Determination of solid fat index by fourier transform infrared spectroscopy

A unique and rapid Fourier transform infrared (FTIR) spectroscopic method for the determination of solid fat index (SFI) of fats and oils was developed, which is capable of predicting the SFI profile of a sample in approximately two minutes, without the need for tempering. Hydrogenated soybean oil samples (n=72), pre-analyzed for SFI by dilatometry, were melted and their FTIR spectra acquired using a 25 μm NaCl transmission flow cell maintained at 80°C. Approximately half the samples were used for calibration, with the balance used as validation samples. Partial least squares (PLS) calibrations were developed from selected spectral regions that are associated with thecis, trans, ester linkage and fingerprint regions of the spectrum and related to the dilatometric SFI values obtained at 50, 70, 80, and 92°F. The calibrations were initially optimized and cross-validated by using the “leave one out” approach, with the accuracy and reproducibility of the calibration models assessed by predicting the validation samples. The overall cross validation accuracy of the PLS calibration models was in the order of ±0.71 SFI units over the four temperatures. Week-to-week validation accuracy and reproducibility was determined to be ±0.60 and ±0.38 SFI units, respectively, the reproducibility being within the specifications associated with the dilatometric reference method. To facilitate routine “on-line” FTIR analyses, a Visual Basic program was written to drive the spectrometer, prompt the user to load the sample, calculate, and print the SFI values determined from the PLS calibrations. As structured, the FTIR method has the potential to serve as a viable substitute for the traditional dilatometric SFI method, with the elimination of the tempering step reducing analysis time from hours to minutes. The FTIR approach should also be applicable to the determination of solid fat content if calibrated against solids data obtained by nuclear magnetic resonance.     

The determination of peroxide value by fourier transform infrared spectroscopy

A rapid method for the quantitative determination of peroxide value (PV) of vegetable oils by Fourier transform infrared (FTIR) transmission spectroscopy is described. Calibration standards were prepared by the addition oft-butyl hydroperoxide to a series of vegetable oils, along with random amounts of oleic acid and water. Additional standards were derived through the addition of mono- and diglyceride spectral contributions, as well as zero PV spectra obtained from deuterated oils. A partial least squares (PLS) calibration model for the prediction of PV was developed based on the spectral range 3750–3150 cm⁻¹. Validation of the method was carried out by comparing the PV of a series of vegetable oils predicted by the PLS model to the values obtained by the American Oil Chemists Society iodometric method. The reproducibility of the FTIR method [coefficient of variation (CV)=5%)] was found to be better than that of the chemical method (CV =9%), although its accuracy was limited by the reproducibility of the chemical method. The method, as structured, makes use of a 1-mm CaF₂ flow cell to allow rapid sample handling by aspiration. The spectrometer was preprogrammed in Visual Basic to guide the operator in performing the analysis so that no knowledge of FTIR spectroscopy is required to implement the method. The method would be suitable for PV determinations in the edible oil industry and takes an average of three minutes per sample.     

Determination of phospholipids in vegetable oil by fourier transform infrared spectroscopy

A method was developed to determine the total phospholipid content in vegetable oil by Fourier transform infrared spectroscopy (FTIR). Calibration curves of I-α-phosphatidylcholine (PC), I-α-phosphatidylethanolamine (PE), and I-α-phosphatidylinositol (PI) in hexane were generated at different concentrations. The optimal phospholipid absorption bands between 1200–970 cm⁻¹ were identified and used for quantitative determination. High R ²≥0.968 were observed between band areas and phospholipid standard concentrations. Phospholipids from crude soybean oil were obtained by water degumming, and purification was performed on a silicic acid column. The phospholipid contents of purified phospholipid extract, degummed and crude soybean oil determined from calibration equations were >90, 0.0113, and 1.77%, respectively. High correlations of determination (R ²≥0.933) were observed between the FTIR method and thin-layer chromatography-imaging densitometry method for the determination of phospholipid content. FTIR was found to be a useful analytical tool for simple and rapid quantitative determination of phospholipids in vegetable oil.     

Quantitative determination of hydroperoxides by fourier transform infrared spectroscopy with a disposable infrared card

Disposable polyethylene infrared cards (3M IR cards) were investigated for their suitability for the quantitative determination of peroxide value (PV) in edible oils relative to a conventional transmission flow cell. The analysis is based on the stoichiometric reaction of triphenylphosphine (TPP) with hydroperoxides to produce triphenylphosphine oxide (TPPO). Preliminary work indicated that the cards, although relatively consistent in their pathlength (±1%), had an overall effective pathlength variation of ±∼5%, caused by variability in loading of the oil onto the cards. This loading variability was reduced to <0.5% by developing a normalization protocol that is based on the peak height of the ester linkage carbonyl overtone band at 3475 cm⁻¹, which allowed one to obtain consistent and reproducible spectra. The standard PV calibration approach, based on the TPPO peak height at 542 cm⁻¹, failed because of unanticipated card fringing in the region where the measurements were being made. However, the development of a partial-least-squares (PLS) calibration provided a means of eliminating the interfering effect of the fringes and allowed the TPPO band to be measured accurately. An alternate approach to the standardized addition of TPP reagent to the oil was also investigated by impregnating the 3M IR cards with TPP, thus allowing the reaction to take place in situ. The spectral analysis protocols developed (normalization/calibration) were programmed to automate the PV analysis completely. The 3M card-based Fourier transform infrared PV methods developed were validated by analyzing oxidized oils and comparing the PV predictions obtained to those obtained in a 100-μm KCI flow cell. Both card methods performed well in their ability to predict PV. The TPP-impregnated 3M card method reproduced the flow cell PV data to within ±1.12 PV, whereas the method with an unimpregnated card was accurate to ±0.92 PV over the calibrated range (0–25 PV). Our results indicate that, with spectral normalization and the use of a PLS calibration, quantitative PV data, comparable to those obtained with a flow cell, can be provided by the 3M IR card. With the analytical protocol preprogrammed, the disposable 3M card provides a simple, rapid and convenient means of carrying out PV analyses, suitable for quality control laboratories, taking about 2–3 min per analysis.     

Quantitative determination of hydroperoxides by fourier transform infrared spectroscopy with a disposable infrared card

Disposable polyethylene infrared cards (3M IR cards) were investigated for their suitability for the quantitative determination of peroxide value (PV) in edible oils relative to a conventional transmission flow cell. The analysis is based on the stoichiometric reaction of triphenylphosphine (TPP) with hydroperoxides to produce triphenylphosphine oxide (TPPO). Preliminary work indicated that the cards, although relatively consistent in their pathlength (±1%), had an overall effective pathlength variation of ±∼5%, caused by variability in loading of the oil onto the cards. This loading variability was reduced to <0.5% by developing a normalization protocol that is based on the peak height of the ester linkage carbonyl overtone band at 3475 cm⁻¹, which allowed one to obtain consistent and reproducible spectra. The standard PV calibration approach, based on the TPPO peak height at 542 cm⁻¹, failed because of unanticipated card fringing in the region where the measurements were being made. However, the development of a partial-least-squares (PLS) calibration provided a means of eliminating the interfering effect of the fringes and allowed the TPPO band to be measured accurately. An alternate approach to the standardized addition of TPP reagent to the oil was also investigated by impregnating the 3M IR cards with TPP, thus allowing the reaction to take place in situ. The spectral analysis protocols developed (normalization/calibration) were programmed to automate the PV analysis completely. The 3M card-based Fourier transform infrared PV methods developed were validated by analyzing oxidized oils and comparing the PV predictions obtained to those obtained in a 100-μm KCI flow cell. Both card methods performed well in their ability to predict PV. The TPP-impregnated 3M card method reproduced the flow cell PV data to within ±1.12 PV, whereas the method with an unimpregnated card was accurate to ±0.92 PV over the calibrated range (0–25 PV). Our results indicate that, with spectral normalization and the use of a PLS calibration, quantitative PV data, comparable to those obtained with a flow cell, can be provided by the 3M IR card. With the analytical protocol preprogrammed, the disposable 3M card provides a simple, rapid and convenient means of carrying out PV analyses, suitable for quality control laboratories, taking about 2–3 min per analysis.     

Determination of iodine value of palm oil by fourier transform infrared spectroscopy

A rapid method for the quantitative determination of iodine value (IV) of palm oil products by FTIR transmission spectroscopy is described. A calibration standard was developed by blending palm stearin and superolein in specific ratios that covered a range of 27.9 to 65.3 IV units. The spectra of these standards was measured in the range between 3050 and 2984 cm⁻¹, corresponding to the absorption band of=C-H cis stretching vibration. A partial least squares calibration model for the prediction of IV was developed to quantify the IV of palm oil products. A validation approach was used to optimize the calibration with a correlation coefficient of R ²=0.9995 and a standard error of prediction of 0.151. This study concludes that the FTIR transmission approach can be used to determine the IV of palm oil products with a total analysis time per sample of less than 2 min for liquid samples.     

Determination of olive oil free fatty acid by fourier transform infrared spectroscopy

A new procedure for determining free fatty acids (FFA) in olive oil based on spectroscopic Fourier transform infrared-attenuated total reflectance spectroscopy measurements is proposed. The range of FFA contents of samples was extended by adding oleic acid to several virgin and pure olive oils, from 0.1 to 2.1%. Calibration models were constructed using partial least-squares regression (PLSR). Two wavenumber ranges (1775–1689 cm⁻¹ and 1480–1050 cm⁻¹) and several pretreatments [first and second derivative; standard normal variate (SNV)] were tested. To obtain good results, splitting of the calibration range into two concentration intervals (0.1 to 0.5% and 0.5 to 2.1%) was needed. The use of SNV as a pretreatment allows one to analyze samples of different origins. The best results were those obtained in the 1775–1689 cm⁻¹ range, using 3 PLSR components. In both concentration ranges, at a confidence interval of α = 0.05, no significant differences between the reference values and the calculated values were observed. Reliability of the calibration vs. stressed oil samples was tested, obtaining satisfactory results. The developed method was rapid, with a total analysis time of 5 min; it is environment-friendly, and it is applicable to samples of different categories (extra virgin, virgin, pure, and pomace oil).     

Quantitative analysis of palm carotene using fourier transform infrared and near infrared spectroscopy

β-Carotene content is usually determined by using ultraviolet (UV)-visible spectrophotometry at 446 nm. In this study, two spectroscopic techniques, namely, Fourier transform infrared (FTIR) and near infrared (NIR) spectroscopy, have been investigated and compared to UV-visible spectrophotometry to measure the β-carotene content of crude palm oil (CPO). Calibration curves ranging from 200 to 800 ppm were prepared by extracting β-carotene from original CPO using open-column chromatography. Separate partial least squares calibration models were developed for predicting β-carotene based on the spectral region from 976 to 926 cm⁻¹ for FTIR spectroscopy and 546 to 819 nm for NIR spectroscopy. The correlation coefficient (R ²) and standard error of calibration obtained were 0.972 and 25.2 for FTIR and 0.952 and 23.6 for NIR techniques, respectively. The validation set gave R ² of 0.951 with standard error of performance (SEP) of 25.78 for FTIR technique and R ² of 0.979 with SEP of 19.96 for NIR technique. The overall reproducibility and accuracy did not give comparable results to that of spectrophotometric method; however, the standard deviation of prediction was still within ±5% β-carotene content over the range tested. Because of their rapidness and simplicity, both FTIR and NIR techniques provide alternative means of measuring β-carotene content in CPO. In addition, these two spectroscopic techniques are environmentally friendly since no solvent is involved.     

Determination of low trans levels in refined oils by fourier transform infrared spectroscopy

A Nicolet 410 Fourier (FTIR) spectrophotometer, equipped with a DGTS detector and a sample cell with NaCl windows (nominal pathlength=50 μm), was used for the development of an FTIR method for routine analysis of low trans levels in physically refined oils. The approach of the study differed from those previously described in that a separate calibration curve was established for each type of oil. Quantitation was established by use of Basic Quant Software® and by measuring the peak height at 967 cm^?1 relative to a baseline drawn between 1002 and 932 cm^?1. The slope of the different calibration curves established in six vegetable oils (soybean, corn, sunflower, high-oleic sunflower, low-erucic rapeseed, and high-erucic rapeseed) was close to 1 (0.9942–1.0041), and correlation coefficients (r ²) were rather good (0.9990–0.9999). FTIR spectra of 20 soybean oil samples were collected and quantitated with the different calibrations. Compared to previous reported literature data, increased accuracy (mean difference=0.05%; standard deviation of difference=0.11%) and reproducibility (r ²=0.09–0.12%) were obtained when the FTIR spectra were quantitated with a calibration curve based on 10 physically refined soybean oil samples.     

Stoichiometric determination of hydroperoxides in fats and oils by fourier transform infrared spectroscopy

A primary Fourier transform infrared (FTIR) spectroscopic method for the determination of peroxide value (PV) in edible oils was developed based on the stoichiometric reaction of triphenylphosphine (TPP) with hydroperoxides to produce triphenylphosphine oxide (TPPO). Accurate quantitation of the TPPO formed in this reaction by measurement of its intense absorption band at 542 cm⁻¹ provides a simple means of determining PV. A calibration was developed with TPPO as the standard; its concentration, expressed in terms of PV, covered a range of 0–15 PV. The resulting calibration was linear over the analytical range and had a standard deviation of ±0.05 PV. A standardized analytical protocol was developed, consisting of adding ∼0.2 g of a 33% (w/w) stock solution of TPP in hexanol to ∼30 g of melted fat or oil, shaking the sample, and scanning it in a 100-μm KCI IR transmission cell maintained at 80°C. The FTIR spectrometer was programmed in Visual Basic to automate scanning and quantitation, with the reaction/FTIR analysis taking about 2 min per sample. The method was validated by comparing the analytical results of the AOCS PV method to those of the automated FTIR procedure by using both oxidized oils and oils spiked with tert-butyl hydroperoxide. The two methods correlated well. The reproducibility of the FTIR method was superior (±0.18) to that of the standard chemical method (±0.89 PV). The FTIR method is a significant improvement over the standard AOCS method in terms of analytical time and effort and avoids solvent and reagent disposal problems. Based on its simple stoichiometry, rapid and complete reaction, and the singular band that characterizes the end product, the TPP/TPPO reaction coupled with a programmable FTIR spectrometer provides a rapid and efficient means of determining PV that is especially suited for routine quality control applications in the fats and oils industry.     

Applications of diffuse reflectance fourier transform infrared spectroscopy to fiber-reinforced composites

Diffuse reflectance Fourier transform infrared (FT-IR) spectroscopy can be used to obtain infrared spectra directly from the surface of composite materials with little or no sample preparation. It is thus of interest as a nondestructive method for industrial inspection. In many cases, the IR spectra provide detailed information concerning the chemical composition and molecular structure of the material. The technique works particularly well for carbon-fiber composites. This paper describes the principles involved, some factors which influence the quality of the spectra, and a number of examples of applications. These include the characterization of epoxy matrices (composition, curing, degradation), the detection of surface contamination, and the determination of the degree of crystallinity in poly(phenylene sulfide)-based composites.     

Determination of anisidine value in thermally oxidized palm olein by fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy with transmission cell is described to predict anisidine value of palm olein. The calibration set was prepared by mixing the thermally oxidized palm olein and the unoxidized palm olein with certain ratios (w/w) covering a wide range of anisidine values. A partial least square (PLS) regression technique was employed to construct a calibration model. This model was further accomplished by a validation step. The standard error of prediction found was 0.51. The precision of this method was shown to be comparable to the accuracy of the American Oil Chemists’ Society method used for measurement of anisidine value, with coefficient of determination (R ²) of 0.99. The study showed that mid-band FTIR spectroscopy combined with a PLS calibration technique is a versatile, efficient, and accurate technique for the estimation of anisidine value of palm olein within about 2 min with less than 2 mL of sample.     

A fourier transform infrared spectroscopy study of the effect of temperature on soy lecithin-stabilized emulsions

The effect of temperature on soy lecithin-stabilized emulsions was studied using Fourier transform infrared spectroscopy (FTIR). Oil-in-water (o/w) 4% (wt/vol) soy lecithin emulsions were prepared in 6% (vol/vol) medium-chain triglycerides and 94% (vol/vol) water using a two-stage homogenizer set at a pressure of 3000 psig. Three types of emulsions were used in this study: emulsions containing Lecigran and Lecimulthin as emulsifiers and a control emulsion, with no emulsifier added. After preparation, the emulsions were cooled to 4°C, held at this temperature, and spectra were collected after 1 h. The emulsions and reference water were raised to room temperature (22°C) and held at that temperature for 1 h and the spectra collected. The temperature was raised 15°C over the temperature range of 22 to 82°C, and spectral data were collected similarly. The four regions used for this determination in the subtracted spectra of the emulsion were those contributing to -OH vibration, -CH₂ stretching, H-O-H bending vibrations, and P=O, C-O-C, and P-O-C vibrations. The control emulsion was greatly affected at temperatures other than room temperature. This was due to the lack of lecithin as an emulsifier, resulting in a destabilization of the emulsion with temperature increases. The vibrational peaks for the emulsion containing Lecimulthin were found to be lower than those for the emulsion made with Lecigran due to greater water bonding. The control had the highest peaks at the -OH regions because of reduced interaction at the oil-water interface. Both of the emulsions with phospholipids remained stable throughout the temperature range. FTIR is a potentially powerful tool that could be used in the rapid determination of emulsion stability in food systems by measuring emulsifier-water interactions.     

Kinetic parameters of the thermoplastic/thermoset epoxy reaction by fourier transform infrared spectroscopy

The melt state reaction, or fusion process of bisphenol-A and the diglycidyl ether of bisphenol-A can produce both linear phenoxy backbone chains and crosslinked network structures. The linear chains can be thought of as thermoplastic polymer, while the crosslinked molecular matrix is a thermoset; therefore, this resin system can be termed a thermoplastic/thermoset epoxy. Fourier transform infrared spectroscopy has been employed to study the chemical kinetics of the urea catalyzed system. The reaction of bisphenol-A and the diepoxide follows first order kinetics with respect to epoxide concentration, through 95 percent consumption of the epoxide for reaction temperatures of 130°C and above while lower temperatures show deviation from first order behavior at 75 percent conversion. When the differential form of the kinetic equation is used for analysis, the system follows first order behavior through 60 percent conversion of the epoxide at which point the order increases to a value of 1.5. Rapid spectral collection techniques have been employed to study this behavior for the temperatures 110 to 160°C. Upon incorporation of 3,4′ bisphenol-A into the system first order behavior still adequately describes the kinetic behavior; however the rate of epoxide consumption and the activation energy are affected. Since the stoichiometric ratio of bisphenol-A to diepoxide was found to affect the rate constant, the reaction mechanisms of linear chain growth and crosslinking cannot be clearly distinguished by the sole use of this technique.     

Diffuse reflectance fourier transform infrared spectroscopy of phospholipid adsorption onto silicic acid

Diffuse reflectance Fourier transform infrared spectroscopy was used to study the mode of adsorption of phosphatidylcholine (PC) in hexane onto silicic acid (SA). PC adsorption was mainly through the charged phosphate group with minimal binding through the ester carbonyl. When the SA surface with adsorbed PC is washed with hexane, containing a small concentration of isopropanol, the desorbed PC is recovered without structural change, i.e., there is no evidence of PC hydrolysis in the adsorption process. Adsorbent misture probably promotes PC adsorption due to the increased availability of surface water hydroxyl groups for interaction with the PC phosphate groups. Isopropanol promoted PC binding by destabilizing PC reverse miscelles in solution, thus promoting its adsorption.     

Upgrading the AOCS infrared trans method for analysis of neat fats and oils by fourier transform infrared spectroscopy

An automated protocol for the direct, rapid determination of isolated trans content of neat fats and oils by Fourier transform infrared (FTIR) spectroscopy was devised, based on a simple modification of the standard AOCS trans method, eliminating the use of CS₂ and methylation of low trans samples. Through the use of a commercially available, heated transmission flow cell, designed specifically for the analysis of neat fats and oils, a calibration (0–50%) was devised with trielaidin spiked into a certified, trans-free soybean oil. The single-beam spectra of the calibration standards were ratioed against the single-beam spectrum of the base oil, eliminating the spectral interference caused by underlying triglyceride absorptions, facilitating direct peak height measurements as per the AOCS IR trans method. The spectrometer was preprogrammed in Visual Basic to carry out all spectral manipulations, measurements, and calculations to produce trans results directly as well as to provide the operator with a simple interface to work from. The derived calibration was incorporated into the software package, obviating the need for further calibration because the program includes an automatic recalibration/standardization routine that automatically compensates for differences in optical characteristics between instruments, instrument drift over time, and cell wear. The modified AOCS FTIR analytical package was evaluated with Smalley check samples for repeatability, reproducibility, and accuracy, producing SD of ± 0.07, 0.13, and 0.70 trans, respectively, the FTIR predictions being linearly related to the Smalley means (r=0.999; SD=± 0.46), and well within one SD of the Smalley sample means. Calibration transfer was assessed by implementing the calibration on a second instrument and reanalyzing the Smalley check samples in cells of two different pathlengths (25- and 50-μm). There were no statistically significant differences between the FTIR trans predictions obtained for the Smalley samples from the two instruments and two cells, indicating that the software was able to adjust the calibrations to compensate for differences in instrument response and cell pathlength. The FTIR isolated trans analysis protocol developed by the McGill IR Group has the benefit of being based on the principles of an AOCS-approved method, matches its accuracy, and allows the analysis to be performed on both neat fats and oils, producing trans predictions in less than 2 min per sample. It is suggested that this integrated approach to trans analysis, which requires a minimum level of sample manipulation and operator skill, be considered as a modification of the proposed Recommended Practice CD14b-95.     

Characterization of model anaerobic adhesive cure using real-time fourier transform infrared spectroscopy and dielectric spectroscopy

A technique has been developed based on Fourier transform infrared spectroscopy (FTIR) in the attenuated total reflectance (ATR) mode, which is suitable for the investigation of the heterogeneous cure of surface-initiated redox polymerizations in thin and thick bondline situations. The results of the investigations into the surface-catalyzed and anaerobically promoted cure of some model adhesives using this method are reported. The systems under investigation were designed to exhibit different levels of cure heterogeneity or “cure through volume” (CTV) so that the influence of the bondline thickness and the formulation variables may be assessed. The analysis of the data provides evidence that supports our concept of the heterogeneous cure distribution existing in the form of a cure gradient. This concept of a cure gradient is developed further using dielectric spectroscopy. Here the cure gradient is characterized in terms of the parameters obtained using an empirical equation to fit the dielectric relaxation data. The results obtained using the infrared and dielectric spectroscopic methods are in satisfactory agreement and are shown to be most informative in characterizing and quantifying the CTV performance of the model anaerobic adhesives. © 1994 John Wiley & Sons, Inc.     

Trans fatty acid determination in fats and margarine by fourier transform infrared spectroscopy using a disposable infrared card

The use of a disposable polyethylene infrared (IR) card as a sample carrier for the quantitative determination of trans content of fats and oils and margarine by Fourier transform IR spectroscopy was investigated. Standards prepared by dissolving trielaidin in a zero-trans oil were used to develop partial least squares (PLS) calibrations for both the IR card and a 100-μm transmission flow cell. These calibrations were then used to predict a series of gas chromatographically-preanalyzed unknowns, the trans predictions obtained using the card being comparable to those obtained with the transmission flow cell. Somewhat improved performance could be obtained when the spectral data from the card were normalized to compensate for inherent variations in path length and variability in sample loading. Both IR methods tracked the gas chromatographic reference trans values very well. A series of margarine samples was also analyzed by the card method, producing results similar to those obtained using a flow cell. For the analysis of margarines, the card method has the advantage that the trans analysis can be performed directly on microwave melted emulsions because moisture is not retained on the card. Overall, the disposable IR card was shown to work well and has the benefit of allowing trans analyses to be carried out without requiring investment in a heated flow cell or attenuated total reflectance accessory.     

Milled rice surface lipid measurement by diffuse reflectance fourier transform infrared spectroscopy (DRIFTS)

Diffuse reflectance Fourier transform infrared spectroscopy was investigated as a method for rice surface lipid determination. Long- and medium-grain rice was milled at four degrees of milling to obtain samples with various levels of residual bran, and total lipids were determined by solvent extraction. Fourier transform infrared spectra were collected between 4000 and 400 cm⁻¹. Weighted regression analysis identified changes in surface chemical functional groups with bran removal. Groups typical of lipids increased with bran content whereas those typical of carbohydrates and protein decreased. Partial least squares (PLS) regression analysis showed a high degree of correlation between the spectra in the 4000–400 cm⁻¹ range and extracted lipids of long-grain rice (R ²=0.96) and medium-grain rice (R ²=0.96); a high degree of correlation was also observed when long- and medium-grain rice data were combined (R ²=0.96). There was a high positive correlation between the spectra and extracted lipids in the 1300–1000 cm⁻¹ range for the long-grain rice (R ²=0.98), medium-grain rice (R ²=0.98), and combined long-/medium-grain rice data (R ²=0.94). PLS selected spectral regions that correlated positively with functional groups of lipid/lipid oxidation products and negatively with functional groups of protein and carbohydrates.