In the beginning were the containment packages, and then came the natural barriers such as leaves and skins, followed by man-made synthetics such as pottery and glass, and manufactured cellulose derivatives such as paper and paperboard. All were essentially passive, that is, materials that kept out moisture, dust, dirt, and pesky creatures. As regular readers well know, the active versions that actually respond to internal or external environmental cues began some half-century ago. Simultaneously, intelligent packaging that senses and signals the surroundings was born, achieving some commercial success and the promise of ultimately controlling the entire universe of food and beverage packaging.
Originally dubbed “smart packaging,” both active and intelligent packaging have been confounded in media reports, peer review articles, and trade journals. The confusion arises from a paucity of intelligent definition and interpretation of package material and structural functionalities. Active packaging responds by sensing and changing some functional aspect. Intelligent packaging responds by sensing and signaling. It is axiomatic among technologists and engineers that when a variable may be sensed and/or measured, it is then possible to convert the signal into some controlling function. Therefore, any intelligence signal is a harbinger of potential eventual active packaging, a positive prediction for commercial success, based, of course, on the current activity in the marketplace.
Contemporary examples of intelligent packaging include temperature indicators/sensors, applied to signal a maximum- or minimum-temperature event; time temperature integrators (TTIs), which are among the more commercially applied of intelligent packaging devices; moisture (i.e., relative humidity) sensors; and gas sensors to indicate excess or shortage of requisite reactant. Gas concentration sensors include those which demonstrate quantitative response to active respiratory gases such as carbon dioxide and oxygen. Biosensors measure biological activities such as, for example, microbiological growth in spoilage indicators. Freshness indicators are claimed to respond to ripeness signals. Package integrity indicators, which try to ensure against contamination, and radio frequency identification (RFID)—perhaps the most promising of all intelligent packaging concepts but still with limitations—also make the list of intelligent packaging options.
High- and low-temperature indicators have been applied in food technology for decades: high temperature to indicate that an entity has reached a specific temperature as in thermal processing—not a packaging application but certainly an adjunct to ensure that a sterilization value has been reached—or in roasting as in a pop-up indicator for a Thanksgiving turkey. And then there are the microwave oven bell signals when the internal steam pressure pushes a package to the limit—equivalent to a maximum temperature. (In contrast to this are self-opening microwave-steamable packages, which are an example of active packaging.)
Low-temperature sensors are sometimes affixed to ice cream or frozen food packages to change color and thus indicate that the frozen temperature has been exceeded. Target temperature indicators are typified by those that offer a signal when a specified temperature has been attained; Coors Light beer packaging with labels that indicate when the beer is at an optimal temperature for drinking is a good example of packaging that features a target temperature indicator. Temperature indicators are signalers of a specific event and so are valuable, but not nearly as much as TTIs.
Time Temperature Integrators
The direct relationship due to Arrhenius responses between product temperature, time, and quality retention has been demonstrated and for many years has been affiliated with on-package sensors. Whether the on-package response is identical to the actual product response to increasing temperature has been debatable, and so commercial acceptance has been relatively limited. Major differences exist between the temperature at the surface of a package and temperature within the product, where the microbiological growth and biochemical activity occurs.
TTIs operate on several different principles including mechanical change, internal enzymatic response, and biochemical activity, all of which, of course, are subject to logarithmic activity increase as a function of temperature increase. By incorporating a visual or analogous reaction that parallels the total thermal input, an integral of time temperature can be signaled.
Although many different TTIs have been developed, patented, and reported throughout their 50 years of existence, only a few have survived the rigors of technical efficacy and satisfying commercial need.
Vitsab’s Checkpoint®, TTI packaging from Vitsab International, Malmo, Sweden, is enzymatic, hydrolyzing a substrate and causing a pH decrease that is signaled by a color change. Fresh-Check® indicators from Temptime, Morris Plains, N.J., are based on a principle of solid state polymerization; the indicator’s color changes with temperature increases from a base temperature, usually the frozen state, although there is some flexibility for the starting temperature. Some commercial applications of this system have been used on the surfaces of chilled prepared food packages and for pharmaceuticals whose distribution temperatures must be controlled. The MonitorMark® TTI from 3M, St. Paul, Minn., is based on the diffusion of polymeric materials in a porous substrate. The three preceding products, ranging in unit price from $0.05 to $0.20, have been in commerce for many years, but none has achieved sufficient commercial application to be considered a success because of cost and precision issues.
Recently, two interesting entries have been making inroads—Ciba Specialty Chemicals’ OnVu™ in which photosensitive crystals are excited and colored as a result of temperature increase and Avery Dennison’s TT Sensor ™, based on a reaction in which a polar compound that changes concentration with temperature rise diffuses between two polymer layers signaling with color change.
Key to the effective application of TTIs is accuracy and avoidance of errors such as first order—signals that the product is spoiled when it is not and the reverse—no indication when the product is actually spoiled. These potential errors and the current economics of the available systems are being obviated in commerce by conservative distribution of chilled products in which expiration dates are short and products are discarded before their actual shelf life has ended.
Semantics appear to further confound this category—biosensors respond to some biological reaction such as enzymatic activity and consequently trigger a signal. Shelf-life sensing and even prediction is often attempting to apply direct measures of variables such as microbiological growth in the product. Spoilage signals derive from biochemical activity such as generation of volatile chemicals. Ripening signals may be produced for climacteric fresh fruit, which may be harvested before full flavor and masticatory properties have been achieved. Which of this diversity of definitions constitutes freshness and which is spoilage or incipient spoilage?
RipeSense freshness or ripeness indicator labels, developed by Auckland, New Zealand–based Jenkins Group, reportedly sense the aromatics and other favorable volatiles such as ethylene that are emitted from ripening fruit. The device indicates ripeness by a change in label color and is particularly useful for fruit that does not change color as it ripens such as pears, melons, and avocados.
Biosensors are one of several terms applied to spoilage sensors that function by detecting changes in volatiles emanating from contained food that indicate metabolites from enzymes or from microbiological growth. Biosensors are also devices that contain antibodies that respond to antigens in growing microbiological cells that make actual contact with the antigen and thus colorimetrically signal the activity. According to the advocates of these devices, the antibody/antigen reaction can be tuned to specific spoilage microorganisms or even to specific pathogenic microorganisms.
Among the volatiles that can be and have been sensed from microbiological growth on protein foods such as fish have been amines such as trimethylamine. The volatiles may be detected chemically and so colorimetrically signal spoilage. The data may be transmitted to electronic transducers such as radio frequency indicators for reading or with a battery to actively communicate the presence of the spoilage to a distributor, retailer, or consumer. In addition to RFID, other communications transducers include chipless sensors with internal paper batteries and smart active level (SAL) with chip. In more advanced transducers, the actual mass of colonies of microorganisms is microweighed to indicate changes in weight that suggest growth.
Predicting the Future
Those who have been wandering the telecommunications corridors have been exposed to a wide range of show stoppers. About 20% of American shoppers already are using self-checkout at retail. The next step is direct communication with your financial institution to debit your account—either as you remove a product from the shelf and scan it though your own personalized hand-held device or via the fully automatic scanner that reads every code in your shopping cart and debits your account for the entire inventory.
On-package codes that interact with your microwave oven so that it heats the product based on algorithms that account for oven age, position in the oven, and size, geometry, heterogeneity, and temperature of the food have been used experimentally. Not quite in operation yet are sensors that read the dates on your in-home food inventory and inform you of the impending expiration date for the product that’s molding in the back of the refrigerator.
And somewhere in the future we can expect packaging that helps track an individual’s fat and calorie consumption and is capable of communicating that information to a healthcare provider. Clearly, we are racing toward a time when the package delivers not only protection and convenience but also functions as a repository of valuable information.
by Aaron L. Brody, Ph.D.,
President and CEO, Packaging/Brody Inc., Duluth, Ga., and Adjunct Professor, University of Georgia