A Non-Thermal Process for Increasing the Shelf-Life and Preserving Functionality of Donor Human Milk

Abstract

There is strong evidence that in the absence of breastfeeding, access to human milk significantly reduces infant mortality and morbidity rate as well as illnesses in later life compared to infant formula. However, apart from unregulated informal sharing, donor human milk (DHM) has a limited shelf-life of three months in frozen storage, and is only available through mostly hospital-based milk banks. Guidelines generally stipulate that DHM should undergo thermal (‘Holder’) pasteurisation; a process known to have detrimental effects on the functionality of bioactive compounds. The aim of this study was to assess the feasibility of a hybrid non-thermal process for the pasteurisation of DHM to tackle these challenges. To this end freeze-drying followed by gamma irradiation was used to extend DHM shelf-life while preserving the functionality of bioactive compounds. The results of analyses by solid-phase microextraction gas chromatography mass spectrometry and gel electrophoresis demonstrated that the overall changes in the free fatty acids, lipid oxidation products and protein profiles in freeze-dried donor human milk (FD-DHM) were comparable to those found after the commonly used thermal Holder pasteurisation. Complete bacterial inactivation was achieved when FD-DHM powder (moisture <2.2 %) was gamma irradiated at 2 kGy. When gamma irradiation was performed at -78.5 °C, a dose of 2 kGy did not significantly change the fat fraction and overall protein profile of the milk. Lipid oxidation gradually increased with higher doses of gamma irradiation (2 kGy: 0.8 ± 0.7; 5 kGy: 2.9 ± 2.5; 10 kGy: 2.8 ± 1.1; 15 kGy: 4.5 ± 4.0), though lipid oxidation reached significance only at high doses (25 kGy: 6.0 ± 4.2 and 50 kGy: 5.3 ± 2.3) when compared to the natural levels found in raw DHM. Gamma irradiation at -78.5 °C in dry ice compared to room temperature irradiation slightly reduced the production of lipid oxidation products across the dose range. Thus, freeze-drying DHM and gamma irradiation at 2 kGy is as efficient at pathogen reduction as Holder pasteurisation with no obvious detrimental effects on the overall composition of the milk. Human milk naturally inhibits the growth of bacterial inoculants, such as Staphylococcus aureus, Salmonella typhimurium and Escherichia coli. Freeze drying (without gamma irradiation) did not significantly reduce this natural growth inhibition. In contrast, Holder pasteurisation significantly reduced the milk’s natural bacteriostatic effect on S. aureus growth after 6 hours (-19.8 % p=0.01). Gamma irradiated FD-DHM showed a strong bactericidal effect across all doses, with only minimal growth of S. aureus observed after 6 hours incubation. Thus, freeze-drying followed by 2 kGy of gamma irradiation may be a suitable non-thermal option for long-term safe storage of DHM. In summary, freeze-drying appears to be a feasible approach to improve the security of supply of DHM, while gamma irradiation offers specific advantages, including terminal, in sealed packaging pasteurisation of bulk amounts of FD-DHM powder. Freeze-drying followed by low dose gamma-irradiation is an efficient technique for the production of pathogen-free DHM powder that can be used by human milk banks and hospitals as an alternative to Holder pasteurisation for the provision of donor human milk to infants intolerant to or at risk of infant formula-associated illnesses. Such pathogen-free DHM can be easily transported to different places and deployed for humanitarian aid from a strategically maintained emergency reserve.