See “Heating-induced Bacteriological and Biochemical Modifications in Human Donor Milk After Holder Pasteurization” by Gómez de Segura et al on page 197.
Growing scientific evidence indicates the benefits of human milk for appropriate growth and development of a newborn infant: the particular nutrient composition, hormonal and enzymatic components, and anti-infective and growth factors of human milk make it a unique and inimitable nutrient. There is no doubt that human milk is the best nutrient for term infants; emerging evidence in the last several decades also has confirmed substantial benefits of the use of human milk for preterm infants. That is why human milk also is the recommended food for preterm infants (1).
Mother's milk is the first choice for all neonates, including preterm infants, and when it is not available or not sufficient, donor human milk is a valid alternative. Abundant information exists comparing the effects of human milk with formula on clinical outcomes, but until recently the evidence focusing specifically on pasteurized donor human milk has been limited. Recent evidence documents clinical benefits in several important areas (2). The main benefit deriving from the use of donor human milk (vs formula) is the reduction of necrotizing enterocolitis incidence as indicated in 3 recent meta-analyses (3–5). Other proven clinical benefits specifically attributed to the use of donor milk are enhanced feeding tolerance, reduced incidence of bronchopulmonary dysplasia, and improvement of long-term outcomes in premature infants (2,6). Long-term follow-up of randomized clinical trials conducted in the United Kingdom in the 1980s showed that adolescents born preterm and randomly assigned to donor human milk, as the sole diet or in addition to their mothers’ milk, had a less atherogenic lipoprotein profile, lower diastolic arterial blood pressure, and lower risk of insulin resistance at 13 to 16 years of age than those assigned to formula (7–9). Culture-dependent and -independent analyses of human milk have revealed that this biological fluid is a source of live bifidobacteria, staphylococci, streptococci, lactic acid bacteria, and enterobacteria for the infant gut (10–14). Some of these bacterial strains transferred to the infant gut through breast-feeding have a high probiotic potential (15) and may contribute to the protective effect of breast milk against infectious diseases.
As a consequence, fresh mothers’ milk is the first choice not only for term infants but also for preterm infants. In these tiny infants, fresh mothers’ milk, when administered within 24 hours, does not require routine culturing or heat treatment (16). Instead, donor human milk needs to be checked microbiologically and should undergo heat treatment and storage procedures. For human milk banks, pasteurization at 62.5°C for 30 minutes (Holder method) is recommended (6). Holder pasteurization allows a good compromise between microbiological safety and nutritional/biological quality of donor human milk; but it is also well known that this method affects some of the nutritional and biological properties of human milk and eliminates the beneficial microbiota present, resulting in a reduction of some bacteriostatic mechanisms that render milk more susceptible to postheating bacterial contamination, and decreases its nutritional value (2).
The article by Gómez de Segura et al in the present issue of JPGN (17) focuses our attention on the effects of the pasteurization process on the bacteria present in the initial fresh samples of human milk and on potential nutritional damage. These authors emphasize that pasteurization at 62.5°C for 30 minutes leads to the loss of beneficial microbiota present in fresh human milk, and that spore-forming bacteria such as Bacillus cereus may survive the heating process. In fact, in contrast to vegetative cells, B cereus spores can survive different heat treatments, including Holder pasteurization (18). The authors state that “a negative result for Bacillus in a postpasteurization culture does not mean this microorganism is absent; it only means that this species is under the limit of the technique.” A reassuring comment in the discussion is that, because the counts of such strains in the positive milk samples were low, the actual presence of the toxins in donor milk is improbable. Another reassuring result from the study is that Holder pasteurization did not significantly modify the concentration of furosine and lactulose, the biochemical indexes used to evaluate the potential nutritional damage of the heat treatment.
This is a study that further reminds us of the inadequacy of the pasteurization method used today in the majority of human milk banks to treat donated human milk and to make it safe for feeding premature infants. To overcome the limitations of Holder pasteurization, different methodologies of human milk treatment are under investigation.
Rapid pasteurization at 72°C for 5 or 15 seconds is a method that seems superior to Holder pasteurization, reaching a better compromise between microbiological safety and nutritional and biological quality of the milk (6); however, this method requires technological investment and is available only at the industrial level. Recently, our group compared this method with Holder pasteurization and showed that rapid pasteurization at 72°C for 15 seconds better retains the protein profile and some biological components of donor human milk, including IgA, lactoferrin, and lipase (19).
The combination of ultrasound and heat (thermoultrasound) is an emerging food preservation technique that retains higher quantities of bioactive components compared with present thermal pasteurization practices (20). The present experimental system is limited to small volumes and scale-up studies would need to be undertaken. Therefore, further experimental evaluation of the efficacy of this technology is essential before implementing in human milk bank.
High-pressure processing is another method that shows promise as an alternative for the pasteurization of human milk. A study conducted by Viazis et al (21) found that total immunoglobulin A immunoactivity and lysozyme activity were significantly higher in high-pressure processing (400 MPa, <30°C) of human milk compared with pasteurized human milk. Another study by Permanyer et al (22) confirmed the same results, showing that after human milk processing at 400 MPa, 100% of IgA content was preserved in milk whey, whereas only 72% was retained in pasteurized milk whey. The method seems to have clear advantages over Holder pasteurization: it results in a product of improved nutritional quality; it is faster and perhaps more convenient than Holder pasteurization; it can be applied to frozen milk samples and can be used on samples of variable size. This treatment has the potential to be developed into a successful pasteurization method for human milk.
Ohmic heating is a new technology under investigation for heat treatment of human milk. Ohmic heating is an advanced thermal processing method wherein the food material, which serves as an electric resistor, is heated by passing electricity through it. Electrical energy is dissipated into heat, which results in rapid and uniform heating. During conventional thermal processing, significant product quality damage may occur because of slow conduction and conventional heat transfer. Instead, ohmic heating volumetrically heats the entire mass of the food material; thus, the resulting product is of far greater quality. Like thermal processing, ohmic heating inactivates microorganisms by heat. The first experimental trials performed on human milk with a pilot plant facility (Alfa Laval, Parma, Italy) have shown no modification of the protein pattern of human milk at a temperature of 72°C, small changes at a temperature of 78°C, and significant changes at a temperature of 85°C (G.E. Moro, S. Arslanoglu, personal communication). More studies are necessary to evaluate the theoretical advantages of this new technology when used to heat human milk.
Because of the actual limitations of Holder pasteurization for the heat treatment of human milk, there is a need to investigate alternative pasteurization methods that are capable of retaining the bioactivity of a wider range of human milk components with the highest level of microbiological safety, including protection against spore-forming bacteria. New technologies are under study, and the results obtained seem promising.
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