Production Of High-Protein Feed By Fermenting Extruded Soy Sauce Residue Of Mixed Strains

Soy sauce residue as raw material was extruded by the single screw extruder to obtain the extruded soy sauce residue which was fermented to produce the protein feed.The effects of extrusion parameters on the various indicators of soy sauce residue were studied and the optimal extrusion parameters were obtained.The physicochemical properties and structural characteristics of the extruded soy sauce residue and soy sauce residue were compared under the optimal extrusion conditions.The optimal extruded soy sauce residue was fermented to explore the influence of medium composition and fermentation process conditions on the indicators of fermentation products,and the optimal medium composition and fermentation process conditions were obtained.The volatile compounds and morphological structure were compared between fermenting extruded soy sauce residue and fermenting soy sauce residue under the optimal fermentation conditions.The aim of this study was to provide certain data reference for the application of extrusion technology to improve the feed quality of fermenting soy sauce residue.The main research results were as follows:(1)The optimal extrusion parameters of soy sauce residue used for fermented feed were determined.Single factor test and central composite design(CCD)test were used to analyze the influence of extrusion parameters(extrusion temperature,moisture,screw speed)on soluble dietary fiber(SDF),crude fiber,protein digestibility and protein solubility of soy sauce residue,and the quadratic regression equation was obtained by response surface analysis.The optimal extrusion parameters were extrusion temperature of 103°C,moisture of 33.7%and screw speed of 98 rpm.Under the extrusion test conditions,the SDF,crude fiber,protein digestibility and protein solubility of soy sauce residue were 3.55%,20.2%,48.7%and 55.43%,respectively.(2)The physicochemical properties and structural characteristics of the extruded soy sauce residue(experimental group)and soy sauce residue(control group)were analyzed under the optimal extrusion conditions.The results showed that the bulk density,water holding capacity,oil holding capacity,water solubility index and swelling capacity of the experimental group were 0.43 g/m L,4.44 g/g,2.59 g/g,9.80%and 6.87 mg/g,respectively,which were significantly higher than those of the control group.According to the particle size analysis,the average particle size of the experimental group had been reduced to 92.67?m,and the specific surface area was significantly increased to 0.15 m²g^-1.Scanning electron microscope analysis showed that the surface of the experimental group was more rough and compact,and existed a lot of irregular fragments.Thermal stability analysis showed that the stability of the experimental group was higher than that of the control group.FTIR analysis showed that the hydrogen bond and C-H bond in the experimental group were strengthened,and the C-O bond in fiber molecules was broken in the experimental group.(3)The optimal medium composition of the solid substrate was determined.The L₉(3⁴)orthogonal test design was used to determine the optimal strain ratio as Aspergillus niger:Bacillus subtilis:Candida utilis:Geotrichum candidum=3:2:1:3.The Plackett-Burman test design was used to screen the impact factors on the true protein,crude fiber,cellulase activity and protease activity,and ammonium chloride,material/water ratio and bran were determined as impact factors.Then the best level of the impact factors was investigated by the steepest climbing test and BBD test.The optimal medium composition were glucose of 1%,sucrose of1%,urea of 1%,ammonium chloride of 1.8%,magnesium sulfate of 0%,disodium hydrogen phosphate of 0%,material to water ratio of 1:1.4 and bran of 23.24%.(4)The optimal fermentation process conditions used for solid medium were determined.Effects of fermentation conditions(inoculation size,fermentation temperature,fermentation time,bottling volume)were analyzed by single factor test and BBD test on true protein,crude protein,crude fiber,cellulase activity and protease activity in the fermented product.According to the quadratic regression equation of response surface analysis,the optimal fermentation process conditions of the extruded soy sauce residue were inoculum size of 16.85%,fermentation temperature of 36°C,fermentation time of 5.67 d and bottling volume of 11.45 g.Under this fermentation conditions,the true protein,crude protein,crude fiber,cellulase activity and protease activity in the fermenting extruded soy sauce residue were 32.17%,36.65%,17.13%,786.53 U/g and 145.18 U/g,respectively.(5)Under the optimal fermentation process conditions,the basic components,enzyme activities,the flavor substances and morphological structure of the fermenting extruded soy sauce residue(experimental group)and fermenting soy sauce residue(control group)were analyzed.The results showed that the true protein content,cellulase activity and protease activity of experimental group were significantly increased compared with control group,while the crude fiber content was significantly decreased.Volatile compounds of experimental group(14)determined by the SPME-GC/MS method was higher than that of control group(4).Scanning electron microscopy analysis showed that there were a large number of folds or ravines on the surface of the experimental group,which was conducive to the attachment of microorganisms.However,the smooth surface of the control group caused a large number of microorganisms to fall off,which was not conducive to the degradation of hemicellulose and lignin in the raw materials,and reduced the digestibility of soy sauce residue.