reaches around 10 – 12%.To reach around 18% ABV of the final product, water, koji and steamed rice are added for additional fermentation.
Generally speaking, the process to add steamed rice and water and koji is conducted in three stages over period of four days. This slow rate of addition is to maintain a steady rate of fermentation and protect from contamination.
Since the starter is protected from contamination by lactic acid, the anaerobic environment caused by the sake yeast’s domination and also alcohol they produce ensures there is little risk of contamination of the starter itself. However, since the starter only accounts for around 8% of the volume of the final product, the lactic acid and yeast-rich environment are diluted if the rice-water-koji mix is added all at once.
Additionally, the pace of saccharification is faster than that of fermentation and yeast is not resistant to osmotic pressure, so if a lot of steamed rice is added, the saccharification impedes the yeast activity and fermentation balance is upset.