| Corncob is rich in cellulose,hemicellulose and lignin and other dietary fiber.Currently,corncob is mainly used in the preparation of xylose、xylitol and oligosaccharide,fermentation of animal feed,production of edible mushrooms and other fields.Corncob contain relatively high levels of insoluble dietary fiber(IDF),generally around 75%,which is sufficient to promote intestinal motility and helps prevent constipation and colon cancer,limiting its functional properties to function due to the tight structure of the corncob.Modification techniques can alter the structural characteristics of corncob IDF,which in turn can change its physicochemical properties and improve its functional properties.In this study,corncob IDF was modified by subcritical water(SW),alkaline hydrogen peroxide(AHP)and subcritical water combined with alkaline hydrogen peroxide(SW-AHP)method to compare the physicochemical properties,functional properties and structural characteristics of IDF before and after modification,and to investigate the effect on the probiotic properties of probiotic bacteria based on this method.The main contents and results of the study are as follows:(1)The effects of SW modification techniques at different temperatures on the physicochemical properties,functional properties and structural characteristics of corncob IDF were explored.The water holding capacity,oil holding capacity,water swelling capacity and glucose adsorption capacity reached the maximum after modification at 160°C,which increased by 83.93%,43.62%,39.09% and 134.25%,respectively.The nitrite adsorption capacity was significantly increased after SW modification(p < 0.05),with the best nitrite adsorption capacity after modification at 220°C.The total phenolic content and the antioxidant capacity significantly increased after SW modification(p < 0.05).The color of IDF became darker and the average particle size decreased with increasing temperature after SW modification as observed by chromatography and laser particle size scanner.The cellular mesh structure appears inside the SW modified corncob IDF structure,and the increase in temperature will lead to the appearance of spherical particles on the surface of the porous structure.Fourier transform infrared spectroscopy(FT-IR)and X-ray diffraction(XRD)patterns indicated that SW treatment degraded part of the hemicellulose in corncob IDF,and the crystallinity of modified corncob IDF was improved.(2)The effects of different mass fractions of hydrogen peroxide on the physicochemical properties,functional properties and structural characteristics of corncob IDF were explored.Compared with unmodified IDF,the water holding capacity,oil holding capacity,water swelling capacity and glucose adsorption capacity reached the maximum values after 2%-AHP modification,which increased by 94.64%,69.14%,49.24% and 113.17%,respectively,and the nitrite adsorption capacity was optimal.The total phenolic content and the antioxidant capacity reduced significantly after AHP modification(p < 0.05).With the increase of hydrogen peroxide mass fraction,the color of corncob IDF gradually became lighter and the average particle size gradually decreased.The surface of corncob IDF was folded and had a porous mesh structure after AHP modification.FT-IR and XRD patterns indicated that AHP could degrade part of the hemicellulose and lignin occurrence in corncob IDF,and the crystallinity of corncob IDF was improved after modification.(3)The effects of different SW-AHP co-modification conditions on the physicochemical properties,functional properties and structural characteristics of corncob IDF were explored.The water holding capacity,oil holding capacity,water swelling capacity and glucose adsorption capacity of corncob IDF after SW-AHP(160℃-1%)modification reached the maximum values,which were increased by 107.59%,94.15%,71.07% and 288.66%,respectively.The nitrite adsorption capacity reached the maximum after SW-AHP(180℃-1.5%)modification.Compared with SW modification or AHP modification,the SW-AHP modified corncob IDF all showed more excellent hydration capacity,glucose adsorption capacity and nitrite adsorption capacity.The total phenolic content and antioxidant capacity of corncob IDF were significantly increased by SW-AHP(160℃-1%,180℃-0.5%,180℃-1%)modification(p < 0.05).The whiteness of IDF increased after SW-AHP(140℃-1.5%)modification,and the average particle size decreased gradually with the enhancement of modification conditions.The irregular bulk structure became loose space structure after the SW-AHP modification,and more pore structures appeared with the enhancement of the modification conditions,but the porosity was reduced.The FT-IR and XRD results demonstrated that the SW-AHP modification did not change the peak position and crystalline structure,large amount of hemicellulose and lignin were degraded,and the crystallinity of corncob IDF increased after the modification.In summary,SW-AHP modification is a successful modification technique as it is more than sufficient to further disrupt the structure of corncob IDF and thus improve its physicochemical and functional properties.(4)The effect of corncob IDF on the growth characteristics of probiotic bacteria before and after modification was explored.Glucose,corncob IDF before and after modification and D-xylose were used as carbon sources for probiotic bacteria.The results of OD,p H and total acid content showed that corncob IDF modified by the above three methods promoted the growth of Lactobacillus rhamnosus,Lactobacillus plantarum,Lactobacillus acidophilus and Bifidobacterium adolescentis.The results of in vitro fermentation experiments showed that the gas production of SW,AHP and SW-AHP modified IDF was significantly higher than that of unmodified IDF(p < 0.05),and the p H of SW,AHP and SW-AHP modified IDF was significantly lower than that of unmodified IDF(p < 0.05).The results of short-chain fatty acid content in fermentation broth showed that SW,AHP and SW-AHP modified IDF produced significantly higher propionic acid and butyric acid content than unmodified IDF(p < 0.05). |
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