Log phase / Exponential phase
Log phase
Japanese: 対数増殖期（ログフェーズ）
The period when microorganisms divide and multiply at maximum speed. With ample nutrition, cell numbers increase exponentially. Metabolic activity is highest and enzyme production is most vigorous.
The log phase is when microorganisms multiply most actively. Cells divide at regular intervals, and population numbers increase exponentially—logarithmically.
During this period, metabolic activity is at its peak and enzyme production is vigorous. In koji-making, the rapid rise in substrate temperature occurs because log-phase koji mold generates enormous amounts of heat. Controlling this heat generation is where the art of koji-making shines.
For bacteria like E. coli with a 20-minute generation time, the population could theoretically increase over a billion-fold in 12 hours. In practice, nutrient depletion and waste accumulation create a ceiling, but this growth potential speaks to the remarkable vitality of microorganisms.
Understanding the log phase directly connects to the ability to read fermentation timing.