Since the beginning of 1970s, Bruker and its partners have been working together on the development of the NMR method to determine Solid Fat Content.
Solid Fat Content (TD-NMR)
- SFC Melting Curves as measured with the minispec TD-NMR system
Official SFC Methods
The first version of an AOCS official method for SFC determination by low resolution NMR was published in 1993 (AOCS Official Method Cd 16b-93). Bruker has fully supported the NMR Committee and its work.
Presently, the AOCS official methods are:
- AOCS Cd 16b-93 revised in 2000; Direct Method
- AOCS Cd 16-81 revised in 2000, Indirect Method
Official methods for SFC in Europe:
- ISO 8292
- IUPAC 2.150
Solid Fat Content (SFC) and Solid Fat Index (SFI) Measurements
In the United States, the traditional method used for measuring the solids content of edible oils is to measure the solid fat index (SFI) by dilatometry.
Solid Fat Index
SFI is an empirical value that is derived from expansion of a fat as a chilled sample is warmed. In the process of melting, previously crystallized parts of the sample become liquefied. Since the fat molecules in a liquid state are less efficiently arranged in space compared to closely packed crystalline regions, liquid fat takes up more volume. Therefore, the degree of expansion is related to the change in the solid content.
Dilatometry does not directly measure the solids content of fat at any given time, rather it measures the change in volume compared to the starting point.
SFI measurements depend on consistent operator skill and judgment for accuracy and reproducibility.
Leaky dilatometer burettes, bubble formation and other artifacts can ruin an entire series of SFI determinations.
SFI values for fats that contain emulsifiers are not very accurate due to some dissolution of emulsifiers into the indicator at the fat/indicator boundary.
A strong impetus therefore exists to adapt an alternative method.
Solid Fat Content
An SFC value is determined by detecting the NMR signal from both liquid and solid components in the fat sample, or by detecting the change in the liquid signal as it is displaced by solid.
The minispec employs the "pulse" method to perform the NMR experiment. This means that a short intense burst of radio frequency (RF) energy is applied to the sample in the static magnetic field to cause excitation of the Hydrogen in the fat.
After excitation, the Hydrogen in the sample generates detectable signal in the minispec receiver. Over time, the Hydrogen relax back to their equilibrium state. The minispec records the time evolution of the NMR response and the resulting signal contains the analytical information from which fractions of Hydrogen in solid and liquid state may be deduced. After the pulse, the signal is detectable for milliseconds to seconds. The NMR signal decays because of relaxation. In solid matter, oscillations are heavily damped, and the signal decays relatively quickly. In a liquid, however, the surroundings are more mobile, thus causing less damping and a slower decay of the signal.
The FID signal shape can therefore be used to distinguish between solid and liquid components of a sample. The initial rapid decline of the signal is due to decay of the solid type components in the sample, which relax quickly to their equilibrium state.
The remaining and slower decaying signal is from the liquid type components, which have slower relaxation.
- Phase a) represents the combined solid + liquid signal following a 90o pulse.
- Phase b) represents the pure liquid signal
Application Methodology for Direct Solid Fat Content Measurements
- The fat is melted at T = 80 to 100°C and held there for 15 minutes.
- The fat must be well-mixed prior to removing a representative and residue-free amount, which is used to fill a 10 mm diameter sample tubes to a height of 4 cm.
- Sample temperature is maintained at T = 60°C for at least 5 minutes but not more than 15 minutes.
- Transfer the sample to T = 0°C and maintain for 60 +/- 2 minutes (must be timed accurately).
- Transfer the sample to the temperature for which a measurement is needed. Maintain this temperature for 30 to 35 minutes (must be timed accurately).
- The creation of a melting curve requires the sample to be measured at many different temperatures (e.g. 10°C / 15°C / 20°C / 25°C / 30°C / 35°C etc).
- Insert the sample tube into the minispec for a 6 second analysis. The direct SFC method consists of the single measurement at the desired temperature. The direct parallel method consists of the consecutive measurement of solid fat content at all desired temperatures.
Note:Choice of tempering method can lead to differences in results. Thus, it is important to specify the method when reporting SFC values.
Options for the SFC Measurement
Two pulsed NMR methods exist for measuring the SFC of edible oils: the direct and the indirect methods. The direct method measures the signal from both the solid and liquid components (see previous page for a diagram); the indirect method measures only the liquid signal and compares it to the signal from a fully melted sample.
The direct method is very fast, reproducible and sample preparation is minimal. Melted oil is simply poured into an NMR tube to a height of approximately 4 to 5 cm and the sample is tempered in the tube. Only one NMR measurement is needed to obtain a SFC value by the direct method.
The indirect method is also very reproducible and accurate and it is not as fast. More care must be taken when preparing samples. Four measurements must be performed at two temperatures in order to calculate the percentage of solids.