Protection and pathogens

(Photo: Louis Pasteur, inventor of pasteurisation) 

A primary function of breastfeeding is protection against infection. Human milk contains both pathogens and protective properties. Protection works best if baby breastfeeds directly at breast, followed closely by drinking untreated, freshly pumped milk that was not chilled nor heated. But direct breastfeeding or consuming fresh milk is not always possible, so some milk may need to be stored and heated. Many professionals feel it should be pasteurized as well. Any kind of storage and all temperature changes will cause some alteration in the protective function of human milk. Multiple studies investigated different ways to pasteurize and store milk. The most common method of pasteurization (Holder pasteurization, 62.5°C, 30 min) turned out to be not only an effective way to destroy all pathogens tested, but also the vast majority of protective factors. Modified pasteurization (57°C, 30 min) in the Czank c.s. study showed to be as effective in the termination of pathogens, but kept far more protection intact. Akinbi et al found that not only  heat treatment, but also storage at −20°C for 4 weeks did lower protective properties, but not as much as heating. Marin c.s. on the other hand did not find significant differences in protective properties after storing milk samples at -20°C for 6 weeks. A remarkable result they did find, however, was that in all milk samples obtained by pumping bacterial counting was higher than in all samples obtained by hand expression.
Akinbi H, Meinzen-Derr J, Auer C, Ma Y, Pullum D, Kusano R, Reszka K, Zimmerly K: Alterations in the Host Defense Properties of Human Milk Following Prolonged Storage or Pasteurization. Journal of Pediatric Gastroenterology & Nutrition: 2010, 51(3):347–352
CZANK, CHARLES; PRIME, DANIELLE K.; HARTMANN, BEN; SIMMER, KAREN; HARTMANN, PETER E.: Retention of the Immunological Proteins of Pasteurized Human Milk in Relation to Pasteurizer Design and Practice. Pediatric Research: 2009, 66(4):374-379
Marín, María L; Arroyo, Rebeca; Jiménez, Esther; Gómez, Adolfo; Fernández, Leonides; Rodríguez, Juan M: Cold Storage of Human Milk: Effect on Its Bacterial Composition. Journal of Pediatric Gastroenterology & Nutrition: 2009, 49(3):343-348