| Low-field nuclear magnetic resonance technology(LF-NMR)mainly uses hydrogen protons as probes to study the mobility of molecules.In the analysis of low-field nuclear magnetic resonance,the transverse relaxation time T2 is often used to reflect the movement of molecules.By measuring the relaxation time of the molecules to characterize the pore structure changes,swelling and phase transition in the sample,the LF-NMR enables non-destructive,fast,dynamic,real-time detection.Due to the advantages of the low-field NMR,it has broad application prospects in many fields such as biomaterials,pharmaceutical industry,food safety and polymer materials.In practical applications,it is often necessary to measure the state of the sample at different temperatures.The existing LF-NMR cannot achieve the online temperature control of the sample,it cannot guarantee the stability of the sample temperature during the measurement,and the structure and mobility of the molecule are affected by temperature changes.Therefore,there is a need for a device to perform accurate temperature control of the sample while detecting,which greatly improves the accuracy of the sample detection.In order to accurately meaure the effect of temperature changes on the sample,in view of the problem that the existing low-field nuclear magnetic resonance system cannot control the temperature of the sample,the LF-NMR online temperature control analysis system is designed and developed in this paper.The analysis system adds a sample temperature control device to ensure the stability and accuracy of the temperature control system.The anti-interference fluorinated liquid and temperature probe are selected,and the temperature of the sample tank is controlled by the super constant temperature circulating water bath.The temperature control analysis system can make the detection accuracy of the sample detection process reach ±0.1?,and the temperature control rate of the sample can reach no less than 1.2?/min,while ensuring the stability of the temperature control system.The LF-NMR on-line temperature control analysis system was implemented to control the temperature of sample.Firstly,the LF-NMR online temperature control analysis system was used to measure the melting process of different varieties of chocolate.The effects of cocoa content on fat motility were studied,and the effects of cocoa and crude fat content on chocolate melting temperature were analyzed.During the sample program temperature increase,therelaxation time(T2)of the movement of liquid oil molecules in chocolate was continuously measured,and the melting processes of four different brands of chocolate were evaluated.The results show that the T2 of four different brands of dark chocolates were highly sensitive to the movement of liquid oil molecules.During the melting process of different varieties of chocolate,the T2 increases significantly with increasing temperature,which was consistent with the increase in fat motility,providing a theoretical basis for the determination of the melting temperature.At the same time,the melting temperature of chocolate was accurately detected through the change in peak area,and it was concluded that the melting temperature of different brands of chocolate gradually increased with the increase of cocoa content and crude fat content.A new method of LF-NMR on-line temperature control analysis system was constructed to accurately characterize the melting temperature of chocolate,which has important reference significance for the production,processing,quality control,storage and transportation of chocolate and other soft foods.The LF-NMR online temperature control analysis system was further detected the gelation process and gelation temperature of the temperature-sensitive chitosan hydrogel material in real time.The analysis system characterized the change of the T2 of water molecules during the gelation process of the temperature-sensitive chitosan hydrogel,and analyzed the phase transition mechanism of the hydrogel from the perspective of water molecule movement.Combined with two other traditional measurement methods including thermogravimetric differential scanning calorimetry(TG/DSC)and rheological methods to analyze the phase transition process and gelation temperature of the chitosan/?-glycerophosphate(CS/GP)temperature-sensitive hydrogel materials.The results show that T2 was very sensitive to the phase transition of the hydrogel,and the pore size of the gel will continue to decrease with continued heating.At the same time,the water inside the hydrogel network will gradually squeeze out of the gel,and the gelation temperature will gradually decrease as the GP concentration increases.Moreover,as the mobility of water molecules varies greatly during the sol-gel phase transition,the LF-NMR measurement was more sensitive and accurate(RSD?0.1%,n=5)compared with DSC(RSD:1.2%-3.7%,n=5)and rheology(RSD:1.1%-2.3%,n=5).In summary,this paper designed and developed an LF-NMR online temperature control analysis system,and the system was used to characterize the melting process of chocolate and the gelation process of temperature-sensitive chitosan hydrogel respectively.From the perspective of molecular motion,the melting process and the behavior mechanism of phase transition were studied.The system measured the melting temperature of chocolate and the phase transition temperature of hydrogel.Its non-invasive and non-destructive properties make the measurement result more accurate and repeatable.It can be seen that the LF-NMR online temperature control analysis system not only provides a powerful tool for the structural change of molecules and the characterization of phase transition behavior,but also the new research ideas have been opened for the research on the melting temperature of chocolate and the gelation temperature of hydrogels.LF-NMR on-line temperature control analysis realizes accurate temperature control of samples while performing nuclear magnetic resonance detection,which will promote the application of low-field nuclear magnetic resonance technology. |
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