Fresh milk contains live maternal cells and the highest amounts of nutrients, growth factors and many other protective components. Over time and with exposure to varying temperatures, these components decrease in potency, while the risk of bacterial contamination and growth of pathogens increases. Fresh human milk is not sterile, but rather contains a wide variety of organisms including non-pathogenic bacteria, pathogenic bacteria, viruses, mycobacteria and fungi. While quantities of bacteria in human milk vary widely, in general, the majority of identified organisms are non-pathogenic, normal skin flora from the mum’s nipple or breast, or are organisms that protect the newborn’s gastrointestinal system after having migrated to the breast via the enteromammary pathway.
Preparation for storage
Breast milk retains most of its immunological properties in glass or hard plastic containers that do not contain polyethylene. In addition, containers made with Bisphenol A (BPA) are no longer used for baby bottles because of its adverse effects. As there may be some concerns about glass breaking, ideal containers for milk storage are therefore food-grade hard plastic, made without BPA, with leak-proof lids. Clean, aseptic or sterile containers may also be acceptable.
*Guidelines for expressing milk under very clean conditions:
Before expressing breast milk, mothers should wash their hands with soap and water or an alcohol-based hand sanitiser. The pump parts, bottles and pumping area must be clean. Breasts and nipples do not need to be washed before pumping.
Use of a medical grade refrigerator with temperature uniformity and constant temperature monitoring is recommended.
These storing and thawing breast milk guidelines are a recommendation. National and hospital internal guidelines and standards may deviate.
Milk can be stored safely at room temperature for a short period of time. Although studies differ in terms of the exact recommended period, in general, warmer temperatures are associated with higher bacterial counts in expressed milk. One key study showed that bacterial growth, which was mainly restricted to non-pathogens, is minimal at 15 °C and remains low at 25°C for the first four to eight hours, but increases rapidly after four hours when stored at 38°C. The authors concluded that milk at 15°C was safe for 24 hours and at 25°C for four hours. As a result, expressed milk can be stored safely for approximately four hours at temperatures of up to 26°C. At lower room temperatures, storage for up to six hours may also be reasonable in clean environments.
Refrigeration at approximately 4°C preserves the integrity of human milk longer than when it is left at room temperature. Refrigeration has been shown to inhibit gram-positive bacterial growth for up to three days.
Freezing breast milk at -20°C for up to three months has been recommended as optimal. At three months vitamins A, E and B, total protein, fat, enzymes, lactose, zinc, immunoglobulins, lysozyme and lactoferrin are maintained, although there may be vitamin C loss after one month. Bacterial growth is not a significant issue for up to six weeks. The antibacterial capacity, however, is generally less than that of fresh milk, due to the loss of live cells such as phagocytes. Up to 9 months in deep freeze at -20°C is considered acceptable, although changes in taste and smell may occur at -80°C as lipase continues to break down fat into fatty acids.
After freezing, milk can be thawed in the refrigerator, or by using a container of warm water or by running it under warm water. Thawing breast milk with very hot water, microwaves and stoves should be avoided since high heating temperatures can reduce the antibacterial and other bioactive properties of milk. Thawing is complete when the frozen milk has become liquid, is still chilled and some ice crystals are still present. The presence of ice crystals is a visible indicator that the milk has not thawed beyond a certain point. Thawed milk should then be refrigerated until immediately before use and not left at room temperature for more than a few hours to prevent bacterial growth. Re-freezing milk after thawing it in the fridge has been shown to maintain a safe bacterial load; however, it has been suggested that milk that has been completely thawed to room temperature should not be re-frozen.