Atomic absorption spectrophotometric determination of tin in canned foods, using nitric acid-hydrochloric acid digestion and nitrous oxide-acetylene flame: collaborative study

Twenty-six collaborators participated in a study to evaluate an atomic absorption spectrophotometric (AAS) method for the determination of tin in canned foods. The 5 foods evaluated were meat, pineapple juice, tomato paste, evaporated milk, and green beans, each spiked at 2 levels. The concentration range of tin in the samples was 10-450 micrograms/g, and each level was sent as a blind duplicate. Statistical treatment of results revealed no laboratory outliers and 6 individual or replicate-total outliers, accounting for 3.3% of the data. Repeatability (within-laboratory) coefficient of variation (CVo) ranged from 2.2 to 48%, depending on the tin level and food evaluated. For samples containing greater than or equal to 80 micrograms/g of tin, repeatability CV averaged 5.6% including outliers and 3.7% after their rejection. Overall among-laboratories coefficient of variation (CVx) varied from 3.3 to 58%; at levels greater than or equal to 80 micrograms/g, it averaged 7.3% with outliers and 5.3% after their rejection. Recovery of tin, based on spiking levels, ranged from 100.0 to 112.8% and averaged 105.4%. Detection limit range is 2-10 micrograms/g, and lower quantitation limit is 40 micrograms/g. This method has been adopted official first action.     

Determination of sulfite in foods and beverages by ion exclusion chromatography with electrochemical detection: collaborative study

A liquid chromatographic (LC) method for determination of total sulfite in foods and beverages by alkali extraction followed by ion exclusion chromatographic separation and electrochemical detection (IEC-EC) was collaboratively studied by 9 laboratories. Blind duplicate samples of starch, diluted lemon juice, wine cooler, dehydrated seafood, and instant mashed potatoes were analyzed without spiking and with added sulfite at 2 levels. The initial sulfite levels varied from 0 to 384 ppm SO2, and the levels added varied from 10 to 400 ppm. The initial sulfite levels determined by the IEC-EC method and the Monier-Williams method were in good agreement. Recovery of added sulfite by the IEC-EC method was generally higher than that by the Monier-Williams method. Within-laboratory repeatability (RSDr) for the IEC-EC method varied from 4.4 to 26.0%, and overall reproducibility (RSDR) varied from 8.5 to 39.3%. The collaborators found the method to be fast, sensitive, and easy to use, which makes it a useful alternative to the Monier-Williams method. The method has been adopted official first action.     

Determination of sulfur dioxide in foods by modified Monier-Williams distillation and polarographic detection

A rapid, sensitive polarographic method is presented for determining sulfiting agents in foods and beverages. The method is based on the modified Monier-Williams distillation followed by polarographic detection by differential pulse polarography or square wave voltammetry. A clearly defined wave is obtained by both techniques, with a current maximum at a potential (E) of about -600 mV vs an Ag/AgCl reference electrode. The reaction is based on the reduction of sulfur dioxide at a dropping mercury electrode. Peak current was linear over the range 0-20 micrograms/mL. Quantitation is done by linear regression analysis of standard addition data or by using a standard calibration graph. Screening levels of less than 1 ppm total SO2 were easily achieved in the foods analyzed, which had levels from less than 1 ppm (cereals) to thousands of parts per million (dried fruit). Recoveries from fortified samples ranged from 70 to 108% at fortification levels of 20, 100, and 1000 micrograms/g.     

Most probable number method for isolation and enumeration of Staphylococcus aureus in foods: collaborative study

Enumeration of Staphylococcus aureus in foods was collaboratively studied by comparing the present AOAC final action method, 46.062, which uses trypticase soy broth with 10% NaCl to a proposed replacement method which uses the same broth with 1% sodium pyruvate added. Fifteen collaborators analyzed uninoculated samples of milk, tuna salad, and ground turkey, as well as samples inoculated with low (10(2) cells/g), middle (10(4) cells/g), and high (10(6) cells/g) levels of S. aureus. The samples were frozen immediately to maintain the inoculated level of S. aureus in the food. A different strain of S. aureus was used for each food; heat-stressed S. aureus cells were used to inoculate the milk samples. The pyruvate-amended broth significantly (alpha = 0.05) increased enumeration of low, middle, and high levels of S. aureus from milk and ground turkey, and from tuna salad at middle and high levels. The pyruvate-amended media method has been adopted official first action to replace method 46.062.     

Collaborative study of the glass wool filtration method for the recovery of virus inoculated into ground beef.

A method for estimating viral population levels in ground beef was studied collaboratively in 7 laboratories. The collaborators recovered virus from 6 inoculated samples. Three samples were replicates of the high virus concentration 050 plaque-forming units (pfu)/g) and 3 replicates represented the low concentration (10 pfu/g). Six of the 7 collaborators recovered acceptable levels of virus from the samples. The per cent of variation was 30.6 for the high concentration and 18.5 for the low concentration. Collaborators did not differ from one another significantly in the results obtained for the 10 pfu/g samples, but results from one collaborator were significantly low for the recovery of virus from the 50 pfu/g samples. The results indicate that the glass wool filtration method is adequate for the detection of a number of viruses that may be found in foods. The method has been adopted as official first action.     

Ion chromatographic determination of sulfites in foods

Ion chromatography (IC) is shown to be a promising technique for the determination of sulfites (SO2, SO2/3-) in foods. Results of a 10 min flash distillation and 10 min IC determination compare favorably with the results from the conventional Monier-Williams method for total sulfite in a variety of food matrices. The IC technique also provides a wealth of additional information, such as (1) sulfite and sulfate (oxidized sulfite) content of the spiking or treatment solution, (2) residual sulfite applied to the food after oxidation losses in the treatment process, (3) free sulfite in foods, and (4) total sulfite in foods. As a further check on the Monier-Williams method, the sulfate content of the trapping solution can be determined by IC. Because the IC technique traps the liberated SO2 in a non-oxidizing rather than an oxidizing medium, it is considered free from interfering sulfides and organic sulfur-containing groups which can give false positives in the Monier-Williams method. IC thus offers a high speed, more sensitive, and cost-effective alternative to conventional techniques for the determination of sulfite in foods.     

Rapid determination of methyl mercury in fish and shellfish: method development

The AOAC official first action method for methyl mercury in fish and shellfish was modified to provide more rapid determination. Methyl mercury is isolated from homogenized, acetone-washed tissue by addition of HCl and extraction by toluene of the methyl mercuric chloride produced. The extract is analyzed by electron capture gas chromatography (GC) on 5% DEGS-PS treated with mercuric chloride solution. The quantitation limit of the method is 0.25 micrograms Hg/g. Swordfish, shark, tuna, shrimp, clams, oysters, and NBS Research Material-50 (tuna) were analyzed for methyl mercury by the AOAC official first action method. All products also were analyzed by the modified method and the AOAC official method for total Hg. In addition, selected extracts obtained with the modified method were analyzed by GC with Hg-selective, microwave-induced helium plasma detection. There was no significant difference between the results for the various methods. Essentially all the Hg present (determined as total Hg) was in the organic form. Coefficients of variation from analyses by the modified method ranged from 1 to 7% for fish and shellfish containing methyl mercury at levels of 0.50-2.30 micrograms Hg/g. The overall average recovery was 100.5%.     

Determination of total sulfite in shrimp, potatoes, dried pineapple, and white wine by flow injection analysis: collaborative study

A method for the determination of total sulfite in shrimp, potatoes, dried pineapple, and white wine by flow injection analysis (FIA) was collaboratively studied by 8 laboratories. In the method, the sample solution is reacted with sodium hydroxide to liberate aldehyde-bound sulfite. The sample stream is acidified to produce SO2 gas, which diffuses across a Teflon membrane in the gas diffusion cell into a flowing stream of malachite green. The degree of discoloration of the malachite green is proportional to the amount of sulfite in the sample solution. Red wine was included in the study but interlaboratory precision for these samples was not satisfactory and correlation with Monier-Williams results was poor. The present method is not recommended for use with these samples. For shrimp, potatoes, dried pineapple, and white wine, average reproducibility (RSDR) of results was 25% for samples at 10 ppm SO2 and 10% for samples at greater than 50 ppm. Overall average reproducibility was 14%. Recoveries of sulfite added to samples averaged 80%. Comparison of FIA with the Monier-Williams method indicated comparable results by the 2 methods. The FIA method has been adopted official first action for determination of greater than or equal to 5 ppm total sulfite in shrimp, potatoes, dried pineapple, and white wine.     

Liquid chromatographic determination of seven antioxidants in dry food

The liquid chromatographic determinative step of the official method for propyl gallate, trihydroxybutyrophenone, tert-butylhydroquinone, nordihydroguaiaretic acid, butylated hydroxyanisole (BHA), 3,5-di-tert-butyl-4-hydroxymethylphenol, and butylated hydroxytoluene (BHT) in fats and oils has been applied to their determination in a number of dry foods. A representative sample (10 g) is homogenized first with hexane (25 mL), then with 5 mL added water, and finally with 75 mL added acetonitrile. The hexane and acetonitrile are decanted, filtered, and separated; the hexane and rehydrated food are reextracted with 2 additional portions of acetonitrile, and the combined acetonitrile extracts are concentrated and diluted to 10 mL. An aliquot is analyzed as described in the official method, using a 150 x 4.6 mm 5 microns C-18 column. The need for rehydration to maximize the recovery of BHA and other antioxidants from marketplace dry food samples such as potato flakes, dry coffee whiteners, and dessert topping mixes was demonstrated. Rehydration was not required for cheese snacks, breakfast cereals, cake mixes, and some other foods. The need for rehydration should be determined by analyzing other foods with and without the addition of water. Potato and corn chips, popcorn and cheese snacks, breakfast cereals, dry beverage mixes, rice, potato flakes, french fried potatoes, and cake mixes were spiked with the above antioxidants at 10-50 ppm. Overall recoveries ranged from 64.3 to 105.6% and repeatabilities ranged from 0.7 to 10.8%. A total of 109 samples of the above foods were analyzed, and 64% contained detectable (greater than 1-2 ppm) antioxidants, mainly BHA and BHT.     

Enzyme immunoassay for determination of gluten in foods: collaborative study

A collaborative study was performed in 15 laboratories to validate a monoclonal antibody-based enzyme immunoassay (EIA) for determination of gluten in foods. The study included 13 samples: maize starch, "gluten-free" baking mixes, wheat flours, cookies, cooked meats, and a soup. Gluten was present in these samples at either zero or 0.02 to 10% by weight, i.e., over almost 3 orders of magnitude. The mean assay values for the foods varied from 88 to 105% of the actual amounts. The assay was quantitative for cereal products and the soup with repeatability (RDS-r, relative standard deviation) and reproducibility (RSD-R) of 16-22% and 24-33%, respectively. The assay was semiquantitative for the processed meat products (RSD-r 14 and 26% and RSD-R 46 and 56%), probably because gluten was unevenly distributed in the small (1 g) samples that were analyzed. The ELISA method produced no false positive results, and false negatives obtained with tannin-containing foods could be avoided by use of a modified sample extractant. None of the collaborators reported problems in following the protocol. The method has been adopted official first action by AOAC for determination of wheat gluten in foods.     

Determination of free (pH 2.2) sulfite in wines by flow injection analysis: collaborative study

A method for the determination of free sulfite in wine by flow injection analysis (FIA) is described. The method involves liberation of sulfur dioxide from the wine at pH 2.2, with detection by decolorization of a malachite green solution. The method was collaboratively studied, and the results indicated an average reproducibility of 12% for white wine samples (average level 12.1 ppm SO2) and 26% for red wine samples (average level 3.1 ppm). When the FIA method was compared to an aeration/oxidation method, the results indicated a high degree of correlation between the 2 methods. The FIA method has been adopted by AOAC official first action.     

Optimized Monier-Williams method for determination of sulfites in foods: collaborative study

A collaborative study was conducted of the Food and Drug Administration (FDA)-optimized Monier-Williams method for determining sulfites in foods. Twenty-one industry and government laboratories participated in the study, which was jointly sponsored by the National Food Processors Association and FDA. Familiarization samples were shipped to each collaborator. Collaborators were permitted to proceed to the main study only after they demonstrated ability to perform the method to ensure that the study tested the performance of the method itself and not that of the individual laboratories. The study design involved 3 food matrixes (hominy, fruit juice, and protein [seafood]). Each matrix was prepared at 3 sulfite levels--the regulatory level, half the regulatory level, twice the regulatory level--and as a blank. All test samples were analyzed as blind duplicates, which gave each collaborator a total of 24 test portions. Collaborative recoveries gave a reproducibility (among-laboratories) coefficient of variation that ranged from 15.5 to 26.6% for sulfite determined as SO2 by weight in the 3 foods at the 10 ppm level. The optimized Monier-Williams method has been approved interim official first action to replace the AOAC modified Monier-Williams method, 20.123-20.125.     

Gas-liquid chromatographic determination of benzoic acid and sorbic acid in foods: NMKL collaborative study

A gas-liquid chromatographic method for the simultaneous determination of benzoic acid and sorbic acid in foods was collaboratively studied by 8 laboratories. Benzoic and sorbic acids are isolated from food by successive extractions with ether, sodium hydroxide, and methylene chloride, converted to trimethylsilyl (TMS) esters, and determined by gas-liquid chromatography. Phenylacetic acid and caproic acid are used as internal standards for benzoic acid and sorbic acid, respectively. Seven samples were collaboratively studied: almond paste, fish homogenate, and apple juice with benzoic and sorbic acid levels from 0.04 to 2 g/kg. Average recoveries (%) for benzoic and sorbic acids were as follows: almond paste, 99.6 and 101.2; fish homogenate, 99.2 and 97.4; and apple juice 98.2 and 106.6. The reproducibility coefficients of variation (%) for benzoic and sorbic acids at 0.5-2 g/kg levels were 3.5-6.1 and 5.2-9.0; and at the 0.04 g/kg level, 14.7 and 23.3, respectively. The method has been adopted official first action at 0.5-2 g/kg levels.     

Determination of fluoroacetate in biological matrixes as the dodecyl ester

A new method for the quantitative determination of fluoroacetate in biological samples was applied to a number of avian samples. Fluoroacetate is isolated as its potassium salt by ion-exchange chromatography and directly converted to its dodecyl ester, using a novel derivatization procedure. The ester is quantified by capillary gas chromatography with a flame ionization detector for the range 1.0-10.0 micrograms/g and by selected ion monitoring GC/mass spectrometry for the range 0.01-1.00 microgram/g. Recoveries from 1 g chicken muscle were about 80%. The method was applied to the determination of fluoroacetate in the crop, stomach, liver, heart, intestine, and breast muscle of 5 Zebra finches (Peophila guttata) that had been fed millet containing 9 micrograms/g of sodium fluoroacetate. Despite a wide variation in dose, the levels in organs and tissues were approximately 1 microgram/g except for heart tissue which was about 2 micrograms/g. The presence of interfering peaks at low levels necessitated the use of selected ion monitoring GC/MS when sample weights were less than 1 g or when levels were less than 1 microgram/g. Samples can be analyzed within hours of receipt; therefore, the method is suitable for routine use in a diagnostic laboratory.     

Bacillus stearothermophilus disk assay for determining ampicillin residues in fish muscle.

The Bacillus stearothermophilus disk assay for penicillin in milk (AOAC official method) was adapted for the determination of ampicillin in fish muscle. The method was evaluated in 2 species of cultured fish: channel catfish and striped bass. Recoveries of ampicillin ranged from 99 to 104% when muscle specimens from both species were spiked at concentrations of 0.025-1.00 micrograms/g. The lower limit of determination (LOD) was 0.025 micrograms/g. The assay was applied to monitor the elimination of ampicillin from the muscle of striped bass after intravascular administration (dosage of 10 mg/kg body weight). The mean concentrations in the muscle declined from 1.160 micrograms/g at 2 h to 0.063 micrograms/g at 18 h. The half-life of ampicillin in the muscle was 3.6 h. Ampicillin concentrations were below LOD at 24 h. No inhibitory activity was observed in the muscle of control fish.     

Colorimetric deoxyribonucleic acid hybridization assay for rapid screening of Salmonella in foods: collaborative study

A collaborative study was performed in 11 laboratories to validate a colorimetric DNA hybridization (DNAH) method for rapid detection of Salmonella in foods. The method was compared to the standard culture method for detection of Salmonella in nonfat dry milk, milk chocolate, soy isolate, dried whole egg, ground black pepper, and raw ground turkey. Samples inoculated with high (0.4-2 cells/g) and low (0.04-0.2 cells/g) levels of Salmonella and uninoculated control samples were included in each food group analyzed. There was no significant difference in the proportion of samples positive by DNAH and culture procedure for any of the 6 foods. The colorimetric DNA hybridization assay screening method has been adopted official first action as a rapid screening method for detection of Salmonella in all foods.     

Headspace gas chromatographic determination of residual 1,3-butadiene in rubber-modified plastics and its migration from plastic containers into selected foods

A headspace gas chromatographic procedure has been developed for the determination of 1,3-butadiene in rubber-modified plastics and in some foods. Polymer solutions or foods are equilibrated in sealed vials at 90 degrees C, and headspace samples are injected into a gas chromatograph. 1,3-Butadiene residues are measured using a flame ionization detector and are quantitated by the method of standard additions or an external calibration curve. Refrigerator tubs, vegetable oil bottles, chewing gum, and foods in contact with this type of packaging were analyzed. Limits of quantitation varied with the matrix, ranging from 2 ng/g (ppb) in chewing gum to 20 ng/g in polymers. 1,3-Butadiene was found in one polymer at 53 ng/g with an 8% coefficient of variation. The procedure yields "apparent" trace levels of 1,3-butadiene, and confirmation by a complementary technique is required.     

Gas chromatographic determination of total iodine in foods

A gas chromatographic (GC) method has been developed for determination of total iodine in foods, based on the reaction of iodine with 3-pentanone. Organic matter of a sample is destroyed by an alkaline ashing technique. Iodide in a water extract of the ash residues is oxidized to free iodine by adding dichromate in the presence of sulfuric acid. Liberated iodine is reacted with 3-pentanone to form 2-iodo-3-pentanone, extracted into n-hexane, and then determined by gas chromatography with an electron-capture detector. Recoveries of iodide from spiked food samples ranged from 91.4 to 99.6%. Detection limit for iodine is 0.05 micrograms/g.     

Microdiffusion and fluoride-specific electrode determination of fluoride in foods

A simple and accurate method was developed for routine determination of fluoride in foods. Hydrogen fluoride is diffused 20 hr at 50 degrees C from fresh or freeze-dried samples (0.1 g dry wt) in polystyrene petri dishes containing 2 mL 40% HCIO4 and 0.3 g Ag2SO4, and is absorbed on the lids, previously spotted with 0.1 mL 0.5M NaOH. The absorbent layer is dissolved in 2 mL buffer solution, and the fluoride is measured potentiometrically. The method was verified by analysis of NBS Standard Reference Materials; recovery from 28 spiked infant foods (average = 99%, range = 75-135%); and comparison of results with colorimetry results for the same diffusates, after modification to handle 1 g samples. Relative standard deviations varied from 4 to 20% day to day. Detection limits were below 0.05 microgram/g dry weight.     

Optical sensor for sulfur dioxide determination in wines

A method for the determination of free and total sulfur dioxide in wines, based on the use of an optical sensor that employs a dichlorobis(diphenylphosphino)methane dipalladium I complex [Pd(2)(dppm)(2)Cl(2)] immobilized in a PVC membrane plasticized with o-nitrophenyloctylether (o-NPOE) is described. A sensing membrane [4.2% Pd(2)(dppm)(2)Cl(2), 20.8% PVC, and 75% o-NPOE] was adapted to the tip of a bifurcated optical fiber bundle to perform reflectance measurements at 550 nm. The detection system consisted of two cells (40 mL), which hold the sample solution (plus reagents) and the optical sensor, respectively. For the determination of free SO(2), a wine sample was mixed with H(2)SO(4) solution in the sample cell, into which N(2) was bubbled, providing mixing of the solutions and conducting the SO(2) formed toward the detection cell. For determination of total SO(2), a KOH solution was mixed with the wine in the sample cell. Afterward, an H(2)SO(4) solution was added to the cell, and then N(2) was bubbled to conclude the measurement. Linear responses up to 50 and 150 mg L(-1) were obtained for free and total SO(2), with detection limits of 0.37 and 0.70 mg L(-1), respectively. The repeatability of the method was evaluated by carrying out 10 measurements using a single wine sample, providing relative standard deviation values of 2.2 and 2.5% for free and total SO(2), respectively. The sensing membrane prepared from 10 muL of the cocktail solution lasted for 80 measurements, whereas those prepared from 200 muL can be used for 250 measurements. The method was applied to free and total SO(2) determination in wines, and the results did not show significant difference from those obtained with the Ripper reference method at a confidence level of 95%.