Breast Milk Storage: What If We Thawed Breast Milk in the Microwave After All? A 2025 Study

Current recommendations, including those of the Academy of Breastfeeding Medicine (ABM Clinical Protocol #8, 2017 revision), consistently advise against using the microwave to thaw or heat breast milk, due to uneven heating, the difficulty of controlling temperature, and documented reductions in the activity of immunological factors above approximately 40 °C. The ABM recommends thawing milk either slowly in the refrigerator, or more quickly by placing the container under a lukewarm running water stream or in a warm water bath. These recommendations are consistent with those of the CDC, which also specify never to use the microwave.

The article by Ito et al. (2025) experimentally explores the possibility of using a domestic microwave oven to thaw human milk while evaluating thermal safety (risk of "hot spots") and the impact on certain immunological and nutritional components. The authors conclude that, under precise conditions, the observed losses remain modest and potentially not clinically problematic.

Background on Breast Milk Storage

This work stems from the observation that milk freezing is common practice at home, in neonatal units, and in milk banks, yet health authorities advise against the microwave due to the risk of burns and degradation of bioactive components. The stated objective is to provide a first step toward "practical" microwave use for thawing human milk, particularly in contexts where balancing breastfeeding and work makes the management of expressed milk more complex.

Methodology in Brief

This is a cross-sectional laboratory experimental study conducted on 35 milk samples from 35 donors at a milk bank, all in good health. The milk, initially frozen and then thawed under running water, was divided into 25 ml samples, subjected to different pre-treatments (none, sonication*, homogenisation) and re-frozen in two types of bags (polypropylene and polyethylene) before microwave exposure (600 W, 30 s).

*Sonication is a technique that uses ultrasound to fragment lipids in order to achieve greater uniformity between components and potentially a more even temperature distribution.

Thawing and re-freezing was performed in order to obtain the different samples.

The authors justify these pre-treatments as a means to test what best allows for greater temperature homogeneity.

Experimental Heating Conditions

The authors used a domestic microwave (600 W, 30 s, samples placed at the centre), then immediately measured temperature by infrared thermography (maximum, minimum, difference) before and after 10 inversions of the bag. Thermal heterogeneity was quantified as the difference between the maximum and minimum temperature within the bag, which provides an indirect assessment of the risk of "hot spots" capable of burning an infant's mouth.

Groups and Materials Studied

Samples from each donor were divided into one unheated control (T0) and six heated conditions (T1 to T6) combining type of pre-treatment (none, sonication, homogenisation) and bag material (polypropylene or polyethylene). This structure allows comparison of both the effect of prior milk structuring (fat/protein distribution) and the thermal properties of the packaging material on temperature distribution.

Biological Measurements Performed

For each condition, the authors measured sIgA and lactoferrin levels, as well as macronutrients (proteins, lipids, carbohydrates, energy).

Temperature Results

Sonication prior to freezing and heating significantly reduced temperature heterogeneity compared to untreated samples, whereas homogenisation at 6000 rpm showed no notable effect. Polyethylene bags also showed a more homogeneous temperature distribution than polypropylene bags, explained by the higher thermal conductivity of polyethylene.

Temperature Values and "Hot Spots"

Under the most unfavourable conditions, median maximum temperatures exceed 60 °C (for example, approximately 64 °C in certain groups), while minima remain close to 4 °C, reflecting a significant gradient before mixing. After 10 inversions, the overall median temperature in all groups is approximately 37–40 °C, close to the recommended temperature for feeding, suggesting that inversion mixing is essential to limit the risk of burns.

Effects on IgA in Breast Milk Heated in the Microwave

IgA decreased in a statistically significant manner in all heated groups compared to the control, with an initial median of approximately 1767 μg/ml and post-heating values generally remaining in the range of 1500–1700 μg/ml depending on the group. The median relative decrease ranged from 0.9% to 16.6%, with small effect sizes, and concentrations remained within the range of values described for mature milk, which the authors considered clinically non-problematic.

Effects on Lactoferrin

Lactoferrin also decreased in all heated groups, with a baseline median of approximately 2600 μg/ml and values reaching as low as approximately 1850 μg/ml in the most affected condition (median decrease 21.3–29.1%). Despite this reduction, levels remained close to typical concentrations described in mature milk, and the authors interpreted the loss as moderate, possibly limited by the brevity of exposure and maximum temperatures below those used in standard pasteurisation.

Impact on Macronutrients

Proteins and carbohydrates showed no significant variation between the control and the heated groups, with small effect sizes, suggesting good stability under these thawing conditions. Lipids and energy decreased significantly in certain groups (particularly homogenised ones), with small to moderate effect sizes, which may reflect phase changes or fat distribution modifications rather than substantial losses.

Authors' Interpretation

The authors attribute the majority of IgA and lactoferrin losses to areas where temperature exceeds approximately 60 °C, consistent with the literature on the thermal sensitivity of these proteins, but note that final concentrations remain within ranges considered adequate. They conclude that, under controlled conditions (duration, power, bag material, and post-heating mixing), the microwave could represent an acceptable option for thawing human milk while limiting thermal risks and the loss of bioactive components.

Scientific Strengths of the Study

This study uses samples from milk bank donors with rigorous inclusion/exclusion criteria, providing a degree of homogeneity in milk profiles and good control of confounding factors related to maternal health. The experimental approach crosses multiple factors (pre-treatment, bag material) and relies on objective measurements through thermography and biochemical analysis, with a statistical plan adapted to non-normal data.

Specific Relevance for IBCLC Practice

For lactation consultants, the primary value of this study lies in the quantification of immunological losses and the demonstration that inversion mixing, following brief microwave heating, can bring the temperature close to that recommended for feeding. The study also opens reflection on the relevance of storage bag material and the possibility of more nuanced recommendations in contexts where conventional thawing methods are difficult to apply.

Major Methodological Limitations

This is a pilot study with 35 samples, which limits the generalisability of results and the detection of more subtle effects. Furthermore, the initial thawing under running water, re-freezing, and subsequent microwave exposure does not exactly replicate the standard clinical scenario (a single freeze/thaw cycle), complicating direct extrapolation to home or clinical service practices.
The use of sonication to reduce thermal heterogeneity is not realistic in daily practice, as the authors acknowledge, and the homogenisation used here did not translate into a clear benefit. Moreover, the specific power/duration conditions and sample volume (25 ml) are very particular, whereas families and clinical services use varying volumes and devices of different power ratings, without systematic control of container positioning or bag condition.
The study does not measure bacterial survival or the effects on other bioactive factors (enzymes, cytokines, oligosaccharides, live cells), nor, most importantly, the actual clinical impact on infant health (infectious morbidity, digestive tolerance, growth). The absence of in vivo data means that recommendations for practice change cannot be based solely on these results, particularly for preterm infants or vulnerable neonates.
The study does not discuss in detail the potential consequences of the 20–30% reduction in lactoferrin observed in some groups for populations at high infectious risk. It also does not compare these losses with those induced by other common preparation methods (bottle warmers, prolonged water baths), which would be useful for IBCLCs comparing microwave use to other standard methods.
The study uses only plastic storage bags subsequently heated in the microwave. Yet an increasing number of mothers are turning to glass containers (repurposed jam jars) for environmental and health reasons.

Key Considerations for IBCLC Lactation Consultants

It should be noted that even if the losses in IgA and lactoferrin are considered "clinically non-problematic" by the authors, these values remain approximations from a limited experimental model. Lactoferrin facilitates iron absorption, and one of the challenges of breastfeeding beyond 6 months is precisely the risk of anaemia in the breastfed infant.

In practice, standard safety recommendations should be prioritised (slow thawing, warm water bath, avoiding the microwave) while awaiting clinical studies confirming the safety of microwave use across different infant populations.

REFERENCES

CDC Breast Milk Storage and Preparation. 2025. Accessed 05/12/2025
ABM Clinical Protocol #8: Human Milk Storage Information for Home Use for Full-Term Infants, Revised 2017 and its correction Accessed 05/12/2025
Ito M, Tanaka M, Date M, Miura K, Mizuno K. Possibility of Microwave Thawing of Human Milk: Effects on Milk Composition and Temperature Distribution. Journal of Human Lactation. 2025;41(4):587-596. doi:10.1177/08903344251365640 Accessed 05/12/2025