What Holder pasteurization protects—and changes
Holder pasteurization is the clearest example of the central problem in donor human milk banking: one treatment can improve microbiological safety while changing heat-sensitive biology. The question is not whether the treatment is simply good or bad. It is how its temperature–time exposure reduces particular hazards, which parts of milk respond to that exposure, and why the evidence supports a balanced recommendation rather than a claim that treated milk remains biologically equivalent to untreated milk.
The comparison with high-temperature short-time processing, or HTST, makes the trade-off easier to see. Both are heat treatments, but they do not expose milk to the same pattern of temperature and duration. That pattern affects both microbial inactivation and the unfolding, aggregation or activity of sensitive molecules. A treatment decision must therefore consider what the bank needs to control, what it is trying to preserve and how confidently the result has been demonstrated.
The temperature–time bargain
Holder pasteurization, often abbreviated HoP or HP, heats donor human milk to 62.5 °C for 30 minutes. It is the reference method in the recent multi-omics review and is widely used because it provides an established safety layer. WHO reports that heat treatment such as Holder pasteurization, HTST or flash heating can reduce microbial loads to below detectable levels in many studies. For a milk bank serving vulnerable infants, that reduction addresses a serious hazard: contamination can expose recipients to bacteria or viruses at a time when infection may have severe consequences.
The result is not sterilization. WHO’s evidence synthesis notes that some studies still found viable cells after heat treatment and that spores of Bacillus species are not destroyed by it. The review also found that heat treatment below the Holder condition did not consistently inactivate viruses. Those limits matter because the word “pasteurization” can imply a final state in which every biological hazard has been eliminated. The actual claim is narrower: under a defined process, many relevant microbial hazards can be reduced, while some organisms or forms require other controls.
A bank therefore has to ask what kind of microbial problem it is trying to control. A process that works against many non-thermally resistant viruses may not have the same effect on a spore-forming organism. A result described as “below detectable levels” also depends on the sample and the test used; it does not mean that every possible organism has disappeared from every part of every batch. Screening, cold-chain handling and post-treatment testing remain necessary because treatment is one intervention against a defined set of risks, not a universal eraser.
The treatment also acts on more than microbes. A temperature-time exposure transfers heat through a complex liquid containing proteins, enzymes, fats, carbohydrates and minerals. The total thermal burden depends on the temperature, how long the milk remains exposed and how heat moves through the container. Molecules differ in their sensitivity. A heat-stable component may remain close to its original concentration while a more delicate protein changes shape or loses activity under the same exposure.
That is why a shorter, hotter process cannot be judged by temperature alone, and a longer, gentler process cannot be judged by total protein alone.