oil extraction and analysis phần 9 pps

Chapter 9

Simple Methods for Measuring Total Oil Content

by Benchtop NMR

P.H. Krygsman and A.E. Barrett

Bruker Optics Ltd., Minispec Division, 555 Steeles Ave., East Milton, Ontario L9T 1Y6,

Canada

Abstract

Low-resolution time domain nuclear magnetic resonance (TD-NMR) is an interna￾tionally recognized and commercially important analytical tool for measuring the

oil content in oilseeds and oilseed residues. It is commonly employed as a simulta￾neous determination of oil and moisture content, as in ISO 10565 or AOCS Ak 4-

95. Because the method is very fast, compared with oven drying to determine

moisture and extraction to determine oil, rapid and more frequent testing is possi￾ble, 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 sun￾flower 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 refer￾ence 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 mea￾sured 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 condi￾tions, 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 predry￾ing 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

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