Introduction
“Intelligent packaging materials” is the term used to represent class of packaging materials that can monitor the condition of either the packaged food or the food environment inside the package which includes temperature, pH, etc., and provides this information to the user. It can be interpreted as an extended function of traditional packaging which is responsible for communication with the consumer depending on its ability to sense and record the changes taking place in the food or its environment. An added advantage of intelligent packaging is its contribution to the improvement of Hazard Analysis and Critical Control Points’ (HACCP) and Quality Analysis and Critical Control Points’ (QACCP) systems and hence onsite detection of unsafe food products, identification of potential health hazards and establishing strategies to abate their occurrence, ultimately leading to improved food quality. The intelligent systems can be classified into three categories; sensors, indicators and radiofrequency identification (RFID) systems.

Sensors
A sensor is a device that can be used in the detection, location and measurement of energy or matter. It responds by giving a continuous output signal which can be interpreted to measure the physical or chemical stimuli to which it responds. Most sensors consist of two basic components; a receptor and a transducer. Sensors may be of several types depending on their response stimuli:

1. Biosensors: These can detect record and convey information relevant to biological systems. The receptors, known as bioreceptors in this case, recognize the target analyte and transducers convert these biochemical signals into measurable electrical signals. The bioreceptors may be organic or biological, like antigens, enzymes, nucleic acids, etc. The transducers used can be of optical, acoustic or electrochemical nature. Most of the commercial biosensors are a combination of antibody based receptor and optical transducer. Sire Technologies Inc. developed an antibody based biosensor with the trade name Food Sentinel System® where a membrane with immobilized antibodies is used as a part of barcode which acts as the sensor. The pathogens interact with the antibodies and a localized dark bar is formed which renders the barcode unreadable. ToxinGuard® developed by Toxin Alert, Canada, is another such system where antibodies are printed on polyethylene based plastic packaging material. The interaction between pathogen and the antibodies results in production of a fluorescent signal which indicates pathogenic attack.

2. Gas sensors:  Gas sensors are employed for the detection of gaseous analytes like oxygen, water vapor, carbon dioxide, ethylene, etc. inside the package. Other than oxygen, carbon dioxide and water vapor sensors, the most commonly used gas sensors are ethanol sensors, piezoelectric crystal sensors, semiconductor field effect transistors, and organic conducting polymers [5,6]. Papkowsky et al. described optical oxygen sensors which were based on the principle of quenching or luminescence upon gaseous analyte contact. The use of pH sensitive dyes like methyl red and curcumin for the detection of basic volatile amine released from rotten meat and fish have been reported.

3. Chemical sensors: Chemical selective coatings which can adsorb a particular chemical on the surface and detect its presence, composition, activity or concentration have been employed as chemical sensors. Since carbon based nanomaterials like graphene, carbon nanotubes and carbon nanofibers have excellent electrical and mechanical properties as well as exceptional surface area, they have been widely applied as chemical sensors. These nano-based sensors are used for the detection of chemical contaminants, pathogens and spoilage, as well as for the tracking of products or ingredients through the processing chain.

4. Electronic Nose: Instruments have been designed to identify and classify the mixture of aromas in an odor on a repeatable basis – a function similar to that of the mammalian olfactory system. The instrument is composed of an array of sensors, either chemical sensors or biosensors, which show partial specificity to each kind of odor. The statistical methods are used to recognize simple and complex odor and produce a unique response towards each one. Successful testing of electronic nose system has been carried out in response to the odor released by fresh yellowfin tuna, vacuum packed beef, fruits and vegetables, and broiler chicken.

Indicators
These are defined as the substances which can determine the presence or concentration of other substance, or the reaction between two or more substances, by giving characteristic optical changes like change in color.

1. Freshness Indicators: These indicators provide information about the product quality by determining the chemical changes resulting from the microbial growth within the product. The microbial growth metabolites react with the indicators integrated inside the food package to give visual information regarding the product quality. Colorimetric indicator labels by the trade name FreshTag® were launched by COX Technologies, USA, which indicated the production of volatile amines by the stored fish and seafood products by means of change in color. Yoshida et al. developed a chitosan based colorimetric pH indicator which was used to determine the presence of metabolites resulting from microbial growth such as n-butyrate, lactic acid, and acetic acid. Indicators for the determination of carbon dioxide produced during spoilage of meat products were also developed. Aqueous solutions of chitosan or whey protein isolates were used which changed transparency in response to the presence of carbon dioxide. However, a major disadvantage of colorimetric freshness indicators is that the color change can occur even in the absence of contaminants and significant deterioration of the product.

2. Time temperature indicators (TTIs): Temperature is the most important environmental factor which determines the spoilage kinetics of food products. The temperature over which the food product tends to spoil is known as threshold temperature. Time temperature indicators are responsible for indicating whether the ambient temperature of the stored food exceeded the threshold temperature and also the minimum time which the food product spent over threshold temperature. TTIs are labels that provide visual indication of the temperature abuse of temperature dependent perishable products, like frozen foods, during distribution and storage from the point of production to the point of consumption. There are three basic types of TTI available in market: critical temperature indicators, partial history indicators and full history indicators. Several commercial TTIs are available which can be diffusion, enzymatic or polymer based systems. 3M Company, USA has commercialized diffusion based TTIs by the trade name 3M Monitor Mark® and Freshness Check®. An example of commercial enzymatic TTI is VITSAB® which is based on color change resulting from a pH drop due to controlled enzymatic hydrolysis of a lipid substrate. Temperature dependent polymerization reaction form the basis of polymer system based TTI commercialized by Lifelines Technology Inc., USA, by the trade name Lifelines Freshness Monitor®. FreshCode (Varcode Ltd.) and Tempix (Tempix AB) are based on barcodes printed with fading inks that disappear due to temperature abuse.

3. Integrity Indicators: Leakage prevention is an important aspect to be considered throughout the production and distribution chain of packaged food. Integrity indicators function to ensure their integrity. Visual oxygen indicators are composed of redox sensitive dyes which change color with change in oxygen concentration in MAP foods. Mitsubishi Gas Chemical Company developed is oxygen indicator tablets by trade name Ageless Eye® which turn pink when oxygen concentration is less than 0.01% and turns blue when it goes beyond 0.5%. The presence of oxygen will be indicated in five minutes or less, while the change from blue to pink may take three hours or more.

Radiofrequency identification (RFID) systems
RFID is a tag or reader based automatic identifications system used for item identification and data accumulation without human intervention. RFID tags have some identification number stored in their databases and are able to accordingly act upon it by retrieving the information about that number from the database. RFID tags are categorized into active and passive. Active tags run on the power supplied by an in-built battery which makes the microchip circuitry functional and sends the signal to the reader. On the other hand, Passive RFID tags function depending on the power supplied by the reader. These tags consist of a coiled antenna which, when comes in contact with the radio waves emitted from the reader, produces a magnetic field and hence generates power to transmit information to the reader.

RFID tags provide the ability to identify, control and manage the goods through supply chain and have been successfully applied for this purpose. These are more advanced, reliable and efficient than the conventional barcode tags for food traceability. RFID tags for monitoring temperature, relative humidity, pressure, pH, and light exposure of the products are already available in the market which aid in enhancing food quality and safety.

Conclusions
The recent years have witnessed development in various intelligent packaging systems. These technologies, when integrated with the food packages can prove to be useful not only for the extension of food shelf life while improving quality, but also can provide useful information regarding the product. More intensive research is still required in the area of intelligent packaging material to develop more economical systems while offering convenience to the consumer.

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About the author

Ruchir Priyadarshi

Head - Biopolymers at Yash papers Ltd

Reserach Scholar at Indian Institute of Technology, Roorkee

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About the author

Anurag Kulshrehtha

Indian Institute of Technology, Roorkee

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About the author

Yuvraj Singh Negi

Indian Institute of Technology, Roorkee

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