By now everybody is familiar with the term "cold chain" – that system of continuous cooling that has to be followed during storage and transportation of certain products from the manufacturer to the wholesaler and retailer right up to the end consumer. It was only with the development of this system that global trade with perishable foodstuffs became possible, and adherence to the cold chain is the precondition for today’s situation where fresh fish can be offered in locations far from the coast. The number of products whose durability is linked to the maintenance of certain temperatures is increasing all the time. Not only are many foods highly sensitive in this respect but also a number of medical and chemical products. And not all of them have to be kept cool: some products require higher temperatures in order to preserve their quality. Instead of using the term "cold chain" it might be more appropriate to say "temperature-controlled transportation chain".
Even in the case of food, different temperatures are required for some product groups. Fresh meat, for example, should not exceed a temperature of 4° C and milk and dairy products must be kept below 8° C. Sometimes, even within an individual product group such as fruit or vegetables different conditions must be met. Whereas apples will keep fresh longest at 1-4° C mangoes prefer temperatures of around 12° C. On its journey from the producer to the consumer frozen fish should be kept at a constant icy temperature of minus 18 degrees Celsius. Already these few examples give an idea of how extensive and how complex logistical requirements are for the storage and transportation of foodstuffs. Specially equipped warehouses, loading units and vehicles are necessary for making sure that a food’s quality and durability, its nutritional value and flavour are not impaired or even spoiled so that it has to be destroyed. Today, refrigerated trucks often have multiple temperature controlled chambers, so that each product can be transported at exactly the temperature that is required by law.
Adhering to the prescribed temperatures alone is not enough, however, because EC Regulation 178/2002 also requires that the cold chain is continuously monitored and recorded at each relevant transfer point. A food’s "history" must be traceable at all the different stages of the process from the taking over of the goods by the carrier to their handing over to the recipient ("levels of responsibility"). Even though all parties generally cooperate closely within the supply chain this goal is not always achieved. All too often, the controls are limited to measuring the temperature of incoming and outgoing goods, i.e. at transfer points. What temperature conditions they may have been exposed to before or after, however, is too rarely considered... despite the fact that there are many ways of eliminating gaps in the documentation of the temperature profile along the supply chain.
Simple and affordable: time temperature indicators
Time temperature indicators (TTI) constitute an elegant and relatively inexpensive solution. They can be stuck onto the product like a label which changes colour if the desired temperature is exceeded. Manufacturers of TTIs make use of some very different principles.
The OnVu technology from BASF and Freshpoint is based on pigments that change their colour over the course of time and in case of temperature variations. The higher the temperature the faster the dye on the OnVu label fades. After the label has been fixed onto the product and activated using UV light the TTI first darkens and then becomes paler over the course of time and as the temperature rises. When the activated colour indicator is the same as the reference colour (which is also printed on the label) the product’s durability has expired.
"Monitor Mark" from the US-based manufacturer 3M is made of coloured wax with a precisely defined melting point, which is covered with an extremely thin protective layer. When the preset temperature is exceeded the wax melts, penetrates to the outside and freezes visibly on the surface. In contrast to numerous competitor products these labels only show that a certain temperature has been exceeded and ignore everything that happens below this critical limit. This simple aid can be very beneficial in the case of foods because where time-sensitive indicators are concerned consumers will unfortunately often prematurely consider a product’s shelf-life to have expired or assume that it is already spoiled.
The U.S. companies Temptime and Lifelines Technologies offer particularly good value indicators which work on the basis of polymers. Polymers are giant molecules that are made of repetitive blocks which, depending on time and the ambient temperature, link together and thereby become darker and darker. A disadvantage is, however, that this polymerization process starts immediately after manufacture, i.e. the TTI is activated directly after production and must therefore be stored at temperatures below minus 25° C until used.
The TTI label from Cox Technologies works with enzymes, whose activity brings about a colour change depending on the temperature. The label consists of two synthetic chambers, which must be broken to activate them. When broken, the liquids they contain – an oil and the enzyme – are mixed. With time the enzyme breaks down the molecules in the oil, whereby the initial green colour turns to yellow. The higher the temperature, the faster this change takes place. A disadvantage here, however, is the fact that temperatures above 40° C destroy the enzyme. Labels that have become unusable due to heat still show the freshness of a product even after it has long been spoiled.
The TTI label for optical temperature control from Topcryo is based on microorganisms that respond "biologically" to temperature fluctuations and their duration. There are 24 standard label variants for 4°, 8° and 10° C in various time increments from 12-192 h so that the user will be able to find a suitable label for a large number of products. The frozen labels are activated by thawing. When the set levels (temperature, duration) are exceeded their colour changes from green to red.
Developed at the University of Münster the "PolyTakSys" chip uses an electrically conductive polymer which after activation by simple moistening begins to move slowly through an organo-electronic layer which is visible by means of the slow growth of a coloured bar. The mechanism is based on a special form of diffusion which is temperature dependent and produces a sharp colour display. This offers various ways of indicating freshness, for example a traffic light (from green / fresh to red / expired), or the appearance of a warning indicating the expiry date of the product. The chip could also be linked to an RFID tag (radio frequency identification) that could transmit other information to a special reader electromagnetically. Foods in the refrigerated section that have passed their expiry date could trigger such a warning, for example.
RFID and cool data loggers for monitoring temperature online
Due to their technical characteristics RFIDs are well suited to documenting temperature profiles. The small chips can be placed directly on the product or even inside larger packaging units. Their capacity is usually sufficient to record the temperature along the entire transport path from manufacturer to retailer without any gaps. Because the information is transmitted electromagnetically, no physical or visual contact between the chip and the reading device is necessary for communicating with the reader, which reads the collected data and transmits them to a computer. This promising technology is, however, as still too interference-prone and not yet ready for practical application. In spite of reusable RFID chips this method is also still fairly expensive. It will be some time before routine use in practice becomes possible.
As an alternative, there is a similar but technically much more sophisticated technology that uses a temperature data recorder, or cool data logger. This small device can be fixed / stationary, for example in cold storage or processing areas, or used for monitoring temperature during transport. The devices trigger an alert visually, acoustically or by cell phone if the cooling technique is not working properly or if the specified temperature limits are exceeded. In the basic version, these systems consist of one or more temperature sensors and a data store. In refrigerated vehicles a printer will often be added to record the temperature profile during transportation so that it can be presented to the customer upon delivery of the goods. On-line transmission of the real-time operating conditions, temperature data and profile is possible. This means that the refrigerated vehicle remains linked to the internet portals of potential interested parties, such as the forwarding company, the supplier or the food manufacturer during transport. The transmitted data give them the chance to optimally integrate the vehicles into their fleet management, or to link the delivery process directly with other operational processes.
The choice of technical solutions available for monitoring temperature in the cold chain is almost overwhelming. In Germany, several well-known manufacturers offer individual products and complete solutions, e.g. Gesytec GmbH (Aachen), whose GesySense systems were developed for device monitoring and wireless data collection via radio. In the GesySense Logger version data are first stored internally and later handed over via radio for data collection. This makes this series particularly suitable for mobile applications and in locations with poor transmission / reception. Gesys systems are also used in the refrigerated sections of supermarkets and for monitoring freezers.
With the temperature data logger TRED30-7 from the LogTag series the company CiK Solutions GmbH (Karlsruhe) has developed a device that allows temperature analysis even without a PC. With the help of its clear display and the integrated statistics memory (up to 7,770 temperature readings can be stored, measuring range from -40 ° C to +99 ° C) daily alarm, minimum, maximum, and alarm duration of the last 30 days are immediately available in addition to the current temperature value. With the LogTag Interface and the intuitive LogTag Analyzer these data can be presented in graphical or tabular format on a PC, sent by e-mail, or uploaded to an FTP server.
The EBI 25 wireless sensor system from ebro Electronic GmbH (Ingolstadt) is among other things suited to the wireless monitoring of temperatures in freezers, refrigerators and refrigeration equipment. Excess temperatures are immediately reported from the central ebro portal via SMS or e-mail.
EU CHILL-ON project expands range of possibilities
Improving the quality, safety and transparency of chilled food in the supply chain was also the objective of the major applied research project CHILL-ON, which was funded by the EU with more than EUR10 million. In the context of this project the TRACE CHILL system was developed. This software combines various technologies to enable communication and data management throughout the entire cold chain. In addition to internet-based Supply Chain Management (SCM) it also includes a Decision Support System (DSS) which is based on predictions concerning the microbiology of individual products. CHILL-ON technologies were tested in practice during the project. The researchers accompanied the transport of cod from Iceland to France, fish from Chile on its way to Spain, and frozen bass within a cold chain in China. All the data that they collected on the journey from the catch to processing, transportation by plane, truck or ship to the final retailers were transmitted wirelessly to a server and evaluated.
What logistical requirements are needed to ensure the cold chain in the global fish business is demonstrated clearly using the example of the Metro Group. The Trade Group Metro Cash & Carry is the largest marketer of fresh fish in Europe and has developed a comprehensive control system which is mandatory for all parties involved in the supply chain. Fish farms and packing stations from Iceland to Thailand must fulfil strict hygiene requirements, and adherence is constantly monitored on site. In the modern logistics centre the fish’s quality and freshness is checked under a constant temperature of 2° C prior to order picking. The vehicles that take the fish from the storehouse to the markets are equipped with automatic temperature controls which continuously transmit the measured temperature values to headquarters via satellite. If a defined threshold is exceeded, those responsible redirect the truck immediately to the nearest cold storage depot.
Fresh fish is displayed in the Metro Cash & Carry markets beneath crushed ice, frozen fish in freezers. Sensors monitor the temperatures and transmit the data to a central server. The system will alert if limits are exceeded so that products can be removed from the shelves as a precaution. At the same time, the control system automatically informs the maintenance service so that a technician can repair the defective cooling system. For its professional customers Metro Cash & Carry provides cool boxes in their wholesale markets with special ice packs for fish purchases. There are automatic dispensers at which used ice packs can be replaced with new ones. At the wholesale market in Düsseldorf Metro is testing a special “chilled parking area” behind the checkout area where major customers can leave their full shopping carts while they fill up the next one.