Aseptic Packaging

Aseptic packaging can be defined as the process in which UHT processed or sterilized milk is filled in pre-sterilized containers under aseptic/sterile environment.This ensures that there is no post processing contamination of the milk so that the product has longer shelf life. Since aseptic packaging systems are complex, great care is needed to prevent contamination. Before the start of product packaging, trial runs are routinely conducted with sterile water. Critical parts of the filling machine and carton forming systems are thoroughly checked. The seal integrity of the package and overall microbial quality of the packaging material are monitored properly. Generally, for a good processing plant permissible spoilage rate is one in every 5000 sterilized, filled and sealed package of one litre carton.

 

i. Types of Sterilizing Medium

Sterilizing mediums to be used in aseptic packaging systems could be broadly classified under two categories: physical sterilization mediums and chemical sterilization mediums.

Physical sterilization mediums: Steam under pressure or hot water is the most simple and reliable sterilant for high sterilization efficiency in short time. In aseptic packaging, its use is however restricted to sterilization of the milk tubes and valve and fittings coming into product contact.

Dry heat/ super heated steam: Hot air is generally used to sterilize the closed space where the filling of milk takes place. Air heated to 300OC may be taken to the areas surrounding electric resistances used for sealing the packages. Dry air at 330-350OC is also used for sterilizing the milk filling tubes. Sterilized air (180-200OC) is used for evaporating residual H2O2 (chemical sterilant) from the package.

Ultraviolet radiation: UV rays (optimum wavelength 250 nm) alone are not a very effective sterilizing medium for aseptic packaging units. Two major reasons for this are: (i) intensity of radiation is not uniform on the entire package surface(ii) bacteria adhering to packages could be protected by dirt/ dust particles present on the surface. UV radiations are therefore used as a complementary sterilizing medium.

Ionizing radiations: Gamma rays are often used for sterilizing packaging materials unable to withstand high temperature. Usually 2.5 Mrad intensity is suitable for sterilizing plastic laminates used in aseptic bag-in-box.

 

ii. Chemical Sterilization Mediums

Ethylene oxide: Ethylene oxide has slow sporicidal action. It is sometimes used as a pre-sterilization agent to reduce microbial load on packaging films so that a shorter time is required for final sterilization.Hydrogen Peroxide (H2O2): H2O2 has poor sporicidal effect at room temperature. However, with increasing application temperature and concentration, sterilization performance improves. H2O2 is the most popular sterilant for aseptic packaging system. H2O2 is applied on the package surface by either dipping or spraying. As its boiling temperature is slightly above 100OC, supply of heat by either sterilized hot air or infrared elements can evaporate the residual H2O2 from the package surface. Thus there is little H2O2 left for contaminating the product. Safety regulation recommended by IDF requires that atmospheric concentration of H2O2 in the packaging hall should not exceed 1 ppm. Further more, residual concentration in milk immediately after filling should not exceed 100 ppb and should reduce to 1 ppb within 24 hours. The most successful combination of sterilizing medium being used in commercial aseptic packaging units are H2O2 coupled with heat supplied by radiant heating element. Some packaging systems also use a combination of H2O2 and UV radiation.

Other sterilizing agents which are rarely used in such applications are sodium hypochlorite and per acetic acid. These agents leave the residues of chloride and acetic acid on the package, which may finally contaminate the product.

 

ii. Type of Packaging Materials

Metal container: Cans made of tin plate or drawn aluminium are generally used for packaging of condensed milk, viscous liquids and chunk-in-gravy type of products.These are expensive and unsuitable for low cost products like liquid milk. They are bulky and require large storage and shipment space. The empty cans are carried in a conveyor to a tunnel for sterilization with steam super heated with gas flame at atmospheric pressure and require about 40-45s. The cans then move to filling chamber for product filling. The can lids are separately sterilized, placed on the cans and seams sealed. The can sterilizing, filling and sealing zones are sterilized before the filling begins with the same mixture of superheated steam and flue gas,which fills them during operation. Cans have been used for in-package sterilization for a long time. Manufacturers of UHT milk who want to impress the consumers with the advantages of the new technology therefore do not prefer to use cans which are so identified with a old technology

Laminates/cartons: Different layers of flexible films of different materials viz.paper, polyethylene and aluminum foil are co-extruded to form a laminate. These materials have specific properties viz. water vapour transmission, burst strength,etc; and hence when co-extruded form an ideal packaging film. Such laminates could be 3, 4 or 5 ply and are generally used for products like, milk, cream, fruit juices, soups, etc. These laminates are supplied as film rolls, which can be mounted on FFS (form-fill-seal) machines. Alternatively, cartons made of laminates are supplied as preformed blanks, which are assembled into cartons for filling and sealing at the top. Plastic films: Black and transparent polyethylene films are co-extruded for packaging of UHT processed milk intended for 2-3 weeks shelf life. The co-extruded film protects the product against light but not oxygen. The packaging machines also need to operate at not more than 45-50OC filling environment. A co-extrusion of polyvinylidine chloride (PVDC) or ethylene vinyl alcohol (EVOH)with black or white polyethylene film is also used as packaging film. Such a combination imparts protection against oxygen as well as light and shelf life of milk can be extended upto 3 months.

Other forms of packaging materials : Preformed packages of different shapes and sizes are also used for aseptic packaging of value added dairy products. Blow-moulded plastic bottles of polyethylene or polypropylene are used as cheap substitutes.However, these are transparent and permeable to oxygen. Multilayer materials with better light and oxygen barrier properties have also been developed. Pre-formed plastic cups of polypropylene (PP) or polystyrene (PS) are now gaining popularity.Bulk filling bags are made of laminates of 3 or 4 layers of which one will be barrier material such as metalized polyester (polyester with a coating of aluminium particles) or ethyl vinyl alcohol (EVOH). The bag with filling valve is sterilized by r-radiation(2.5 Mrad dose) before shipping. Bags remain sealed and internal surface therefore remains sterile. At the filling station, the sterilized bags are opened, filled and sealed under aseptic condition. All product contact surfaces in the filler however need to be sterilized with steam before the filling operation begins.

 

iii. Description of the Packaging System

Most of the aseptic packaging machines being used in the country are of form-fill-seal (FFS) type. Packaging material used generally is laminate of polyethylene –paper – polyethylene – Al foil – polyethylene. Packaging film in the form of a roll is mounted on the packaging machine. The film moves continuously downward in the form of a strip and a shaping roll gives it a cylindrical shape. Heat sealing forms an overlapping longitudinal seal. Simultaneously extra polythene strip is heat bonded along inside of longitudinal seam. This is done to prevent filled product penetrating the paper layer. As this continuous cylinder moves downward, jaws at the bottom make transverse heat seal. The product is filled instantly and another jaw seals the package at the top. Depending on the type of machine, different shapes can be given to the package. The most popular is brick shaped package. Tetrahedron shapes were also being used some times back. Some new innovations that are now being used for packaging of fruit juices are Fino packs. To cut down on costs some dairies have introduced pillow packs for packaging of milk.