Milk powder is defined as a milk product obtained by the extraction of water from milk. By removing moisture, the potential for microbial growth is reduced. As a result, milk powder provides transportation, storage convenience and shelf life benefits not found in liquid milk. Milk powder usually provides a lower cost milk alternative to liquid milk and has many applications.

There is a range of milk powders available, each with varying levels of protein, fat and water. These milk powder types include whole milk powder, nonfat dry milk powder and skimmed milk powder. Each milk powder has commonly used abbreviations of WMP, NFDM and SMP respectively. The defining characteristics of each milk powder are milkfat by weight on a milk-solids-not-fat (MSNF) basis, which ranges between 1.5% and 40% depending on the type of milk powder. The second characteristic is moisture, which ranges from 5% moisture by weight to less than 1%. NFDM and SMP are further defined by milk protein content. SMP has a minimum milk protein content of 34%, whereas NFDM has no standardized protein level.1

WMP, NFDM and SMP milk powders are classified for use as ingredients according to the heat treatment used in their manufacture. This treatment determines the solubility of the milk powder and thus, its selected application. These uses include blending it with pastry dough to make the dough crispier, use as an egg substitute, a freshness aid in the baking of breads, and in the production of milk chocolate, ice cream and infant formula. Milk powder is also used in humanitarian relief efforts, is stored in emergency shelters and is carried by hikers and campers.

Characteristics and Challenges:

While the low water content of milk powder eliminates much of the potential for microbial growth, most forms of milk powder still have a limited shelf life. WMP maintains a premium freshness level for approximately six months. Skim milk powder, which lacks the fat molecules found in WMP, has an average shelf life of three years.2

Stringent hygienic conditions are required in most countries for the handling of milk powder. In the United States, the 3-A Sanitary Standards Inc. has established criteria for the materials, fabrication, and installation of milk powder and other dairy product processing systems. 3-A sanitary standards cover topics which include, equipment design standards that facilitate proper hygiene such as design concepts, product contact surface finish specifications and permissible materials that may be used in the construction of the equipment.3 Consultation with a qualified and experienced manufacturer of 3-A design and constructed equipment is recommended when choosing a system to convey milk powder.

Milk powder is often a very highly processed product that requires special treatment during handling. Highly fat-enriched milk powders may leave a product buildup in the convey line, which could affect the quality of the end product and will also create hygienic risks. Milk powders readily take up moisture from the air, leading to agglomerations in the convey line, as well as a rapid loss of product quality. The fat in milk powder can react with oxygen in the air to give off-flavors, especially at higher storage temperatures above 85°F (30°C).4

Milk powder has an average bulk density of 35 lb/ cu ft.5 It is aeratable, meaning air movement may cause the milk powder to take

Milk Powder

flight and settle as dust. Some milk powder dusts will fluidize, or take on the characteristics of a liquid. Milk powder dust is also potentially explosive. Experienced handlers of powdered milk have observed that bulk densities tend to change without notice as shipments of dry milk powder may arrive at the processing facility from a multitude of suppliers.

If the milk powder has been transported to the facility in bulk bags, care must be taken to ensure that the bag discharge frames are constructed to 3-A standards as well as being able to completely empty the bags. Bag activating devices which compress the sides and bottom corners of the bulk bag are generally effective promoting a better flow. Complementary devices will also stretch the bags, elongating them to remove pockets in the bag that may retain milk powder. Some of these flow promotion devices can incorporate clamping features which provide an airtight seal between the bulk bag and the receiving hopper. This seal will preserve the integrity of the product as well as the containment of dust.

As the milk powder fills a receiving hopper beneath the bulk bag, the air inside the vessel is forced out. Unless this air passes through a filter, airborne dust particles can escape into the surrounding atmosphere. A dust collector mounted on the discharger frame will contain the milk powder inside the conveyance system. This not only lowers risk of potentially dangerous dusting, it also reduces the time necessary to perform the routine cleaning and sanitation required in a 3-A environment.

In addition to the design and construction of process vessels for sanitary purposes, to ensure proper material flow of milk powder to downstream processes such as a conveyor, other factors must also be considered. As fat and moisture content varies, so do the flow properties of different grades of milk powder. Geometries of storage hoppers along with flow promotion devices such as vibrators and mechanical agitators must be considered and applied appropriately.

If the milk powder is packaged in smaller bags, a bag breaking station with a dust hood, and remote dust collection cartridges are likely sufficient to support the manual unloading of the material. A hopper grate will protect operators from moving equipment components and prevent the introduction of foreign objects

If the milk powder is being pneumatically conveyed into a processing system, the conveying air must be properly filtered to prevent the introduction of contaminants. Further, the blower used to move milk powder through the conveying line must be sized to meet the demands of system.

In certain applications, the use of a dryer to remove the moisture from the air may be necessary to prevent agglomeration of the milk powder in the conveying line.

As previously described, milk powders have a wide MSNF range. As this can result in a wide variability of material flow and compression properties, when handling milk powders with a flexible screw conveyor, it is important that a range of spiral geometries are available in order to optimize the performance of the conveyor. This will allow for the proper pairing of powder properties with equipment design.

Should your milk powder application feature the loading of material into bulk bags, the bag capacity will be maximized by use of vibratory densification decks to de-aerate the milk powder as it fills the bag and by the application of load cells to ensure the desired weight. Seals and other dust containment devices will ensure a dust-tight operation.

Flexicon Applications:

Flexicon is pleased to be a holder of a 3-A symbol (authorization number 631) since 1991. Flexicon was the first manufacturer authorized to build "mechanical conveyors for dry milk and dry milk products," as listed under section 41-00 of the 3-A Symbol Council's Sanitary Standards. Flexicon has many more years and application experience providing equipment built to 3-A standards than any competitor.

Flexicon designed and built a floor hopper and flexible screw conveyor system that helped a contract packer package milk powder at a high rate. Installation of a Flexicon flexible screw conveyance system resulted in an expanded product line and doubled production for a malted milk drink producer.

Flexicon's Sanitary Mobile Flexible Screw Conveyor is designed, constructed and finished to 3A Dairy Accepted standards, making it an ideal choice for milk powder handling.

Flexicon's dust suppression and collection systems ensure proper containment of milk powder throughout the conveyance process.

Consultation with a Flexicon specialist will help you decide if a flexible screw or pneumatic solution best fits your milk powder application.

Flexicon's expert design and engineering staff will weigh each parameter and recommend the best solution for you. Upon request, Flexicon's test lab will simulate your milk powder handling functions before the system is installed in your plant.

Flexicon's product line of advanced flow promotion conveyors, high flow hoppers, deaeration/densification decks and a host of other components and accessories are proven performers that promote flow while reducing degradation, dusting and/or the separation of blends comprised of disparate particles.

Related Articles on Milk Powder:

1 US Dairy Export Council, Milk Powder
2 Ibid
3 United States Dairy Association, Guidelines for the Sanitary Design and Fabrication of Dairy Processing Equipment
4 New Zealand Dairy Research Institute
5 Source: Powder and Bulk. Com - Bulk Density Chart

Sources: Where noted. All other information courtesy of Flexicon Corporation.