Aseptic packaging has been with us now for more than 45 years, and for much of that time the benefits of the technology were only really available to dairy and juice processors. But that didn’t stop the Institute of Food Technologists voting it “the most significant food science innovation of the last 50 years”, back in 1989. Since then, steady development has opened up more and more applications for aseptic packaging technology and a diverse group of manufacturers is now exploiting its advantages. The latest innovations seem set to take the technology still further into new markets. Throw in some strong green credentials and a rosy future for aseptic packaging looks assured.

The term aseptic packaging essentially means filling pre-sterilised packaging with pre-sterilised product in a hygienic environment. That doesn’t sound terribly exciting at first hearing, but turning that concept into reliable technology has had profound consequences for the food and drinks industries. Aseptic packaging offers the possibility of presenting highly perishable food in a convenient form that remains in good condition for at least six months without refrigeration. Of course, traditional canning technology can do that too, and using a much simpler procedure. So what is so special about aseptic packaging?

A revolutionary technology is born
In conventional canning, the non-sterile product is filled into the non-sterile can, which is then sealed. The whole thing is sterilised in a retort, producing a robust shelf-stable package of ready-to-eat product with a shelf life of months, or even years. The problem is, that in order to completely sterilise the can contents, a lot of heat must be applied over a comparatively long period. Low-acid products that could support the growth of dangerous Clostridium botulinum bacteria must be heated to an equivalent of 121oC for at least three minutes at the slowest heating point, which is usually the centre of the can. This means that the material closer to the can wall receives far more heat than it needs before the centre of the can heats up sufficiently. The result is that, even in sophisticated modern retorts, delicate products become so overheated that their quality can become unacceptable, as anyone who has tasted whole milk sterilised in a conventional retort will probably confirm.

The key feature of aseptic packaging is that the sterilisation of the product and the packaging are separate processes. This means that the sterilisation of the product can be optimised to protect its taste, colour and nutritional content, usually by heating rapidly in a heat exchanger, or by direct steam injection, to high temperatures (140 - 150 oC) for just a few seconds. This produces a safe heat process, but has a much less detrimental effect on the quality of the product. This gives aseptic packaging the potential to make a whole new range of foods available in a shelf-stable and convenient format.

That was exactly what Sweden’s Tetra Pak Company recognised when it launched the world’s first aseptic food package in 1961. That first pack was the Classic Aseptic carton, developed from the company’s clever tetrahedron-shaped Tetra Classic milk carton, a design intended to minimise packaging material and maximise hygiene. The Classic Aseptic looked outwardly identical to the older carton, but had one crucial addition. A layer of aluminium was built into the walls of the carton to provide an effective oxygen barrier and adequate protection for a much longer shelf life. Initially, the new packaging system was used only for milk. A high temperature short time (HTST) sterilisation process was applied to minimise the heat damage to the milk and the cartons were formed, sterilised, filled and sealed in a closed, ‘clean room’ environment. The result was an acceptable quality product at a reasonable cost that would keep for months at ambient temperature, and the start of a dramatic change in milk distribution across Europe. So successful was the idea that the Classic Aseptic system is still in use today, especially in developing countries, where the cold chain infrastructure for supplying chilled products to distant locations is not yet in place.

The next big step came in 1968 with the launch of the box-shaped Tetra Brik Aseptic carton that has since become so familiar. Constructed from a laminate of paper, polyethylene and aluminium foil layers supplied in a flat roll, the Tetra Brik carton is formed by shaping the laminate into a continuous tube after sterilising it with hot hydrogen peroxide. The sterilised product is then filled into the tube and each carton is shaped and sealed below the product level and then separated from the tube leaving a filled carton with no headspace. The convenience and cost efficiency of the Tetra Brik has seen it used in huge quantities all over the world for aseptic packing of milk and drinks, and it is still the mainstay of the aseptic packaging sector.

Now available in a multitude of shapes and sizes and with a range of different openings, the Tetra Brik has been refined and improved over the years – the very latest version is the Tetra Gemina Aseptic gable-top shaped package – but the original concept remains the same.

Limitations to be overcome
Any packaging system that has remained largely unchanged for nearly 40 years clearly has a lot going for it, but the Tetra Brik does have its limitations. Firstly, although the cost of producing an individual carton is very low, the capital investment needed to install the equipment is substantial for a smaller business, when compared with a simple hot-fill system, for example. The level of technical expertise needed to run an aseptic packaging system successfully is also quite high, although long experience of the process has helped with this. Consumer attitudes to aseptic cartons have also held back development in some markets, especially North America and the UK, though less so elsewhere in Europe. Milk and drinks in aseptic cartons have been regarded as less attractive than their ‘fresh’ chilled equivalents. But the major limitation is the fact that the Tetra Brik carton is formed by effectively ‘sealing through’ the product. This means that it is only really suitable for homogeneous liquids that are not viscous and do not contain particulates, because any particles trapped in the seal could cause the carton to leak and the contents to become contaminated. This is not an issue for milk and drinks, but it limits application to other liquid foods like soups and sauces, which could otherwise benefit from being packed in aseptic cartons.

Paradoxically, one possible solution to this problem is a canning process. Various aseptic canning systems have been developed, most notably the Dole process from the USA. This has been around for many years – almost as long as the Tetra Pak carton – and it can handle viscous and particulate products. Despite this, aseptic canning processes have had fairly limited success in Europe. An alternative approach first appeared in the 1980s, when Swiss-based SIG launched its combibloc aseptic packaging system. First used in 1985 for soups containing small particles, the combibloc carton looks very similar to the Tetra Brik but comes as a sleeve already cut to shape and with a longitudinal seal already in place. This means that each carton can be formed and sterilised and then individually filled, rather like filling a can or jar, and then ultrasonically sealed at the top above the product without the risk of trapping material in the seal. Today’s combibloc systems can fill quite viscous liquids containing particles up to 15 mm in diameter, with up to 50% of the product being made up of particles. It is also possible to fill two product components independently into the same carton using a ‘two phase’ filling system that can handle separate liquid, viscous, or ‘chunky’ components. This allows different heat processes to be applied to each component, thus optimising quality and safety, and ensures that each carton contains the correct proportions of liquid and solid ingredients. The combibloc has been widely used for soups and sauces for 20 years, but SIG says it is increasingly being employed to pack desserts and even baby foods.

Despite its versatility, the combibloc system doesn’t eliminate all the drawbacks of aseptic packaging. Capital cost and engineering complexity are still factors, and although the system can fill particles, these still have to be heat processed effectively. The simple plate heat exchangers used to sterilise liquids cannot deal with high viscosity products or particles, and designing a safe process for products containing large chunks of meat and vegetables using tubular heat exchangers is problematic because of factors like flow characteristics and residence times of particles in the heat exchanger. Product ‘burn on’ to the heated surfaces can also be a problem, as can serious overheating of the liquid phase. A number of technological solutions to this problem have been developed, including various heat exchanger designs that ensure proper mixing and desirable ‘turbulent flow’ through the heating tube. An example is the Contherm scraped surface heat exchanger from Alfa Laval. This is specially designed to handle thick liquids and products containing particles and is fitted with rotating scraping blades that continuously remove product from the heated surfaces and produce enough turbulence to heat the product evenly, while minimising burn-on.

An example of an alternative to the heat exchanger is the RotaTherm aseptic system developed by Australian company Gold Peg International. Based on the company’s own RotaTherm continuous direct steam injection cookers, the system claims to be able to handle particulates up to 25 mm in size, as well as viscous products, and to eliminate product burn-on. It is able to deliver an aseptically processed product direct to a filling system and is said to be easy to operate and clean. There has also been a lot of interest in ohmic heating as an alternative approach to processing viscous and particulate products. This technology has been around for some 20 years and relies on the heating effect of passing an electrical current through a conducting material via an electrode. Ohmic heating has the major advantage that liquid and particles of up to 25 mm are heated almost simultaneously, so that the liquid phase is less overcooked. Also, there are no hot surfaces and so no burn-on. The downside is that the product being heated has to have sufficient electrical conductivity for the process to work and the electrodes can release electrolytic decomposition products into the food product. Commercial ohmic heating plants have been in operation in Europe, Japan and the USA since the early 1990s, but the process hasn’t been widely taken up. This may change following more recent developments by Wild-Indag in Germany, combining ohmic heating with microwave and radio frequency heating to achieve much faster heating of particulates.

Development helps aseptics gain new ground
A major recent development in the drinks sector is the rapid growth of cold aseptic filling into PET bottles. This is being driven by the rising demand for healthier, non-carbonated, preservative-free drinks that need to be processed to remove microbial contamination, but which are heat sensitive and may be damaged by traditional hot-filling processes. One solution for supplying this expanding market is to pasteurise or sterilise the product in a heat exchanger, cool it rapidly and then fill aseptically into sterilised bottles. This provides a safe, high quality product that is shelf-stable for months, but does not contain any preservatives. A number of equipment manufacturers have products designed for this application, including Tetra Pak itself, SIG Combibloc, KHS and Krones. For example, the German company Krones offers the PET-Asept process, which is a fully aseptic bottle sterilising, rinsing, filling and capping system, all isolated in a sterile area that can be integrated into existing bottling lines. These cold aseptic systems are also becoming easier to operate and more compact, with smaller sterile isolators being fitted around the filling section of a line, so that there is no requirement for the aseptic filling to be housed in a separate cleanroom area. It seems likely that aseptically filled PET bottles will provide stiff competition for the ubiquitous Tetra Brik in years to come. The aseptic can is also making a comeback in the health drink market. Ball Packaging produces a 0.25 litre two-piece tinplate steel aseptic can equipped with a tear-off tab and designed for use with health drinks, milk, coffee and yoghurt products. It is now even possible to add probiotic microbial cultures to aseptically packaged drinks. Tetra Pak’s stand-alone aseptic FlexDos system is placed between the processing unit and the filler and allows manufacturers to add very heat sensitive ingredients, such as probiotics, Omega 3, natural colours and vitamins to the product after the heat process has been applied.

A very recently launched product in the UK may give an indication of where aseptic packaging will go in the future. Veetee Rice launched its Dine-In range of cooked “restaurant quality” rice in June this year, using natural white rice rather than the easy-cook varieties usually used in convenience rice products. The range of four products, including Thai Jasmine and Pilau varieties, is sold in shelf-stable plastic trays sealed with a clear plastic film that can be reheated in a microwave in two minutes. The quality is claimed to be far superior to the competition and the product has a long shelf life at ambient temperature. The secret is reported to be a patented aseptic packaging process originally developed in Japan and adapted by Veetee. If it is possible to apply this technology to produce other shelf-stable meal components and complete meals, they might at last be able to compete with chilled ready meals in terms of quality.

Aseptic packaging clearly has a lot of advantages and the potential for further growth in new markets. But that is not all, because the technology can also lay claim to some pretty impressive environmental benefits. For example, SIG Combibloc points out that aseptic cartons minimise the amount of packaging needed to contain a set volume of product. This means that a truck loaded with cartons is carrying a load consisting of only 5% packaging and 95% product. This is obviously an efficient use of the transport and results in relatively fewer carbon emissions. At a time when excessive packaging is coming under pressure, not just from environmental campaigners, but also from consumers and governments, this is a major green advantage. Aseptic cartons are also completely recyclable and systems to recover the paper, plastic and aluminium components are already in place in many countries. Finally, there is a considerable energy benefit in the fact that aseptically packaged products do not need to be refrigerated and will keep for many months. This does away with the need for an energy-hungry cold chain and also minimises waste. Put all this together and it is hard to imagine a greener form of packaging.