Pittcon technical sessions: A focus on food analysis

The technical program of Pittcon 2000 encompassed a wide range of technologies, emphasizing instruments with applications from biotechnology to environmental to food analysis, forensics, and toxicology. Sensor technology was especially “hot”. The contributed environmental sessions covered biomonitoring, organics, particulates and inorganics in air, microwave sample preparation, waste materials, water and waste water, and complex molecules. Several technical sessions examined capillary electrophoresis with respect to bioanalytical experiments, neurochemistry, separation, and DNA-related experiments. Also involving capillary electrophoresis were the symposia “Ultrasmall Capillary Electrophoresis: From Cells to Vesicles” and “Separation-Based Biosensors Based on Capillary Electrophoresis”. Pittcon offers so many poster presentations and technical sessions that it is impossible to do justice to the amount of material presented.

The food industry was well represented in technical sessions with “Food Analysis I-V”. Richard Durst of Cornell University coordinated the “Bioanalytical Techniques for Food Safety” symposium, featuring new techniques for the rapid detection and characterization of pathogenic microorganisms and their toxins. The conventional method of plating out microorganisms to classify them is slow and inconsistent, creating a demand for new methods, such as impedance and conductance, bioluminescence, flow cytometry, immunoassays, and nucleic acid hybridization. For more rapid detection of microorganisms, a combination of these and molecular typing methods is being used.

Flow cytometry and cell-labeling techniques are based on fluorescent labeling of single intact cells. The labels are then available for total viable count and for determining the presence of specific organisms. Flow cytometry does not require any culture step and employs specific labels and antibody tags. However, if the cell wall bursts, the detection is lost. Other methods for molecular typing include nucleic acid hybridization and amplification by PCR. To select an analytical method, a researcher must consider speed, specificity, precision, accuracy, reliability, and reproducibility.

Novel Biosensing Systems. Presentations also included novel biosensing systems for the detection of Cryptosporidium parvum and Aeromonas hydrophila and the monitoring of mycotoxins, insecticides, and proteins of genetically modified crops. Bruce Hammock of the Departments of Entomology and Mechanical Engineering at the University of California, Davis, suggested using accelerator mass spectrometry as a detection system for immunoassays because of its exceptional level of sensitivity (attomole). Equally fascinating, Hammock also talked about using the high-density, digital surface of a compact disc as a means of formatting immunoassays, because thousands of individual spots can be analyzed on a CD, and laser detection systems can determine absorbance, reflectance, fluorescence, or interference.

Thermal Analysis. In addition to the contributed session, “Thermal Analysis: Applications and Specialized Instrumentation”, Jonathon A. Foreman of Mettler-Toledo, Inc. spoke about the

The Eleventh James L. Waters Annual Symposium This symposium is designed to explore the origin, development, commercialization, and key applications of established scientific instrumentation. This year’s symposium covered the history of X-ray diffraction of powders and thin films. The four speakers and their respective presentations were: • Dr. Ronald Jenkins: Landmarks in the Development of Powder Diffraction Instrumentation • Dr. Herbert E. Goebel: Selected Highlights of Contributions to X-ray Powder Diffraction • Dr. Thomas Ryan: Instrumentation for Thin-Film X-ray Diffraction • Dr. Jimpei Harada: The Role of the Development of the Rotating Anode X-ray Generator and the Use of the Imaging Plate in Powder Diffraction Instrumentation

characterization of phase transition using thermal analysis during “Food Analysis I”. Thermal analysis measures the affects of processing and storing food products by measuring the changes of energy as a function of temperature. The technique analyzes the changes in phase transitions of the food product, providing information about the crystal structure—a property directly related to the texture and feel of the food product in your mouth. Measuring the cooling rate of chocolate and cocoa butter provides information about the phase transitions of those particular food products. For instance, fine chocolates melt at 36 °C—very close to the temperature of your mouth (37 °C)—and not at room temperature, which would create a mess. The taste and texture of a given sample of chocolate depends on the means of processing, and specifically, the cooling process. As determined by varying the cooling rates of three samples of chocolate, attaining fine chocolate requires a slower net cooling time and/or longer annealing times. This seminar made it obvious that time-dependent processes such as storing and aging, as well as cooling rates, ultimately affect food behavior and quality.

Food and Water Contamination. The growing concern over food safety necessitates rapid testing of food products and water. Several presentations addressed advancements in biosensor technology, such as the use of an artificial microorganism model called “Bugbead”, to evaluate new devices and methods without risking potential hazards by using real microorganisms. This particular Bugbead consists of microspheres coated with proteins via biotin–avidin bonding, which makes immunoassays possible. The protein antigens can be chosen according to availability, cost, desired size, and epitope density. The use of electrochemical detection of these Bugbeads is convenient, because it does not require radioactivity, is very sensitive, takes advantage of enzyme amplification, and is highly specific.

Because contamination by Escherichia coli O157:H7 is a grave concern of the food industry, Naser Elayan of Southern University demonstrated the use of an amperometric immunosensor for its detection. The device also proved beneficial when used with Salmonella typhimariun, Renibacterium salmoninarum, Listeria, and Campylobacteria species.

Test by Taste and Smell. Large volume static headspace (LVSH), in conjunction with gas chromatography, has managed to duplicate olfactory detection limits in food and flavor analysis. (Now, it’s not only the nose that “knows”.) Based on gas chromatography (GC) and GC coupled with mass spectroscopy (MS) technology, LVSH allows the visualization of contaminants, impurities, and other compounds. LVSH requires a 3-stage preconcentrator: automating GC analysis of gas phase samples, cold trapping of the analyte, and complete heating of the tube. Lower temperatures and milder adsorbents reduce analyte loss and chemical rearrangements versus the classical adsorbent trapping, which requires higher temperatures and causes material loss.

This technology proved useful for the analysis of coffee and yogurt, and the comparison of garlic versus garlic powder and chocolate versus carob. A 3-step process involves analysis of 100cc of the “good” food product(s); analysis of 100cc of the bad or spoiled food product(s); and picking the markers or those compounds that differ or change between the good and bad food materials. When good grapes were compared with spoiled grapes, there were two obvious compound changes—ethyl acetate increased by 120% in the spoiled product and the production of limonene dropped off. Overall, LVSH maintains the sample in a more natural state and allows the measurement and detection of those compounds responsible for odor and taste.

Sol-gel Waveguide Fluorescence Sensors. Modified-atmosphere packaged food encompasses a mixture of nitrogen, oxygen, and carbon dioxide in a gas-flushing process, with the specific gas concentration depending on the food type. When foods are packaged, their gaseous environments are selected to prolong the lifetime of food by inhibiting bacterial growth or to enhance the food’s visual aspect. Normally, oxygen sensing is performed by conventional electrochemical means and carbon dioxide sensing is performed by infrared absorption, both of which require destructive testing of the package. To prevent any damage to the package, on-package sensor strips for optical sensing have been developed.

While attached to packaged food, sensor strips change color when the atmosphere changes. The immobilization matrix made of sol-gel contains an analyte-sensitive dye. Oxygen-sensing works by the fluorescent quenching of ruthenium polypyridyl complexes entrapped in the sol-gel matrix. A change or shift in the excitation versus luminescence signal occurs in the presence of oxygen. The phase sensor changes as a function of time with oxygen and lends itself well to a lifetime-based sensor. Because carbon dioxide forms carbonic acid in the presence of water, carbon dioxide can be detected by a fluorescent pH indicator. Using fluorescence resonance energy transfer, a lifetime-based indicator can also be designed for carbon dioxide. These are just a couple of ways in which the sol-gel process can be manipulated to optimize the response of both the sensing film and wavelength structures.

Flash EA 1112. CE Elantech and Thermoquest described a new automatic oxygen dosing system to improve the determination of nitrogen and protein content in food material. For over 100 years, researchers used the Kjeldahl method as the way to determine the protein content of a wide variety of products. However, the Kjeldahl method uses toxic chemicals, produces hazardous waste, and lacks automation. As a result, there has been great interest in simplifying the determination of protein in food.

Thermoquest designed their instrument as an extension of the dynamic flash combustion of the sample, which requires igniting the sample with oxygen and the appropriate catalyst to determine protein content. The Flash EA 1112 protein analyzer can rapidly analyze large sample sizes in any material form with reproducibility. The instrument also supplies the proper amount of oxygen needed for quantitative combustion and tailors the reaction to the sample weight and type of food, such as rice, milk, and cheese.

Quantifying Contamination in Food Packaging. Because we use so much plastic packaging, it’s necessary to master the packaging process or improve the polymers. Researchers at Alpha Mos are trying to advance the electronic nose technology by developing sensors that measure the odor and taste quality of food in plastic packaging. Sensor selection removes noisy information and reduces the complexity of systems. The automated conversion of the sensor array system reduces the constant need for new quality control protocols.

The Electronic Nose
Another hot topic at Pittcon 2000, and an irresistible topic to cover in this article, concerned the recent developments in sensor array technology and the electronic nose.

Researchers currently use GC and GC-MS to analyze volatile organic compounds (VOCs). The development of multi-array sensors such as the electronic nose—a low-cost alternative technique to analyze VOCs—allows qualitative identification of odiferous compounds, even where certain limitations still exist with respect to slow kinetics and inadequate quantitative information for the analytes of interest. Miriam M. Masila from the State University of New York at Binghamton discussed how most limitations come from insufficient understanding of the mechanism of molecular interactions. She proposed using quantitative sensor–analyte activity relationships for enhancing the quantification of volatile compounds in electronic nose systems.

The specificity and selectivity of polypyrrole-based gas sensors depends on the top layer polymer and the counter-ion combination used during the electrochemical polymerization of the top layer to polypyrrole (PPy) base layer. The combination of the counter-ion and top layer polymer also creates sensor diversity. Researchers at Osmetech have been studying the relationship between these combinations and the sensor’s response to various analytes, in order to predict the response of specific sensors to an analyte.

Because federal grain inspectors “nose” samples of grain, including wheat, corn, sorghum, and soybeans, to ensure quality—the sniffing potentially exposes workers to a variety of microbiological, fungal, and chemical hazards—Alexander McNeish of Osmetech introduced an in-line preconcentrator for use with a conducting polymer array. By incorporating a preconcentrator to minimize the influence of moisture changes and to concentrate the odor prior to analysis, the use of instruments facilitates the process by automating the analysis to become an objective, quick, and impartial test.

Other lectures presented a miniaturized device—based on a smart array of conducting polymers gas sensors—coupled with a hydrodistillation pilot unit, for quantifying the qualitative variations of essential oils and volatile compounds during the distillation process, as well as quantifying the chemical mixtures in perfumed matrices using mass sensing and multi-sensor array. Finally, Thomas M. Hawkins of Osmetech discussed the application of structure-property relationships in conducting polymer gas sensors (see figure below) to investigate solvent/water partition and drug activity.

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Jennifer B. Miller is an assistant editor with Today’s Chemist at Work. Comments and questions for the author may be addressed to the Editorial Office by e-mail at tcaw@acs.org, by fax at 202-776-8166 or by post at 1155 16th Street, NW; Washington, DC 20036.