Packaging of dried milks

Milk powder packaging should be carried out in a dry atmosphere. This is because lactose is present in glass (i.e. non-crystalline/amorphous) state in dried milks and the dried milk in this state is very hygroscopic. If exposed to humid atmosphere, it would readily absorb moisture, become sticky and cake in storage.

The skimmed milk powder is packed in containers that retain product quality during storage as well as result in better handling. The successful packaging of whole milk powder involves the additional step inert gas packaging in which oxygen entrapped in the air cells of the milk powder particles is removed and thus reduce the oxygen concentration to as low a level as possible. The oxygen permeates the walls of air cells and diffuses out until equilibrium is attained between the interior of air cells and head gas space in the can. Attainment of equilibrium in the gas packed cans is reached in about 5-7 days. The general procedure is to immediately remove oxygen by subjecting the product to 28” of vacuum within 24 hours of drying with final packaging within a few days. Less than 2% final oxygen in the headspace of the package is considered satisfactory for most storage conditions. This is accomplished by vacuumizing and gas packing with an inert gas. The specific procedure selected for gas packaging is governed by final oxygen limit desired in the package, and by the equipment-filling rate in relation to oxygen evacuation rate. As per Prevention of Food Adulteration Act, The spray-dried milk shall be packed in nitrogen or mixture of nitrogen and carbon dioxide in hermetically sealed containers.

i. Bulk Packaging

Traditional packaging of 25 kg milk powder, irrespective of type, has been in polyethylene lined multi-wall paper sacks. This type of packaging is the most versatile of all the packs for milk powders and it is due to this versatility that these are likely to remain the major bulk container for milk powder. Their key features are that they are robust, relatively low in cost and are capable of being packed at high production rates i.e. 9 to 10 ton/hr with modern filling/sealing equipment. Automatic dispensing machines are readily available to dispense correct weight of powder in one bag before shifting the product flow into the next bag. Bags are sewn 3 to 3.5 stitches per inch automatically or by a manually operated sewing machine suspended and

counterbalanced within easy reach of the filling area. These are convenient pack for major end users such as recombining plants and repacking stations.Specifications for multi-wall bags vary from country to country, but generally a 4-5- ply bag with a minimum weight of 420 g/m2 and an inserted low-density polythene (LDPE) liner of 75 micron is considered to be adequate. A major improvement in the design of the multi-wall bags is the development of a “heat sealable style.” This has been achieved by incorporating the polythene liner into the bag as the innermost ply during bag manufacture.

For packing whole milk powder (WMP), the package is gas-flushed (nitrogen). In 1991, Colby-packaging systems introduced a new filling machine for milk powder manufactured by standard methods. “COLBYPAC-25” discharges the product in a 25 kg disposable barrier laminate bags. Machines incorporate a “Pre” or “Postgassing” step, which extends the shelf life of milk powder, O2 is removed (pre or post filling) and replaced with CO2 or N2. Gas-flushed packs are required to be more sturdy and with considerably improved barrier properties so as to maintain the modified atmosphere within the package. Some markets require the milk powder to be delivered in 300 kg Bulk bins, gas-flushed. These systems require specialized filling/gassing/sealing equipment.

i) Bulk bin: Considerable interest in the bulk handling of milk powder has been generated by the increased costs of handling and transportation. Concept of bulk bin packaging is relatively new and is proving to be of value to the repacking and reprocessing plants. Following are the major types of bins:

a) Tote bin: The bins are for 800 and 1000 Kg of powder and fabricated from different metals or alloys including stainless steel. Aluminium is most often used by the dry milk industry, costing about one-half as much and,being lighter than those made from stainless steel. It is well suited to the internal transfer of powder to central packing stations. These bins are often used as a container for shipment to industrial users. The advantages are mainly in reduction of labour and bag costs with possibly a reduction in dust problem. Automatic conveying equipment is available to fill a group of these bins, each in succession when properly positioned. They are airtight and can be stacked. The bins are emptied into a hopper by elevating and tilting. Its major disadvantages include the high initial cost of construction and the cost of returning empty bins to the milk powder manufacturers.

b) Big bag/Bag in box: Both styles are superseding the tote bin. Their basic concept is that of a large sized multi-wall bag in that a large polythene bag is filled with milk powder and closed by tying. The filled bag is protected from outside influences by woven plastics such as polypropylene or rigid twin or triple laminated corrugated board. They are normally transported on pallets and are emptied by suction hose. Apart from potential savings in filling and transportation costs, the relatively low cost of packs permits them to be exported on a nil return basis.

ii. Retail Packaging

The retail pack market is dominated by the metal can but there is increasing competition from a number of plastic laminates that are being formed into a variety of pack styles.

i) Metal can: The metal can is the nearest to a complete container for milk powder. However, apart from cost, the major drawback of the can is its tendency to rust through moisture condensation when changes in temperature are experienced during storage and transportation. The incidence and severity of this problem has been reduced by the adoption of aluminium ends for the cans and improved lacquers for the can bodies. The can is commonly of threepiece construction and is manufactured from electrolytic tin plate. The features include:

•  A cylindrical body that is either welded or soldered.
•  A base that is double seamed on to the body.
•  A lid that is double seamed on to the body immediately after the vaccum/ gassing cycle has been completed. Sealing compounds in lid and base ensure that the can is hermetically sealed 

Equipments for semi automatic and automatic packaging of whole milk powder which can vacuumize and N2-pack cans, have been developed by leading companies. The most of the machines used for packaging of whole milk powder operate with gravity feed and vibrating hopper. The latest model is a combination of Auger and vacuum feed, which combines speed and accuracy of filling with compact, clean and dustless packaging by vacuum method. The cans are then finally sealed by a double seaming operation on an automatic closing machine.

a) Filling the cans: The cans used for milk powder packaging abroad are commonly of 1 pound, 2.5 pound and 5 pound weights. There is also a 4.5 ounce-can. A limited number of 25 pound and 100 pound cans are also used.

b) Vaccum packing: To confine the oxygen content in the headspace of can to 3% or less requires a vacuum of about 28.5 inches on a 30-inch barometer at the time of vacuuming.

c) Nitrogen packing: It consists of drawing a high vacuum on the cans of dried whole milk (within 1/4 inch of the barometer), then dissipating the vacuum with an inert gas such as nitrogen or carbon dioxide, and sealing the cans.

d) Double vaccumizing: Some manufacturers double vaccumize the powder before gassing. In this case, the dried milk is transferred from the sifter to large drums (usual capacity 200 pounds). The filled drums are immediately vacuumized. This temporary form of package is held for several hours or overnight or even as long as 48 hours. After the holding period, the powder is transferred to final containers, in which it is again vacuumized followed by gassing and sealing.

e) Double gassing: To obtain a low level of headspace oxygen in WMP, a double gassing technique is applied. The customary procedure is the collection of filled cans on trays to be conveyed into the vacuum chamber.The air is removed rapidly (60 seconds) with the gauge indicator decreasing to 29” of vacuum. After a 2 to 5 min hold, the pressure is restored with nitrogen to 0.5 to 1.0 psi above atmospheric pressure. Nitrogen may be replaced with a mixture of nitrogen and CO2, the latter being restricted to 5 to 20%. After removal from the chamber, the hole in the lid of cans is soldered. Containers are held for oxygen desorption. When oxygen equilibrium has been attained in the head space, usually within a week,but at the most ten days, the cans are punctured and the vacuum treatment,pressure restored with nitrogen, and sealing steps are repeated.

ii) Sachet packs: Flexible packaging has evolved from polyethylene bags to more sophisticated packs. Sachet pack styles vary from the simple flat pack, which is heat-sealed around the edges to the complex free standing, block bottom form. In most cases, the sachet of milk powder is enclosed for distribution and sale inside cardboard boxes. In recent times, metallized polyester/liner lowdensity polyethylene laminates have emerged. Synthetic materials such as Nylon tend to generate static electricity during packaging operation, which may cause faults in vital seal areas.

A development that could well have useful application is the introduction of ‘soft can’. In a fully automated system, a sachet is formed from a heat sealable composite foil and a robust carton formed around the sachet. The system then follows the standard filling, vacuum and gas packing practice of the metal can.As the composition of the laminate is virtually a perfect barrier to moisture and gas, it is considered that this system could offer a suitable alternative to the can. With the development of aluminium/plastic laminates with considerably improved barrier properties, the quantity of WMP and special products in this type of packaging is showing a rather marked increase. A typical composition of sachet material for whole milk powder is polyester 12 micron/Al-foil 9 micron/PE 50 micron. which is satisfactory in relation to oxygen transmission. Bags with 250 g, 500 g and 1000 g are used. Material combinations that exclude foil are being used successfully on the lower fat powders.