Validation of the Hygicult E dipslides method in surface hygiene control: a Nordic collaborative study

A collaborative study with Enterobacteriaceae was conducted to validate Hygicult E dipslides by comparison with violet red bile glucose agar (VRBGA) contact plates and swabbing, using stainless steel surfaces artificially contaminated with microbes at various levels. Twelve laboratories participated in the validation procedure. The total number of collaborative samples was 108. The microbial level in each sample was assessed in triplicate by using the 3 above-mentioned methods. No Enterobacteriaceae were used at the low inoculation level. At the middle inoculation level, the percentages detached from the test surfaces were 16.6 with the Hygicult E method, 15.3 with the contact plate method, and 14.6 with swabbing; at the high innoculation level, the percentages were 14.5, 15.8, and 9.8, respectively. The percentage of acceptable results after the removal of outliers was 97.2. Repeatability relative standard deviations ranged from 33.4 to 44.9%; reproducibility relative standard deviations ranged from 45.2 to 77.1%. The Hygicult E dipslide, VRBGA contact plate, and swabbing methods gave similar results at all 3 microbial levels tested: <1.0 colony-forming units (CFU)/cm2 at the low level, 1.2-1.3 CFU/cm2 at the middle level (theoretical yield 8.0 CFU/cm2), and 1.2-2.0 CFU/cm2 at the high level (theoretical yield 12.5 CFU/cm2).     

Drops down the hill: theoretical study of limiting contact angles and the hysteresis range on a tilted plate

The limiting inclination angle (slip angle), for which a two-dimensional water drop may be at equilibrium on a chemically heterogeneous surface, is exactly calculated for a variety of cases. The main conclusion is that, in the cases studied, the contact angles at the upper and lower contact line do not always simultaneously equal the receding and advancing contact angles, respectively. On a hydrophobic surface, the lowest contact angle (at the upper contact line) tends to be approximately equal to the receding contact angle, while the highest contact angle (at the lower contact line) may be much lower than the advancing contact angle. For hydrophilic surfaces, the opposite is true. These conclusions imply that the hysteresis range cannot in general be measured by analyzing the shape of a drop on an inclined plane. Also, the limiting inclination angle cannot in general be calculated from the classical equation based only on the advancing and receding contact angles.     

Biosensor analysis of beta-lactams in milk: comparison with microbiological, immunological, and receptor-based screening methods

Two recently developed surface plasmon resonance biosensor assays for detection of beta-lactams in milk were used to screen raw producer milk samples. Both assays use a beta-lactam receptor protein with carboxypeptidase activity for detection. The results of the biosensor assays were compared with those of various commercial screening tests, i.e., the Delvotest SP, Penzym S, Beta-STAR, SNAP, and Parallux. The results of the 2 biosensor assays showed good agreement with those of the other screening tests. Of 195 analyzed milk samples, the results of only 5 samples differed between the assays. Additionally, 30 milk samples with both negative and positive results in the screening assays were analyzed by liquid chromatography for identification and quantification of any beta-lactam residues. All screening tests showed 0% false-negative results with 15 incurred samples containing between 4.0 and 268 microg/kg penicillin G. The biosensor assays showed 27% positive results (false violatives) with 15 producer milk samples containing penicillin G concentrations between 0 and 3.6 microg/kg, i.e., below maximum residue limit. This figure varied between 27 and 53% for the other screening tests.     

3M Petrifilm Staph Express Count plate method for the enumeration of Staphylococcus aureus in selected types of meat, seafood, and poultry: collaborative study

The 3M Petrifilm Staph Express Count plate method was compared with AOAC Official Method 975.55 for the enumeration of Staphylococcus aureus in selected foods. Four foods--cooked, diced chicken; cured ham; smoked salmon; and pepperoni--were analyzed for S. aureus by 12 collaborating laboratories. For each food tested, the collaborators received 8 blind test samples consisting of a control sample, a low inoculation level, a medium inoculation level, and a medium inoculation level with background flora, each in duplicate. The mean log10 counts for the methods were comparable for all 4 foods. The repeatability and reproducibility variances of the 24 h Petrifilm Staph Express Count plate method were similar to those of the 72 h standard method.     

3M Petrifilm enterobacteriaceae count plate method for enumeration of enterobacteriaceae in selected foods: collaborative study

The practice of detecting and enumerating all oxidase-negative, glucose-fermenting-Gram-negative rods (i.e., the family Enterobacteriaceae) is used to indicate unsanitary or inadequate food processing conditions. The objective of this interlaboratory collaborative study was to evaluate and compare the methods described in Standard Methods for the Examination of Dairy Products (SMEDP) and the Compendium of Methods for the Microbiological Examination of Foods (Compendium) with a commercial product, the 3M Petrifilm Enterobacteriaceae Count Plate, for the recovery of Enterobacteriaceae in foods. Six foods--cheddar cheese, milk, flour, frozen prepared meals, frozen broccoli, and nut pieces--were analyzed for Enterobacteriaceae by 12 collaborating laboratories. For each food tested, the collaborators received 8 blind test portions consisting of a control test portion and 3 levels of inoculated test portion, each in duplicate. Each test portion was tested by the Petrifilm Enterobacteriaceae Count Plate method as well as the SMEDP or Compendium methods. The precision estimates (repeatability or within-laboratory variation, and reproducibility or between-laboratory variation) were calculated with standard statistical techniques.     

Design and use of quality control samples in a collaborative study of trace metals in daily diets

Summary A series of six simulated diets consisting of mixtures of a limited number of foodstuffs have been produced, primarily to be used as external quality control samples in the ongoing WHO/UNEP HEAL-project. Lead and cadmium in the simulated diets have been determined by several reference laboratories. Zinc, copper, iron, manganese, chromium, nickel, calcium, aluminium, cobalt and arsenic have so far been analysed by at least two different laboratories. Examples of reference values and of the practical use of the diets in analytical quality assurance are given.

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Planung und Verwendung von Qualitätskontrollproben bei einem Ringversuch für Spurenmetalle in der täglichen Nahrung

     

Enumeration of total aerobic microorganisms in foods by SimPlate Total Plate Count-Color Indicator methods and conventional culture methods: collaborative study

The relative efficacy of the SimPlate Total Plate Count-Color Indicator (TPC-CI) method (SimPlate 35 degrees C) was compared with the AOAC Official Method 966.23 (AOAC 35 degrees C) for enumeration of total aerobic microorganisms in foods. The SimPlate TPC-CI method, incubated at 30 degrees C (SimPlate 30 degrees C), was also compared with the International Organization for Standardization (ISO) 4833 method (ISO 30 degrees C). Six food types were analyzed: ground black pepper, flour, nut meats, frozen hamburger patties, frozen fruits, and fresh vegetables. All foods tested were naturally contaminated. Nineteen laboratories throughout North America and Europe participated in the study. Three method comparisons were conducted. In general, there was <0.3 mean log count difference in recovery among the SimPlate methods and their corresponding reference methods. Mean log counts between the 2 reference methods were also very similar. Repeatability (Sr) and reproducibility (SR) standard deviations were similar among the 3 method comparisons. The SimPlate method (35 degrees C) and the AOAC method were comparable for enumerating total aerobic microorganisms in foods. Similarly, the SimPlate method (30 degrees C) was comparable to the ISO method when samples were prepared and incubated according to the ISO method.     

Structure of a dipolar fluid in contact with a solid: Adsorption and surface potential

The structure of a dipolar fluid in contact with a solid is investigated using the optimized cluster theory. The solid-liquid interaction is described by an effective one-dimensional potential. The model of a perfect impenetrable wall is dropped. An adsorption potential similar to the non-electrostatic potential appearing in the classical models of the double layer is introduced. We show that the effect of a reasonable adsorption potential is localized in the first monolayer in contact with the solid although the total effect of the solid is non localized in this monolayer. No states of orientation predominate in such a way that we can justify a two- or three-state model. The potential drop g^s(dip) across the interface and the change in the surface Gibbs energy due to the adsorption potential are calculated. An adsorption potential of magnitude 1kT gives g^s(dip) - ?0.6 V. In order to obtain the values of g^s(dip) generally accepted in the literature, no dielectric constant or clusters should be introduced. Because of the competition between the dipolar interaction and the adsorption potential, an increase of the dipole moment does not necessarily increase g^s(dip).     

3M Petrifilm Staph Express Count plate method for the enumeration of Staphylococcus aureus in selected dairy foods: collaborative study

The 3M Petrifilm Staph Express Count plate method was compared with AOAC Official Method 975.55 for the enumeration of Staphylococcus aureus in selected foods. Five foods--ice cream, raw milk, yogurt, whey powder, and cheese--were analyzed for S. aureus by 12 collaborating laboratories. For each food tested, the collaborators received 8 blind test samples consisting of a control sample, a low inoculation level, a medium inoculation level, and a medium inoculation level with background flora, each in duplicate. The mean log10 counts for the methods were comparable for all 5 foods. The repeatability and reproducibility variances of the 24 h Petrifilm Staph Express Count plate method were similar to those of the 72 h standard method.     

Application of emanation and surface labelling methods in the study of thermal processes

The emanation method and the method of surface labelling have been applied to study thermal processes. Using these methods, processes which are not connected with thermal effects can be analysed. In certain cases, for example in the study of poorly crystalline, or amorphous phases, these methods are even more sensitive than thermographic and X-ray techniques. The method of surface labelling is advantageous in cases when it is impossible to activate the samples by the parent isotope of the gas during the process of their preparation. This work was performed during the author's study leave to Moscow State University, Department of Radiochemistry.     

Nanodrop on a nanorough hydrophilic solid surface: contact angle dependence on the size, arrangement, and composition of the pillars

A two-dimensional nanodrop on a hydrophilic solid surface decorated with nanopillars is examined using a nonlocal density functional theory. It is shown that, in contrast to the commonly used Wenzel formula, even an extremely small roughness can considerably increase the contact angle. The contact angle depends on the distance between pillars, their height and width, as well as their composition. It was found that for all selected pillar heights and compositions, the largest contact angle is obtained when the distance between pillars acquires a size at which the liquid molecules can no longer penetrate between them. The further decrease in the interpillar distance decreases the contact angle, in qualitative agreement with the Cassie-Baxter formula. Considering pillars of various compositions, the role of the gradient of the fluid-solid interaction potential is examined. The presence of such a gradient does not allow the formation of a stable nanodrop on the surface. However, asymmetrical metastable nanodrops can be formed.     

Calculating the surface tension between a flat solid and a liquid: a theoretical and computer simulation study of three topologically different methods

We discuss three topologically different methods for calculating the surface tension between a flat solid and a liquid from theoretical and computer simulation viewpoints. The first method, commonly used in experiments, measures the contact angle at which a static droplet of liquid rests on a solid surface. We present a new analysis algorithm for this method and explore the effects of line tension on the contact angle. The second method, commonly used computer simulations, uses the pressure tensor through the virial in a system where a thick, infinitely extended slab of liquid rests on a solid surface. The third method, which is original to this paper and is closest to the thermodynamic definition of surface tension, applies to a spherical solid in contact with liquid in which the flat solid is recovered by extrapolating the sphere radius to infinity. We find that the second and third methods agree with each other, while the first method systematically underestimates surface tension values.     

Nanodroplets on a planar solid surface: temperature, pressure, and size dependence of their density and contact angles

The shape and the density of a liquid drop on a planar solid surface, embedded in an inert gas at constant temperature and pressure, were examined on the basis of a microscopic density functional approach that accounts for the entropic (temperature-dependent) and energetic contributions to the free energy of the system. Integro-differential equations describing the profile and the density of a cylindrical (2D) drop were derived by the variational minimization of the Gibbs free energy with respect to both the drop profile and density under the assumption of uniform density. The equations were solved numerically using the constraint of a constant number of molecules N(l) per unit length of the drop. It was shown that for temperatures lower than a certain temperature Tw the free energy against density has generally two minima, representing a stable equilibrium state and a metastable one. One of those minima is located at a density corresponding to the density of a normal liquid, whereas the other one is located at a density comparable to the density of the surrounding inert gas. For this reason, the latter state of the drop cannot be stable. For T > Tw, the minimum corresponding to the liquid state disappears, and no drop can be formed on the surface. The temperature Tw depends on N(l) and the external pressure p and increases when N(l) and p increase. The true wetting angle theta0 that the drop profile makes with the solid surface depends on the parameters characterizing the microscopic interactions, the density, and the surface densities. If in the thermodynamically stable state absolute value(cos theta0) > 1, then no drop is formed on the surface. If in that state absolute value(cos theta0) < 1, then at any pressure the true contact angle decreases when the temperature increases and approaches Tw. However, theta0 does not reach a zero value for T < or = Tw but has for T = Tw a discontinuity from a finite to a zero value. The true contact angle also depends on the number of molecules in the drop and on the external pressure. For all considered values of N(l), p, and microscopic parameters of the intermolecular interactions, the density of the drop decreases with increasing temperature. The rate of decrease is constant for temperatures sufficiently far from Tw and increases when T approaches Tw. At a given temperature and pressure, the density of the drop decreases with decreasing N(l). For relatively large drops (N(l) approximately = 10(14)-10(20)), the rate of decrease is very small, whereas for small droplets (N(l) approximately = 10(12)) it becomes much larger.     

Enumeration of total yeasts and molds in foods by the SimPlate Yeast and Mold-Color Indicator method and conventional culture methods: collaborative study

The relative effectiveness of the SimPlate Yeast and Mold-Color Indicator method (Y&M-CI) was compared to the U.S. Food and Drug Administration's (FDA) Bacteriological Analytical Manual (BAM) method and the proposed International Organization for Standardization (ISO) method, ISO/CD 21527, for enumerating yeasts and molds in foods. Test portions were prepared and incubated according to the conditions stated in both the BAM and ISO methods. Six food types were analyzed: frozen corn dogs, nut meats, frozen fruits, cake mix, cereal, and fresh cheese. Nut meats, frozen fruits, and fresh cheese were naturally contaminated. All other foods were artificially contaminated with either a yeast or mold. Seventeen laboratories throughout North America and Europe participated in the study. Three method comparisons were conducted. In general, there was <0.3 mean log count difference in recovery between the SimPlate method and the 2 corresponding reference methods. Moreover, mean log counts between the 2 reference methods were also very similar. The repeatability (Sr) and reproducibility (SR) standard deviations were comparable between the 3 method comparisons. These results indicate that the BAM method and the SimPlate method are equivalent for enumerating yeast and mold populations in foods. Similarly, the SimPlate method is comparable to the proposed ISO method when test portions are prepared and incubated as defined in the proposed ISO method.     

Multi-analytical study of patination methods on steel substrates: a full insight into surface chemistry and morphology

Patination of metals has been used for decorative or protective purposes, and several methods aimed to create coloured films on metal surfaces have been developed. This work describes a multi-analytical approach to characterize artificial blue patinas created on mild steel substrates by means of traditional recipes and methods for colouring ancient objects and artefacts. We suggest the combined use of secondary ion mass spectrometry, focused ion beam, X-ray diffraction spectroscopy, white light interferometry and reflectance spectroscopy to characterize blue patinas on steel substrates and to investigate the relationship between the developed colour and the patina layer microstructure and composition. Therefore, the analysis of the oxide films produced by either thermal or chemical colouring methods has been successfully performed, providing information about the film morphology, the surface composition and in-depth elemental distribution within the coloured layers, and the origin of the colour developed on the surface.     

Salmonella in selected foods by VIDAS immuno-concentration Salmonella plus selective plate method (Hektoen enteric,xylose lysine desoxycholate,bismuth sulfite): collaborative study

The VIDAS Immuno-concentration Salmonella (ICS) plus selective plate method (Hektoen enteric, xylose lysine desoxycholate, bismuth sulfite) method for the detection of Salmonella was compared to the Bacteriological Analytical Manual (BAM)/AOAC culture method in a collaborative study. Thirty-two laboratories participated in the evaluation. Each laboratory tested one or more of the 6 test products: milk chocolate, nonfat dry milk, dried whole egg, soy flour, ground black pepper, and ground raw turkey. The 2 methods were in agreement for 1,297 of the 1,455 samples. Of the 158 samples not in agreement, 82 were VIDAS ICS plus selective plate-positive and BAM/AOAC-negative, and 76 were VIDAS ICS plus selective plate-negative and BAM/AOAC-positive.