S16_NMR

Investigation on Powdered Milk composition by NMR Relaxation Properties

Aurelio Dregni and Tiance Xia

University of Minnesota, MXP II, Spring 2016

Introduction

Nuclear magnetic resonance was discovered in 1946 by Edward Purcell at Harvard and Felix Bloch at Stanford. [1] The researchers placed the sample in a uniform magnetic field and observed the response of the nuclei to an additional continuous RF magnetic field as it swept over the resonance frequency. This discovery opened up a new type of spectroscopy which became an important tool available to physicists, chemists and biologists. NMR was widely used to characterize materials, since it probed the nuclei and their immediate surroundings.

There were two types of NMR relaxation time. T1 reflected the characteristic time the magnetization needed to return to the equilibrium state; T2 reflected the decaying time of the magnetization in the direction perpendicular to the applied magnetic field. The relaxations times were influenced by the molecular size, and the physical and chemical environment of the nucleus, such as the viscosity of the solution. Our goal in the experiment was to establish the relationship between the relaxation time and the relative content of the lactose and protein (mainly casein) in the powdered milk.

Experimental Setup

The NMR apparatus contained three components, the permanent magnet, the PS2 controller and the “Mainframe”. (Fig 6.) The permanent magnet provided a constant B field in the z-direction. There was a coil in the region between the magnets, which produced the oscillating magnetic field (the “pulse”). A RF sample probe was mounted inside to detect the frequency of the oscillating field since it was necessary to match it with the Larmor frequency of the sample. The PS2 controller provided temperature control to the permanent magnet and also the homogeneity of the magnetic field. The mainframe was composed of Receiver, Synthesizer, Pulse Programmer, Lock-in Amplifier, and built-in DC Power supply. The mainframe had the function of modulating the RF signal sent to the coil, receiving and amplifying the magnetization information of the sample. Then the signal would be sent to a digital oscilloscope. The devices were connected by BNC cables.

Fig. 6. The apparatus diagram of the experiment.

In the experiment, skim powdered milk was used since it could prevent the influence of the fat, which had comparable molecular size as the proteins. The sample used in the experiment could be illustrated by the following graph.

Fig. 7. The distribution of the powered milk samples. (0%, 0%) represented the distilled water.

The relative content of protein was controlled by the concentration of milk, which meant the amount of water added to the powdered milk. (compared to the recommended ratio provided by the manufacturer) Each vertical line in the graph represented the same lactose content so that the influence of proteins could be measured, and each horizontal line indicated the same protein content so that the effect of lactose could be studied.

Results and analysis

Since the data points were insufficient, we did not perform linear fit on the results. However, it was obvious that as the relative content of the protein or lactose increased, the T1 and T2 of the sample decreased, which coincided with the theoretical prediction related to the viscosity. [5]

Conclusion

The T1 and T2 were affected by the tumbling rate of the protons. As the viscosity and the molecular size of the sample increased by adding protein and lactose, the nucleus would rotate slower, which caused the fast decay of T1 and T2. Future improvement included collecting more data and establishing the precise relationship between the relaxation time and relative content of the component, so that the NMR could be an effective method to determine the amount of protein in the milk sample.

Reference

[1] Bloch F: Nuclear Induction. Physical Review 70:460‐474, 1946

[2] Teachspin Instructional Pulsed NMR Apparatus Instruction Manual. Department of

Physics and Astronomy, The University of Tennessee.

[3] Fundamentals of NMR. THOMAS L. JAMES. Department of Pharmaceutical Chemistry

University of California

[4] Milford D, Rosbach N, Bendszus M,Heiland S (2015) Mono-Exponential Fitting in T2-Relaxometry: Relevance of Offset and First Echo.PLoS ONE 10(12): e0145255. doi:10.1371/journal.pone.0145255

[5] 1H NMR Diffusometry Study of Water in Casein Dispersions and Gels. FRANC¸ OIS MARIETTE. J. Agric. Food Chem. 2002, 50, 4295−4302