Simple Methods for Measuring Total Oil Content by Benchtop NMR

Low-resolution time domain nuclear magnetic resonance (TD-NMR) is an internationally recognized and commercially important analytical tool for measuring the oil content in oilseeds and oilseed residues. It is commonly employed as a simultaneous determination of oil and moisture content, as in ISO 10565 or AOCS Ak 495. Because the method is very fast, compared with oven drying to determine moisture and extraction to determine oil, rapid and more frequent testing is possible, and dry-weight oil is easily reported. The NMR method is widely used in European countries, especially France and Germany, for oil in sunflower, rape, and soybean. A related method used in Spain is oil in olives for assigning commercial value. In North America and South America, official oil content in canola and sunflower is determined by the NMR method. All other oilseeds are amenable to the method, including soy, flax, corn, cotton, and peanut. A large part of the reason for the success of the method is the simplicity of calibration. In cases in which rapid and accurate screening of oil content is the primary concern, simple calibration maintenance is desired. Calibration for oil can involve as few as two or three reference oilseed samples. Often 5–20 calibration samples are used to satisfy statistical variation in the natural product. Repeatability of measurement is usually limited by subsampling error. If the same sample is measured repeatedly, the SD is typically ≤0.02. Agreement between duplicate samples taken from the same lot depends on the oilseed and size of sample used. It was found to be ~0.1–0.2 for soybean measured in ~22-g samples. TD-NMR measures oil by detecting the hydrogen in the liquid phase of the sample. The NMR signal for oil is well isolated and normally free of interference from other components of the oilseed; under normal conditions, the signal per gram increases in a simply linear fashion with increasing oil content. Moisture present in the seed <10% by weight (14% in soy) is strongly associated with solid components, and relaxes at least an order of magnitude faster than oil. If excess water is present in the sample, it must be removed by a predrying step. Water in various degrees of “free” states may relax with a time constant similar to that of oil, or even longer. Vegetable oils contain a mixture of fatty acids Copyright © 2004 AOCS Press in triacylglycerides, and the fatty acid profile can vary considerably. If unsaturation is introduced, the hydrogen content (%) decreases. This decrease also will result in a lower estimate of total oil content because hydrogen is used as a reporter in the NMR measurement. The hydrogen content (%) can easily be determined in isolated oil (extracted or expressed) to ensure that it fits within an acceptable range.