| Soybean residue is the main by-product of making soybean milk and tofu, and there is considerable interest in its recovery, recycling and upgrading. It was rich in dietary fiber, but it contained most of insoluble dietary fiber, that didn’t meet the requirement of high quality dietary fiber. The aim of the present study was to investigate how lactic acid bacteria (LAB) and dynamic high pressure microfluidization (DHPM) could affect the dietary fibre in soybean residue, which was characterized by the content, composition, surface topography and physicochemical properties of dietary fibre. The main research contents and the conclusions were listed as follows:(1) The elementary composition of soybean residue was carbohydrate (72wt.%) and protein (16wt.%). IDF was the main fibre fraction of dietary fibre in soybean residue. The ratio of insoluble to soluble dietary fibre was11.6. SDF of soybean residue was mainly constituted of uronic acid, galactose, and arabinose; IDF was characterized by its high content in glucose, galactose and uronic acid.(2) Surface topography and crystal structure analysis:the tissue of samples was seen as an irregular and massy shape with fibrous structure and amorphous structure. The samples showed broad diffraction peaks, with a low relative crystallinity.(3) We discussed the effects of fermentation on the content of dietary fibre and monosaccharide composition in soybean residue. The results showed that when soybean residue was fermented, there was a significant increase in TDF (80.0to85.1g/100g) and SDF (6.4to9.7g/100g) content. The ratio of insoluble to soluble dietary fibre was decreased from11.6to7.8. The decrease in IDF produced by fermentation was apparently due to a reduction of insoluble polymers containing arabinose, uronic acid and xylose.(4) We discussed the effects of DHPM on the content of dietary fibre and monosaccharide composition in soybean residue. The results showed that DHPM treatment made SDF increase and IDF decrease significantly, and both varied with the differing DHPM pressure. The increase in SDF became more evident for samples DHPM-treated after fermentation, and the most increase was found at200MPa in samples with combined fermented and DHPM-treated. In this case, well balanced composition of IDF and SDF (IDF:SDF=2.5) was obtained for soybean residue. The mechanism similar to that of hemicellulose degradation turned into some small molecular substances was responsible for the reduction of IDF. The increase in solubility produced by DHPM was due to a shift of insoluble to soluble fibre, most probably from hemicellulose and pectic polysaccharides containing arabinose, galactose and uronic acid.(5) We studied the effect of fermentation and DHPM on microstructure ofdietary fibre in soybean residue. The results showed that fermentation resulted in the modification of the fibrous structure. Fermented samples had a lower relative crystallinity than the original sample and showed an amorphous profile. This meant that fermentation changed the fibre crystalline regions. DHPM damaged the structure and caused it to be rugged. The higher DHPM pressure applied, the more sample’s structure was puffed. DHPM did not result in the advanced degradation of fibre polymer, and only amorphous regions were damaged preferentially. Combined fermentation and DHPM could change microstructure of dietary fibre in soybean residue more easier.(6) We studied the effect of fermentation and DHPM on physicochemical properties of dietary fibre in soybean residue. The results showed that they both modified hydration properties and oil holding capacity greatly, but did not affect cation exchange capacity significantly. Sample showed a lower bile acid binding capacity after fermentation, but a higher capacity after DHPM treatment.(7) Dietary fibre from soybean residue was evaluated for binding capacity for three heavy metals, Hg, Cd, Pb in vitro experiments under the physiological conditions (37℃, pH=2.0and pH=7.0). The results showed that soybean residue dietary fibre had binding capacity for three heavy metals, and the capacity decreased in the following order:Pb2+>Hg2+>Cd2+. The binding process completed in20min and the binding capacity under the small intestine environment was far stronger than the gastric environment. The fermented dietary fibre and dynamic high pressure microfluidization-treated dietary fibre showed higher binding capacities for three heavy metal cations.Therefore, fermentation with LAB and DHPM could be considered good methods to improve the functionality of dietary fibre in soybean residue. |
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