NMR Method for Measuring the Oil Content of Oil Seeds
As a nondestructive testing technology, there have perfect research system and mature achievement for the detection of Oil &Moisture content detection field based on the relaxation time of the low field NMR. Oil &Moisture rate detection also plays an important role in the agriculture and food industry. At present, international standards ISO10565:1995< oilseeds, Simultaneous determination of Oil &Moisture content and pulsed NMR measurement> and national standard GB / T 15690:1995< Oil content of oilseed NMR measurement>.It is suitable for the oilseed containing content below 10%, such as rapeseed, soybean, flaxseed, sunflower seeds, sesame, cotton seed and others. If the moisture content is higher than 10%, drying treatment will be needed, so that the moisture down to the required range.
The hypothesis is that the NMR signals from the seeds are only derived from water and oil. There have a significant difference for the hydrogen proton NMR signal of the water and oil seeds. Specifically, the rate of decay is different, which is also different in the relaxation time. Therefore, the T2 of the two is different. For the solid and bound water, the bound water relaxation time is shorter, and the relaxation time of oil and free water are longer. The water inside the seed can be regarded as bound water, so T2 water is much less than T2 oil. By this time, M value is a result that the hydrogen protons in water and oil working in concert and without the effect of T2 water and T2oil. After a half echo time, M value will be P oil（-e-t/T2oil）+ P water（-e-t/T2water）.
When τ value is far greater than T2water but less than T2oil, the first item in the formula was approximately zero. At this time, water has been completely attenuated, the peak of the echo signal is provided by the oil hydrogen. At this time, FID signal is the mixture of oil and water signals. The first echo peak is the signal of oil, total FID signal minus oil signal will get the water signal. The ISO10565:1998 specified that testing the moisture content of the oil seed when τ value is 3.5ms.
Fig. 1.the schematic diagram of measuring the Oil &Moisture content in the seed by CPMG sequence
The signal strength of oil (or water) is linearly depend on the quality of oil (or water).So we can get the corresponding relationship between the signal strength of oil (or water) and the quality of oil (or water) through calibration , so as to get the oil content (or moisture) of the test sample.
Table 1: Sunflower seeds oil& moisture content accuracy test
NMR method VS Suo’s extraction