Koji is an intermediate fermentation agent, made by inoculating known microorganisms in grains, such as rice, beans, and barley, to hydrolyze starch or protein. The quality of koji can influence the final quality of doenjang. This study aimed to investigate changes in the physiochemical and enzymatic activities of doenjang prepared with different amounts of rice koji during a 30-day fermentation period. Three doenjang samples were prepared with varying levels of rice koji: K1 (11% reduced), K2 (control), K3 (11% increased). Physiochemical characteristics including pH, TA, acid value, moisture content, color, sugar and reducing sugar content, and enzymatic activities including α- and β-Amylase, acidic and neutral protease activities. Samples were taken every 5 days for 30 days of fermentation period. The doenjang with a high content of rice koji had higher levels of total sugars, reducing sugars, alcohol, and protein enzyme activity than the doenjang samples with a lower content of rice koji (p < 0.05). However, no differences in the physiochemical and enzymatic activities were found between the doenjang made with a lower amount of koji and the control doenjang during fermentation (p > 0.05).
Keywords: enzymatic activities; fermentation; koji; physiochemical quality characteristics.