Effect of soy protein on the quality of Qianye tofu in different storage periods

In this paper, the effects of different storage periods on the structure of soy protein isolate (SPI) were studied, and the effects of storage period on the quality of Qianyetofu were discussed. The content, subunit, sulfhydryl group, gel texture and sensory evaluation were determined for SPI in different storage periods, and the SPI secondary structure, disulfide bond configuration and side chain structure were analyzed by Raman spectroscopy. The measurement was carried out, and the microstructure of Qianyetofu was observed by scanning electron microscopy. The results showed that with the prolongation of storage period, the content of SPI carbonyl increased gradually; the content of disulfide bond in free sulfhydryl group and total sulfhydryl group decreased gradually; the content of secondary structure changed; the hardness of gel showed a downward trend. The sensory evaluation of Qianyetofu deteriorated in terms of taste, color and flavor; the gel network structure became looser, the degree of crosslinking decreased and the network structure was rough. This indicate that the SPI has undergone an oxidation reaction during the storage period, resulting in a change in the structure of the soybean protein, which ultimately reduces the gel properties. According to Raman spectroscopy, the analysis of soy protein isolates in different storage periods showed that the α-helix and β-sheet configurations were higher during the storage period of 0-18 days, and the intermolecular disulfide bonds were converted into intramolecular disulfide bonds. During the storage period of 18-30 days, the random coil configuration increased significantly, and the disulfide bond was converted from intramolecular to intramolecular. At this time, the secondary structure of soy protein was loose and the protein gel strength was low. During the whole storage process, the carbonyl content increased gradually, SPI may react during storage to produce reactive oxygen species or free radicals, which attack proteins, causing breakage of the main peptide chain or oxidation of side chain groups, resulting in elevated carbonyl content, protein carbonylation leading to changes in protein structure, the content of sulfhydryl groups and disulfide bonds decreased gradually, oxidation can change the redox state of cysteine in proteins and the equilibrium constant of thiol/disulfide bond interactions, thereby changing distribution of sulfhydryl and disulfide bonds in proteins, and the subunits of 65, 62, 57 and 20 kDa belonged to the bands, it indicates that long-term storage leads to the formation of insoluble high molecular weight oxidized aggregates of soy protein, making them unable to enter the electrophoresis gel, and non-disulfide bonds participate in the formation of oxidized aggregates, which were related to the large aggregate formed by oxidative attack of SPI. Observing the combination of the microstructure of Qianyetofu and the sensory evaluation of texture characteristics, it is concluded that the soy protein is oxidized and attacked to form oxidized aggregates, which affects the processing characteristics of the protein, which is the reason for the decrease of gel strength and poor taste. This indicates that the SPI has undergone an oxidation reaction during the storage period, resulting in a change in the structure of the soybean protein and a decrease in its gel properties.