Innovative packaging for UHT

In the field of food packaging it is particularly important to guarantee correct preservation of the product. This means that the food product, during its shelf life and prior to consumption, must retain its nutritional properties and organoleptic characteristics.

Also, consumers prefer practical packaging. When considering liquid food, for example juice and milk, consumers favour a light pack with a large opening so that the product pours easily without splashing. Methods that enable the package to be re-sealed when a product is not to be consumed at once are also preferred.

In addition, marketing people have their own requirements for packaging:

• A novel shape for their products
• The possibility to change the shape at any time, or for different products
• A great number of colours for the labels with high quality printing, in order to achieve a significant impact on consumers

Finally, the transformer must implement packaging machines with an increasingly high level of automation; better value and productivity; be easy to manage and, of course, at a competitive level of investment.

Problems to be solved

In the case of sterilised dairy products, a major problem with the packaging is the sensitivity of the product to external influences such as bacterial contamination or oxidation due to light or oxygen. Therefore, in order to give the product the desired length of storage time without refrigeration, it must be packed in sterile containers, in a sterile environment and be protected against UV radiation and visible light and from oxygen.

Light radiation can generate two effects:

Photochemical effect

In the sun light violet and ultra-violet radiations, i.e. those having the lower wavelength and thus higher energy, are more efficient and have enough energy to activate some molecules of the food product and produce undesirable chemical reactions.

Ionising effect

Ionisation is the formation of ions through subtraction of electrons from the atoms of the molecules that are struck by the light radiation as target. The ionising radiations perform an action on the food product similar to that of the ultra-violet rays; the extension of the action depends on the radiation energy.

The above degradation phenomena are linked to the amount of light (light intensity and exposure duration) to which the product is exposed; the wavelength of the radiation the exposure temperature and the absorbing power for the considered radiation of the molecules of the product itself (different products have different sensitivities).

In milk, in particular, greater alterations are found for exposures to radiations having a wavelength lower than 550nm.

Additionally, as mentioned before, products destined for a long shelf life, for example UHT milk, can be subjected to oxidation reactions which, although developing slower than the degradation induced by light, produce further undesirable alterations to the taste.

Solutions

The established technique to pack a food product of this type provides several solutions.

The most usual solution is to use polycoupled containers, i.e. containers with several layers (also of different materials), of which at least one is opaque to light. Familiar examples are containers wherein an aluminium sheet is interposed between a plastic film and a cardboard layer.

Bottle containers are another possibility. The most popular method for protecting liquid food from light radiation are bottles produced by co-extruding and blowing high density polyethylene (HDPE), or bottles of polyethylene terephthalate (PET) obtained from multilayer injection molded pre-forms. In both cases, between the internal and external surface of the container, an opaque layer is included to create a barrier protecting the product from light.

Other possibilities are to use bottles made of PET obtained from injection molded pre-forms with a large quantity of additives or colour added, e.g. titanium dioxide, or to create a barrier to the oxygen or light with a coating process on the blown bottle.

The process used to obtain bottles in HDPE is extrusion of polymer pellets. This process is generally characterised by low productivity and requires very specific skills. This means that a food manufacturer risks becoming a transformer of plastic materials. Therefore, they must use ready formed bottles. This can be achieved by ‘door to door’ agreement with the supplier, otherwise it causes severe impact on the transport costs (high volume needed for a low number of containers) and to the logistical organisation of the process in which the bottles have to be used.

To use a great amount of additives or colour into PET pre-form, or to realise a multilayer pre-form to obtain containers in PET, can render the blowing process of the pre-forms less reliable and can have a negative impact on the mechanical properties of the bottles, including risking product quality. In addition, it is important for the supplier and the client to implement the necessary systems to detect in line that the inside layer of every pre-form obtained by a multilayer injection molded process meets the required specifications. Lastly, the cost of multilayer pre-forms is still high for ‘poor’ products. Different coating processes are currently under development.

Moreover, the use of PET in the manufacturing of containers for food products produces important advantages.

Importantly, PET has a better barrier to oxygen than HDPE. The transformer can begin the packaging process blowing pre-forms. This process creates high productivity and does not require any particular skill as manufacturing such a product is within anyone’s means. Of course, to transport pre-forms brings significant advantages compared to transporting formed bottles.

The container can form a large mouth-piece and the precise definition of the bottle thread (realised by injection molded process and not by extruding and blowing) provides a hermetic seal owing to a perfect couple with the cap. In this way it is possible to avoid using an aluminium foil over the opening to achieve hermetical closure after the filling process and the closure remains hermetical after each subsequent opening.

PET bottle covered by a PET metalised full-sleeve

Taking this into account, Centrale del Latte di Brescia (Brescia dairy) has recently adopted, as a new pack for UHT milk, PET bottles produced by blowing PET pre-forms with a UV barrier. A PET film sleeve with a metalised side is thermoshrunk onto the bottles after filling.

This complete label, printed with the latest technology in a variety of colours, enables the product to be displayed to optimum visual advantage but, most importantly, improves the barrier properties against light radiation.

The metalisation process consists of depositing a reduced amount of aluminium powder on one side of the PET film in a vacuum environment in order to make the film opaque. The thickness of this powder is less than 300 Å (a normal aluminium foil, by contrast, is thicker by at least two powers of ten).

Such a limited thickness is negligible in comparison with the plastic material, so the thermoshrinking film can be considered as a mono-material film.

With regard to environmental benefits, this means that the PET film can also be recycled along with the bottle and, moreover, the energy consumed during the metalisation process is largely reduced.

If the fullsleeve label also covers the cap it ensures complete closure, in addition to the safety ring on the cap.

Research

Because UHT milk in PET constitutes a first application and a very important change, Brescia dairy worked with the Netfood Centre in Torino and the PDC Srl in Milan on a packaging and shelf life study, respectively. Exhaustive checks were run on the barrier and other properties of PET and on possible interactions between milk and PET, before deciding to proceed with the project and the investment.

The same milk, filled once in the traditional pack and once in the PET bottle covered with the metalised sleeve, was compared over a period of four months’ storage time at ambient temperature with exposure to sunlight. In parallel, a warm test at 40°C up to 45 days was conducted. A great number of parameters were measured at regular intervals; the aromatic substances were analysed and sensory taste tests were conducted. The following are examples of the analysis that has been made:

• Oxygen permeability (OTR)
• Light transmission (Haze)
• Appearance analysis
• Shelf-life test
• Chemical and microbiological analysis (Vitamins, Casein, Protein, Ammonia, etc.)
• Interaction pack/product
• Sensorial analysis (rif. UNI 10192)
• Acetaldeide migration to milk

The warm test and those conducted at ambient temperature produced the same persuasive results: There were no significant differences between the UHT milk in the traditional package and that filled in the PET bottle in chemical and microbiological terms; there was no interaction between PET and milk; the blind tastings were favourable and the labelled PET bottles scored well in terms of oxygen barrier and pressure stability. The light-permeability was far below the maximum value that the Milk International Federation requires for packaging milk with a long shelf life. In summary, all the pre-trials indicated that PET was the right option.

Filling technology

Once verified with laboratory tests that UHT milk in PET bottles covered with a metalised sleeve could last the required shelf life, selection of the appropriate aseptic filling process for PET bottles was needed.

Recent improvements in aseptic filling – in particular, the introduction of the Isolator Technology – made selection much easier. This technology enables a special rotary filler machine with a ‘white chamber’ inside to be housed in a ‘standard’ environment. Prior to this, the practice was to house a standard filler machine in a ‘white room’. The difficulties involved in attempting to bring an entire room to aseptic conditions and guarantee such conditions throughout the production process are obvious. Operators must enter the room following special procedures and wearing aseptic clothes, and if a maintenance action is needed the risk of losing the aseptic condition is greatly increased. Isolator technology, however, overcomes these limits.

The final decision was made to use the Krones PET-Asept system with IT. The compact size of this filler (only one hundred square metres for the aseptic filler rated at 12,000 containers an hour) features a BLOC comprising isolator, rinser and filler. The isolator enables the containers to be disinfected both inside and outside over a defined period with peracetic acid and steam, before being rinsed with sterile water in the downstream double-channel rinser.

At the Volumetic 50-valve filler, the volume of the product being filled is precisely specified using an inductive flowmeter. The electronic control system provides for pre-programmed filling steps reproducible at will.

The filling operation is performed without contact between the valve and bottle – enabled by the valve in neck-handling mode – without lift cylinders. In this way it is possible to optimise hygiene levels and maximise the filling angle. In fact, as soon as the bottle has been transferred to the filler, the electro-pneumatic control system opens the filling valve, thus triggering the filling operation. The closures are disinfected separately in a bath of peracetic acid and arrive directly, in aseptic condition, to the capper inside the block.

The filler design features a greatly reduced number of functional parts inside the machine allowing a separation from the sterile environment of the pneumatic, electrical and motorised components. In this way, risk of product contamination is minimised and also cleaning is made easier through improved access to the critical parts of the system. A differential pressure chamber allows regulations of the airflow, ensuring the sterility of the process, as well as preventing the peracetic acid vapour to escape.

Automatic CIP cups also support the cleaning function.

The installation further included the entire peripherals, with the chemical store, the dosing systems and the sterile water supply.

Final pack and results

The installation and validation of the line (made by a third party laboratory) took around five months. Following this, after a short market test, the industrial production of UHT milk in PET bottles began in September 2004 with the three variants of milk: full-fat, reduced-fat and skimmed milk. These products are packed in 6, 8 and 12 bottle packs, and in 1 litre and 1/2 litre bottles.

The bottle has a square bottom that is the same size for both 1 litre and 1/2 litre sizes – so that the set up is negligible. The shape of the bottle means that no protection is needed for the neck and cap, and packaging of a thermoshrinking film and carton tray is all that’s required.

In summary, the UHT milk in PET project has been very successful for Brescia Dairy. As with the fresh milk, PET bottles containing UHT milk have been offered on the shelves at the same price of the previous pack. The production volumes have increased by more than 50 per cent in just a few months and new markets are now served.