10.5.2. Nuclear Magnetic Resonance
NMR has been used to measure the solid content of food emulsions for many years (Walstra and van Beresteyn 1975, van Boekel 1981, Dickinson and McClements 1995). NMR instruments are capable of rapidly analyzing emulsions that are concentrated and optically opaque, without the need for any sample preparation. For this reason, they have largely replaced the more cumbersome and time-consuming dilatometry method in laboratories which can afford the relatively high initial cost of an NMR instrument. The NMR technique utilizes interactions between radio waves and the nuclei of hydrogen atoms to obtain information about the solid content of a material. The fundamental principles of this application have been described elsewhere (Dickinson and McClements 1995), and so only a simplified description of the technique will be given here. Basically, a radio-frequency pulse is applied to an emulsion, which causes some of the hydrogen nuclei to move into an excited state, which leads to the generation of a detectable NMR signal. The frequency, amplitude, and decay time of this signal depend on the ratio of solid to liquid material in the sample. Thus, by analyzing the characteristics of the NMR signal, it is possible to obtain information about the solid content of the material.
A variety of NMR instruments which can be used to measure the solid content of emulsions are commercially available. These instruments vary in their operating principles, the types of information they are capable of providing, and their cost. The more sophisticated instruments can measure a large number of different physicochemical characteristics of emulsions, but because they are often very expensive and require highly trained operators, their application is restricted to a small number of research laboratories. A number of less sophisticated instruments are available that have a more limited range of applications but which are considerably less expensive. These instruments are the most widely used to determine the solid content of foods, and therefore only they will be considered here.
A typical pulsed NMR instrument consists of a static magnet, a probe coil, electronics to generate and receive electromagnetic pulses, and a computer to regulate the measurement procedure and analyze and store the data (Figure 10.21). Nuclei in the sample are excited to
FIGURE 10.20 Temperature dependence of the density of a hexadecane oil-in-water emulsion. For food oils, melting and crystallization normally occur over a wider range of temperatures.
a higher energy level by applying a radio-frequency pulse via the probe coil, and the excited nuclei are detected by the same coil once the radio-frequency pulse is removed. The detected signal is digitized by a digital-to-analog convertor and stored in the computer for data analysis. The solid content is determined by analyzing the decay rate of the detected signal after the application of the radio-frequency pulse.
The decay of the NMR signal is much more rapid for a solid than a liquid (Figure 10.22). Consequently, the solid and liquid phases in a sample can be differentiated by measuring the decay of the NMR signal with time. Immediately after the radio-frequency pulse is switched off, the NMR signal (S0) is proportional to the total number of nuclei in the liquid and solid phases. After a certain time, the contribution from the solid phase has completely decayed, and the signal (Sf) is then proportional to the number of nuclei in the liquid phase. Assuming that the NMR signal per gram of the solid and liquid phases is similar, the solid content can simply be determined: <c = (S0 - Sf)/S0. In practice, it is not possible to measure the signal at zero time because the NMR receiver takes a short time to recover after the radio-frequency
pulse is applied. Consequently, it is necessary to use a correction factor that accounts for the slight decay of the signal before the first measurement is made. The solid content can also be determined by only making measurements of the signal from the liquid phase, since St is proportional to the mass of liquid phase present (ML). The mass of liquid in a sample is deduced by measuring St and using a previously prepared calibration curve of ML versus St. If the total mass of the sample analyzed (MT) is known, then <c = (MT - ML)/MT. The solid content can also be determined by measuring St for a partially crystalline sample and then heating it to a temperature where all of the solid phase melts and measuring it again: <c = (S' - St)/S' where the prime refers to the measurement at the higher temperature. This value has to be corrected to take into account the temperature dependence of the NMR signal of the liquid phase.
NMR has been used to determine the extent of droplet crystallization in emulsions in a number of fundamental studies and commercial products (Walstra and van Beresteyn 1975, Waddington 1980, van Boekel 1981, Dickinson and McClements 1995). Benchtop instruments are available which are extremely simple and rapid to use. A tube containing the sample is placed in the NMR instrument and the solid content is given out in a few minutes or less. The technique is therefore particularly useful for quality control purposes when many samples have to be rapidly analyzed. Attempts have been made to develop on-line versions of these NMR instruments, but their application is limited because the sample cannot be analyzed within a metal pipe because of its distorting influence on the magnetic field. An additional limitation is that the technique cannot be used to accurately determine the solid contents when the degree of crystallization is small (<5%).
Before leaving this section, it should be noted that there are a variety of other NMR techniques which can also be used to determine droplet solid contents (Dickinson and McClements 1995). The most powerful of these is NMR imaging, which can be used to determine the solid content at any location within a material, so that it is possible to obtain a three-dimensional image of the droplet crystallinity (Simoneau et al. 1991, 1993). These imaging techniques provide food scientists with an extremely powerful method of monitoring and predicting the long-term stability of food emulsions, which will undoubtedly help to identify the most important factors that determine emulsion properties. Nevertheless, these techniques are extremely expensive and require highly skilled operators and therefore are currently available to a small number of research laboratories.
10.5.3. Thermal Analysis
Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are two thermal analysis techniques that can be used to monitor melting and crystallization of emulsion droplets (Walstra and van Beresteyn 1975, McClements et al. 1993a, Davis 1994a). These techniques are based on measurements of the heat released or adsorbed by a sample when it is subjected to a controlled temperature program. A material tends to release heat when it crystallizes and absorb heat when it melts. The major difference between the two techniques is the method used to measure the heat adsorbed or released by the sample.
Differential Thermal Analysis. DTA records the difference in temperature between a substance and a reference material when they are heated or cooled at a controlled rate. A DTA
Differential Thermal Analysis
Was this article helpful?