The Effect of Deep Freezing on Human Milk Macronutrients Content

Long-term storage of human milk (HM) requires freezing at low temperatures, the consequences of which upon macronutrients are unclear. The conflicting results of the few studies on freezing-induced changes on HM macronutrients led to different storage recommendations. We examined differences in macronutrients content of HM after long-term storage at -20°C compared with -80°C.

Mature HM samples from 25 mothers of term and preterm infants were collected. Each sample was divided into six aliquots for storage at -20°C and -80°C for 4, 12, and 24 weeks. After thawing and homogenization, energy and macronutrients content were measured using an HM infrared spectroscopy analyzer.

A total of 150 HM samples were available for analysis. Thirteen samples were removed due to calibration errors. The final analysis was performed on 137 samples with validated results. Fat and energy content were consistently higher in the -80°C samples compared with the paired -20°C samples at each time point (p < 0.05). Comparison of the differences in macronutrients content over time (4 versus 24 weeks) revealed a significant loss of fat (0.3 g/100 mL [7.9%], p = 0.001) and energy (2.3 kcal/100 mL [3.3%], p = 0.03) in the -20°C group. Fat and protein were also significantly decreased over time in the -80°C group (by 0.14 g/100 mL [3%], p = 0.009, and 0.06 g/100 mL [6.4%], p = 0.02, respectively).

Long-term storage of HM at -80°C is associated with better fat and energy preservation compared with storage at -20°C. These results may help construct evidence-based recommendations and guidelines for optimal HM storage.