Characterization And Rapid Quantitative Detection Of Water And Protein Changes During Tofu Formation

Tofu is the gel formed by the coagulant of soybean protein and treated under pressure to remove the excess water,which has a three-dimensional network structure.It is rich in nutrition and delicate in taste,and is a favorite traditional food.The three-dimensional network structure of tofu is mainly composed of water and protein.The content and distribution of water and protein play a vital role in the formation of tofu,and are closely relate to the quality of the final product.However,there are few reports on the the relationship between the changes of water state and protein structure and the quality of tofu,meanwhile,the traditional detection methods are time-consuming and laborious,and are difficult to monitor the changes of water and protein during the whole process of tofu formation,As a result,the mechanism of water and protein affecting tofu quality is still unclear.In this study,tofu was prepared by using different ratio of grinding slurry to liquid,amount of coagulant,and coagulation temperature.The texture characterization of Tofu was conduct using the nuclear magnetic resonance,Fourier transform infrared spectroscopy?FTIR?and circular dichroism technology to see the changes of water composition,secondary structure of protein in the four different formation states?soy milk,hot soy milk,soy gel and tofu?,respectively.The relationship between water components,protein secondary structure and tofu quality were also systematica lly studied.The hyperspectral imaging technology was also used to quantitatively predict and visualize the water/protein distribution and content in the four formation states.This study provided a theoretical basis for the online monitoring and quality control of water and protein changes in tofu processing.The main research conclusions are as follows:?1?Study on texture and physicochemical properties of tofuThe texture characteristics,water retention,as well as the water and protein contents in the four states of different tofu were measured.The results showed that in the single-factor test,grinding slurry to liquid ratio as 1:7,coagulant amount as 14 mL,and coagulation temperature as 80 ^oC has the best quality characteristics of tofu,which has the elasticity as 12.72 nm,the chewability as 440.15 mJ,the water retention as74.21%,and the maximum water content as 82.21%.?2?Analysis of water composition during tofu formation using low-field nuclear magnetic resonance technologyLow-field nuclear magnetic resonance technology was used to detect changes of the water composition in soy milk,hot soy milk,gel,and tofu.It was found that there are two types of water components in soy milk and hot soy milk,and three types of water components in gel and tofu.According to the difference s in relaxation time,the water components in soy milk,hot soy milk,gel and tofu were classified,during the changing of hot soy milk to gel,free water gradually transferred to the network structure of the gel,and formed weakly bound water in the protein network structure.After the gel transformed to tofu,the T₂₂ relaxation time was decreased from 330 ms to 108 ms,and the degree of freedom of the weakly bound water was also reduced.The analys is of peak area of the T₂₂ component indicated that the T₂₂ component of tofu with good water retention has a larger peak area and more weakly bound water.?3?Secondary structure analysis of protein during tofu formation based on Fourier transform infrared and circular dichroism spectroscopyAfter freeze-drying of the soy milk,hot soy milk,gel,and tofu,the powder were analysised by Fourier transform infrared and circular dichroism spectroscopy.The experimental results indicated that the content of?-helix decreased and the content of?-turn and?-sheet of protein increased after hotting the soybean milk.After the addition of the coagulant,the protein in the hot slurry gradually aggregated and formed the protein gel,the content of?-helix in the gel sample was increased.On the other hand,for the tofu samples,the secondary structure of protein mainly composed of?-helix and?-sheet.Among different kinds of tofu,the?-helix and?-sheet content was higher in the tofu with better quality.Meanwhile,the protein network structure is uniformly and tightly arranged in the microstructure of tofu,and with the small pores.?4?Rapid detection of water and protein content and distribution during tofu formation based on hyperspectral technologyThe image of soymilk,hot soy milk,gel and tofu with five different grinding slurry liquid ratios were collected by hyperspectral,and the spectral information was extracted.Five pretreatment methods were used for the four extracted spectra pretreatment respectively.Three quantitative models including partial least squares regression?PLSR?,principal component regression?PCR?and least squares support vector machine?LS-SVM?models were established.Comparing with the LS-SVM model,the prediction abilities of PLSR and PCR models are lower.Continuous projection algorithm?SPA?was used to screen the characteristic wavelengths of soy milk,hot soy milk,gel and tofu samples,13,9,8 and 9 characteristic wavelengths were selected to establish the LS-SVM model.The results showed that the characteristic wavelength based SPA+LS-SVM model has a better prediction effect compared with the LS-SVM model in the full wavelength range.The results of prediction set?R_P?for the water content were 0.9212,0.9381,0.9772 and 0.8813,respectively.The R_P for the protein were 0.9952,0.9582,0.9978 and 0.8688,respectively.Based on the optimal SPA+LS-SVM model,the water and protein contents of each pixel in the images of soy milk,hot paste,gel and tofu were calculated to detect the distribution of water and protein in different states during the formation of tofu.In this study,the relationship between the water/protein content and distributio n,as well as the tofu processing parameters were established,which provids a new idea for the automated control of tofu processing.At the same time,hyperspectral image technology was utilizhed for the rapidly detecting the concentrations and distributio n of water and protein during the formation of tofu,which provided a theoretical basis for the quality control of tofu processing.