ang. 2-Methyl-1- butanol, HEMF, and 4-EG were not produced and the
Table 3. Organic acid contents in doenjang made of SB-, 0.8SB-0.2R-, 0.6SB-0.4R-meju.
Table 4. Antioxidative compounds and activity in doenjang made of SB-, 0.8SB-0.2R-, 0.6SB-0.4R-meju.
production of 3-methylbutanol, propionic acid, and furfuryl alcohol commonly decreased by the rice addition to the meju dough as shown in Table 5.
3.6. Bacterial Community Variation in Doenjang
16S-rDNA was normally amplified but 18S-rDNA was not. This may reflect the maintenance or growth of specific bacteria during meju fermentation and doenjang ripening, while fungal hyphae may lose their physiological activity. The number of DNA bands detected in the TGGE gels for SB-doenjang was more than 50% lower than that for SB-R-doenjang, and the TGGE patterns for 0.8SB-0.2Rand 0.6SB-0.4R-doenjang were completely identical with each other as shown in Figure 1. Bacteria belonged to Bacillus genus were commonly identified in all doenjang preparations [26-28]. In addition, bacteria of the genera Staphylococcus and Virgibacillus were commonly identified in 0.8SB-0.2Rand 0.6SB-0.R-doenjang as shown in Table 6.
The quality of doenjang can be evaluated based on the contents of various nutritional ingredients that include free amino acids, organic acids, volatile compounds, minerals, fatty acids, and antioxidative compounds [29, 30]. Free amino acids, volatile compounds, and organic acids are biochemically produced by microorganisms responsible for meju fermentation. However, antioxidative compounds (polyphenol and total phenolic compounds) and minerals originated from the soybean ingredient. Practically, contents of free amino acids and organic acids measured for 15 doenjang preparations were markedly varied from 1.70% to 5.36% and from 0.5% to 1.65%, respectively. Meanwhile, the contents of polyunsaturated fatty acids and unsaturated fatty acids measured based on area% were slightly varied from 51.52% to 64.91% and from 82.28% to 85.22%, respectively . Variations of the free amino acid contents were greatly higher than those of fatty acids, considering that variation of contents of crude protein and lipid were from 11.8% to 16.8% and from 7.1% to 8.6%, respectively, in 15 doenjang preparations. This indicates that contents of microbial metabolites in doenjang may be an index to evaluate doenjang quality because those are produced depending on metabolic and physiological activity of microorganisms responsible for meju fermentation. Accordingly, the effect of rice on doenjang quality may be evaluated on the basis of the biochemical metabolites contained in doenjang. However, the chemical ingredients originating
Table 5. Volatile organic compounds in doenjang made of SB-, 0.8SB-0.2R-, 0.6SB-0.4R-meju.
Table 6. The homologous bacteria with DNAs extracted from the numbered bands in TGGE gel were arranged in the order of the band numbers in Figure 1.
from soybean and rice must be analyzed for comparison of the nutritional value of rice-supplemented doenjang.
Adding rice to the dough used in the preparation of meju may alter the percentage balance of organic carbon and nitrogen sources for microbial growth. A prior study reported protein contents in SB-, 0.8SB-0.2R-, and 0.6SB-0.4R-meju to be about 40%, 34%, and 27% (w/w), respectively, and carbohydrate contents to be about 35%, 46%, and 57% (w/w), respectively . These contents are 5 - 8 times higher than the total free amino acids (Table 1) and 11 - 15 times higher than the total organic acids (Table 3) in doenjang. The higher and appreciably higher ratios of free amino acids and organic acids to soybean content, respectively, in SB-R-doenjang as compared with SB-doenjang is a clue that that rice addition to meju dough may not be a cause to limit protein content for free amino acid production but may be a factor to activate microbial metabolism for fermentation of organic acids . The present observation of higher bacterial diversity in SB-R-doenjang than in SB-doenjang (Figure 1 and Table 6) is another clue that the rice addition to meju dough be a factor to nutritionally activate bacterial growth and ecologically increase bacterial diversity . Increase of bacterial diversity may cause to
Figure 1. TGGE pattern of 16S-rDNA variable region amplified with genomic DNA extracted from doenjang made of DFS- (1), DFS-0.2R- (2), DFS-0.4R-meju (3).
increase specific metabolites in fermenting meju and ripening doenjang, resulting in the increased ratios of free amino acids and organic acids to soybean content, while the production of volatile compounds may be limited or inhibited . Certain volatile compounds that cause fragrant flavors and off-flavors may be generated by biochemical conversion of amino acids and metabolic fermentation of sugars during meju fermentation. Increase of the percentage balance of carbohydrate in meju dough may induce the active microbial catabolism of carbohydrates while limiting the catabolism of protein. As a consequence, off-flavors or unwanted flavors (such as peppermint, caramel, bacon, spice, camphor, sweet, and smoky) for doenjang may be produced in limited quantity. Contents of free amino acids and organic acids, especially, citric and lactic acid, are major factors to the improved taste and nutrients of doenjang . Volatile compounds, such as 2-methyl-1-butanol, HEMF, 4-EG, 3-methylbutanol, propionic acid, furfuryl alcohol, and HDMF detected in SBand SB-R-doenjang are improper or unwanted factors for doenjang [36,37]. Accordingly, it can be suggested that the addition of 20% - 40% (w/w) rice to meju dough may induce nutritional conditions that are favorable for bacterial growth and fermentation, with consequent improvement of doenjang quality.
Generally, minerals including Mg, Ca, Mn, Fe, Zn, and Cu, and antioxidative compounds including polyphenol and total phenolic compounds  contained in soybean and rice become chemical ingredients of doenjang. Adding rice to meju dough was a cause the minerals and the antioxidative compounds to be maximally 30% and 40% decreased, respectively, in proportion to the percentage balance of soybean and rice. However, nature of doenjang may be not influenced by the reduction of the minerals and the antioxidative compounds because doenjang has not been used as mineral source for nutritional requirement or as antioxidant for functional reinforcement of food but has been used as a sauce for seasoning to taste of various foods. Generally, doenjang quality may be commercially fortified by the supplementation or content of antioxidative compounds [38,39]. From the nutritional standpoint, polyphenol and total phenolic compounds contained in doenjang may be helpful for the long-term preservation of foods, which may be substituted by other antioxidative compounds biochemically generated by microorganisms responsible for meju preparation and ripening of doenjang, considering that the antioxidative activity (DPPH scavenging and ferric ion reduction activity) was not decreased by addition of rice and was not proportional to the contents of the antioxidative compounds in doenjang. It is very possible that various metabolic intermediates, reduced metabolites, and coenzymes originating from chemically or biochemically lysed microbial cells can function as antioxidative agents, which may be proportional to biomass and diversity of microorganisms .
Rice may be useful as a supplement for meju dough when evaluated on the basis of free amino acid production, organic acid production, volatile compounds, and bacterial community diversity. Especially, DPPH scavenging and ferric ion reduction activity of doenjang may possibly be intimately related to the rice addition because the proper environmental conditions for bacterial growth may induce increased bacterial community diversity and activated bacterial physiology. The environmental condition of doenjang may be converted to the biophile condition. Positive interrelationship between bacteria and doenjang condition may be a factor to improve doenjang quality by activation of fermentation metabolism or enzymatic reaction. The use of rice in meju preparation may be helpful to activate rice consumption and improve doenjang quality, because the fermentation and ripening process may positively influenced by the chemical and nutritional balance between protein and carbohydrate. Especially, a maximum of 40% (w/w) rice may be substituted for soybean in the meju process according to the comparable quality of SB-, 0.8SB-0.2R-, and 0.6SB-0.4Rdoenjang.
This research was supported by Seokyeong University in 2009.
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