| Soybean meal is a byproduct of Soy which is used to extract oil, containing about 50% crude protein with high nutritional value.As a source of high protein material, soybean meal is widely used in feed and feed processing industry. In order to make full use of soybean meal, increasing domestic and foreign scholars are studying how to exploit high economic value of soybean meal. Peptides produced by soybean meal have antioxidant, anti-hypertension, weight lossand other significant effects. Therefore, people are very concerned about the use of soybean meal in producing edible proteinand functional peptides. Firstly, the seed and fermentation culture were optimized in this paper. Then ultrasound stimulation was added in the process of soybean meal fermentation by Bacillus subtilis, exploring the best combination of ultrasonic time, power density and frequency parameters. It also studied the protein structure of fermentation products to reveal the mechanism and effects of ultrasound on the process. Finally, in order to illustrate the ultrasonic effect on the fermentation product, the specific processing properties of protein and the activity of antioxidant and anti-hypertension of peptide were analyzed to obtain the relationship between soluble protein structure and functional activity of fermentation products during ultrasound process. The main researches and conclusions are as follows:(1) The optimum conditions are obtained by the single factor experiments that optimized the component of seed culture and the culture condition: 1% yeast extract, 1% corn powder, 1% KH2PO4 and 36oC fermentation temperature. The biomass, neutral and alkaline protease activity was enhanced by 210.10%, 459.20% and 67.50% respectively, compared to conventional LB peptone medium. Then on this basis, using orthogonal experiment to optimize the fermentation culture, the optimal conditions were as follows: 20% soybean meal, 2% maize powder, 1% KH2PO4, 10% inoculation size and 36oC culture temperature. The biomass, soluble protein and peptide content was 40.90%, 28.60% and 40.90% higher than those of the culture medium containing only 22% soybean meal. The results showed that the optimum seed culture and fermentation culture could significantly improve the efficiency of the fermentation of liquid soybean meal by Bacillus subtilis.(2) The parameters of ultrasonic assistant fermentation of soybean meal were optimized and the effect of ultrasonic on the structure of protein was studied. The results illustrated that the effect of low intensity pulsed ultrasound could promote the fermentation efficiency at the third day after inoculation. Under the following ultrasonic parameters: 0.08 W/mL, 33 kHz and 1 h of ultrasonic time, the biomass, the content of soluble protein and peptide were improved by 141.97%, 15.06% and 24.42% respectively, compared to that without ultrasound. The results of UV absorption spectra, the fluorescence emission spectra, the infrared spectraand atomic force microscope indicated that the ultrasound had lowered the height of protein particles and altered the relative content of secondary structure. In a word, ultrasound treatment could change the structure of soluble protein, making the protein molecules looser and stretch.(3) The protein processing properties and the functional activity of the fermented product were studied. By measuring the water/oil holding capacity andemulsifying properties, it is found that ultrasound can improve the processing characteristics of the soluble protein. By determining 4 kinds of in vitro antioxidant assay and ACE inhibition rate, it is shown that ultrasound can improve the functional activity of peptide. Pearson’s correlation coefficient analysis showed that the water holding capacity with Rq, oil-absorbing capacity with random coil, α-helix, Rq, Ra, emulsifying with β-sheet, have significant linear correlation. Meanwhile, the ·OH scavenging rates of peptide with β-sheet, β-turn, the Fe2+ chelating abilities with β-sheet, ACE inhibition rate with β-turn, were significantly linear correlated. The least square fitting methods can better reflect the relationships between the structure and property. |
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