Pasteurised milk is heat treated to kill bacteria, yeasts and moulds. After treatment it is rapidly cooled and must be kept refrigerated. Dried milk is pasteurised before drying but can be stored at room temperature until reconstituted. After reconstituting it must be treated as fresh milk; dried milk powder is not sterile, and may contain bacteria, which will begin to grow after the product is opened.
While pasteurised milk needs refrigeration and lasts only days, dried milk can be stored for up to four years at 10 degrees Celsius. Care must be taken, however, to keep dried milk airtight and away from sunlight and high temperatures. Exposure to hot sunlight in tropical countries during disaster relief for example, greatly reduces dried milk's lifespan.
Weight and Volume
Dried milk is lighter and takes up less space. This has storage and shipping advantages, and makes it ideal for backpackers. Reduced weight and volume, combined with longer shelf life and greater economy, make dried milk a staple for emergency relief and disaster preparedness. The same qualities make dried milk a staple of food production, especially baked goods. Using fresh milk would increase the cost of food production.
If liquid milk leaks, it damages adjacent goods and soaks into absorbent materials leaving unpleasant odours. Dried milk powder can be swept up cleanly.
Safety and Taste
Dried milk powders require a safe water supply for mixing. This can be a problem in disaster relief situations--water may be contaminated and power supplies to boil water limited. There have been problems with supplying baby formula to third world countries without good water supplies, where breast feeding would be much safer. Due to their dusty texture, traditional dried milks mix poorly with water and retain a slightly gritty texture, leading some to dislike the taste. Newer agglomerated products have improved solubility, while taste is a question of familiarity---reconstituted dried milk tastes "different," rather than "bad." Using dried milk for cooking, but fresh milk for drinking is one solution.
Although the calcium, protein and riboflavin content of milk is unaffected by drying, vitamin C, thiamine and vitamin B-12 are lost. Powdered milk is higher in sodium and sugars than fresh pasteurised milk. For shelf-life enhancement, dried skimmed milk is best. Milks with higher fat content do not store so well, but because dried skimmed milk has no fat, it also lacks the fat-soluble vitamins. Children under 2 years must have full milk because they need the fat and vitamins.
It requires much energy to remove all moisture from milk; transporting dried milk internationally increases the carbon footprint. Sodium hydroxide and nitric acid are used in the production process and must be disposed of safely. Consuming milk closer to the source and closer to the form in which it leaves the cow is more environmentally friendly.
Cases involving the contamination of dried milk with bacteria, hormones and chemicals have cast doubts over its safety. In China in 2008 dried milk was found to contain melamine. Later China banned imports of US dried milk containing high levels of nitrates.