Historical perspectives on methodology to detect Listeria monocytogenes

While recognized as a causative agent of illness in animals and humans for some time, the foodborne role of Listeria monocytogenes is a new and emerging one. This review briefly summarizes the historical developments in methodology used to detect the presence of L. monocytogenes. Although clinical procedures exist, these procedures do not consider isolation of Listeria from heavily contaminated environments. Federal agencies such as the U.S. Food and Drug Administration and the U.S. Department of Agriculture have defined protocols for the isolation of Listeria from dairy and meat products, respectively. Each of these protocols, and current problems common to all methods for the isolation of Listeria from food products, are discussed. Finally, future challenges with respect to improvement in our abilities to recognize, isolate, and rapidly identify Listeria in foods are presented.     

Symposium on microbiology update: old friends and new enemies. Listeria monocytogenes

Listeria monocytogenes, one of the "new enemies" in food microbiology, is a human and animal pathogen that is widespread in nature. The organism is a transient constituent of the intestinal flora excreted by 1-10% of healthy humans. It is an extremely hardy organism and can survive for many years in the cold in naturally infected sources. L. monocytogenes has been isolated from a wide variety of foods, including dairy products, meats, and fish. Although most of the foodborne listeriosis outbreaks have been linked to the consumption of dairy products, recent sporadic cases have been associated with meats, as well as other foods. It is now recognized that listeriolysin 0, a 60-kilodalton protein, is one of the major virulence factors of the organism. All strains of L. monocytogenes are pathogenic by definition although some appear to be more virulent than others. There have been recent reports of hemolytic isolates of L. monocytogenes, which are nonpathogenic for mice. Attachment to and penetration of cells also appear to be prerequisites for human infection. Cultural methodology for isolating the organism from foods has been in a state of flux since 1985. Rapid methods using both ELISA and DNA technology have been developed. Because of the widespread nature of the organism, it is nearly impossible to eliminate it from the food supply. However, by using a hazard analysis-critical control point approach, the health hazard associated with this organism can be reduced to a minimum.     

Comparative studies of nucleic acid hybridization assay for Listeria in foods

A nucleic acid hybridization assay has been developed for Listeria spp. in dairy foods and environmental samples. The assay is based on detection of unique Listeria 16S rRNA sequences by using a 32P-labeled synthetic DNA probe. Inclusivity and exclusivity of the probe were confirmed with 139 Listeria isolates representing all known species, and 73 non-Listeria bacterial strains. In this paper, we present results from our preliminary studies comparing the hybridization assay with conventional culture on a total of 575 specimens that represent a variety of inoculated and uninoculated foods and environmental samples. The assay, which is done in a filter manifold format after 2 days of cultural enrichment, requires a total assay time of less than 2.5 days. The false-negative rate for all sample groups tested using the GENE-TRAK hybridization assay was less than the rate for culture. Thus, the new assay allows rapid screening of the indicated product groups and provides reliable numerical results.     

Concentration of bisphenol A in highly consumed canned foods on the U.S. market

Metal food and drink cans are commonly coated with epoxy films made from phenolic polymers produced from bisphenol A (BPA). It is well established that residual BPA monomer migrates into can contents during processing and storage. While a number of studies have reported BPA concentrations in foods from foreign markets and specialty foods on the U.S. market, very few peer-reviewed data for the BPA concentrations in canned food from the U.S. market were available. This study quantified BPA concentrations in 78 canned and two frozen food products from the U.S. market using an adaptation of a previously reported liquid chromatography-tandem mass spectrometry method. The tested products represented 16 different food types that are from the can food classifications that constitute approximately 65% of U.S. canned food sales and canned food consumption. BPA was detected in 71 of the 78 canned food samples but was not detected in either of the two frozen food samples. Detectable BPA concentrations across all foods ranged from 2.6 to 730 ng/g. Large variations in BPA concentrations were found between different products of the same food type and between different lots of the same product. Given the large concentration ranges, the only distinguishable trend was that fruits and tuna showed the lowest BPA concentrations. Experiments with fortified frozen vegetables and brine solutions, as well as higher BPA concentrations in canned food solids over liquid portions, clearly indicated that BPA partitions into the solid portion of foods.     

Vitamin k contents of meat, dairy, and fast food in the u.s. Diet

The purpose of this study was to determine the contents of three forms of vitamin K [phylloquinone, dihydrophylloquinone, and menaquinone-4 (MK-4)] in representative samples (including different samples within the same food category) of meat (n = 128), dairy and eggs (n = 24), and fast foods (n = 169) common to the U.S. diet. The findings of our analysis indicate that no single food item in these categories is a rich dietary source of any one form of vitamin K. However, these foods are often consumed in large quantities; hence, they may be of importance in overall contribution to total vitamin K intake. The presence of MK-4 in meat, eggs, and dairy foods could be important as physiologic functions unique to MK-4 are identified.     

Improved procedure for determination of acrylonitrile in foods and its application to meat

The previously described headspace-gas chromatographic procedure for the determination of acrylonitrile (AN) in several foods, with N/P selective detection, has been modified to include packaged luncheon meats. The loss of AN during equilibration at 100 degrees C in meat samples as well as the previously described loss in cold pack cheese and peanut butter has been studied. The loss of AN could be prevented by the addition of 10% phosphoric acid, which increases the acidity of the food-acid-salt slurry to pH 1.2-1.5. This acidification permits detection of AN at 2 ppb (5% FSD at 16 X 10(-2) amp/mV) in all foods studied. AN was not detected in 10 samples of luncheon meat packaged in AN-based plastic which contained up to 2.6 ppm AN.     

Food and Drug Administration pesticide residue monitoring of foods: 1978-1982

Pesticide residues in foods are reported for the 5-year period 1978-1982 [fiscal years (FY) 78-82]. Results were compiled from the 2 complementary elements that comprise the Food and Drug Administration's (FDA) program for monitoring pesticide residues in foods. Under regulatory monitoring, which focuses on residues in raw agricultural commodities, a total of 49,877 samples (30,361 domestic and 19,516 import) that included fresh fruits and vegetables, grains, milk and dairy products, seafoods, and a variety of processed foods were analyzed. No residues were found in about 55 and 44% of the domestic and import samples, respectively. About 3% of the domestic and 7% of the import samples were classed as violative. Data from the Total Diet Study, which is conducted to determine dietary intakes of a variety of chemicals, showed that residues of 42 pesticides were found in 1044 composites of table-ready foods. Results of FDA's monitoring for FY78-82 demonstrate that pesticide residue levels in the U.S. food supply were generally well below regulatory limits, and dietary intakes were manyfold lower than the Acceptable Daily Intakes established by international agencies.     

Food and Drug Administration pesticide residue monitoring of foods: 1983-1986

Pesticide residues in foods are reported for the 4-year period 1982-1986 [fiscal years (FY) 83-86]. Results were summarized from the 2 complementary approaches that make up the Food and Drug Administration's (FDA) pesticide residue monitoring program. Under regulatory monitoring, which focuses on residues in raw agricultural commodities, a total of 49,055 samples (27,700 domestic and 21,355 import) that included fresh fruits and vegetables, grains and grain products, milk and dairy products, seafoods, and a variety of processed foods were analyzed. No residues were found in 60 and 48% of the domestic and import samples, respectively, compared with 55 and 44% in FY78-82. About 3% of the domestic and 5% of the import samples were violative. In FY78-82, about 3 and 7% were violative, respectively. The other FDA monitoring approach, the Total Diet Study, was revised in April 1982 to expand coverage of age/sex groups, use updated diets, and provide for analysis of individual foods. Results from monitoring under this modified approach and from regulatory monitoring continued to demonstrate that pesticide residues in the U.S. food supply were well below regulatory limits, and dietary intakes were many times lower than the Acceptable Daily Intakes established by international agencies.     

Fluorescence spectroscopy: a rapid tool for analyzing dairy products

This paper gives a critical evaluation of the use of fluorescence spectroscopy for measuring chemical and physical changes in dairy products caused by processing and storage. Fluorescence spectroscopy is able to determine various properties of foods without use of chemicals and time-consuming sample preparation. This is shown by examples where the measurement of a given chemical parameter has been appropriately described and validated, as well as situations showing potential applications, but where further research and validation is required. The interpretation of fluorescence spectroscopic data is complex due to absorbance by other molecular groups, changes caused by variation in the sample matrix, etc. It is illustrated how advanced data analytical techniques are required to obtain optimal interpretation of the data. Even though the review focuses on examples from the dairy industry, the principles are broader and can be applied to other fields of food and agricultural research.     

Formation of the mutagen IFP in model systems and detection in restaurant meats

Mixtures of the free amino acids, creatine and glucose, were dry-heated to model the potential formation of heterocyclic amines in meats. The formation of the mutagenic amine IFP (determined to be 2-amino-(1,6-dimethylfuro[3,2-e]imidazo[4,5-b])pyridine) was investigated by varying heating time, heating temperature, and precursors. With an optimized mixture of glutamine, creatine, and glucose, heated at 200 degrees C for 60 min, 2 mg of IFP was purified for studies to define its structure. Trideuteriomethyl-IFP was made from trideuteriomethylcreatinine in the model system for use in LC-MS detection of IFP in foods. Analysis of well-done meats purchased from restaurants showed about half to contain IFP at levels from 1.4 to 46 ng/g of cooked meat, demonstrating human exposure to this mutagen.     

Liquid chromatographic determination of natural and synthetic colorants in lyophilized foods using an automatic solid-phase extraction system

Five synthetic and five natural colorants were determined in lyophilized dairy products and fatty foods using an automatic method based on lixiviation and a solid-phase extraction process that includes cotton and RP-C(18) columns for the sequential retention of synthetic colorants and natural colorants, respectively. The lyophilization of the sample coupled with the separation procedure provides clean extracts despite the complexity of the matrices studied. In addition, the lyophilization process preserves the sample for at least 2 months without changes in the concentrations of the colorants. Identification and determination of synthetic and natural colorants were carried out using a liquid chromatograph equipped with a diode array detector. The detection limits achieved for all of the colorants (0.03-75 microg/g of lyophilized sample) allowed their determination within the limits established by the European Union, with good precision (approximately 4.5%). In addition, colorants spiked to different foods provided average recoveries (spiked at three concentration levels in four types of dairy samples and in three types of fatty foods) near 94 +/- 4%.     

Automated fluorometric determination of vitamin C in foods.

Two automated fluorometric methods were compared with the official AOAC methods for determining vitamin C in fortified ready-to-eat cereals, fruits, vegetables, baby foods, flour products, pet foods, meats, frozen dinners, juices, and nutritional health bars. Each sample was analyzed in duplicate on 2 days and included a recovery study on all products. Egberg's semiautomated method (Method 1) and Roy's automated method (Method 2) were compared with the manual AOAC tritrimetric and fluorometric methods. The correlation factor for Methods 1 and 2 were 0.999 and 0.979, respectively, when compared with the AOAC methods. The recovery study showed average recoveries of 97.8% for Method 1, 99.3% for Method 2, and 100.6% for the AOAC methods. The results suggest that Method 1 is the method of choice for the majority of the products analyzed.     

Rapid prediction of gross energy and utilizable energy in cereal food products using near-infrared reflectance spectroscopy

Near-infrared (NIR) spectroscopy has been used in foods for the rapid assessment of several macronutrients; however, little is known about its potential for the evaluation of the utilizable energy of foods. Using NIR reflectance spectra (1104-2494 nm) of ground cereal products (n = 127) and values for energy measured by bomb calorimetry, chemometric models were developed for the prediction of gross energy and available energy of diverse cereal food products. Standard errors of cross-validation for NIR prediction of gross energy (range = 4.05-5.49 kcal/g), energy of samples after adjustment for unutilized protein (range = 3.99-5.38 kcal/g), and energy of samples after adjustment for unutilized protein and insoluble dietary fiber (range = 2.42-5.35 kcal/g) were 0.053, 0.053, and 0.088 kcal/g, respectively, with multiple coefficients of determination of 0.96. Use of the models on independent validation samples (n = 58) gave energy values within the accuracy required for U.S. nutrition labeling legislation. NIR spectroscopy, thus, provides a rapid and accurate method for predicting the energy of diverse cereal foods.     

Improved enrichment for recovery of Shigella sonnei from foods

Shigella species were recovered from foods by the procedure described in the Bacteriological Analytical Manual, 5th Ed. The method is effective if Shigella species are present at about 10(6) cells/g. A 25 g food portion was incubated in Gram-negative (GN) and selenite cystine broths for 16 h at 35 degrees C and streaked onto MacConkey, Levine's eosin methylene blue, desoxycholate citrate, and xylose lysine desoxycholate agars. S. sonnei cells were recovered quantitatively at 44.5 degrees C, and along with other Shigella species, were grown with Escherichia coli in a tryptone broth under anaerobic conditions. Shigella species were also grown in a mixed microflora from foods. S. sonnei cells were inoculated into an enrichment broth containing 20 g tryptone, 2 g K2HPO4, 2 g KH2PO4, 1 g glucose, 5 g NaCl, 1.5 mL Tween 80, and 0.5 mg novobiocin/L (pH 7.0) and incubated for 20 h at 44 degrees C. Enrichments were streaked onto MacConkey agar and the plates were incubated 20 h at 35 degrees C. Suspect Shigella colonies were screened in glucose, tryptone, and lysine broths and in triple sugar iron and motility agars. The sensitivity varied from 0.3 to 1000 bacteria/g. The method has been examined with artificially inoculated lettuce, celery, brussels sprouts, mushrooms, and hamburger. It is also applicable to S. flexneri if incubation is conducted at 42 degrees C.     

Fluoride content of foods made with mechanically separated chicken

The goal of the present study was to determine the extent to which foods made with mechanically separated chicken can contribute to total fluoride intake. Fluoride content of each blended sample was determined with a fluoride combination electrode following perchloric-acid-facilitated diffusion of hydrogen fluoride. Infant foods had the highest fluoride content followed by chicken sticks, luncheon meats, and canned meats. A single serving of chicken sticks alone would provide about half of a child's upper limit of safety for fluoride. Fluoride content of foods made with mechanically separated chicken was significantly correlated with calcium content, which is consistent with the possibility that the mechanical separation process was the source of the extra fluoride. Foods made with mechanically separated turkey were not a major source of fluoride.     

矿物养分来源与缺乏影响人体健康与营养的研究进展

Mineral nutrients are fundamentally metals and other inorganic compounds.The life cycle of these mineral nutrients begins in soil,their primary source.Soil provides minerals to plants and through the plants the minerals go to animals and humans;animal products are also the source of mineral nutrients for humans.Plant foods contain almost all of the mineral nutrients established as essential for human nutrition.They provide much of our skeletal structure,e.g.,bones and teeth.They are critical to countless body processes by serving as essential co-factors for a number of enzymes.Humans can not utilize most foods without critical minerals and enzymes responsible for digestion and absorption.Though mineral nutrients are essential nutrients,the body requires them in small,precise amounts.We require them in the form found in crops and they can be classified into three different categories:major,secondary,and micro or trace minerals.This classification is based upon their requirement rather than on their relative importance.Major minerals such as potassium(K)and phosphorus(P)are required in amounts of up to 10 g d~(-1).The daily requirement of secondary and micro minerals ranges from 400 to 1500 mg d~(-1)and 45μg d~(-1)to 11 mg d~(-1),respectively.To protect humans from mineral nutrient deficiencies,the key is to consume a variety of foods in modest quantities,such as different whole grains,low fat dairy,and different meats,vegetables and fruits.For insurance purposes,a supplement containing various mineral nutrients can be taken daily.     

Rapid monoclonal antibody-based enzyme-linked immunosorbent assay for detection of Listeria in food products

A rapid diagnostic test for the detection of Listeria in food products has been created. This test, known as Listeria-Tek, uses 2 monoclonal antibodies specific for Listeria in an enzyme-linked immunosorbent assay (ELISA) format. The test requires only 40 h of broth enrichment with no culturing on solid media. It is extremely simple to perform and easy to interpret, and is at least as sensitive and accurate as the best of the culture methods. The test can be used with dairy products, meat products, and environmental samples. The ELISA test is safely performed on the open bench of the laboratory because no live cultures, no radioactivity, no phage, etc., are necessary. There is no need for special licenses or reserved laboratory space, and no waste disposal problems are encountered. If necessary, one technician could easily perform hundreds of assays per day. A printed data sheet is available for permanent records.     

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.     

Near-infrared reflectance model for the rapid prediction of total fat in cereal foods

AOAC method 996.01, used in cereal foods to determine total fat as defined by the U.S. Nutrition Labeling and Education Act (NLEA), is laborious and time-consuming and utilizes hazardous chemicals. Near-infrared (NIR) reflectance spectroscopy, a rapid and environmentally benign technique, was investigated as a potential method for the prediction of total fat using AOAC method 996.01 as the reference method. Near-infrared reflectance spectra (1104-2494 nm) of ground cereal products (n = 72) were obtained using a dispersive spectrometer, and total fat was determined according to AOAC method 996.01. Using multivariate analysis, a modified partial least-squares model was developed for total fat prediction. The model had a SECV of 1.12% (range = 0.5-43.2%) and a multiple coefficient of determination of 0.99. The model was tested with independent validation samples (n = 36); all samples were predicted within NLEA accuracy guidelines. The results indicate that NIR reflectance spectroscopy is an accurate means of determining the total fat content of diverse cereal products for nutrition labeling.     

Accelerated solvent extraction and confirmatory analysis of sulfonamide residues in raw meat and infant foods by liquid chromatography electrospray tandem mass spectrometry

This paper describes a new method for the rapid extraction and unequivocal confirmation of 13 sulfonamides (SAs) in raw meat and infant foods. The highly automated extraction procedure is based on accelerated solvent extraction followed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) as a confirmatory analysis. After 1 g of food matrix was blended with 2 g of C18 as a solid support material, the mixture was packed into the extraction cell and the SAs were extracted with 10 mL of hot water at 160 degrees C and 100 atm; 100 microL of the extract was directly injected into the LC-MS system. The analytes were ionized in an electrospray interface operating in the positive ion mode and were identified by selecting two multireaction monitoring transitions, which guaranteed method specificity. Typical recoveries from crude meat and baby food samples ranged from 70 to 101% at a fortification level of 100 ppb, corresponding to the maximum residue limits established by the European Union and the U.S. Food and Drug Administration. The interday method precision was less than 8.5%, and the limits of detection were below 2.6 ppb. This study has taken matrix-induced suppression of ionization into account, by comparing standard and matrix-matched calibration curves. Four of the 13 monitored SAs have been detected in some baby foods and raw meat samples, bought from Roman supermarkets and butchers' shops, using the described methodology.