The present invention relates to techniques for determining the amount of at least one component of a sample and, more specifically, performing time-domain nuclear magnetic resonance measurements on food and related samples that are substantially dry (i.e., if they contain water, the majority of it is bound water).
Time-domain nuclear magnetic resonance measurements (time-domain NMR or TD-NMR) may be used to determine the amount of specific components in foods or animal feed. For example, the determination of fat (and oil) content in such food products can be of particular interest to commercial producers of processed food. Variation in fat and oil content during the production process can be detrimental to product quality or adversely affect production economics. The fat content of a sample also provides useful information about food products such as texture, heat resistance, mouth feel, and flavor release. Additionally, many foods are subject to various statutory and regulatory labeling and content requirements with respect to the fats and oils they contain. Information about fat and oil content is often valuable or necessary in controlling various food processing techniques.
Those skilled in the art know that the primary distinction between fats and oils is that fats are solid at room temperature and oils are liquid. Accordingly, the terms “fat” and “oil” may be used interchangeably herein.
Furthermore, variation in the moisture content of foodstuffs can be detrimental to product quality. For example, to extend the shelf life of dry products, the moisture content of the product should typically be minimized. Accordingly, information about moisture content is also valuable or necessary in controlling food processing techniques.
Traditional methods for determining the moisture and fat content of foodstuffs are time consuming and include oven drying and solvent based extractions. Therefore, the use of traditional methods for purposes of production process control is inefficient and in many cases not practical. For example, many food testing applications in high volume production plants require rapid analysis so that products may be tested before moving on to the next processing stage. Accordingly, time-consuming traditional methods are generally unacceptable. Furthermore, many methods require solvents that are expensive, often hazardous, and pose disposal challenges. Accordingly, scientists have sought alternatives for determining fat and oil content in samples.
Scientists have proposed using NMR as an alternative means of determining the fat and moisture content of foodstuffs. NMR analysis is essentially a spectroscopic method that measures a phenomenon that occurs when nuclei of certain atoms are placed in a first static magnetic field and then exposed to a second oscillating electromagnetic field. The theory and operation of NMR analysis are well understood in the art and will not be discussed in detail herein other than as necessary to describe the invention. In somewhat simplistic terms, however, during NMR analysis a substance is placed in a magnetic field that affects the “spin” of the atomic nuclei of certain isotopes of elements. The nuclei orient themselves in a specific way in response to the magnetic field. If a second radio frequency (RF) magnetic field is passed over the nuclei, the protons in the nuclei will reorient when the RF field reaches a specific frequency. When the RF field is turned off, the nuclei relax, reorient themselves again, and release energy that provides data on the molecular structure of the substance.
Under proper circumstances, NMR can distinguish not only between liquids and solids, but also between chemical compounds. Theoretically, in abstract circumstances, all protons should resonate at the same frequency or relax over the same time period. Surrounding electrons, however, interfere with the magnetic field acting upon a given proton, and thus each proton will resonate at a slightly different frequency, or relax over a different time period, depending on the electronic density around it. As a result, different compounds (and different functional groups within compounds) have different resonance frequencies and different relaxation times.
As mentioned previously, NMR has long held promise as an alternative to solvent extraction and conventional over drying for quantitatively determining the fat and moisture content of a sample. Efficiently utilizing NMR in this regard, however, has proven difficult. This difficulty is especially prevalent in determining the moisture, fat, and oil content of foodstuff samples.
For example, NMR resonance occurs over a narrow band for liquids and this narrow window of NMR resonance is used to easily distinguish liquids from solids. Traditional fat and oil analysis takes advantage of this by melting all the fat and oil in a sample prior to NMR analysis. Because many foods have a relatively high moisture content, and because high moisture content usually makes NMR analysis unfeasible, food samples typically must be thoroughly dried prior to NMR analysis.
After the sample is dried, the sample is usually heated until all the fat and oil present in the sample is assumed to have melted, with the further assumption that the only liquid remaining in the sample is fat. Such heating is typically referred to as thermal equilibration because NMR instruments typically have a set or chosen temperature of operation and samples are heated to approximately the same temperature as the NMR instrument's operating temperature. If aggressive heating techniques, such as convection ovens, microwave ovens, or high temperature heating blocks, are used to speed drying or thermal equilibration of the sample, the chemical structure of the sample may be altered (e.g., the sample may be cooked) which may alter the NMR results and provide a less accurate—or even highly inaccurate—analysis.
To this end, a variety of techniques have been employed to achieve thermal equilibration of an NMR sample. For example, a simple technique involves placing the sample in an NMR instrument and setting the interior temperature of the NMR instrument to the desired operating temperature. The sample is heated by the atmosphere within the instrument until it reaches thermal equilibration, and then the NMR measurement is performed. Although simple, this technique is very time-consuming because of the time required to achieve thermal equilibration for each sample.
U.S. Pat. No. 6,768,305 discloses a convective heating technique that requires a vertical axial bore NMR spectrometer. Such convective heating techniques often involve costs which preclude implementation of time-domain NMR instruments such as those used in the food quality control industry, because specialized NMR hardware is required which would allow flow of thermostated gases over the sample.
U.S. Pat. No. 6,218,835 discloses a method of heating a sample within an NMR instrument that includes applying a set of heating radio-frequency pulses to the sample before NMR analysis. Such RF-heating techniques involve the application of RF energy to a metal-coated sample tube to inductively heat the sample tube which then heats the sample through conductive heating. RF-heating techniques can be relatively expensive and time-consuming, and RF-heating parameters are highly sample-type dependent.
U.S. Pat. No. 7,002,346 discloses a technique that applies temperature correction factors to compensate for sample temperature differences at the NMR measurement time. The correction factors are sample-type dependent and involve relatively complicated calculations making the disclosed technique less reliable and, again, sample-type dependent.
Additionally, a variety of techniques have been employed to achieve more reliable, accurate time-domain NMR analysis. For example, U.S. Pat. No. 6,972,566 discloses a time-domain NMR technique that utilizes magnetic gradient fields to measure the fat and water content of a hydrous sample (i.e., a sample with a significant amount of free water). The magnetic gradient fields are used suppress the signal contributions from water, so that the fat and water may be measured simultaneously. The application of such magnetic gradient fields during the NMR measurement increases the complexity of the technique, the machinery required to employ the technique, and the analysis of the generated data.
U.S. Pat. No. 7,397,241 discloses another time-domain NMR technique that measures water, fat, and protein in samples. The magnetization of the sample is initially saturated using RF pulse sequences, and additional RF pulse sequences are applied to the sample while signal amplitudes are measured. The time parameters and number of RF pulses in the technique are matched to the sample. Thus, this NMR technique is sample-type dependent. Furthermore, the number of saturation and measurement sequences required makes the disclosed technique more time-consuming and complex.
Thus, there exists a need for a thermal equilibration technique that reduces (i) the risk of burning the sample, (ii) the cost of NMR equipment required to employ the technique, and (iii) the time necessary to achieve thermal equilibration. Additionally, there exists a need for a method of determining the amount of a component of a sample (e.g., a dry sample) that does not depend upon sample-particle-size and that reduces the cost of NMR equipment required to employ the technique and the time required to perform a measurement.