Anal. Chem. 1995, 67, 127R-153R
Food Sam K. C. Chang,*stl* Edna Holm,t Jurgen Schwarz,**fand Patricia Rayas-Duarte* North Dakota State University, Fargo, North Dakota 58105 Review Contents
Nitrogen and Proteins Protein Contents Infrared OR), Near-IR, and Fourier Transform IR
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(FT-IR)
Electrophoresis and Isoelectric Focusing High-Performance Liquid Chromatography (HPLC) Enzyme-Linked ImmunoadsorbentAssay (ELISA) Protein Digestibility Carbohydrates Mono- and Disaccharides Lactose and Sucrose in Milk Products Reducing Sugars Use of Capillary Electrophoresis Oligosaccharides Dietary Fiber Starch Inorganics (Minerals) Sample Preparation for Atomic Absorption Applications
Enzymes Enzyme Biosensors Flow Injection Analysis Immunosorbent Assays Flavor and Odor Chirality Wine
Beer Meat
Fish Dairy Lipids Coffee Vegetables Fruit Herbs and Spices
Flavor Enhancers Vanillin
Adulteration Irradiation Packaging
Color Anthocyanins Carotenoids Meat Pigments
Other Pigments Lipids Fat Content/Fatty Acids Food Lipid Oxidation
Vitamins Vitamin C (AscorbicAcid) Carotenes and Vitamin A
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This review was written based on the literature searches using Chemical Abstracts covering the date between October 1992 and 0003-2700/95/0367-0127$15.50/0 0 1995 American Chemical Society
October 1994. The developmentsof new methods for measuring eight major food compositions/properties were selected. New applications of developed methodologiesalso were included. The selected abstracts were primarily those published in English. Electrophoresis, HPLC, and enzyme-linked immunoassays continued to be the major methods for identification and quantifications of proteins in food materials. Microwave heating has been used on several occasions to prepare samples for mineral analysis. HPLC methods have continued to be used for analyzing amino acids, vitamins, sugars and carbohydrates. NITROQENAND PROTEINS Protein Contents. Bullock and Moore (AI) reviewed Kjeldahl, Dumas, and near-IR reflectance spectroscopy for determination of corn proteins. Feinberg (A2) developed an openvessel microwave digestion system for Kjeldahl nitrogen determination to shorten digestion times. Microwave digestion steps included decomposition of the organic matrix with sulfuric acid without catalyst, followed by oxidation with hydrogen peroxide. Digestion by microwave can reduce the time of sample preparation compared with official methods. Chan (A3) determined cereal protein using a m o d ~ e drapid bicinchoconinic acid. Solid samples were directly suspended in the BCA reagent. The reaction was carried out under sonication. The solid-phase assay showed that the method was a rapid alternative comparison with the Kjeldahl assay if numerous insoluble proteinaceous materials require screening. Reichardt (A4 reviewed the methods for W determination of protein content in foods, body fluids, or aqueous solutions by absorbance measurements in the region between 185 and 260 nm or by employment of double-wavelength spectroscopy. The differences in absorbance at 205-233, 210-233, 215-233, and 235-280 nm as well as measurements at a wavelength between 205 and 215 nm are suitable for biological samples. A simple and reliable W method for rapid determination of total tryptophan in intact proteins was developed (As). The method is based on the solubilization of proteins in sodium hydroxide and transformation of the W (285.5 nm) absorption spectrum data. The method is particularly recommended for everyday analysis of a large number of samples (As). Mapping protein foulants, including peptides, which were stained with Coomassie blue and viewed on the active surface of ultrafiltration membranes with a reflectance microspectrophotometer at 580 nm, was easy and quick compared to other methods such as SDS-polyacrylamide gel electrophoresis and scanning electron microscopy (A@, The protein concentrationin milk was determined fluorometrically by diluting samples into an aqueous solution of 0.01%bromophenol blue and illuminating with a laser light at 337.1 nm and an output of 2-3 mW. Fluorescence was
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measured at 540 nm (A7). A direct colorimetric method based on the reduction of Ellman’s reagent by thiol groups was used on thiol groups and disulfide bonds in corn meal-based materials. This method is rapid and convenient for screening thiol and disulfide contents in insoluble proteinaceous materials (AS). Mared (IR), Near-IR,and Fourier Transform IR (FT-IR). De La Roza and Martinez (AS) used near-IR reflectance spectroscopy to determine crude protein, dry matter, and cellulose organic matter digestibility. Bacheiner ( A l O ) measured total protein, nonprotein N, protein N, urea N, and fat contents of milk samples by IR spectrometry (Milko-Scan; MSC) . The various compositions of casein, urea, and nonprotein nitrogen caused different deviations in results when compared with that obtained from Kjeldhal methods. Liu (All)used a rapid milk analyzer (Milkoscan), which was based on IR spectroscopy, to measure grain legume protein. The Milkoscan was a rapid method for determining grain protein and was compared with the Kjeldahl procedure. There were differences between the two methods if pH 2 years by the Finnish customs authorities to detect previous irradiation of many imported foods. Sharifzadeh and Sohrabpour (E157)and Mamoon et al. (E158)showed it was possible to detect irradiation of spices 4-9 months postirradiation by the use of a thermoluminescence method. Kispeter et al. (E159)used thermoluminescence and ESR spectroscopy in conjunction with dynamic viscosity measurements to detect radiation-induced changes. These and other methods were evaluated by Schreiber et al. (E160)for detection of irradiation in several foods. In 1993, a workshop on recent advances in detection of irradiated foods was held by the Committee of European ComAnalytical Chemistry, Vol. 67, No. 12, June 15, 1995
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munities. The Proceedings report several studies on detection of irradiated foods (E161-E165). In an organoleptic study, Lefebvre et al. (E266) found that the odor and flavor of irradiated cooked ground beef was slightly disliked while no difference was detected in the color and texture. A low dose (1 kGy) was recommended. Packaging. Potey et al. (E167) used DCI-platine coupled to GC to measure the transfer of ethyl acetate vapor through a vegetable parchment paper and a coated paperboard. Stenstroem et al. (E168) used accelerator mass spectrometry to determine the I4C-dopedethyl acetate content of low-density polyethylene. When evaporation was avoided, the degree of absorption (44 ng of ethyl acetate/mg of polyethylene) was lower than that obtained by SFE with GC (58 ng/mg). Steele et al. (E169) used a dynamic heated headspace purgeand-trap extraction technique with selected ion monitoring capillary G U M S to measure styrene levels in 12 commodities. The limit of detection was generally less than 2 ng/g. Concentrations of styrene ranged from 39 200 ng/g in cinnamon to no detectable level in tomatoes, milk, and chicken. As a technique to determine migration of volatiles into microwaved foods, Jickells and Castle (El 70) incorporated 15 model substances with a boiling range of 77-440 "C into susceptorsvia the vapor phase or by formulation into the adhesive. Migration to the susceptor was detected by GC-FID or GC/MS. Testing migration to microwaved foods and test simulants Tenax and a semisolid simulant of olive oil and water absorbed on an inert support showed migration to both simulants was higher than to foods, indicating that both simulants could be used for screening. Boccacci et al. (El 71) reported a rapid HPLC method for the quantitative determination of tetramethylammonium chloride in four aqueous matrixes used as food simulants in migration tests. The method has been applied to test migration of tetramethylammonium chloride residues from epoxy resins, which are used as food-packaging materials. COLOR Gennaro et al. (F1)compiled a review discussing HPLC of natural and synthetic colors. The addition of photodiode-array (FDA) detection to the HPLC technology has enhanced the ability to separate and identify pigments in foods. Several new techniques are being applied to the study of color and pigments such as pseudoelectrochromatography negative ion electrospray MS (FZ),near-IR analysis in the visible range (F3), and derivative spectrometry (F4). Berzas Nevado et al. (F4) employed derivative spectrometry with zero-crossing measurements in the first-derivative spectra and using first derivatives of ratio spectra to determine the components of binary synthetic colorant mixtures in food products. Berzas Nevado et al. (F5) applied similar techniques with the addition of second-derivative spectrophotometry to resolving ternary mixtures of synthetic yellow colorants. Quantitative TLC was used to determine erythrosine content of hard gelatin capsules (F6). Erythrosine and synthetic yellow colorants were the subject of a study in Turkey in which chewing gums and jellies were found to contain concentrations that exceeded specified acceptable daily intake values (F7). Zeng et al. (F8) describe a method for qualitative and quantitative determination of synthetic pigments in food by using 142R
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a spectrophotometric method combined with the branch and bound algorithm. No preseparation of pigments was required. Cserhati et al. (F9) tested many types of TLC plates with both adsorption and reversed-phase chromatography to separate the color pigments of paprika (Capsicum annuum). No single development was able to separate the pigments satisfactorily. Anthocyanins. HPLC continues to be the method reported to be most useful in analyzing anthocyanins. Gao and Mazza (F10)reported a rapid method for complete chemical characterization of simple and acylated anthocyanins by HPLC and capillary GLC. Biswas et al. (F11)used HPLC to analyze pigment in Indian black grapes. Terahara et al. (F12)analyzed the pigments in aromatic red rice and its wine. Shi et al. (F13)combined liquid secondary-ion MS and HPLC to determine the structure of the main anthocyanin from Tabeluia pallida. In other types of studies, Maccarone et al. (F14)investigated the effect of phenolic copigments on the flavylium-chromenol equilibrium of anthocyanins. Bakker and Arnold (F15)analyzed sensory and spectrophotometric data for evaluation of color in 39 port wines. The lighter and browner ports were associated with higher quality ratings. Carotenoids. Two papers on carotenoids (3'16, F17) review the natural sources of carotenoids and analytical techniques that have been applied to their study. HPLC has been a major technique applied to carotenoid separation and quantification according to reports in the recent literature. Khachik et al. (F18) compiled a review with 20 references on HPLC methods. Scott and Hart (F19) caution those who use HPLC that column temperature can have a dramatic effect on results, with 20-22.5 "C showing optimum resolution in their work. Biacs and Daood (FZO)developed an HPLC system with PDA detection that permitted the isocratic separation of -45 components in less than 50 min. The method was tested with carotenoids from several selected fruits and vegetables. The same system was applied to studying paprika (F21).HPLC separation was used by Takahata et al. (FZZ)for studying carotenoids in sweet potatoes, by Epler et al. (F23) for studying carotenoids in human serum and in food, and by Minguez-Mosquera and Homero-Mendez (FZ4)for separation and quantification of carotenoids in red peppers, paprika, and oleoresin. Personnel at the National Institute of Standards and Tecnology studied a variety of bonded-phase parameters for their effect on column retention and selectivity toward carotenoids (FZ5). One of the purposes for the study was to study quantification of p-carotene in dietary supplements which are under consideration as agents for cancer intervention/prevention. There is increasing interest in using colorimetry as a nondestructive measurement of ripeness in fruits and vegetables. Thiagu et al. (F26),e.g., studied tomatoes during ripening by light reflectance measurements from a tristimulus photovoltaic color instrument. A near-IR method operating in the visible range has been applied to measuring pigment in durum (FZ7). Meat Pigments. Several new techniques have been proposed for studying meat pigments. The potential of electrospray MS for meat pigment identi6cation from various species was investigated by Taylor et al. (FZ8).Izumimoto and Ozawa (FZ9) proposed a reflectance spectrophotometric method for determining the relative proportions of oxygenated, reduced, and metmyoglobin in raw beef meat. Tsuruga et al. (F30) used a tissue
spectrophotometer 'IS200 to analyze meat and rice flour pastes containing known amounts of myoglobin and to analyze intact beef and pork samples. Garrido et al. (F3I) determined the total pigment content of pork meat by phase partitioning in the nonionic detergent Triton X-114 and oxidizing the pigments to metmyoglobin with sodium nitrite. The authors indicate the method can be used to measure pigment in other meat types, even those with low pigment levels. Other Pigments. Cochineal detection in food products was enhanced by methylation with ethereal diazomethane in THF prior to TLC analysis (F32). Nondestructive testing of naturally white food products on-line was explored by Marszalec et al. (F33). Rocha et al. (F34) found that chlorophyll pigments in mint and basil were preserved better with a short blanching period before drying or by using a low drying temperature if the product was not blanched. Ward et al. (F35) compared several methods of measuring chlorophyll content in canola seed and oil. HPLC appears best. It was suggested that a standardized method of chlorophyll pigment measurement by HPLC needs to be developed. Castellar et al. (F36) used a preparative HPLC procedure to isolate picrocrocin from saffron. LIPIDS
Fat Contenflatly Acids. Due to the complexity of food composition, most methods of fat analysis have to be matrix specific. Now, with the FDA's newly adopted fat definition, fat determination for labeling should be more consistent. Rathbone ( G I , G2) explained this definition of total fat as the sum of all fatty acids obtained from a total lipid extract of food expressed as triglycerides. Ngeh-Ngwainbi and Lane (G3) discussed methods of fat analysis and how the choices of solvent and treatment of food before solvent extraction cause differences in the quantity and character of the lipid material extracted. It is clear that no extraction method is universal for all lipids. The performance of the approved Babcock and modified Mojonnier ether extraction methods for the determination of the fat content of raw milk was evaluated by Lynch et al. (G4). Data indicated that the statistical protocol for collaborative studies is effective as the basis for multilaboratory quality assurance programs and the performance can be maintained. Ackman (G5) reviewed the procedures for extraction and analysis of 0 - 3 fatty acids and mentioned how microprocessorcontrolled analytical equipment with direct numerical data output have slowly lessened the need for sample understanding and total lipid recovery; however, chloroform/ methanol extraction typically employed in these methods poses health risks. Considerable advances in lipid analysis have been made recently due to the increasingpower of the analytical methodology available, especially the chromatographic techniques. HPLC has been used in combination with many techniques in lipid analysis. Christie (G6) recently reviewed silver ion chromatography and chiral chromatography in the structural analysis of triacylglycerols. Nikolova-Damyanova et al. (G7) proposed to learn more about the silver ion complexation in chromatographic systems by determining the retention characteristics of phenacyl and phenethyl with reversed-phase HPLC and silver ions in the mobile phase. Equivalent and fractional chain length values were calculated, and they indicated that the silver ions and double bonds form simple 1:lcomplexes in the mobile phase. Toschi et al. (G8) compared a rapid silver ion HPLC technique plus GC with Fourier
transform IR spectroscopy for the determination of trans double bonds in unsaturated fatty acids. Values were a little higher with the IR method, presumably due to the additional detection of trans unsaturation in di- and polyunsaturated fatty acids. Konishi et al. (G9) correlated reversed-phaseHPLC with flame ionization detection and gas/liquid chromatography for fatty acid composition of some vegetable oils. The results show good agreement between fatty acid composition as calculated from HPLCdetermined triacylglycerol and as determined by GLC. This supports findings that triacylglycerol molecular species could be identified on the basis of theoretical carbon numbers and that fatty acids in soybean oil can be obtained from the data of HPLC with flame ionization detection. Monoacylglycerolwas determined using HPLC with evaporative light-scattering detection and results agreeable with GC and supercritical fluid chromatography (GIO) were obtained. Conforti et al. ( G I I ) also applied HPLC with evaporative light-scattering detection in the analysis of wheat flour lipids. The technique was found to be useful in identifying the lipids and their relative mass percentages. Light-scattering detection gave a flat stable baseline, reproducible results, and eliminated solvent fronts in which peaks of interests would have coeluted. Kermasha et al. (G12) claim HPLC with laser lightscattering detection has a high sensitivity for standard triacylglycerols whereas poor sensitivity was observed with UV detection. Reversed-phaseHPLC with short-wavelengthUV detection was found by Aitzetmueller et al. ( G I 3 ) to be a useful alternative to conventional separation systems with RI detection for analysis of triacylglycerolof highly unsaturated vegetable oils. Characteristic peak area ratios help in the identification of the oils. Statistical experimental design and multivariate optimization was applied in the optimization of a reversed-phase HPLC method for the analysis of triacylglycerol molecular species of natural oils. The molecular species was idenaed using a version of the equivalent carbon number concept, utilizing the Synder polarity index (G14). Froehlich (G15) reviewed the use of capillary gas chromatography in fat analysis and explained the advantages due to ease of handling, resolution, and stability over existing methods. An accurate method for the determination of sterols by capillary gas chromatographywas developed by Rodriguez-Palmer0 ( G I @ and applied to foods. Alonso (GI 7) proposed a rapid gas chromatographic method for the analysis of triacylglycerols in cheese on a capillary column using programmed-temperatureinjection; fat was extracted with hexane. Bartle et al. ( G I 8 ) reviewed supercritical fluid chromatography/Fourier transform IR spectrometry and included solvent elimination methods, on-line detection, sensitivity, and applications. Van de Voort and co-workers published a series of research papers on the application of Fourier transform IR in lipid determinations. Rapid, automated IT-IR methods were developed for fat or moisture determination in high fat foods ( G I 9 ) , peroxide values in vegetable oils (GZO), protein and fat in meat ( G Z I ) , and free fatty acids in fats and oils (G22). Reproducibility was superior to conventional chemical methods and limited only by sample homogeneity and the reference chemical methods used to standardizethe calibration powders. Luinge et al. (G23) also used FT-IR to determine the fat, protein, and lactose content of milk. They compared calibration techniques such as classical inverse least-squares regression, principal component regression, and Analytical Chemistty, Vol. 67, No. 12,June 15, 1995
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partial least-squares regression. The methods appeared comparable and are equivalent to the conventional filter-based method. Kaufmann (G24) discusses the applications of chemometrics to the complex lipid systems and need for several modes of chromatography for complete resolution. He explains the utilization of experimental design to develop methods, the use of multivariate classificationand chromatographic data in identification, and the use of compositional data in multivariate regression. Near-infrared reflectance spectroscopy calibrations were developed by Krishnan et al. (G25) to measure the oil content of oat cultivars. A strong correlation in comparison to oil content determined by the AACC method 30-20 was found. The performance of homogenizers in IR milk analyzers was surveyed to determine whether they achieve a
