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Breast milk freezing – baby, it’s cold outside


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Freezing breast milk – whether you’ve seen it done during your NICU time, or done it yourself as a parent, it’s probably something that we’ve all had to consider. General advice is that it’s ok to freeze your breast milk for 6-9 months. But is that really true, or does freezing damage breast milk?

A study published online this month in the Journal of Pediatrics, assesses whether or not freezing breast milk is safe. Do we have to change our practice?

Who freezes breast milk?

In NICU, breast-feeding of premature infants is encouraged, and breast milk is frequently frozen for an extended period of time. Similarly, at home, mums often express and freeze milk for later use, particularly as they return to work. The idea is that even though the mum cannot breastfeed her child herself, the child will still get all the benefits of breast milk.

What are the current guidelines for safe milk storage?

The American Academy of Pediatrics recommends that breast milk be refridgerated for 24 hours and then frozen, and then can be stored for 3-6 months if it remains at -20oC. The NHMRC Infant Feeding Guidelines recommends refridgerating and then freezing within 72 hours, and then can be stored for up to 12 months if it remains at -20oC.

What were the study aims?

This study aimed to measure qualities in the milk after freezing – including pH, bacterial counts, and nutrients.

Who were the study participants?

The study included 40 mothers with infants in NICU. Only those with excess milk supply were included.

Mothers were excluded if they showed signs of a breast infection, or if they had received antibiotics in the previous 7 days.

Each mother expressed 100ml breast milk and this was split into 9 x 10ml volumes. 4 of these were refridgerated (4oC) for 72 hours and then frozen (-20oC) and 4 were frozen immediately (-20oC). One of each was thawed at 1, 3, 6, and 9 months and analyzed.

One 10ml sample was used as a control and was frozen immediately to -80oC.

How did they analyse the thawed breast milk samples?

The authors tested for pH, total bacterial colony count, gram-positive and gram-negative colony count, lipids, osmolality, and finally concentrations of protein, lactoferrin, and secretory IgA.

What were the findings?

Milk pH declined and fatty acid concentration increased over 9 months and was not affected by prior refridgeration.

Bacterial count decreases during the time when milk is frozen too, and this decline happens more rapidly in milk that was refridgerated prior to freezing.

Concentrations of protein, lactoferrin and secretory IgA were unaffected.

What does the declining bacterial count mean?

The significance of the bacterial count isn’t really made clear. The authors suggest that the decline over time means that the milk is still biologically active, which is good. However, a decline to zero bacteria may remove some of the benefits of the breast milk. As refridgerating the milk first seems to accelerate the decline of bacteria, then this should be considered when storing breast milk for pre-term infants where we know that bacteria can be beneficial for neonatal health.

In conclusion: the nutritional content of milk does not change with freezing, and that is the same even with prior refridgeration. The bacterial count does decline but the significance of this seems less certain. Overall, freezer storage of milk for up to 9 months is acceptable.

About the authors

  • Tessa Davis is a Consultant in Paediatric Emergency Medicine at the Royal London Hospital and a Senior Lecturer at Queen Mary University of London.


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