As with acidity, the amino acid level of a sake or its amino sando levels are a measure of how much amino acid is in the sake. Amino sando along with the sugar levels largely determine the viscosity and chewiness of a sake.
Amino sando levels tend to vary between about 0.7 and 1.4. The lower the amino sando value the thinner the sake tends to be; this opens up the sake. Higher levels of amino sando are accompanied by higher viscosity and rounder flavors.
Umami, an important flavor component tied to the amino acid glutamate. As amino sando rises and falls, glutamate also tends to rise and fall, so reaching an ideal umami level is achieved by adjusting the amino sando levels. Researchers from Akita Prefectural University found that of the 20 or more amino acids in sake four are most important.
They found four amino acids that are strongly related to sake taste components and that the total concentrations are less important for taste than is the balance of these four. They demonstrated that through the manipulation of the choice of rice, milling levels, koji production and both yeast aerobic and anaerobic processes the balance could be controlled to create high quality sake.
As a side note related to amino acids: The reason the existence of iron in water to be use for sake is such a problem is that it, the iron, attaches to the center of a compound attached to an amino acid from koji. In this position it darkens the sake and changes the taste and aroma for the worse. In addition, iron speeds up a reaction between residual sugars and amino acids that changes the taste and aroma of the sake over time.