Processes for Packaging Fluid Milk

i. Processes for single-use packaging

The pouch-filling machine is an integral part of a modern fluid milk plant. Several types of plastic materials are used in forming the pouches. The material generally comes in the form of rolls, which are loaded onto the machine. The layer of plastic is folded vertically and sealed. A horizontal sealer seals the bottom of the pouch.Simultaneously, milk from an overhead float tank is siphoned into the formed cylinder.It is then again horizontally sealed at the top and cut off to form a pouch filled with milk. These machines come with adjustable filling heads capable of filling several volumes of milk as the need be.

 

ii. Processes for Multiple-use Packaging

The filling of milk in glass bottles was an important operation in the fluid milk factory till recent times when the sachet-packing systems gained popularity for several reasons. However, inspite of the weight of the bottle and the problems regarding the return and cleaning of bottles before refilling, it has several advantages such as ease of cleaning and ease of visual detection of spoilage and impurities.

The bottle-filling process is a cycle of events that follow one another. Clean bottles are filled in the bottle-filling machine and capped (generally with aluminium foil caps). The filled bottles are stacked in crates and sent for cold storage/refrigeration.The crates of empty bottles, after selling the product are returned to the factory.The crates are emptied and cleaned separately. The bottles go to the bottle washer,where the broken/chipped bottles are discarded and the clean, disinfected bottles are returned to the bottle-filling machine.

Bottles require the following properties: The shape of the bottle should be such that it facilitates easy cleaning and allows brushes and jets (of water and detergents) to act on the entire inner surface. The neck of the bottle should be joined to the body smoothly with no sharp and protruding angle. The base should be concave so that the sediments and residues collect in the center of the bottle rather than on the periphery, thus making cleaning easier.

The resistance of the bottle to shocks is influenced by shape, consistency of quality and the thickness of the glass. The bottles should also be able to resist the high internal pressures and temperatures created during in-bottle sterilization.Optical defects such as irregularities in the composition of glass, presence of air bubbles, deformations and coarseness in the surface or extraneous matter in the glass also decrease the resistance to mechanical and thermal shocks.The capacity of the bottle should be constant and consistent and should match with the capacity of the filling machine. The most common volumes in the market are one litre, 500 ml and 200 ml.

 

iii. Processes for Bulk Supply

The milk for bulk supply is generally processed and filled in containers manually.These may be cans of PET containers. Bulk milk is also sometimes supplied in small tankers or vans fitted with SS tanks.In the modern dairying countries where the bag-in-box-type of containers is used,the bags, which have a separate bung with a flexible tube attached, are supplied to the producers. The dairy owners use semi-automatic filling machines to fill directly into the box. When delivered to the caterers the filled box fits into a custom-built refrigeration unit with a prefitted, simple on-off tap. The tube on the bag is fed around the tap and cut off to open the bag and permit the product to be dispensed.

 

iv. Processes for Long-life Milk

Long-life milk is that milk from which most of the spoilage bacteria are removed so that the milk could be stored for a longer period than normal milk. Specific processes such as bactofugation, microfiltration, ultra-pasteurisation and ultra-high-temperature (UHT) processing help to increase the shelf life of milk.

Bactofugation is a method by which the bacteria in the milk are removed by centrifugation. This uses the theory that bacteria have higher density than milk and thus will separate out into the outer orbit during centrifugation and can be removed after the process. The bacteria-rich portion is separately sterilized and added to the centrifugally sterilized bulk of the milk. Any packaging systems for fluid milk packaging may be employed to pack and market the product. However, the packaging system has to be clean and the seals preferably hermetic, in order to avoid recontamination during packaging and storage.

Microfiltration is a non-thermal, pre-pasteurization step that can extend the shelf life of milk up to 45 days. The microfiltration process uses a ceramic filter membrane to remove spoilage bacteria from milk, thus extending shelf life. Because it is a non-thermal process, milk is less susceptible to heat-related sensory defects with a 90-day shelf life. Packaging systems that prevail for fluid milk packaging may be used to pack and sell the product.

Aseptic packaging is a technology wherein the product and package are separately sterilized, and the product is then filled into the package and the package sealed in a sterile environment. The product is commercially sterile (meaning that any pathogenic or other spoilage microorganisms have been destroyed) and shelf stable(does not require refrigeration or freezing). Containers for aseptic filling have traditionally been aluminum cans, high-barrier pouches and multi-layer, foil barrier boxes. Aseptic packaging using flexible materials is also employed where extended shelf life is required. Many aseptic packaging systems are based on form-fill-seal technologies that eliminate the need to ship preformed containers to the processor.That is, the processor procures the packaging material in the form of rolls or stacks and they are formed (shaped) during the filling process.

Plastic materials used in aseptic packaging of milk products are polyethylene,polypropylene, polystyrene as tubes, bottles or plastic film laminates with paperboard or aluminum in the form of cartons. High-pressure steam is used to sterilize product lines and hydrogen peroxide with heat of UV radiation for container materials. The popular commercial systems available for aseptic packaging of milk are Tetra Pak,Tetra Brick, Brick Pack, Combi Block, Pure Pak, Hind Pak, etc. Tetra Pak/ Tetra Brick packs are used to pack UHT-treated milk into pre-sterilized package in aseptic conditions. The first aseptic carton was the ‘Tetra Classic’. It was made from a roll of packaging material that had been sterilized in hydrogen peroxide,formed into a tube, filled with liquid, sealed transversely and cut into tetrahedron-shaped containers. Tetra Pak uses paperboard laminated with 10m LDPE from outside and 70-75m LDPE from inside. The Tetra Brick uses aluminum foil of 7-9m in addition to above laminates. The machinery needed for this system is very expensive.

Ultra-pasteurized products are produced under slightly less extreme conditions than aseptic processing. However, heat processing and clean packaging still play important roles. Ultra-pasteurized milk beverages are usually packaged in barrier-coated paperboard cartons, or HDPE or PET bottles. To prevent light degradation, PET bottles, which are clear as compared to HDPE bottles that are opaque, can be tinted and/or covered with full-body labels. A window in the label allows consumers to see the product.

Ultra-pasteurized bottles include a hermetic foil seal on the bottle mouth. This prevents contamination and enables the product to achieve a 90-day refrigerated shelf life. Some processors who want to obtain a slightly longer shelf life than the standard 14 days, but do not want to ultra-pasteurize the milk, may opt for an intermediate thermal process referred to as higher-heat-shorter-time. The shorter heat exposure can leave the milk free from the sensory defects associated with aseptic and ultra-pasteurization temperatures. The refrigerated shelf life for such products is about 30 days.