Study of volatile organic acids in freeze-dried Cheonggukjang formed during fermentation using SPME and stable-isotope dilution assay (SIDA)

Volatile organic acids in freeze-dried Cheonggukjang were quantified using a stable-isotope dilution assay (SIDA) according to the fermentation period. Five organic acids, acetic acid, propanoic acid, 2-methylpropanoic acid, butanoic acid, and 3-methylbutanoic acid, were identified using solid-phase microextraction (SPME) in conjunction with gas chromatography-mass spectrometry (GC-MS). The contents of volatile organic acids in Cheonggukjang were highly dependent on the fermentation period and they increased during fermentation. Moreover, the branched-chained organic acids (namely 2-methylpropanoic acid and 3-methylbutanoic acid) were formed earlier and were present at much higher contents than the corresponding straight-chained organic acids during Cheonggukjang fermentation.