INTELLIGENT PACKAGING- PART -VI

SENSOR

     

  A sensor is defined as a device used to detect, locate or quantify energy or matter, giving a signal for the detection or measurement of a physical or chemical property to which the device responds (Kress-Rogers, 1998a). To qualify as a sensor, a device must be able to provide continuous output of a signal. Most sensors contain two basic functional units a receptor and a transducer. The transducer is a device capable of transforming the energy carrying the physical or chemical information about the sample into a useful analytical signal.

        Chemical sensor and biosensor technology has developed rapidly in recent years. The main types of transducers with potential use in packaging systems include electrical, optical, thermal or chemical signal domains. Sensors can be applied as the determinant of a primary measurable variable or, using the marker concept, as the determinant of another physical, chemical or biological variable (Kress-Rogers, 1998a).

Gas sensors

        Gas sensors are devices that respond reversibly and quantitatively to the presence of a gaseous analyze by changing the physical parameters of the sensor and are monitored by an external device. Systems presently available for gas detection include amperometric oxygen sensors, potentiometric carbon dioxide sensors, metal oxide semiconductor field effect transistors, organic conducting polymers and piezoelectric crystal sensors (Kress-Rogers,1998b). Conventional systems for oxygen sensors based on electrochemical methods have a number of limitations (Trettnak, Gruber, Reiniger, & Klimant, 1995). These include factors such as consumption of analyte (oxygen), cross-sensitivity to carbon dioxide and hydrogen sulphide and fouling of sensor membranes (Gnaiger & Fortsner,1983). Such systems also involve destructive analysis of packages.

        Conventional systems for oxygen sensors based on electrochemical methods have a number of limitations (Trettnak, Gruber, Reiniger, & Klimant, 1995). These include factors such as consumption of analyte (oxygen), cross-sensitivity to carbon dioxide and hydrogen sulphide and fouling of sensor membranes (Gnaiger & Fortsner, 1983). Such systems also involve destructive analysis of packages

        In recent years, a number of instruments and materials for optical oxygen sensing have been described (Papkovsky, Ponomarev, Trettnak, & O’Leary, 1995; Thompson & Lakowicz, 1993; Trettnak et al., 1995). Such sensors are usually comprised of a solid-state material, which operate on the principle of luminescence quenching or absorbance changes caused by direct contact with the analyte. These systems provide a non-invasive technique for gas analysis through translucent materials and as such are potentially suitable for intelligent packaging applications.

        The solid-state sensor is inert and does not consume analyte or undergo other chemical reactions (Wolfbeis, 1991). Optochemical sensors have the potential to enhance quality control systems through detection of product deterioration or microbial contamination by sensing gas analytes such as hydrogen sulphide, carbon dioxide and amines (Wolfbeis & List, 1995).      

Biosensors      

        Other approaches to freshness indication, which may find commercial application in intelligent packaging systems are based on recently developed biosensor technologies.

        Biosensor is a device which is use to determine the concentration of the particular analyte present in the sample .Biosensors are compact analytical devices that detect, record and transmit information pertaining to biological reactions (Yam, Takhistov, & Miltz, 2005). These devices consist of a bio receptor specific to a target analyte and a transducer to convert biological signals to a quantifiable electrical response. Bio receptors are organic materials such as enzymes, antigens, microbes, hormones and nucleic acids. Transducers may be electrochemical, optical, calorimetric, etc., and are system dependent. Intelligent packaging systems incorporating biosensors have the potential for extreme specificity and reliability. The majority of available biosensor technology is not yet capable of commercial realization in the food sector. At present two biosensor systems are commercially available.

Commercially available Biosensors

        Toxin GuardTM by Toxin Alert Inc. (Ontario, Canada) is a system to build polyethylene-based packaging material, which is able to detect the presence of pathogenic bacteria (Salmonella sp., Campylobacter sp., Escherichia coli O157 and Listeria sp.) with the aid of immobilized antibodies. As the analyte (toxin, microorganism) is in contact with the material it will be bound first to a specific, labeled antibody and then to a capturing antibody printed as a certain pattern (Bodenhamer, 2000). The method could also be applied for the detection of pesticide residues or proteins resulting from genetic modifications.

        Another example of microbial indicators for the detection of specific microorganisms like Salmonella sp., Listeria sp. and E. coli is Food Sentinel SystemTM. This system is also based on immunochemical reaction, the reaction taking place in a bar code (Goldsmith, 1994). If the particular microorganism is present the bar code is converted unreadable.

The use of biosensor in food industries can subdivide into two groups

·      Enzyme biosensor for detecting food components

Biosensors are used in industries. As wine, beer, soft drinks etc. For detecting or measuring the Carbohydrates from alcohol, amino acids, amines, amides, phenol etc.

·      Biosensor for detecting bacteria in food

The detection of pathogenic bacteria can be done in two ways

DIRECT DETECTION

·      These biosensors are those in which Biospecific reactions are directly measured in real time by measuring the physical changes induced by the complex formation.

·      Optical Biosensor: These biosensors are used for direct detection of bacteria. These sensors are  able to detect the small change in refractive indices, when cell bind to receptor which are  immobilized on the transducer[3].The common optical biosensor which are used now a days are  Elapsometric, Ewascent wave interferometer, resonant mirror, and piezoelectric biosensor.

·      Bioluminescence Biosensor. The use of photons as a byproduct of reaction for Bio analytical  sensor led to the so called bioluminescence biosensor which may be use to detect the presence or  physiological state of cell. E.g. Luciferase reporter phage in which the gene encoding Luciferas incorporated into the genome of bacterial viruses.

·      Electrical impedence biosensor: In this Biosensor microbial metabolism bring increase in both conductance & capacitance and thus cause a decrease in impedance.

·       The biosensor is based on impedance measure of adherently growing cells on inters digitized electrode structure. I.e., Cell density, growth of cells on electrode changes the impedance of Biosensor.

INDIRECT DETECTION

·      These biosensors are those in which a preliminary bio chemical reaction  takes place and the product of the reaction are detected by a sensor.

·      Fluorescence labeled biosensor: Microorganism have protein & polysaccharides in their outer coat which permit development of bioassays for bacterial detection. In Fluoro immunoassay,  Fluoro chrome molecules are used to label immunoglobulin. These Fluoro chrome molecules absorb short wavelength light and emit higher wavelength light which detect by the fluorescent microscopy. FITC, RITC, BSA are most useful Fluorochrome used to tag the antibody eg: Using an Antibody to the protective protein express with the anthrax toxin for use of detection of Anthrax.

·      Microbial metabolism based biosensors: Microorganism is able to transducer their metabolic Redox reaction in to quantify electric signals by using Oxido Reductase reaction and mediator.

·      Flow immune sensors: Now days, many of the assays for microbes are based on Elisa using micro titration plates on completion of Chromogenic reaction, the quantitative determination is  done using Elisa reader e.g. E.coli detection.

Fluoresence-based oxygen sensors

        Flourescence-based oxygen sensors represent the most advanced and promising systems to date for remote measurement of headspace gases in packaged meat products. Reiniger, Kolle, Trettnak, & Gruber (1996) first introduced the concept of using luminescent dyes quenched by oxygen as non-destructive indicators in food packaging applications.

        The active component of a fluorescence-based oxygen sensor normally consists of a long-delay fluorescent or phosphorescent dye encapsulated in a solid polymer matrix. The dye-polymer coating is applied as a thin film coating on a suitable solid support (Wolfbeis, 1991). Molecular oxygen, present in the packaging head space, penetrates the sensitive coating through simple diffusion and quenches luminescence by a dynamic, i.e. collisional mechanism. Oxygen is quantified by measuring changes in luminescence parameters from the oxygen-sensing element in contact with the gas or liquid sample, using a pre-determined calibration. The process is reversible and neither clean, neither the dye nor oxygen is consumed in the photochemical reactions involved, no by-products are generated and the whole cycle can be repeated.

        Materials for oxygen sensors must meet strict sensitivity and working performance requirements if they are to prove suitable for commercial intelligent packaging applications. They must also have fluorescent characteristics suited to the construction of simple measuring devices. Fluorescence and phosphorescence dyes with lifetimes in the microsecond range are best suited to oxygen sensing in food packaging. Other necessary features include suitable intensity, well resolved excitation and emission long wave bands and good photo stability characteristics of the indicator dye. Such features allow sensor compatibility with simple and inexpensive optoelectronic measuring devices (LEDs, photodiodes, etc.), minimize interference by scattering and sample fluorescence and allow long-term operation without recalibration (Papkovsky, 1995). Materials using fluorescent complexes of ruthenium, phosphorescent palladium( II)– and platinum(II)–porphyrin complexes and related structures have shown promise as oxygen sensors (Papkovsky et al., 1991; Papkovsky et al., 1995; Wolfbeis, 1991).

        The combination of indicator dye and the encapsulating polymer medium in which oxygen quenching occurs determine the sensitivity and effective working range of such sensors. Other polymers with good gas barrier properties such as polyamide, polyethylene terephthalate and PVC are not suitable for oxygen sensing gas oxygen quenching is slow in such media (Comyn, 1985). The use of plasticized polymers is also unsuitable due to toxicity concerns associated with potential plasticizer migration.

        Sensor fabrication involves a simple process of dissolution of lipophilic indicator dye and appropriate polymer support in an organic solvent. This cocktail is applied to a solid substrate such as a polyester film or glass and allowed dry to produce a fluorescent film coating or spot. A number of coating techniques that lend themselves to large scale, continuous production (casting, dipping, spin coating, drop dispensing and spraying) offer possibilities for commercial production. Sensors, normally 1–2 cm in diameter, are coloured (due to the dye) and are readily visible on different support materials.

        Working range of most oxygen sensors work effectively within the range from 0 to 100 kPa of oxygen, or at least 0–21 kPa (0–21%) with detection limits of 0.01–0.1 kPa (where, in simple terms, kPa corresponds to percentage oxygen pressure (at room temperature and ambient air pressure).

Intrinsic toxicity: Sensor materials, i.e. dyes, polymers, residual solvents and additives are the main cause for concern in terms of potential toxicity issues. In general, the total quantity required to produce a single pack sensor is normally less than 1 mg, of which the encapsulating polymer represents >95%. The amount of dye per sensor usually varies to within a few micrograms. For most organic dyes, such quantities are far below established toxicity levels. It is advisable that solvents normally used in the food industry be used in sensor manufacture in order to avoid dangers associated with residual solvents.

        OxySense is the first commercially available fluorescence quenching sensor system for measurement of headspace or dissolved oxygen in transparent or semi-transparent, sealed packages. The system uses an oxygen sensor (O2xyDote) placed in the package before filling and is non-destructive, rapid (measurements take less than 5 s) and able to withstand pasteurization temperatures without loss of sensitivity

REFERENCES

·      AHVENAINEN, R., 2003: Novel Food Packaging Techniques. Cambridge UK: Wood head Publishing, 400 p. ISBN 978-1-85573-675-7.

·      COLES, R., MCDOWELL, D. and KIRWAN, M. J., 2003: Food Packaging Technology. Oxford, UK: Blackwell Publishing, 346 p. ISBN 978- 0849397882.

·      C. Suzuki, J. Chem. Ed. 68, 588–589 (1991).

·      FQSI International, FreshQt smart sensor label web information. Available at http://www.fqsinternational.com/products.htm. Accessed March 2008.

·      International published Patent WO 2006/032025 A1, J. R. Williams, K. E. Myers, M. M. Owens, and M. A. Bonne (to Food Quality Sensor International, Inc.).

·      R. Want, ‘‘Enabling Ubiquitous Sensing with RFID,’’ Computer 37, 84–86 (2004).

·      S. Nambi, S. Nyalamadugu, S. M.Wentworth, and B. A. Chin, ‘‘Radio Frequency Identification Sensors,’’ 7th World Multiconference on Systemics, Cybernetics and Informatics, Dubna, Russia, July 30–August 2, 2003.

·      SUMMERS, L., 1992: Intelligent packaging for quality. So_ Drinks Management International, Vol. 36, p. 32–33. ISSN 0953–4776.

·      RODRIGUES, E. T. and HAN, J. H., 2003: Intelligent packaging. In: Heldman, D. R. and Moraru, C. I. (ed.). Encyclopaedia of Agricultural, Food and Biological  Engineering. 2nd edition, New York: Marcel Dekker, pp. 528–535. ISBN 978-1439811115

·      HAN, J. H., HO, C. H. L. and RODRIGUE, E. T., 2005: Intelligent packaging. In: Han, J. H. Innovation in food packaging. UK, London: Elsevier Academic  Press, p. 138–155. ISBN 978-0123116321.

·      Gontard, N. Active packaging for food processing and preservation. In: International Congress on Engineering and Food: ICEF 9, 7e11 March 2004, Montpellier, France.

To be continued