Determination of Solid Fat Content by Nuclear Magnetic Resonance

The adoption of NMR methods as a standard way to measure SFC is largely due to the work of the American Oil Chemists Society (AOCS). Over the past decades they have conducted a series of interlaboratory collaborative trials to revise and improve methodologies and have published their recommendations as Official Methods and Recommended Practices of the AOCS (Firestone, 1998). The methods described here were based on methods Cd 16b-93 and Cd 16-81 of this book, and readers are encouraged to consult the original text. The important advantages of NMR for this application are that it can rapidly and nondestructively measure the number of hydrogen nuclei in the solid or liquid phase and thus the ratio of the amounts of each phase. 

CAUTION No one with a pacemaker should approach within 30 cm of an NMR instrument. Higher specification instruments require further caution. If in doubt, consult a doctor and the equipment manufacturer. Keep magnetic storage devices (including credit cards) away from the measurement coil. 

DETERMINING SOLID FAT CONTENT BY A DIRECT PARALLEL NMR METHOD 

The direct methodology separately measures the NMR signals from nuclei in the solid and liquid phases. The solid fat content is calculated from their ratio. 

Water baths (or similar apparatuses) capable of maintaining temperatures listed in Table D3.1.1 

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D3.1 Determination of Solid Fat Content by Nuclear Magnetic Resonance... 

D3.1 Determination of Solid Fat Content by Nuclear Magnetic Resonance Basic Protocol Determining Solid Fat Content by a Direct Parallel NMR D3.1.1... 

The dilatometric method is time-consuming and subject to the bias introduced by the convention described. More recently pulsed Nuclear Magnetic Resonance (pNMR) has been used to measure the relative amounts of liquid and solid fat in a sample, based upon the difference in rates of relaxation of protons in the two phases after the sample has been pulsed (AOCS Method Cd 16-81). With proper calibration this gives a direct determination of the percentage of solid fat, and the results are termed sohd fat content (SFC). The analysis takes less time than dilato-metry, but the equipment is more expensive. 

Haughton, A.J., K. VanPutte, and L.F. Vermas, Determination of the Solid Content of Fats by Wide Line Nuclear Magnetic Resonance The Signal of Liquid Oils, Ibid. 49 153-156(1972). 

Typical solid fat contents of cocoa butters, determined by pulsed NMR (nuclear magnetic resonance), are given in Table 3.26. 

The fat in margarines and shortenings consists of liquid and solid components. The solids consist of fat crystals that incorporate the liquid oil in a crystal network. The amount of crystals present or, in other words, the amount of solids, the crystal size, and the strength of the network are important in determining the texture. The solid fat content (SFC) is determined by either nuclear magnetic resonance spectroscopy (NMR) or dilatometry. The dilatometry method is old and cumbersome but is still in use. NMR can also be used to estimate the SFC in the finished product by transferring a tube of the finished product into the NMR tube by means of a stainless steel open probe [3]. The manner in which the solid fat content is determined is very important. There are two official methods, the American Oil Chemists Society (AOCS) method, which is used in the Americas, and the lUPAC method, which is used mainly in the rest of the world. In the AOCS method the fat is cooled at 0°C for 15 min, tempered at 26.1°C for 30 min, and cooled again to OX for 15 min, and then measurements are taken at the various temperatures after the samples have been left for 30 min at those temperatures. In the lUPAC method samples are not tempered instead, the fat is cooled at 0°C for 30 min and then measurements are taken as in the AOCS method. There is quite a difference in the SFC values obtained at temperatures... 

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