Traditional Nuruk, one of the basic ingredients to brew Korean alcoholic beverages, is pointed out for its quality inequality and lack of stability. To address these problems, improve the quality of Korean alcoholic beverages and develop more practical technology in the field, studies on standardization and quality improvement of Nuruk are necessary. In this study, pellet-type Nuruk was produced by using Aspergillus oryzae 83-10, Rhizopus oryzae CN084, and Lichtheimia ramosa CN044 selected as effective brewing microorganisms from traditional Nuruk. By doing so, this study reviewed the characteristics of pellet-type Nuruk and its brewing characteristics according to raw material conditions and inoculated strains.
As a result, as for raw material conditions, using sterilized and heated raw materials, rather than unprocessed raw materials, was more stable in making pellet type Nuruk and more appropriate in improving enzyme activity.
In the pellet-type Nuruk production experiment, the enzyme activity (unit/g) of commercially available traditional Nuruks was compared with that of S-AO, S-LR, and S-Mix Nuruk (32-hour cultivation) which showed excellent Nuruk enzyme activity according to raw material conditions and cultivation hours. As a result, the glucoamylase activity, the quality standard of Nuruk, was found in order of SS (450.29 unit/g) > S-AO (325.92 unit/g) > SH (297.44 unit/g) > S-LR (294.43 unit/g) > S-Mix (238.44 unit/g) > SJ (167.02 unit/g). As for the α-amylase activity, S-AO Nuruk recorded 18.98 unit/g, far more excellent than other Nuruks recording 1.75±0.94 unit/g on average. Of the commercially available Nuruks, SH Nuruk showed a relatively higher value at 3.38±0.01. Concerning the acidic protease activity, S-AO Nuruk also recorded the highest. In general, pellet type Nuruk showed higher performances than commercial Nuruks.
Makgeolli brewed with pellet type Nuruks and commercial Nuruks were analyzed for alcohol, pH, acidity, and solid contents during fermentation. During the fermentation process pH, acidity, solids content, amino acids, and alcohol content was characterized by the difference between the two type of Nuruk used for Makgeolli brewed. Free sugar contents were analyzed. Each of the processed specimens showed a total amount less than 100 ppm, respectively. The pellet-type Nuruk showed 0.72～4.02 ppm, extremely small amount. But the commercial Nuruk showed 5.90 ppm SH mash, 51.54 ppm SS mash, and 81.96 ppm SH mash, presenting relatively large gaps. Organic acid examination found that lactic acid accounting for the largest part, was 309.93～336.96 mg% in pellet-type Nuruk Makgeolli and 636.03～650.72 mg% in commercial-Nuruk Makgeolli, double the gap in a maximum case. The malic acid of pellet-type Nuruk ranged 111.51～176.27 mg% and that of commercial Nuruk ranged 3.29～14.96 mg%, presenting a huge gap. Based on such findings, it is deemed that the commercial Nuruk consumed more sugar and produced more acid due to diverse microorganisms from Nuruk during fermentation.
Enzyme activity in the mash showed a tendency the higher the activity of the Nuruk is also high reproducibility.
As a result of the sensory test of Nuruk-specific Makgeolli, the overall preference was found S-AO (4.15) > S-LR (3.92), S-Mix (3.15), SS (3.00) > SH (2.69), SJ (2.69) in order, indicating that pellet type-Nuruk-based Makgeolli had more excellent sensory characteristics than commercial Nuruk-based ones.
Given these findings, if the pellet-type based Nuruk is used to brew Korean alcoholic beverages, product quality would be stable while not compromising the quality of existing traditional Nuruk-based products. Traditional Nuruk usually takes 20～30 days for fermentation and 36～40 hours from koji start to finish. However, pellet-type Nuruk takes about 32 hours to koji finish. From these study findings, it is expected that if pellet-type Nuruk is produced based on sterilized whole wheat and used to cultivate koji mold, it would save the fermentation time considerably while not compromising the quality of existing traditional Nuruk products.