Dairy Engineering

Module 6. Sterilizing & packing equipment

Lesson 24
ASEPTIC PACKAGING AND EQUIPMENT

24.1 Introduction

The product from any UHT sterilizer has to be filled into individual containers for storage, distribution and sale, for commercial use. This process of filling should not lower the sterility level. Satisfactory aseptic filling is an essential part of any system to give a commercially sterile product from a UHT process. In fact most of the spoilage is due to contamination after UHT processing.

Scientists suggest that an overall spoilage level after incubation of all filled containers of 1 in 5000 should be attainable in commercial practice. It is observed that most of the contaminations are due to failure in aseptic filling system. It is found to be more difficult to design and operate a good and reliable aseptic filling system than a good and reliable UHT sterilizer.

An aseptic filling system must meet a series of requirements, each of which must be satisfied individually before the whole system can be considered satisfactory. These are:

1. The container and method of sealing must be suitable for aseptic filling, and must not allow the passage of organisms into the sealed container during storage and distribution.

2. The container, or that part of which comes into contact with the product, must be sterilized after it is formed and before being filled.

3. No contamination should occur while container is being filled.

4. If any sealing is needed, it must be sterilized immediately before it is applied.

5. The closure must be applied and sealed within the sterile zone.

24.2 Methods of Container Sterilization

The sterilization performance required for different containers depends on the probable number of organisms existing in the container before sterilization. A sterilizing process giving about 3 to 4 decimal reductions of resistant spores is adequate to give a single survivor in about 5000 containers of 0.5 to 1 litre capacity.

Different methods of sterilizing agents are there, like saturated steam, Dry heat, Hydrogen Peroxide, other chemical sterilants, UV irradiation, and combination of above.

24.2.1 Saturated steam

It is one of the most reliable sterilant with minimum of objections. It has however to be under pressure to be at high temperatures that are required here. A pressure chamber must therefore be used, with the container or container material to be sterilized entering and leaving the chamber through suitable valves. Air, if present in a steam pressure chamber interferes with the transfer of heat from the steam to the container surface. Hence, any air entering the containers must be removed and not allowed to accumulate. Condensation of the steam during heating of the container surface produces condensate which may remain in the container and dilute the product.

In addition to cans being sterilized, filling systems using steam have also been designed in which polystyrene thermoformed cups and polypropylene preformed cups are sterilized by steam under pressure.

24.2.2 Dry heat

Dry heat can be applied either in the form of a hot gas or as hot non-aqueous liquid such as glycol. Dry heat can reach high temperatures at atmospheric pressure, which simplifies the mechanical design of sterilizer system. However, dry heat desiccates microorganisms and makes them more resistant. Much higher temperatures are therefore needed for thermal inactivation by dry heat than for inactivation in similar time by wet heat. Temperatures of the order of 200°C may be needed if the sterilization time is not to be very long. The high temperatures needed for satisfactory sterilization by dry heat mean that containers which are heat sensitive cannot be used.

24.2.3 Hydrogen peroxide

It is well known that Hydrogen peroxide is lethal to microorganisms, including heat-resistant spores. Many manufacturers are using a combination of hydrogen peroxide and heat for the sterilization of the surface of the container material. Sterilizing performance increases with both peroxide concentration and temperature. Some aseptic filling systems use a bath of hot peroxide to sterilize the container or the container material. However, the majority of the systems apply the peroxide solution (usually at 30-35% concentration) to the surface of the container material by dipping or by a finely dispersed spray. The surface is then subjected to heat, either from radiant heating elements or from hot air jets. The peroxide solution on the surface is therefore heated and evaporated, to sterilize the surface at the same time and remove the peroxide solution to prevent it contaminating the product after filling. Hydrogen peroxide is a poison, and in some countries there are strict limits to the concentrations which may remain in the filled container, and which are allowed in the atmosphere surrounding aseptic filler in operation and which could be inhaled by operators.

24.2.4 Ultraviolet (UV) irradiation

The optimum wavelength for UV radiation to be effective is about 250 nm. The effectiveness falls rapidly away at shorter or longer wavelengths than the above. The effect seems to arise from direct absorption of the radiation by DNA of the bacterial cell. However, UV irradiation is not as effective as Hydrogen Peroxide.

24.3 Types of Aseptic Filling System

The principal controlling factor in design of aseptic filling system is the type of container which is to be filled, with subsidiary factors being container material and whether the container is preformed one or whether it is formed during the aseptic filing process.

24.3.1 Cans

Though they are the earliest aseptic fillers to be used, presently they are relatively expensive, with cheaper alternatives being available. It is especially so, for a low-cost product such as milk. They are also bulky to transport and store before use.

The cans may be of tinplate or drawn aluminum, with the solder being of higher melting point than normal to withstand the can sterilization temperatures. The cans are sterilized in the tunnel at atmospheric pressure by steam at 200 to 220 °C, super heated with gas flames. The sterilizing time is about 40 seconds. The can lids are sterilized, by superheated steam, in a separate unit. When the cans have passed through the sterilizing tunnel, they continue through the filling chamber where they are filled, often using a simple in-line filler of the slit or multi-port type. The filling quantity is determined by the product flow rate and speed of travel of the can under the filler.

24.3.2 Cartons

These are commonly used for aseptic filling systems for milk, cream, soya-based milks and fruit juices. The filling systems are mostly in which the carton is formed within the filler from a continuous reel of material, though cartons are also supplied as preformed blanks, folded flat, and assembled into cartons in the filler. The carton material is a laminate of paperboard coated internally and externally with polyethylene, which makes the carton impermeable to liquids and allows thermal sealing of both the internal and external surfaces. An oxygen barrier is provided by a thin aluminum foil incorporated in the laminate. The structure of paperboard laminate is shown in 24.1:

Fig. 24.1 Cartons (Source: Tetrapak manual)

In the fillers, the container material moves continuously downward in a strip, and is formed by shaping rolls into a cylinder. An overlapping longitudinal seal is formed by heat sealing. An additional thin polythene strip is heat bonded along the inside of the longitudinal seam at the same time. As the continuous cylinder moves downwards, a series of transverse heat seals are made by jaws which move down at the same speed as the cylinder. These seals have the effect of closing the bottom of the cylinder, so that it can be filled with product. This is done through a filling tube from above, and a float-operated filling valve at the outlet of the tube maintains the liquid level above the sealing level (as shown in diagrams 24.2).

Fig. 24.2 Forming cartons from continuous strips

Different forms of seals were evolved, the earlier ones giving shape of tetrahedral, while the later ones are molding into rectangular blocks.

In some of the designs a headspace is provided in the carton by arranging a float-controlled filling valve to give a product level below the top transverse seal. The filled volume is then determined by the carton dimensions and the position of the valve. The cartons are sterilized with hydrogen peroxide and heat, before they are filled as above.

Depending upon the application of Hydrogen Peroxide, and stage at which it is exposed to sterile air, it is done in two alternative methods, as shown fig. 24.3.

Fig. 24.3 Alternative methods of sterilization of carton material from a reel