Quality evaluation of Korean soy sauce fermented in Korean earthenware (Onggi) with different glazes

Summary The aim of this study was to investigate the effect of porosity‐controlled earthenware as fermentation vessels for Korean soy sauce. Porosity of fermentation vessels was controlled by glazing the surfaces of Korean earthenware. Three kinds of onggis – the outside glazed, inside and outside glazed, and unglazed onggi – were made and used to investigate the effect of glazing on the fermentation of soy sauce. During fermentation of soy sauce in porosity‐controlled earthenwares at 30 °C for 4 months, physical, chemical, microbiological and sensory quality attributes were monitored. Compared to other vessels, soy sauce fermented in onggi with both inside and outside surfaces glazed showed less water loss (10.7%), salt content (17.6%) and pH (pH4.4) after the fourth month. It also produced higher total acidity (1.43%), protease activity (810  μ g mL −1  min −1 ) and microbiological changes that included total aerobic bacteria [4.3 log(cfu mL −1 )], lactic acid bacteria [3.8 log(cfu mL −1 )] and yeast [4.2 log(cfu mL −1 )]. The contents of total nucleotide (200–255 mg per 100 g sample) and free amino acids (4634–4848 mg per 100 g sample) in soy sauce were not consistent with glazing, which may be more affected by other factors, such as water loss, than the porosity of vessels. However, the percentage of glutamic acid among total free amino acids was 23.6% in onggi with both surfaces glazed, which was a little higher than the 21.9% in the outside glazed and 21.5% in the unglazed. These positive physicochemical and microbiological changes during fermentation in onggi with both sides glazed also resulted in higher sensory quality.