Nuclear magnetic resonance device and measuring the percentage of solids in oils and fats

The low-resolution nuclear magnetic resonance (NMR) device (Low-Resolution NMR) The cornerstone of quality laboratories in ghee, margarine and shortening oils (Shortening) factories. Its importance lies in its ability to measure The percentage of solids in fat (Solid Fat Content - SFC) Within a few seconds, it is a physical feature that directly and decisively affects the quality of the final product in terms of:

• Plasticity
• Individual ability (Spreadability).
• Mouth sensation (Mouthfeel).

Despite the rapid results, the physical principle by which the device works is somewhat complex. In the following lines, we provide a simple and accurate explanation of the components of the device and its working idea.

First: the main components of the device

The NMR device used in this type of measurement consists of four main units that integrate each other:

1. Main magnet (Main magnet):
2. It is a high intensity fixed magnet (usually starting from 20 MHz and above). Its function is to provide a stable and accurate magnetic field to align the hydrogen nuclei within the sample.
3. Radio frequency files (RF coils):
4. These files are responsible for sending radio pulses (Pulses) to the sample to stimulate the hydrogen nuclei, and then receive the feedback signal (Signal) from them after interacting with the magnetic field.
5. Temperature control system (Tempering System):
6. A vital unit that controls and stabilizes the temperature of the sample with extreme accuracy at the required measurement temperature, as the percentage of solids in oils is very sensitive to thermal changes.
7. Electronic processing unit:
8. The mastermind of the device is responsible for operating and controlling the components, in addition to processing the received signals and converting them into a digital reading representing the percentage of solid fat (%SFC).

Second: the idea of work and the principle of measurement

1. Magnetic moment function (Nuclear Spin)

The device is based on the physics of some atoms. Nuclei that contain an odd number of protons (such as hydrogen) have a nuclear spin (Spin). This rotation results in a small magnetic field that makes the nucleus behave as if it were a precise magnet.

Because hydrogen atoms are a key component of organic chemistry (and oils and fats in particular), the device relies on a hydrogen proton, which has a clear magnetic field.

2. Magnetic field compatibility

When the sample is inserted into the device, hydrogen nuclei (micromagnets) are exposed to a strong magnetic field ($B$). They tend to align in a direction parallel to the field and begin to rotate around its axis ($z-axis $) with a specific frequency called “Larmor frequency” (Larmor frequency), which depends directly on the strength of the magnetic field.

3. Radio pulse and resonance

The device emits a powerful radio pulse with a frequency equal to that of “Larmor”, with enough energy to flip the magnetic output of the cores at a 90-degree angle to the level ($XY$). Here, the nuclei enter into a state of “resonance” and become synchronized in phase (Phase Coherence). All protons rotate in the same direction and moment, generating an oscillating magnetic field that stimulates an electrical current in the receiving coil.

4. Free fading (FID)

Once the pulse is stopped, the nuclei begin to gradually lose synchronization (Dephasing) and return to equilibrium along the main field. During this return, the resulting electrical current gradually decreases, and this signal is known as the “Fading Free Signal” (FID — Free Induction Decay).

Third: the difference between solid and liquid fats in readings

The device distinguishes between solid and liquid states based on the speed of signal loss (relaxation time):

• In solid fat: Molecular movement is very restricted, so nuclei lose their synchronization very quickly, and the occasional relaxation time ($T_2$) is very short (about 10 microseconds).
• In liquid fat: Molecules are freer, relaxation occurs slowly, and the time ($T_2$) is longer (up to 1000 microseconds).

The device exploits this large variation in ($T_2$) values to separate solid and liquid part signals and calculate the SFC ratio accurately.

Fourth: the suppression time (Dead Time) and the accuracy of the measurement

Device protection:

When the high-energy radio pulse is released, the device stops receiving signals for a very short period (usually no more than 10 microseconds) known as “dead time” (Dead Time). This pause is necessary to protect sensitive receiver circuits from damage or high power saturation.

Make up for the lost signal:

During the suppression time, the solid fat signal has already started to fade (due to the short time of $T_2$), which means that a large part of the “solid” data is in the unreadable area.

To solve this problem:

1. The device starts recording immediately after the end of the suppression time (at 11 microseconds, for example), where the signal is a mixture of solid and liquid.
2. The device is used Calibration Factor To make up for the missing part mathematically, this coefficient is adjusted daily using approved reference materials.
3. The device takes a second reading at a longer time (such as 70 microseconds) where the solid signal is completely gone and only the liquid signal remains.
4. Subtracting the liquid signal from the total signal (after compensation), we get the net solid signal and the electronic unit calculates the percentage.

Fifth: Calibration and sample preparation and their effect on results

To ensure the accuracy of the results, the analysis is based on two main pillars:

1. Calibration:

• Daily Validation: It is done in the laboratory using standard samples to make up for lost signals during suppression time and to ensure the validity of SFC readings.
• External Calibration: It is done by a competent authority (annually or semi-annually) to control the device temperature, resonance frequency, suppression time, and file sensitivity.

2. Sample preparation (Sample Preparation):

The sample should be professionally prepared to ensure the crystallization of the fat in the stable crystalline form $\ beta-'$ (beta-prime):

• The sample is completely thawed to erase the “fat memory” (Fat Memory) and remove any previous crystals.
• The sample is conditioned and cooled at 0 °C for an hour (or according to the standard method) to achieve crystal stability.

ATTENTION: If this process is not done accurately, $\ alpha$ (low-melting) or $\ beta$ (high-melting) crystals may form due to the phenomenon of crystal polymorphism (Polymorphism), which changes the relaxation time ($T_2$) and gives inaccurate SFC results.

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