| Mao-tofu is a nutritious Chinese traditional fermented soybean food. People all like it due to its delicious taste, unique flavor and good nutrition. Mucor, as the main strain of Mao-tofu, secretes protease, which can obviously degraded protein of Mao-tofu into peptides and amino acids. The antioxidant and ACE-inhibitory activity of Mao-tofu had changed during fermentation. In our study, Mao-tofu fermented for 3, 5, 7, 9 days were extracted by five extraction solvents. The extracting yield of the proteins and the degree of hydrolysis (DH) of the proteins in the extracts were analyzed, and especially for their antioxidant activity and ACE-inhibitory activity. The aim of our study was to reveal the impact of fermentation time and solvent types on these activities of the extracts of Mao-tofu. The results obtained in this study are as follow.(1)During the fermentation days, from 0~9 days, moisture content of Mao-tofu decreased, while protein content increased, and water-soluble protein content also increased.(2) Mao-tofu fermented for 3, 5, 7, 9 days were extracted by five extraction solvents, including 20, 40, 60% ethanol solution, pH 4.5 and pH 6.5 water, respectively, the results showed that the longer the fermentation time was, the higher the extracting yield in the extracts were, the highest extracting yield of of soluble protein fractions was 36.1%. pH 6.5 water could extract more protein fractions from Mao-tofu than those of pH 4.5 water. When the concentration of ethanol increased, the solvent would extract much less soluble protein fractions. The DH of soluble protein fractions in the extracts increased from about 17 to 45% as fermentation progressed, revealing that soluble protein fractions in the extracts might mainly be small peptides or amino acids. The DH of pH 4.5 water extract was higher than that of pH 6.5 water extract. As the concentration of ethanol increased, the DH of protein in the ethanol extracts increased.(3) The results showed that the longer fermentation time was, the higher antioxidant activity of the extracts was. The free radical scavenging activities of the extracts on DPPH, ABTS radicals, hydroxyl radicals and superoxide radical-scavenging activity, iron (III)-reducing activity and iron (II)-chelating activity were evaluated. No matter what methods was, the results showed the consistent law that the extract prepared with 60% ethanol solution behaved the highest activity while the extract prepared with pH 4.5 water showed the lowest activity, as the concentration of ethanol increased, the activity of the ethanol extracts increased.The results also showed that extraction solvents had effects on the antioxidant activity of Mao-tofu extracts, but further research need to be done.(4) The determination of the ACE-inhibitory activities of the extracts of Mao-tofu indicated that longer fermentation time did not increase the activity. When the Mao-tofu was fermented for 5 days, its extracts had the highest ACE-inhibitory activity. The ACE-inhibitory activity of pH 4.5 water extract was the highest and that of pH 6.5 water extract was the lowest. The ACE-inhibitory activity of the 60% ethanol solution extract was higher than that of other ethanol extracts, while The ACE-inhibitory activity of 20% solution ethanol extract was the lowest.(5) The analysis results from size exclusion chromatography confirmed that the composition of the five extracts obtained from Mao-tofu fermented for 5, 9 days was mainly peptides and amino acids. The different composition of the five extracts maybe the reason for the difference of these properties.The analysis results from high performance liquid chromatography also confirmed that the composition of the five extracts obtained from Mao-tofu fermented for 5, 9 days had changed, by comparing the different profiles of water extracts and ethanol solution extracts. The profiles need to be further analyzed and identified.(6) The amino acid compositions of five extracts of Mao-tofu fermented for 5 days. Analysis results showed that the 60% ethanol solution extract contained more total hydrophobic amino acids (THAA) than other extracts, while pH 4.5 water extract was a little smaller than that of the 60% ethanol solution extract. The results showed that five extracts of 9 days had similar THAA profiles. Meanwhile, it could be seen that three ethanol solution extracts had a higher level of Arg, Cys and Met, compared with two water extracts. It could be assumed that the enrichment of sulfur-containing amino acids (Met and Cys) in Mao-tofu extracts of ethanol solution might account for their better antioxidant properties. |
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