Study Of Water Distribution Of Soybean And Tofu By Using Low Field Proton NMR Transverse Relaxation Measurements

Water?fat and starch are very important ingredients in food.They are closely related to sensory attributes,textural properties and shelf life of food.In recent decades,low-field nuclear magnetic resonance(LF-NMR)is attracting attention as a rapid,non-destructive,and low-cost technique in the field of food science.The distribution and mobility of protons in food products are well reflected by LF-NMR,which is meaningful for us to get deep understanding of the water and macromolecules states in food.At present study,the variations of water distribution of soybean during soaking and blanching were studied by using proton NMR transverse relaxation.In the meanwhile,the different water fractions in tofu were assigned and the dynamic states of water were studied on the basis of diffusive exchange model.The effects of high pressure homogenization on water distribution of tofu were evaluated.The main research contents and results are as follows:1.The soaking and blanching of soybean can be well reflected by NMR relaxometry and magnetic resonance images(MRI).Water absorbed by soybean during soaking mainly existed in intercellular space between cotyledon cells.NMR images revealed that water firstly penetrated into soybean from its upper half,which may be related to the substructures such as radical,umbilici and micropyle.From the perspective of water distribution,10 hours is sufficient for soaking of soybean at 20°C.Due to the cell structure variations of cotyledon cells,T'22 fraction increase significantly during blanching.Soybean still absorbed water in blanching even it has been soaked for 12 hours.Soybean blanching should last at least for 10 min before its water distribution began to change.2.In the present study,LF-NMR was introduced into the research of different kinds of tofu for the first time.Three water components were observed in all five kinds of tofu samples.Principal component analysis was performed to analyze factors significantly affected tofu's water distribution.The results revealed that the solubility of coagulant and anions had important impacts on tofu's water distribution.Transverse relaxation times of tofu showed high sensitivity and good linear correlation with tofu' water-holding capacity(WHC).Ten linear equations which could provide simple linear numerical relationships between T2 relaxation time and WHC of tofu were firstly reported.Furthermore,a new water fraction was observed during microwave heating and it might origin from the special microstructures constituted by thermal denatured protein.3.On the basis of the diffusive exchange model,the water diffusive domain size of two traditional types of tofu was estimated by T2 measurements.The CaSO4 and MgC12 tofu exhibited similar minimum size of water molecule diffusive domains of about 3 ?m.The maximum diffusive distances of water in CaSO4 and MgC12 tofu were 34 and 26 ?m,respectively.The effects of HPH on water distribution and microstructure of final tofu matrix could be well reflected by LF-NMR.From the perspectives of water distribution and WHC,we are of the opinion that 50 MPa is considered to be the appropriate pressure for pretreatment of soymilk.T21 relaxation time was considered to be the best T2 parameter to monitor the changes of water distribution and morphology of tofu.LF-NMR was suggested to be a rapid method to study the state of water and morphology of tofu.